physicsworld--2020-08-26--Intricate sugar printing helps build blood vessels - Physics World

Late one night I found myself on a Pinterest board trying to understand the appeal of isomalt for candy-making and edible cake decorations .
Why ?
A Twitter post announcing the latest research out of an interdisciplinary collaboration had caught my attention : complex networks of blood vessels can be 3D printed using isomalt , a sugar substitute usually used for making candies and decorative edible sculptures .
Such work would open avenues for 3D printing personalized organs , reducing the human organ transplant shortage .
“ Each year , thousands of patients die simply waiting for donor organs to @ @ @ @ @ @ @ , assistant professor at Rice University and the principal investigator leading the study .
“ These critical medical challenges motivate us every day to apply engineering principles to biology to build tissues and organs that can one day solve this challenge .
Our own field of biomanufacturing , though still in its infancy , is rapidly gaining capabilities we couldn ’ t imagine a decade ago.
” Simply put , the researchers wanted to develop complicated vascular systems that can sustain engineered tissues and organs .
And as it turns out , as @ @ @ @ @ @ @ Engineering paper , this is a really complex problem .
The role of blood vessels is deceptively simple : to deliver oxygen and nutrients to tissues and carry away some waste .
But blood vessels are an intricate network of incredibly extensive branching – they even grow in response to certain signals – all of which researchers must consider when 3D printing functioning tissues .
Conventional 3D printing methods aren ’ t suitable for replicating these delicate vascular webs .
Take extrusion 3D printing , for example , in which structures are formed @ @ @ @ @ @ @ through a nozzle .
Similar to the balsawood bridge you built in physics class , blood vessels created with this method will deform or collapse under their own weight unless supported by more 3D-printed material .
Vessel networks that can be made with extrusion printing are therefore limited .
As if this wasn ’ t enough , 3D-printed blood vessels also need to keep engineered tissues alive .
Any 3D-printed tissue will need to be chock-full of living cells to function , and vessels need to deliver oxygen and nutrients to the furthest @ @ @ @ @ @ @ 3D printing can not easily create dense networks of blood vessels , this task is also difficult .
Such engineering challenges required the researchers to rethink the way in which they 3D print tissues and blood vessels .
The researchers realized that they could apply a 3D printing technique already widely used for polymers and metals to build 3D-printed blood vessels that overcome these restrictions .
The technique , called selective laser-sintering , uses a focused laser to melt and fuse small powder grains into pre-defined shapes .
They found that these powdered @ @ @ @ @ @ @ much testing , they 3D printed a sugar template with extensive branching and unsupported overhangs using an isomalt mixture .
The team collaborated with the Nervous System design studio to create the blood vessel network shape designs .
Next , they created a tissue by filling the space around the 3D-printed sugar template with a cell-infused gel .
When this cell-infused gel solidified , the researchers dissolved the template using water and flushed it away , leaving empty channels primed to deliver oxygen and nutrients to cells .
It takes only five minutes @ @ @ @ @ @ @ , compared with hours or days for conventional 3D printing techniques .
This gives researchers time to send liquids containing oxygen and nutrients through the vessels and learn more about how cells function when supplied by these vessels .
“ A major focus for this paper was to build spatial ‘ maps ’ of how cells function based on their position relative to the 3D-printed vascular network , to quantify how effective or efficient a particular vascular design is and better replicate what we see in the body , ” says lead author @ @ @ @ @ @ @ Miller ’ s lab at Rice University .
The researchers ’ 3D printing technique , developed in collaboration with researchers at the University of Washington , Seattle , allowed them to control the engineered tissue environment , including tissue composition , cell types and blood flow , to better understand how cells retain their function over time .
They demonstrated that their 3D-printed blood vessels could keep large and dense engineered liver tissues alive outside the body for at least a week .
What ’ s next for the tech The researchers surmounted @ @ @ @ @ @ @ for this work to become reality .
“ The sugar materials presented major fabrication challenges and required creative solutions , like developing a new kind of powder dispenser and laser control system , ” says Kinstlinger .
“ There were virtually no blueprints for do-it-yourself , open-source selectively laser-sintered systems , and nobody had successfully shown selective laser-sintering with sugars.
” Currently , the researchers are working on a high-precision laser control system that will help them 3D print even smaller vessels ( they can currently print vessels around 300 microns in size @ @ @ @ @ @ @ of engineered tissue and keep cells alive .
There ’ s still much to learn from a biology perspective , including how various types of cells respond to environments engineered with this method .
And , armed by advances in biology research and their 3D printing technique , the researchers hope to extend the size and lifetime of engineered tissues and deploy the technology more widely .
The 3D printing technique might also be useful to create medical imaging test devices like tissue phantoms , for example .
“ Our long-term vision is @ @ @ @ @ @ @ and replacement of damaged organs with new manufactured ones that are perfectly matched to each patient in need , ” says Miller .
“ I am hopeful that efforts like ours will yield manufactured organs for human patients within my lifetime.
” As for me ?
I need to pay an online visit to a local bakery and partake in some sweet treats…for science ’ s sake .

Ian Kinstlinger prepares the selective laser sintering system to print blood vessel templates from a powdered sugar blend. The templates allow bioengineers to make lab-grown tissues that have sufficient blood flow to sustain densely packed cells. (Courtesy: Jeff Fitlow/Rice University)
Late one night I found myself on a Pinterest board trying to understand the appeal of isomalt for candy-making and edible cake decorations.
Why? A Twitter post announcing the latest research out of an interdisciplinary collaboration had caught my attention: complex networks of blood vessels can be 3D printed using isomalt, a sugar substitute usually used for making candies and decorative edible sculptures. Such work would open avenues for 3D printing personalized organs, reducing the human organ transplant shortage.
“Each year, thousands of patients die simply waiting for donor organs to become available,” says Jordan Miller, assistant professor at Rice University and the principal investigator leading the study. “These critical medical challenges motivate us every day to apply engineering principles to biology to build tissues and organs that can one day solve this challenge. Our own field of biomanufacturing, though still in its infancy, is rapidly gaining capabilities we couldn’t imagine a decade ago.”
Simply put, the researchers wanted to develop complicated vascular systems that can sustain engineered tissues and organs. And as it turns out, as the researchers explain in their Nature Biomedical Engineering paper, this is a really complex problem.
3D printing the lifeblood of tissues
The role of blood vessels is deceptively simple: to deliver oxygen and nutrients to tissues and carry away some waste. But blood vessels are an intricate network of incredibly extensive branching – they even grow in response to certain signals – all of which researchers must consider when 3D printing functioning tissues.
Conventional 3D printing methods aren’t suitable for replicating these delicate vascular webs. Take extrusion 3D printing, for example, in which structures are formed by melting strands of material that flow through a nozzle. Similar to the balsawood bridge you built in physics class, blood vessels created with this method will deform or collapse under their own weight unless supported by more 3D-printed material. Vessel networks that can be made with extrusion printing are therefore limited.
As if this wasn’t enough, 3D-printed blood vessels also need to keep engineered tissues alive. Any 3D-printed tissue will need to be chock-full of living cells to function, and vessels need to deliver oxygen and nutrients to the furthest reaches of engineered tissue. Because extrusion 3D printing cannot easily create dense networks of blood vessels, this task is also difficult.
Such engineering challenges required the researchers to rethink the way in which they 3D print tissues and blood vessels.
Printing with sugar
The researchers realized that they could apply a 3D printing technique already widely used for polymers and metals to build 3D-printed blood vessels that overcome these restrictions. The technique, called selective laser-sintering, uses a focused laser to melt and fuse small powder grains into pre-defined shapes. They found that these powdered grains could be sugars, and after much testing, they 3D printed a sugar template with extensive branching and unsupported overhangs using an isomalt mixture. The team collaborated with the Nervous System design studio to create the blood vessel network shape designs.
A sample of blood vessel templates 3D printed using a special blend of powdered sugars. (Courtesy: Brandon Martin/Rice University)
Next, they created a tissue by filling the space around the 3D-printed sugar template with a cell-infused gel. When this cell-infused gel solidified, the researchers dissolved the template using water and flushed it away, leaving empty channels primed to deliver oxygen and nutrients to cells.
It takes only five minutes to generate vascular tissues using this approach, compared with hours or days for conventional 3D printing techniques. This gives researchers time to send liquids containing oxygen and nutrients through the vessels and learn more about how cells function when supplied by these vessels.
Keeping cells alive with laser-welded sugar channels
“A major focus for this paper was to build spatial ‘maps’ of how cells function based on their position relative to the 3D-printed vascular network, to quantify how effective or efficient a particular vascular design is and better replicate what we see in the body,” says lead author Ian Kinstlinger, a graduate student in Miller’s lab at Rice University.
Co-authors Sarah Saxton and Ian Kinstlinger successfully begin their studies of how well 3D-printed blood vessels sustain cells. (Courtesy: Kelly Stevens/University of Washington, Seattle)
The researchers’ 3D printing technique, developed in collaboration with researchers at the University of Washington, Seattle, allowed them to control the engineered tissue environment, including tissue composition, cell types and blood flow, to better understand how cells retain their function over time. They demonstrated that their 3D-printed blood vessels could keep large and dense engineered liver tissues alive outside the body for at least a week.
What’s next for the tech
The researchers surmounted numerous feats of hardware and software engineering for this work to become reality.
“The sugar materials presented major fabrication challenges and required creative solutions, like developing a new kind of powder dispenser and laser control system,” says Kinstlinger. “There were virtually no blueprints for do-it-yourself, open-source selectively laser-sintered systems, and nobody had successfully shown selective laser-sintering with sugars.”
Currently, the researchers are working on a high-precision laser control system that will help them 3D print even smaller vessels (they can currently print vessels around 300 microns in size) so that vessels reach every piece of engineered tissue and keep cells alive.
There’s still much to learn from a biology perspective, including how various types of cells respond to environments engineered with this method.
And, armed by advances in biology research and their 3D printing technique, the researchers hope to extend the size and lifetime of engineered tissues and deploy the technology more widely. The 3D printing technique might also be useful to create medical imaging test devices like tissue phantoms, for example.
“Our long-term vision is an end to the donor organ shortage and replacement of damaged organs with new manufactured ones that are perfectly matched to each patient in need,” says Miller. “I am hopeful that efforts like ours will yield manufactured organs for human patients within my lifetime.”
As for me? I need to pay an online visit to a local bakery and partake in some sweet treats…for science’s sake.


physicsworld--2020-08-25--Good vibrations: acoustic technologies in cars - Physics World

We can ’ t see them .
In fact , we often can ’ t hear them either .
But mechanical vibrations , whether sound or ultrasound , play a key role in modern cars .
They ’ re perhaps most familiar in the form of those ultrasonic sensors that safely guide us into and out of tight spaces when we park or drive off .
But more importantly , the vast majority of internal-combustion engines that power today ’ s vehicles use an adaptation of an ultrasonic transducer invented by the French physicist Paul Langevin in 1916 .
Over @ @ @ @ @ @ @ road vehicles have been fitted with fuel-injection systems , of which roughly a third contain the Langevin-inspired devices .
Relying on the tiny mechanical vibrations of piezoelectric crystals , they inject precise , computer-controlled amounts of fuel into an engine ’ s cylinders .
Indeed , fuel-injection systems have been so successful that cars , lorries and trucks are no longer the polluting monsters they were 50 years ago .
Today ’ s vehicles give off practically no lead , black soot or hazardous un-burnt petrol fumes , though they still emit lots @ @ @ @ @ @ @ .
As a physicist , what fascinates me with fuel-injection systems is that they rely on piezoelectric vibrations , in this case at frequencies of 1–5 kHz .
That ’ s well within the range of human hearing , which extends up to about 20 kHz .
Above that limit lies “ ultrasound ” , though the precise upper frequency you can hear depends on your age and the quality of your ears .
But in my profession , whether a vibration is audible sound or ultrasound is mostly immaterial : the physics @ @ @ @ @ @ @ in mind , let ’ s begin our sonic car journey with a technology that ’ s vital before you ’ ve even started driving .
Look closely at most modern cars and you ’ ll see a few small , round discs about 20 mm in diameter set into the front and rear bumpers .
Hundreds of millions of vehicles have been fitted with these electro-mechanical transducers since they first entered the market in the mid-2000s .
Emitting and receiving ultrasonic pulses , they ’ re echo-locaters , acting not only like @ @ @ @ @ @ @ like their ears .
Pulses of ultrasound travel away from the parking transducer until they bounce off a nearby object .
Some of the reflected energy returns to the transducer where it is converted into electrical signals and then sent for signal processing .
The time , Δt , for a pulse to make this round trip is 2L/c where c is the speed of sound in air ( roughly 330 m/s ) and L is the distance to the object .
With electronic digital timers able to make timing measurements of this @ @ @ @ @ @ @ μs ) , ultrasound sensors are perfect for measuring distances of a metre or two .
Simple and cheap to make , parking transducers are structured a bit like a sandwich ( figure 1 ) .
Metal , usually aluminium , is first extruded to form a flat-bottomed cup with walls about 1 mm thick .
A flat piezoelectric disc made from polarized lead zirconate titanate is then glued to the inside base of the cup .
About 1 mm thick and 1 cm in diameter , the disc has two metal electrodes @ @ @ @ @ @ @ flat surfaces .
The open end of the cup is sealed with a polymer layer that has two metal pins electrically wired to the two electrodes .
Applying an electrical voltage to the transducer ’ s pins deforms the piezoelectric disc , which expands and contracts as it follows the electrical drive voltage ( usually a short burst of sine waves ) .
But there is also a smaller , radial deformation that depends on the material ’ s specific piezoelectric properties .
With the disc glued to the inside of the device @ @ @ @ @ @ @ surface therefore buckles , creating ultrasonic vibrations at a frequency of about 40 kHz .
Sandwich transducers have many resonating frequencies , corresponding to different modes of vibration , just like the skin of a drum .
Manufacturers , however , like to tune the frequency of the driving voltage so that the entire , flat surface of the transducer oscillates in phase , moving up and down like a piston .
This vibration maximizes the electro-acoustic coupling , creating waves that are in phase over a wide area .
In fact , @ @ @ @ @ @ @ roughly equals the diameter of the piezoelectric disc ( 10 mm ) , the sound waves propagate as a hemisphere with a radius expanding at the speed of sound .
Any piezoelectric transducer can also work in reverse , generating an electric charge when deformed by an incoming ultrasonic wave .
Obviously , the transducer can not transmit and receive ultrasound waves at the same time , so it receives waves only when the drive voltage is switched off ( there is no timing problem because the echo arrives many microseconds later ) @ @ @ @ @ @ @ of the transducer is spatially matched to receive planar echo-waves , which the outgoing spherical waves will have become by the time they return .
Sandwich transducers are incredibly common as ultrasonic parking sensors , with 10 or so fitted to a typical car .
They have a maximum range of about 5 m – perfect for parking – while their sealed construction means they can withstand rain and much of the grime that accumulates on road vehicles .
However , their range is too short to detect other vehicles moving at speed @ @ @ @ @ @ @ cameras are used instead for collision-avoidance purposes .
One unexpected benefit of ultrasonic sandwich transducers is that they can ward off rodents , which love nothing better than climbing into car engine bays and gnawing at tasty electrical cables and rubber hoses .
Rats can hear ultrasound up to 60 kHz , while mice have ears that operate up to 100 kHz , making ultrasound transducers installed under your bonnet the ideal , low-cost rodent-scarer .
The animals hear the sound , but we don ’ t .
And you don ’ t @ @ @ @ @ @ @ that well : all you need is a loud , intermittent ultrasound and the rodents will leave your cherished vehicle alone .
So let ’ s assume those ultrasound parking sensors have helped you negotiate your way out of that tight parking space and you ’ re ready to hit the road .
To get moving , you press your foot down on the accelerator pedal , which sends a signal to the car to start burning more fuel .
But for it to combust as efficiently and as effectively as possible , @ @ @ @ @ @ @ just the right amount of air .
In an internal combustion engine , this was originally done with a carburettor – essentially a pipe through which air passes into the engine .
Fuel enters from the side , with a “ float valve ” controlling how much gets in .
Problem is , the valve has to be correctly oriented with respect to gravity .
Turn it upside-down and it won ’ t work at all .
Corner your car too fast and the float valve can be disturbed by strong inertial forces @ @ @ @ @ @ @ and preventing it from generating mechanical power .
This drawback of carburettors applies to aircraft engines too , as Britain ’ s Royal Air Force found to its cost during the Second World War .
Its Spitfire and Hurricane planes were fitted with Merlin engines , which had carburettors that were prone to cutting out when the pilots performed steep dives .
The German Messerschmitt 109 , in contrast , used one of the first fuel-injection systems and could perform dives and manoeuvres under full power .
It was a superior fighting aircraft @ @ @ @ @ @ @ s ability to build new planes faster than the Germans gave Britain the decisive military edge .
Since they entered the car market in the 1970s and 1980s , fuel-injectors have transformed how we drive .
With a traditional carburettor , a classic four-stroke internal combustion engine works by the piston first moving down as air and fuel enters the cylinder .
On the second stroke , the piston moves up to squeeze the mixture , which ignites ( spontaneously in a diesel engine ; with spark plugs in a petrol engine ) @ @ @ @ @ @ @ piston back down again .
Finally , the piston moves back up and expels the burnt gases , each stroke taking half a revolution of the engine ’ s main crank-shaft .
But with individual fuel-injectors for each cylinder , the four strokes are slightly different .
On the first , the piston moves down as air ( rather than an air/fuel mixture ) enters the cylinder .
On the second stroke , the air is compressed and only now – towards the end of this stroke – is the fuel injected .
@ @ @ @ @ @ @ the cylinder down .
Finally the piston moves back up as the gases are expelled .
By injecting fuel towards the end of the second stroke , the engine becomes both more efficient and able to run at a lower temperature .
Driven by increasingly stringent emissions regulations over the last 50 years , fuel-injection systems have now largely replaced carburettors in most road vehicles .
Key to their operation are tiny piezoelectric transducers , which open and close the valve to let precise amounts of fuel into the engine ’ s cylinders @ @ @ @ @ @ @ these fuel-injection systems allow modern cars to run much efficiently and with far less fuel for the same power output .
However , manufacturers can ’ t use the sandwich transducers found in the humble ultrasound parking sensor : they can ’ t generate displacements of more than 0.01 μm in air and are unable to work with liquid fuel , which is far harder to compress than air .
Instead , most high-power ultrasonic transducers in fuel-injection systems are based on the Langevin transducer , which was invented during the First World @ @ @ @ @ @ @ down enemy submarines by detecting the echoes from the transducer itself .
Rather than having a single piezoelectric disc as with the ultrasonic parking sensors , a modern Langevin transducer uses a stack of them sandwiched between two metal rods .
Roughly 5 mm thick and 30 mm in diameter , each lead-zirconate-titanate disc has a cylindrical 10 mm hole in the centre , through which a threaded metal screw passes .
The screw holds the assembly into a single mechanical unit , which vibrates by expanding and contracting along its length ( @ @ @ @ @ @ @ does two main things in a fuel-injection system .
First , it actuates a “ pintle valve ” that allows fuel into a small chamber near to the nozzle at the tip of the injector .
Second , it pumps the fuel out of the small chamber into the car engine ’ s cylinder at high pressure .
In fact , the transducer is clamped at the end farthest from the injection nozzle by the outside body of the injector , meaning one end of the stack is stationary but the other end @ @ @ @ @ @ @ displacement at the tip so that fuel can be pumped into the engine as effectively as possible .
We know that the displacement of the piezoelectric material is proportional to the strain , which depends largely on the applied electric field , E , across each disc .
So to maximize the displacement we need to maximize E = V/d , where V is the voltage across each disc and d is the thickness of the disc .
The best solution is to use lots of thin discs rather than one , single @ @ @ @ @ @ @ the same overall thickness .
The multiple thin discs will give the same total deformation but with a fraction of the voltage , which is handy as smaller voltages are easier to generate , safer to use and require thinner insulation .
In practice , most fuel-injectors have between 20 and 100 piezoelectric discs in a stack , with each disc being 10–20 mm in diameter and just 0.5 mm thick .
They can achieve displacements of about 10 μm with drive voltages of about 150 V and boast a relatively low mechanical @ @ @ @ @ @ @ they do not resonate and you can get a single mechanical pulse rather than a packet of pulses as you would with a conventional Langevin stack .
In simple systems , cars use just one injector to fire fuel into the air-intake manifold , ahead of the air-intake valves for each cylinder , thereby directly replacing a carburettor .
But many cars now have one injector per cylinder , each delivering a precisely optimized amount of fuel to each cylinder to suit the quantity of air received .
Nearly all the fuel gets @ @ @ @ @ @ @ piston – practically eliminating the carbon deposits that would build up were combustion imperfect – and ensuring almost no micro-particles of carbon leave the exhaust .
As for how quickly a fuel injector needs to work , an engine running at top speed turns at about 6000 revolutions per minute , which is 100 revolutions per second .
The injector therefore has to open and close in less than half a cycle of the piston , which would be 5 ms in this example , though typical opening and closing times are usually @ @ @ @ @ @ @ transducers , in contrast , operate at 25–60 kHz and so have a period of oscillation of 15–40 μs , which is much less than the 1 ms needed for fuel injection .
One final trick of a Langevin transducer is that it converts incoming low-pressure fuel into liquid at high pressure .
This not only overcomes the compression pressure in the cylinder but also turns the fuel into a mist of fine droplets roughly 10 μm in diameter .
Much more of the fuel surface is therefore exposed to air , allowing @ @ @ @ @ @ @ The Langevin design does this thanks to its vibrations having a high “ acoustic impedance ” ( density × wave speed ) .
When it comes to delivering fuel to your car engine , fuel injectors can adjust the precise amount in three ways .
First , they control how long the injector is open .
This method is more effective when an engine is running faster ( at low speeds , where strong acceleration is needed , the fuel-pumping action can not sustain a high pressure of fuel for the entire time @ @ @ @ @ @ @ Second , fuel injectors can control the voltage to the piezoelectric stack of discs ( the peak voltage is about 150 V but lower values will narrow the nozzle ’ s aperture , thereby restricting the quantity of fuel entering the cylinder ) .
Finally , fuel injectors can adjust fuel levels by repeatedly pulsing the voltage so the nozzle opens and closes repeatedly within one stroke of the piston .
As a pulsed system , fuel injectors therefore allow the modern internal combustion engine to be highly controlled .
Indeed , some @ @ @ @ @ @ @ how well they can do this by running their engines without a load on a test bed and programming them to play tunes such as “ God Save the Queen ” .
The digital control of the engine , provided by the fuel-injectors , allows the engine speed – and hence the audible musical note heard from the engine – to be synthesized precisely .
It ’ s a gimmick , but impressive nonetheless .
In reality , your car has its own computer system to calculate how much fuel to inject – @ @ @ @ @ @ @ , the “ engine-control unit ” collects data from several sensors on the engine and exhaust as well as the accelerator pedal , which it then compares with a table of values it holds to find what fuel-injector settings to use .
Manufacturers can therefore precisely control their car engines to give drivers a choice of operation – such as economy , sport , race-track , engine longevity and low-pollution – or to meet specific pollution-monitoring tests .
Clearly , some of these modes conflict with one another or are even mutually exclusive @ @ @ @ @ @ @ not fit well with sport or race-track modes , while the desire to maximize performance while beating pollution-monitoring tests can also conflict , as one large car manufacturer ( Volkswagen ) has found to its cost .
But we should not lose sight of the fact that fuel-injection systems have transformed motoring .
Apart from letting internal-combustion engines run more efficiently and more reliably , they use less fuel and cut pollution compared with carburettors .
They also let engines last longer and – by lowering the amount of pollution from exhaust gases @ @ @ @ @ @ @ .
Fuel-injection systems have not , however , cut CO emissions from road vehicles and surely the future of road transport will involve burning hydrogen in internal combustion engines ( producing water instead of CO ) or switching wholesale to electric vehicles .
Both changes would eliminate CO generation , provided that the hydrogen and electricity in each case can be supplied without emitting its own greenhouse gases .
In the meantime , however , we can thank the vibrations of tiny piezoelectric crystals for at least helping us to burn fuel better @ @ @ @ @ @ @ comes from the action of car brakes and tyres than from the exhaust fumes of current internal-combustion engines – a testament to the effectiveness of catalytic converters and to modern fuel-injection systems .
Those vibrations may be small , but their benefits are big .

Physics World, where it appeared under the headline "Good vibrations". Members of the Institute of Physics can enjoy the full issue Taken from the August 2020 issue of, where it appeared under the headline "Good vibrations". Members of the Institute of Physics can enjoy the full issue via the Physics World app
Without mechanical vibrations, a modern car wouldn’t get far down the road – in fact it wouldn’t start. David R Andrews examines the role of sound and ultrasound in fuel injection and parking sensors
Sound solution Fuel-injection systems, which rely on the vibrations of piezoelectric transducers, enable modern cars to burn fuel cleanly and efficiently. (Courtesy: iStock/Pointimages)
We can’t see them. In fact, we often can’t hear them either. But mechanical vibrations, whether sound or ultrasound, play a key role in modern cars. They’re perhaps most familiar in the form of those ultrasonic sensors that safely guide us into and out of tight spaces when we park or drive off. But more importantly, the vast majority of internal-combustion engines that power today’s vehicles use an adaptation of an ultrasonic transducer invented by the French physicist Paul Langevin in 1916.
Over the last 30 years about 1.5 billion road vehicles have been fitted with fuel-injection systems, of which roughly a third contain the Langevin-inspired devices. Relying on the tiny mechanical vibrations of piezoelectric crystals, they inject precise, computer-controlled amounts of fuel into an engine’s cylinders. Indeed, fuel-injection systems have been so successful that cars, lorries and trucks are no longer the polluting monsters they were 50 years ago. Today’s vehicles give off practically no lead, black soot or hazardous un-burnt petrol fumes, though they still emit lots of climate-warming carbon dioxide (CO 2 ).
As a physicist, what fascinates me with fuel-injection systems is that they rely on piezoelectric vibrations, in this case at frequencies of 1–5 kHz. That’s well within the range of human hearing, which extends up to about 20 kHz. Above that limit lies “ultrasound”, though the precise upper frequency you can hear depends on your age and the quality of your ears. But in my profession, whether a vibration is audible sound or ultrasound is mostly immaterial: the physics is all the same. With that in mind, let’s begin our sonic car journey with a technology that’s vital before you’ve even started driving.
Sensing danger
Look closely at most modern cars and you’ll see a few small, round discs about 20 mm in diameter set into the front and rear bumpers. Hundreds of millions of vehicles have been fitted with these electro-mechanical transducers since they first entered the market in the mid-2000s. Emitting and receiving ultrasonic pulses, they’re echo-locaters, acting not only like the vocal cords of bats but also like their ears.
Pulses of ultrasound travel away from the parking transducer until they bounce off a nearby object. Some of the reflected energy returns to the transducer where it is converted into electrical signals and then sent for signal processing. The time, Δt, for a pulse to make this round trip is 2L/c where c is the speed of sound in air (roughly 330 m/s) and L is the distance to the object. With electronic digital timers able to make timing measurements of this kind very accurately (Δt ~ 300–30,000 μs), ultrasound sensors are perfect for measuring distances of a metre or two.
Simple and cheap to make, parking transducers are structured a bit like a sandwich (figure 1). Metal, usually aluminium, is first extruded to form a flat-bottomed cup with walls about 1 mm thick. A flat piezoelectric disc made from polarized lead zirconate titanate is then glued to the inside base of the cup. About 1 mm thick and 1 cm in diameter, the disc has two metal electrodes that have been evaporated onto its two flat surfaces.
The open end of the cup is sealed with a polymer layer that has two metal pins electrically wired to the two electrodes. Applying an electrical voltage to the transducer’s pins deforms the piezoelectric disc, which expands and contracts as it follows the electrical drive voltage (usually a short burst of sine waves). But there is also a smaller, radial deformation that depends on the material’s specific piezoelectric properties. With the disc glued to the inside of the device, the cup’s outer flat surface therefore buckles, creating ultrasonic vibrations at a frequency of about 40 kHz.
1 Parking sensors: the usefulness of ultrasound Ultrasonic parking sensors consist of a metal cup with a piezoelectric disc in the base. Known as a “sandwich transducer”, the structure is shown here with one part cut out for clarity and without its electric pins or the polymer sealant that covers the top. The colours, which are the results of a finite-element computer model, show the displacement of different parts of the structure, ranging from dark blue (zero displacement) to brown (maximum displacement of 10–13 m) at a resonating frequency of 39,006 Hz. The piezoelectric disc, which moves up and down like a piston, has the largest displacement; its zero-displacement position is given by the black wireframe pie. (Courtesy: David Andrews/Cambridge Ultrasonics; iStock/Chesky_W)
Sandwich transducers have many resonating frequencies, corresponding to different modes of vibration, just like the skin of a drum. Manufacturers, however, like to tune the frequency of the driving voltage so that the entire, flat surface of the transducer oscillates in phase, moving up and down like a piston. This vibration maximizes the electro-acoustic coupling, creating waves that are in phase over a wide area. In fact, because their wavelength (8 mm) roughly equals the diameter of the piezoelectric disc (10 mm), the sound waves propagate as a hemisphere with a radius expanding at the speed of sound.
Any piezoelectric transducer can also work in reverse, generating an electric charge when deformed by an incoming ultrasonic wave. Obviously, the transducer cannot transmit and receive ultrasound waves at the same time, so it receives waves only when the drive voltage is switched off (there is no timing problem because the echo arrives many microseconds later). In fact, the plane surface of the transducer is spatially matched to receive planar echo-waves, which the outgoing spherical waves will have become by the time they return.
Sandwich transducers are incredibly common as ultrasonic parking sensors, with 10 or so fitted to a typical car. They have a maximum range of about 5 m – perfect for parking – while their sealed construction means they can withstand rain and much of the grime that accumulates on road vehicles. However, their range is too short to detect other vehicles moving at speed, which is why microwave sensors or cameras are used instead for collision-avoidance purposes.
One unexpected benefit of ultrasonic sandwich transducers is that they can ward off rodents, which love nothing better than climbing into car engine bays and gnawing at tasty electrical cables and rubber hoses. Rats can hear ultrasound up to 60 kHz, while mice have ears that operate up to 100 kHz, making ultrasound transducers installed under your bonnet the ideal, low-cost rodent-scarer. The animals hear the sound, but we don’t. And you don’t even need to measure the return signals that well: all you need is a loud, intermittent ultrasound and the rodents will leave your cherished vehicle alone.
Injection of interest
So let’s assume those ultrasound parking sensors have helped you negotiate your way out of that tight parking space and you’re ready to hit the road. To get moving, you press your foot down on the accelerator pedal, which sends a signal to the car to start burning more fuel. But for it to combust as efficiently and as effectively as possible, the fuel has to be mixed with just the right amount of air.
In an internal combustion engine, this was originally done with a carburettor – essentially a pipe through which air passes into the engine. Fuel enters from the side, with a “float valve” controlling how much gets in. Problem is, the valve has to be correctly oriented with respect to gravity. Turn it upside-down and it won’t work at all. Corner your car too fast and the float valve can be disturbed by strong inertial forces, potentially starving the engine of fuel and preventing it from generating mechanical power.
This drawback of carburettors applies to aircraft engines too, as Britain’s Royal Air Force found to its cost during the Second World War. Its Spitfire and Hurricane planes were fitted with Merlin engines, which had carburettors that were prone to cutting out when the pilots performed steep dives. The German Messerschmitt 109, in contrast, used one of the first fuel-injection systems and could perform dives and manoeuvres under full power. It was a superior fighting aircraft, though in the end Britain’s ability to build new planes faster than the Germans gave Britain the decisive military edge.
Since they entered the car market in the 1970s and 1980s, fuel-injectors have transformed how we drive. With a traditional carburettor, a classic four-stroke internal combustion engine works by the piston first moving down as air and fuel enters the cylinder. On the second stroke, the piston moves up to squeeze the mixture, which ignites (spontaneously in a diesel engine; with spark plugs in a petrol engine). The burning fuel then pushes the piston back down again. Finally, the piston moves back up and expels the burnt gases, each stroke taking half a revolution of the engine’s main crank-shaft.
But with individual fuel-injectors for each cylinder, the four strokes are slightly different. On the first, the piston moves down as air (rather than an air/fuel mixture) enters the cylinder. On the second stroke, the air is compressed and only now – towards the end of this stroke – is the fuel injected. Third, the mixture burns, pushing the cylinder down. Finally the piston moves back up as the gases are expelled. By injecting fuel towards the end of the second stroke, the engine becomes both more efficient and able to run at a lower temperature.
Burning advantages
Driven by increasingly stringent emissions regulations over the last 50 years, fuel-injection systems have now largely replaced carburettors in most road vehicles. Key to their operation are tiny piezoelectric transducers, which open and close the valve to let precise amounts of fuel into the engine’s cylinders. Compared to old-fashioned carburettor engines, these fuel-injection systems allow modern cars to run much efficiently and with far less fuel for the same power output.
However, manufacturers can’t use the sandwich transducers found in the humble ultrasound parking sensor: they can’t generate displacements of more than 0.01 μm in air and are unable to work with liquid fuel, which is far harder to compress than air. Instead, most high-power ultrasonic transducers in fuel-injection systems are based on the Langevin transducer, which was invented during the First World War to help the French navy track down enemy submarines by detecting the echoes from the transducer itself.
Rather than having a single piezoelectric disc as with the ultrasonic parking sensors, a modern Langevin transducer uses a stack of them sandwiched between two metal rods. Roughly 5 mm thick and 30 mm in diameter, each lead-zirconate-titanate disc has a cylindrical 10 mm hole in the centre, through which a threaded metal screw passes. The screw holds the assembly into a single mechanical unit, which vibrates by expanding and contracting along its length (figure 2).
2 Fuel-injection systems: powered by piezoelectrics Fuel-injection systems in cars contain an ultrasonic piezoelectric transducer, the vibrations from which open and close a valve, allowing precise, computer-controlled amounts of fuel to be pumped as droplets into the engine’s combustion chamber. The transducers are based on a design invented by the French physicist Paul Langevin during the First World War to spot echoes from submarines. a This cut-away diagram of a classic “Langevin transducer” has four ceramic piezoelectric discs (black rings) inside a metal rod (purple). The screw running through the centre holds the parts together and compresses the discs, preventing them from shattering. b This finite-element computer model shows the transducer in motion, with the displacement ranging from zero (blue) to a maximum (red). The central region is a node, while the two ends move axially in antiphase in a half-wave mode. c Fuel injectors in cars use an adaptation of this transducer, with many more piezoelectric discs (200 in this example) and much less rod. The discs are connected at the top to a stiff outer casing and at the bottom to a spring (here a polymer ring). As this finite-element computer model shows, the stack moves up and down like a piston with the biggest displacement (~30 nm in this model) at the end near the spring. (Courtesy: David Andrews/Cambridge Ultrasonics)
The Langevin transducer does two main things in a fuel-injection system. First, it actuates a “pintle valve” that allows fuel into a small chamber near to the nozzle at the tip of the injector. Second, it pumps the fuel out of the small chamber into the car engine’s cylinder at high pressure. In fact, the transducer is clamped at the end farthest from the injection nozzle by the outside body of the injector, meaning one end of the stack is stationary but the other end moves.
Manufacturers want is to maximize displacement at the tip so that fuel can be pumped into the engine as effectively as possible. We know that the displacement of the piezoelectric material is proportional to the strain, which depends largely on the applied electric field, E, across each disc. So to maximize the displacement we need to maximize E = V/d, where V is the voltage across each disc and d is the thickness of the disc. The best solution is to use lots of thin discs rather than one, single fat disc even if it is of the same overall thickness. The multiple thin discs will give the same total deformation but with a fraction of the voltage, which is handy as smaller voltages are easier to generate, safer to use and require thinner insulation.
In practice, most fuel-injectors have between 20 and 100 piezoelectric discs in a stack, with each disc being 10–20 mm in diameter and just 0.5 mm thick. They can achieve displacements of about 10 μm with drive voltages of about 150 V and boast a relatively low mechanical “quality factor”, which means they do not resonate and you can get a single mechanical pulse rather than a packet of pulses as you would with a conventional Langevin stack.
In simple systems, cars use just one injector to fire fuel into the air-intake manifold, ahead of the air-intake valves for each cylinder, thereby directly replacing a carburettor. But many cars now have one injector per cylinder, each delivering a precisely optimized amount of fuel to each cylinder to suit the quantity of air received. Nearly all the fuel gets burnt in each power stroke of the piston – practically eliminating the carbon deposits that would build up were combustion imperfect – and ensuring almost no micro-particles of carbon leave the exhaust.
As for how quickly a fuel injector needs to work, an engine running at top speed turns at about 6000 revolutions per minute, which is 100 revolutions per second. The injector therefore has to open and close in less than half a cycle of the piston, which would be 5 ms in this example, though typical opening and closing times are usually 1 ms or less. Conventional Langevin transducers, in contrast, operate at 25–60 kHz and so have a period of oscillation of 15–40 μs, which is much less than the 1 ms needed for fuel injection.
One final trick of a Langevin transducer is that it converts incoming low-pressure fuel into liquid at high pressure. This not only overcomes the compression pressure in the cylinder but also turns the fuel into a mist of fine droplets roughly 10 μm in diameter. Much more of the fuel surface is therefore exposed to air, allowing combustion to be more fully completed. The Langevin design does this thanks to its vibrations having a high “acoustic impedance” (density × wave speed).
Fuel for thought
When it comes to delivering fuel to your car engine, fuel injectors can adjust the precise amount in three ways. First, they control how long the injector is open. This method is more effective when an engine is running faster (at low speeds, where strong acceleration is needed, the fuel-pumping action cannot sustain a high pressure of fuel for the entire time the injector valve is open). Second, fuel injectors can control the voltage to the piezoelectric stack of discs (the peak voltage is about 150 V but lower values will narrow the nozzle’s aperture, thereby restricting the quantity of fuel entering the cylinder). Finally, fuel injectors can adjust fuel levels by repeatedly pulsing the voltage so the nozzle opens and closes repeatedly within one stroke of the piston.
As a pulsed system, fuel injectors therefore allow the modern internal combustion engine to be highly controlled. Indeed, some Formula 1 car manufacturers like to show how well they can do this by running their engines without a load on a test bed and programming them to play tunes such as “God Save the Queen”. The digital control of the engine, provided by the fuel-injectors, allows the engine speed – and hence the audible musical note heard from the engine – to be synthesized precisely. It’s a gimmick, but impressive nonetheless.
In reality, your car has its own computer system to calculate how much fuel to inject – and when. Performing simple tasks quickly, the “engine-control unit” collects data from several sensors on the engine and exhaust as well as the accelerator pedal, which it then compares with a table of values it holds to find what fuel-injector settings to use. Manufacturers can therefore precisely control their car engines to give drivers a choice of operation – such as economy, sport, race-track, engine longevity and low-pollution – or to meet specific pollution-monitoring tests.
Maximized performance Fuel injectors have made combustion engines more efficient and less polluting, as well as making the engines last longer. (Courtesy: iStock/fermate)
Clearly, some of these modes conflict with one another or are even mutually exclusive. Economy, for example, does not fit well with sport or race-track modes, while the desire to maximize performance while beating pollution-monitoring tests can also conflict, as one large car manufacturer (Volkswagen) has found to its cost. But we should not lose sight of the fact that fuel-injection systems have transformed motoring.
Apart from letting internal-combustion engines run more efficiently and more reliably, they use less fuel and cut pollution compared with carburettors. They also let engines last longer and – by lowering the amount of pollution from exhaust gases – extend the lifetime of catalytic converters. Fuel-injection systems have not, however, cut CO 2 emissions from road vehicles and surely the future of road transport will involve burning hydrogen in internal combustion engines (producing water instead of CO 2 ) or switching wholesale to electric vehicles.
Both changes would eliminate CO 2 generation, provided that the hydrogen and electricity in each case can be supplied without emitting its own greenhouse gases. In the meantime, however, we can thank the vibrations of tiny piezoelectric crystals for at least helping us to burn fuel better. In fact, more small-particle pollution comes from the action of car brakes and tyres than from the exhaust fumes of current internal-combustion engines – a testament to the effectiveness of catalytic converters and to modern fuel-injection systems. Those vibrations may be small, but their benefits are big.


physicsworld--2020-08-28--Standing on the outside looking in: X-rays through glass - Physics World

As a medical physicist at a large public trauma hospital in Melbourne , Australia , I ’ m not a frontline worker , but many of my colleagues in radiology are face-to-face with COVID-19 positive or suspected patients .
As staff get more fatigued and confront the many challenges of the pandemic , I wanted to support them by making their work safer and easier .
We heard about a clever technique for taking chest X-rays through glass .
In theory , this allows the X-ray unit to remain outside an isolated patient ’ s room – conserving personal protective @ @ @ @ @ @ @ risk and speeding up the process .
We first saw details about the technique on Twitter .
Also , authors of a 2016 article have explained about imaging and the infection-control aspects for the Ebola virus ( Spectrum 2016 23 18 ) , which seemed equally applicable to COVID-19 .
The first step was to convince our staff this was a safe and reasonable way to take chest X-rays , but it was not easy to find publications that explained the radiation-safety or image-quality aspects of the technique .
This motivated us to @ @ @ @ @ @ @ transmission through glass , safety aspects caused by scattered radiation and the technique parameters that would be most useful .
We ’ ve published our findings recently in Physical and Engineering Sciences in Medicine .
We found that 110 kV and 5.5 mAs were the most commonly used factors to account for the glass and additional distance to the patient ( this could be up to 3 m between the X-ray tube and detector ) .
The glass was typically equivalent to about one half-value-layer ( i.e .
it reduced the intensity of @ @ @ @ @ @ @ and also increased the beam quality or average energy of the X-ray beam ) .
Our radiologists were satisfied with the image quality , with 90 % of images taken through glass considered diagnostic .
With our parameters , the typical radiation dose to patients is the same whether the X-ray is taken through glass or not .
The technique has been established for use in the current COVID-19 pandemic , and we assessed the radiation safety accordingly .
The radiographers and/or nurses do not need to wear a lead apron .
This @ @ @ @ @ @ @ PPE they were already wearing and the infection-control risks associated with sharing lead aprons .
The radiation dose to staff standing 1 m away while a chest X-ray is taken through glass is equivalent to approximately three hours of natural background radiation in Australia ( less than 0.5 µSv ) and is even lower further away .
We encourage staff to maximize their distance from the patient being imaged and also from the backscatter off the glass outside the room .
We have now performed several hundred chest X-rays using this technique .
@ @ @ @ @ @ @ rounds , one radiographer and a nurse tend to stay in the room , while the second radiographer operates the X-ray unit from outside the room .
In the emergency department , where the rooms are not as large , staff leave the room while the X-ray is performed .
This technique should never be used through lead glass .
Staff need to consider whether the technique is appropriate for each individual patient and also the clinical question .
For example , it is not going to be useful where a large patient @ @ @ @ @ @ @ tube position .
In these cases , a supine in-room technique will answer the clinical question more quickly and with less chance of needing to repeat the X-ray .
One of the highlights of implementing this technique was the teamwork and collaborative spirit displayed .
Radiologists , radiographers , physicists , infection-prevention nurses , ICU nurses and researchers all worked together .
Adaptability is key in our response to the pandemic .
This technique strengthens our defence against COVID‑19 .
All of us have stretched resources coping with the pandemic and we hope @ @ @ @ @ @ @ enough information for others to put the “ chest X-ray through glass ” technique into practice .
By the way , in case you are curious about the origin of the first part of the headline for this article , it comes from Cold Chisel ’ s song “ Standing on the Outside ” , which appears in the Australian band ’ s 1980 album called “ East.
” • This article was originally published on AuntMinnieEurope.com © 2020 by AuntMinnieEurope.com .
Any copying , republication or redistribution of AuntMinnieEurope.com content is expressly @ @ @ @ @ @ @ AuntMinnieEurope.com .

As a medical physicist at a large public trauma hospital in Melbourne, Australia, I’m not a frontline worker, but many of my colleagues in radiology are face-to-face with COVID-19 positive or suspected patients. As staff get more fatigued and confront the many challenges of the pandemic, I wanted to support them by making their work safer and easier.
We heard about a clever technique for taking chest X-rays through glass. In theory, this allows the X-ray unit to remain outside an isolated patient’s room – conserving personal protective equipment (PPE), reducing radiographer risk and speeding up the process.
The radiographer inside the cubicle positions the bed close to the glass door and places the digital detector behind the patient for an erect anterior–posterior chest X-ray before stepping away from the patient during the exposure. (Courtesy: ACPSEM)
The radiographer outside the cubicle positions the X-ray tube head close to the glass door and steps laterally away to initiate the X-ray while maintaining visual contact with the patient. The subject in the figure is not a patient but the author of this publication. (Courtesy: ACPSEM)
We first saw details about the technique on Twitter. Also, authors of a 2016 article have explained about imaging and the infection-control aspects for the Ebola virus (Spectrum 2016 23 18), which seemed equally applicable to COVID-19.
The first step was to convince our staff this was a safe and reasonable way to take chest X-rays, but it was not easy to find publications that explained the radiation-safety or image-quality aspects of the technique. This motivated us to answer questions about the magnitude of X-ray transmission through glass, safety aspects caused by scattered radiation and the technique parameters that would be most useful.
An example of how the technique can obtain good image quality (110 kV, 5 mAs). (Courtesy: ACPSEM)
We’ve published our findings recently in Physical and Engineering Sciences in Medicine. We found that 110 kV and 5.5 mAs were the most commonly used factors to account for the glass and additional distance to the patient (this could be up to 3 m between the X-ray tube and detector). The glass was typically equivalent to about one half-value-layer (i.e. it reduced the intensity of the X-ray beam by about 50% and also increased the beam quality or average energy of the X-ray beam).
Our radiologists were satisfied with the image quality, with 90% of images taken through glass considered diagnostic. With our parameters, the typical radiation dose to patients is the same whether the X-ray is taken through glass or not.
The technique has been established for use in the current COVID-19 pandemic, and we assessed the radiation safety accordingly. The radiographers and/or nurses do not need to wear a lead apron. This was important because of the amount of PPE they were already wearing and the infection-control risks associated with sharing lead aprons.
The radiation dose to staff standing 1 m away while a chest X-ray is taken through glass is equivalent to approximately three hours of natural background radiation in Australia (less than 0.5 µSv) and is even lower further away. We encourage staff to maximize their distance from the patient being imaged and also from the backscatter off the glass outside the room.
We have now performed several hundred chest X-rays using this technique. In intensive care unit (ICU) rounds, one radiographer and a nurse tend to stay in the room, while the second radiographer operates the X-ray unit from outside the room. In the emergency department, where the rooms are not as large, staff leave the room while the X-ray is performed. This technique should never be used through lead glass.
Staff need to consider whether the technique is appropriate for each individual patient and also the clinical question. For example, it is not going to be useful where a large patient is semi-erect to confirm the wide-bore nasogastric tube position. In these cases, a supine in-room technique will answer the clinical question more quickly and with less chance of needing to repeat the X-ray.
Zoe Brady is chief physicist in radiology and nuclear medicine at Alfred Health, Melbourne.
One of the highlights of implementing this technique was the teamwork and collaborative spirit displayed. Radiologists, radiographers, physicists, infection-prevention nurses, ICU nurses and researchers all worked together. Adaptability is key in our response to the pandemic.
This technique strengthens our defence against COVID‑19. All of us have stretched resources coping with the pandemic and we hope our “how to” guide provides enough information for others to put the “chest X-ray through glass” technique into practice.
By the way, in case you are curious about the origin of the first part of the headline for this article, it comes from Cold Chisel’s song “Standing on the outside”, which appears in the Australian band’s 1980 album called East.
This article was originally published on AuntMinnieEurope.com © 2020 by AuntMinnieEurope.com. Any copying, republication or redistribution of AuntMinnieEurope.com content is expressly prohibited without the prior written consent of AuntMinnieEurope.com.


physicsworld--2020-08-28--Our perilous planet - Physics World

Our perilous planet - Physics World

“ Comfort ’ s in Heaven , and we are on the Earth , ” says the Duke of York in William Shakespeare ’ s Richard II , a dire bit of philosophy that many writers have touched upon .
Ellen Prager , marine scientist , writer and science adviser to Celebrity Cruises in the Galapagos , gives her own take on the subject in her latest book Dangerous Earth : What We Wish We Knew about Volcanoes , Hurricanes , Climate Change , Earthquakes , and More .
It ’ s a dangerous world out there , she writes @ @ @ @ @ @ @ our planet that we do not understand .
Most of us are going to die due to old age , or common causes such as heart disease , stroke and dementia .
Every so often , though , it is the Earth itself that poses a significant danger to life and limb .
Natural disasters – including earthquakes , tsunamis and hurricanes – garner the world ’ s attention and can even mark the passage of history : the volcanic eruption of Mount Vesuvius ; the 1906 San Francisco earthquake ; the Boxing @ @ @ @ @ @ @ a quarter of a million people .
Despite their enormous destructive power , these natural phenomena are of significant interest to scientists .
They appear possibly predictable but are mostly not ; they ’ re massive in scale , dwarfing the human realm .
A mature hurricane can release about 200 times the total electrical generating capacity of civilization , according to NASA .
In the open ocean a tsunami wave can travel at more than 800 km/h .
Anthropogenic warming is heating the ocean by the energy equivalent of over five Hiroshima @ @ @ @ @ @ @ and rescue communities all over the world learn from each such event , as Prager explains .
Yet there is much we do not know .
She describes what is known about these various cataclysms , usually with specific examples from recent decades , then covers the “ wish-we-knews ” – what scientists and others would like to know .
Indeed , science still can ’ t predict when a volcano will erupt , the exact path a hurricane will take , or the value of climate sensitivity ( the average amount of @ @ @ @ @ @ @ carbon-dioxide levels in the atmosphere double ) .
The latter may be one of the most important numbers in human history , and requires much better understanding of , for example , the dynamics of the 15 melting planetary ice sheets , the rate at which permafrost will melt ; and how the resulting climate change will affect extreme weather , extreme rainfall , corals and marine life , algae blooms and ocean dead zones .
Prager is a strong writer with an excellent narrative sense .
As an Oregonian I know a @ @ @ @ @ @ @ Mount St Helens volcano in the Pacific Northwest of the US , and I was captivated by her telling of the story of the 1991 eruption of Mount Pinatubo in the Philippines .
Although more than 840 people died from the 15 June eruption in the Philippines ’ Luzon Volcanic Arc , the largest in those months and the second-largest of the 20th century , many more were saved by the work of geologists who were closely monitoring the mountain for seismic activity and magma movement .
Some of the best work was @ @ @ @ @ @ @ massive eruptions and pushing local officials to make thorough evacuation plans .
Then , Prager writes , pulling the reader steadily along , “ a defining moment occurs ” .
A geologist named Rick Hoblitt accompanies an air force general on an aerial survey of the mountain .
Hoblitt points out the current hazards and notes how the imminent pyroclastic flow will lead right into the nearby Clark Air Base .
“ The general turns to his colonel and orders an evacuation , ” and the next day 25,000 people leave ; soon @ @ @ @ @ @ @ Within days the main eruption bursts forth and the base is obliterated , but many lives have been saved .
Climate change appropriately features heavily in Prager ’ s book , being the mother of all disasters now and for the next few centuries .
I found the chapter on volcanoes the best in Dangerous Earth , for both science and narration .
The last chapter – on rogue waves , landslides , rip currents and sinkholes – is refreshingly novel , but the final subsection on sharks seems shoehorned in , perhaps @ @ @ @ @ @ @ .
The midsection of the book contains an attractive collection of colour photographs and figures .
Prager ’ s paragraph on coral bleaching – about which , for some reason , I ’ ve always had somewhat of a mental block – is the most succinct , useful explanation I ’ ve ever come across .
Despite its many positives , the book does have issues .
Tornadoes only receive two pages , and then only in the context of hurricanes .
In fact , in the US more people are killed annually @ @ @ @ @ @ @ hurricanes .
In several places in the book , a presumably asymmetric 3D object is specified by only two numbers .
A more serious problem , in my view , is the lack of an index or even a useful notes section .
Each chapter has its own bibliography , but almost no effort was made to link claims , quotations and topics in that chapter with specific sources in the chapter ’ s bibliography .
The reader is left to guess which reference might apply based solely on the paper ’ s @ @ @ @ @ @ @ s title .
It makes the book less useful and turns it into something to be read instead of studied .
This book deserves more .

Physics World. Members of the Institute of Physics can enjoy the full issue Taken from the August 2020 issue of. Members of the Institute of Physics can enjoy the full issue via the Physics World app
Unsafe science There is a lot we still don’t know about natural disasters, such as exactly when a volcano will erupt. (Courtesy: iStock/Beboy_ltd)
“Comfort’s in Heaven, and we are on the Earth,” says the Duke of York in William Shakespeare’s Richard II, a dire bit of philosophy that many writers have touched upon. Ellen Prager, marine scientist, writer and science adviser to Celebrity Cruises in the Galapagos, gives her own take on the subject in her latest book Dangerous Earth: What We Wish We Knew about Volcanoes, Hurricanes, Climate Change, Earthquakes, and More. It’s a dangerous world out there, she writes, and there is still much about our planet that we do not understand.
Most of us are going to die due to old age, or common causes such as heart disease, stroke and dementia. Every so often, though, it is the Earth itself that poses a significant danger to life and limb. Natural disasters – including earthquakes, tsunamis and hurricanes – garner the world’s attention and can even mark the passage of history: the volcanic eruption of Mount Vesuvius; the 1906 San Francisco earthquake; the Boxing Day tsunami in 2004 that killed almost a quarter of a million people.
Despite their enormous destructive power, these natural phenomena are of significant interest to scientists. They appear possibly predictable but are mostly not; they’re massive in scale, dwarfing the human realm. A mature hurricane can release about 200 times the total electrical generating capacity of civilization, according to NASA. In the open ocean a tsunami wave can travel at more than 800 km/h. Anthropogenic warming is heating the ocean by the energy equivalent of over five Hiroshima bombs a second.
Scientific, policy and rescue communities all over the world learn from each such event, as Prager explains. Yet there is much we do not know. She describes what is known about these various cataclysms, usually with specific examples from recent decades, then covers the “wish-we-knews” – what scientists and others would like to know. Indeed, science still can’t predict when a volcano will erupt, the exact path a hurricane will take, or the value of climate sensitivity (the average amount of surface warming the Earth will experience if carbon-dioxide levels in the atmosphere double). The latter may be one of the most important numbers in human history, and requires much better understanding of, for example, the dynamics of the 15 melting planetary ice sheets, the rate at which permafrost will melt; and how the resulting climate change will affect extreme weather, extreme rainfall, corals and marine life, algae blooms and ocean dead zones.
Prager is a strong writer with an excellent narrative sense. As an Oregonian I know a lot about the 1980 eruption of the Mount St Helens volcano in the Pacific Northwest of the US, and I was captivated by her telling of the story of the 1991 eruption of Mount Pinatubo in the Philippines. Although more than 840 people died from the 15 June eruption in the Philippines’ Luzon Volcanic Arc, the largest in those months and the second-largest of the 20th century, many more were saved by the work of geologists who were closely monitoring the mountain for seismic activity and magma movement. Some of the best work was done educating the local population about past massive eruptions and pushing local officials to make thorough evacuation plans.
Then, Prager writes, pulling the reader steadily along, “a defining moment occurs”. A geologist named Rick Hoblitt accompanies an air force general on an aerial survey of the mountain. Hoblitt points out the current hazards and notes how the imminent pyroclastic flow will lead right into the nearby Clark Air Base. “The general turns to his colonel and orders an evacuation,” and the next day 25,000 people leave; soon a total of 200,000 are evacuated. Within days the main eruption bursts forth and the base is obliterated, but many lives have been saved.
Climate change appropriately features heavily in Prager’s book, being the mother of all disasters now and for the next few centuries. I found the chapter on volcanoes the best in Dangerous Earth, for both science and narration. The last chapter – on rogue waves, landslides, rip currents and sinkholes – is refreshingly novel, but the final subsection on sharks seems shoehorned in, perhaps at the suggestion of the publicity department. The midsection of the book contains an attractive collection of colour photographs and figures. Prager’s paragraph on coral bleaching – about which, for some reason, I’ve always had somewhat of a mental block – is the most succinct, useful explanation I’ve ever come across.
Despite its many positives, the book does have issues. Tornadoes only receive two pages, and then only in the context of hurricanes. In fact, in the US more people are killed annually by tornadoes, on average, than hurricanes. In several places in the book, a presumably asymmetric 3D object is specified by only two numbers.
A more serious problem, in my view, is the lack of an index or even a useful notes section. Each chapter has its own bibliography, but almost no effort was made to link claims, quotations and topics in that chapter with specific sources in the chapter’s bibliography. The reader is left to guess which reference might apply based solely on the paper’s, site’s or book’s title. It makes the book less useful and turns it into something to be read instead of studied. This book deserves more.
2020 University of Chicago Press 272pp £20hb


physicsworld--2020-08-27--Molecular Trojan Horse breaches blood–brain barrier - Physics World

Nanoparticles doped with molecules derived from a neurotransmitter can smuggle chemical cargoes across the blood–brain barrier ( BBB ) .
A team at Tufts University in the US created such nanoparticles and used them to deliver a diverse range of therapeutic substances into the brains of mice .
The technique could one day be used to treat neurological conditions such as infections and neurodegenerative disorders , while avoiding the side effects that accompany other methods for penetrating the BBB .
The BBB comprises a layer of endothelial cells that line the brain ’ s blood vessels .
This layer of @ @ @ @ @ @ @ brain from toxins and pathogens that might be circulating in the blood .
Unfortunately , it also keeps out most drugs and other therapeutics , making certain brain diseases difficult to treat by conventional methods of drug delivery .
Finding ways to breach or circumvent the BBB is the subject of active research .
Although some techniques have been shown to deliver drugs with a degree of success , methods that disrupt the barrier by chemical or physical means can simultaneously let other – potentially harmful – substances sneak across alongside the intended @ @ @ @ @ @ @ Feihe Ma , Liu Yang and colleagues report an approach that is more selective .
To develop their technique , the team experimented with three chemicals that are naturally present in the brain : tryptamine , phenethylamine and phenylethanolamine .
These neurotransmitters are members of the special class of molecules that can cross the BBB in a process that is thought to occur via active transport ( rather than passive diffusion ) across the endothelial cells ’ membranes .
Ma , Yang and colleagues coupled the functional amines in the neurotransmitters to lipid @ @ @ @ @ @ @ NT–lipidoids ) that are hydrophobic at one end and hydrophilic at the other .
In an aqueous solution , such amphiphilic molecules bunch together to form spherical particles called micelles that can encapsulate other chemical species within their cores .
By using NT–lipidoids in this process , the researchers created nanoparticles with surfaces studded with the neurotransmitter-derived amines .
When they injected NT–lipidoid nanoparticles carrying fluorescent dye into live mice , the researchers found that the tryptamine-derived formulation ( NT1 ) successfully breached the BBB and accumulated in the animals ’ brains .
@ @ @ @ @ @ @ of the nanoparticles apparently triggered the transport process that lets the neurotransmitter pass into and through endothelial cells .
A mystery that the researchers are currently investigating is why the phenethylamine- and phenylethanolamine-derived formulations failed to achieve the same result .
To test the technique with a more therapeutically relevant cargo , next the team used NT1 nanoparticles to encapsulate amphotericin B , an antifungal drug to which the BBB is usually impermeable .
Like the fluorescent dye , this substance was also carried across the BBB successfully .
However , the researchers @ @ @ @ @ @ @ animals ’ brain tissue was even higher when they added lipidoids that included phenylboronic acid to the mixture .
This component makes the lipidoid more soluble in water , thus decreasing the size of the nanoparticles from 800 nm to 100 nm as a result .
For some treatments , merely getting the drug across the BBB is not enough , however .
Antisense therapy , for example , could be used to limit production of the tau-protein tangles associated with Alzheimer ’ s disease , but the antisense oligonucleotides ( ASOs ) @ @ @ @ @ @ @ into the neurons themselves .
To achieve this , the researchers incorporated into the nanoparticles yet another lipidoid , 306-O12B-3 , which they had previously shown could deliver ASOs into cells in the liver .
They found that nanoparticles that combined NT1 and 306-O12B-3 lipidoids transported the tau ASOs across both the BBB and the membranes of the neurons , yielding a reduction in tau mRNA .
Although the results are promising , the technique is still some way from the clinic .
“ More studies and clinical trials , including pharmacokinetics , @ @ @ @ @ @ @ needed to determine the utility and safety of the technology in humans , ” explains corresponding author Qiaobing Xu .
There are also still fundamental questions about the phenomenon at the heart of the work .
The ineffectiveness of lipidoids derived from phenethylamine and phenylethanolamine is unexplained , but even the way in which the tryptamine-doped nanoparticles work is unclear .
“ We think they cross the BBB by way of a transporter-mediated process , ” says Xu , “ but we don ’ t know the mechanism exactly .
”

In vivo delivery of a gene-editing protein (Cre-recombinase) into the mouse brain, visualized as red fluorescence (scale bar 100 μm). Inset: transmission electron microscopy image of lipid nanoparticles containing Cre-recombinase (scale bar 0.1 μm). (Courtesy: Qiaobing Xu, Tufts University)
Nanoparticles doped with molecules derived from a neurotransmitter can smuggle chemical cargoes across the blood–brain barrier (BBB). A team at Tufts University in the US created such nanoparticles and used them to deliver a diverse range of therapeutic substances into the brains of mice. The technique could one day be used to treat neurological conditions such as infections and neurodegenerative disorders, while avoiding the side effects that accompany other methods for penetrating the BBB.
The BBB comprises a layer of endothelial cells that line the brain’s blood vessels. This layer of cells is selectively permeable and protects the brain from toxins and pathogens that might be circulating in the blood. Unfortunately, it also keeps out most drugs and other therapeutics, making certain brain diseases difficult to treat by conventional methods of drug delivery.
Finding ways to breach or circumvent the BBB is the subject of active research. Although some techniques have been shown to deliver drugs with a degree of success, methods that disrupt the barrier by chemical or physical means can simultaneously let other – potentially harmful – substances sneak across alongside the intended molecules. Writing in Science Advances, Feihe Ma, Liu Yang and colleagues report an approach that is more selective.
Left to right: study authors Feihe Ma, Liu Yang and Qiaobing Xu. (Courtesy: Qiaobing Xu)
To develop their technique, the team experimented with three chemicals that are naturally present in the brain: tryptamine, phenethylamine and phenylethanolamine. These neurotransmitters are members of the special class of molecules that can cross the BBB in a process that is thought to occur via active transport (rather than passive diffusion) across the endothelial cells’ membranes.
Ma, Yang and colleagues coupled the functional amines in the neurotransmitters to lipid chains. This created composite molecules (NT–lipidoids) that are hydrophobic at one end and hydrophilic at the other. In an aqueous solution, such amphiphilic molecules bunch together to form spherical particles called micelles that can encapsulate other chemical species within their cores. By using NT–lipidoids in this process, the researchers created nanoparticles with surfaces studded with the neurotransmitter-derived amines.
When they injected NT–lipidoid nanoparticles carrying fluorescent dye into live mice, the researchers found that the tryptamine-derived formulation (NT1) successfully breached the BBB and accumulated in the animals’ brains. The presence of tryptamine on the surfaces of the nanoparticles apparently triggered the transport process that lets the neurotransmitter pass into and through endothelial cells. A mystery that the researchers are currently investigating is why the phenethylamine- and phenylethanolamine-derived formulations failed to achieve the same result.
To test the technique with a more therapeutically relevant cargo, next the team used NT1 nanoparticles to encapsulate amphotericin B, an antifungal drug to which the BBB is usually impermeable. Like the fluorescent dye, this substance was also carried across the BBB successfully. However, the researchers found that the concentration delivered to the animals’ brain tissue was even higher when they added lipidoids that included phenylboronic acid to the mixture. This component makes the lipidoid more soluble in water, thus decreasing the size of the nanoparticles from 800 nm to 100 nm as a result.
For some treatments, merely getting the drug across the BBB is not enough, however. Antisense therapy, for example, could be used to limit production of the tau-protein tangles associated with Alzheimer’s disease, but the antisense oligonucleotides (ASOs) that produce the beneficial effect must penetrate into the neurons themselves. To achieve this, the researchers incorporated into the nanoparticles yet another lipidoid, 306-O12B-3, which they had previously shown could deliver ASOs into cells in the liver. They found that nanoparticles that combined NT1 and 306-O12B-3 lipidoids transported the tau ASOs across both the BBB and the membranes of the neurons, yielding a reduction in tau mRNA.
Although the results are promising, the technique is still some way from the clinic. “More studies and clinical trials, including pharmacokinetics, pharmacodynamics, toxicity etc, will be needed to determine the utility and safety of the technology in humans,” explains corresponding author Qiaobing Xu.
There are also still fundamental questions about the phenomenon at the heart of the work. The ineffectiveness of lipidoids derived from phenethylamine and phenylethanolamine is unexplained, but even the way in which the tryptamine-doped nanoparticles work is unclear. “We think they cross the BBB by way of a transporter-mediated process,” says Xu, “but we don’t know the mechanism exactly.”


physicsworld--2020-08-28--Water polo’s eggbeater kick defies biomechanics, supercomputer says disposable masks are best - Phys

Water polo’s eggbeater kick defies biomechanics, supercomputer says disposable masks are best - Physics World

Water polo is a gruelling sport and even staying in one place requires the continuous effort of treading water .
To extend their reach for the ball while stationary , players use a kick called the “ eggbeater ” to raise their upper bodies above the surface of the water .
The kick is so called because the legs make large circles in the water .
Now Hideki Takagi of the University of Tsukuba ’ s swimming laboratory and colleagues have discovered that the efficacy of the eggbeater exceeds the predictions of conventional biomechanical theories based on Newton ’ s @ @ @ @ @ @ @ hints that water polo players are actually taking advantage of complex physics , including unstable vortices , to achieve this increased efficiency , ” explains Takagi .
The Japan-based team describes their surprising finding in Sports Biomechanics .
What sort of mask is best for curbing the spread of Covid-10 ?
Researchers in Japan have used the Fugaku supercomputer to compare the efficacy of disposable medical style masks with reusable masks made from woven fabrics – specifically cotton and polyester .
The calculations suggest that the disposable masks , which are made from @ @ @ @ @ @ @ cough droplets that woven masks .
The disposable masks are particularly good at stopping smaller droplets , while the woven masks are not – suggest the calculations .
Fugaku has recently earned the title of the world ’ s most powerful supercomputer and was also used to model the flow of droplets in an office and on a railway carriage .
You can read more about the study in The Guardian .

Complex physics: a water polo goalkeeper uses the eggbeater kick to rise up in the water. (Courtesy: Ryanjo/CC BY-SA 3.0)
Water polo is a gruelling sport and even staying in one place requires the continuous effort of treading water. To extend their reach for the ball while stationary, players use a kick called the “eggbeater” to raise their upper bodies above the surface of the water. The kick is so called because the legs make large circles in the water.
Now Hideki Takagi of the University of Tsukuba’s swimming laboratory and colleagues have discovered that the efficacy of the eggbeater exceeds the predictions of conventional biomechanical theories based on Newton’s laws and hydrodynamics.
“Our study hints that water polo players are actually taking advantage of complex physics, including unstable vortices, to achieve this increased efficiency,” explains Takagi. The Japan-based team describes their surprising finding in Sports Biomechanics.
Mask calculations
What sort of mask is best for curbing the spread of Covid-10? Researchers in Japan have used the Fugaku supercomputer to compare the efficacy of disposable medical style masks with reusable masks made from woven fabrics – specifically cotton and polyester.
The calculations suggest that the disposable masks, which are made from a non-woven polypropylene fabric, stop more cough droplets that woven masks. The disposable masks are particularly good at stopping smaller droplets, while the woven masks are not – suggest the calculations.
Fugaku has recently earned the title of the world’s most powerful supercomputer and was also used to model the flow of droplets in an office and on a railway carriage. You can read more about the study in The Guardian.


physicsworld--2020-08-28--Diamond defects reveal viscous currents in graphene - Physics World

A team led by researchers from Harvard University and the University of Maryland in the US has used defects in diamond to map the magnetic field generated by electrical currents in graphene .
Their experiments reveal that currents in this atomically-thin form of carbon flow like a viscous fluid – a result that could provide fresh insights into the collective behaviour of electrons in strongly-interacting quantum systems .
Graphene has many exceptional electrical properties .
Among them is the fact that , at the point where its conduction and valence bands just touch each other ( the Dirac point ) @ @ @ @ @ @ @ electrons and an equal number of positively-charged holes , rather than electrons alone .
In the present work , Ronald Walsworth , Amir Yacoby and colleagues set out to establish whether these electron-hole plasmas ( or Dirac fluids , as they are also known ) flow smoothly , like electrons travelling through a metallic wire , or unevenly like water running through a pipe .
Since two-dimensional systems like graphene exhibit a one-to-one relationship between current density and magnetic field , Walsworth , Yacoby and colleagues knew they could extract the current flow @ @ @ @ @ @ @ their graphene sample .
To do this , they turned to nitrogen-vacancy ( NV ) centres in diamond .
NV centres are defects that arise when two neighbouring carbon atoms in diamond ’ s crystalline lattice are replaced by a nitrogen atom and an empty lattice site .
They possess numerous quantum properties , including spin and the ability to emit single photons .
Their energy levels are also highly sensitive to magnetic fields – meaning that when an NV centre is excited with a laser pulse , the intensity of the fluorescence @ @ @ @ @ @ @ fluctuations in the local magnetic field .
The team exploited these properties of NV centres in two ways .
First , they ran a nanostructured diamond tip containing a single NV centre across a narrow ( 1-1.5 micron wide ) conducting channel in graphene .
This enabled them to characterize the magnetic field along a narrow line across the graphene device , revealing changes in the pattern of current flow with a spatial resolution of around 50 nm .
While this approach gave them a detailed view of current flow , it was @ @ @ @ @ @ @ use in mapping the flow across the experiment ’ s entire two-dimensional field of view .
To get a wider perspective , the team placed a graphene sample on a diamond sheet that contained many NV centres near its surface .
They then used a camera to image the magnetic field distribution in the sample , as recorded by the fluorescence of the NV centres .
This technique produced a complete , two-dimensional snapshot of the pattern of current flowing through the graphene at a single moment in time .
While the snapshot @ @ @ @ @ @ @ 400 nm ) than the image obtained from a single NV centre , it gives an overview of how the current is flowing in the entire sample , rather than in a single narrow channel , at a given moment .
“ Our two methods have now produced images revealing that the Dirac fluid current density develops a parabolic profile in which flow is fastest at the centre of the graphene sample and decreases at the edges – just as it is for water flowing in a pipe , ” say study lead @ @ @ @ @ @ @ While previous studies had shown that electrons can flow like water in graphene , they add , these other studies failed to confirm the presence of viscous flow in currents composed of Dirac fluids , which are characterized by strong interactions between the component particles .
The researchers also note that they observed the viscous behaviour at room temperature , while previous experiments were restricted to colder temperatures .
“ The study of viscous hydrodynamic flow in highly pure electronic systems ( like graphene ) is important in condensed-matter physics because such behaviour @ @ @ @ @ @ @ strongly-interacting quantum matter in general , ” Ku tells Physics World .
“ Our work opens up the exciting possibility of exploring phenomena like ( hitherto unobserved ) electronic turbulence , which is thought to play a role in the unusual electronic properties of high-temperature superconductors , for example.
” The study , which is detailed in Nature , also shows that magnetic field imaging with NV centres can reveal novel transport phenomena .
This means that NV defects could be used to study other high-quality materials that may display electron hydrodynamics , @ @ @ @ @ @ @ electronic transport behaviour ( such as topological superconductivity ) .
Understanding viscous electronic flow may even allow researchers to exploit this behaviour in new microelectronics devices made from graphene and related materials .

Electrical current in graphene shows fluid-like flow. Courtesy: Walsworth and Yacoby research groups, Harvard and University of Maryland
A team led by researchers from Harvard University and the University of Maryland in the US has used defects in diamond to map the magnetic field generated by electrical currents in graphene. Their experiments reveal that currents in this atomically thin form of carbon flow like a viscous fluid – a result that could provide fresh insights into the collective behaviour of electrons in strongly interacting quantum systems.
Graphene has many exceptional electrical properties. Among them is the fact that, at the point where its conduction and valence bands just touch each other (the Dirac point), it can support currents composed of electrons and an equal number of positively charged holes, rather than electrons alone. In the present work, Ronald Walsworth, Amir Yacoby and colleagues set out to establish whether these electron–hole plasmas (or Dirac fluids, as they are also known) flow smoothly, like electrons travelling through a metallic wire, or unevenly like water running through a pipe.
Mapping the local magnetic field
Since two-dimensional systems like graphene exhibit a one-to-one relationship between current density and magnetic field, Walsworth, Yacoby and colleagues knew they could extract the current flow pattern by mapping the magnetic field near their graphene sample. To do this, they turned to nitrogen-vacancy (NV) centres in diamond.
NV centres are defects that arise when two neighbouring carbon atoms in diamond’s crystalline lattice are replaced by a nitrogen atom and an empty lattice site. They possess numerous quantum properties, including spin and the ability to emit single photons. Their energy levels are also highly sensitive to magnetic fields – meaning that when an NV centre is excited with a laser pulse, the intensity of the fluorescence it emits will change in response to fluctuations in the local magnetic field.
The team exploited these properties of NV centres in two ways. First, they ran a nanostructured diamond tip containing a single NV centre across a narrow (1–1.5 µm wide) conducting channel in graphene. This enabled them to characterize the magnetic field along a narrow line across the graphene device, revealing changes in the pattern of current flow with a spatial resolution of around 50 nm.
While this approach gave them a detailed view of current flow, it was also time-consuming, and therefore impractical for use in mapping the flow across the experiment’s entire 2D field of view. To get a wider perspective, the team placed a graphene sample on a diamond sheet that contained many NV centres near its surface. They then used a camera to image the magnetic field distribution in the sample, as recorded by the fluorescence of the NV centres.
This technique produced a complete, 2D snapshot of the pattern of current flowing through the graphene at a single moment in time. While the snapshot has a lower spatial resolution (around 400 nm) than the image obtained from a single NV centre, it gives an overview of how the current is flowing in the entire sample, rather than in a single narrow channel, at a given moment.
Dirac fluid current density develops parabolic profile
“Our two methods have now produced images revealing that the Dirac fluid current density develops a parabolic profile in which flow is fastest at the centre of the graphene sample and decreases at the edges – just as it is for water flowing in a pipe,” say study lead authors Mark Ku and Tony Zhou. While previous studies had shown that electrons can flow like water in graphene, they add, these other studies failed to confirm the presence of viscous flow in currents composed of Dirac fluids, which are characterized by strong interactions between the component particles. The researchers also note that they observed the viscous behaviour at room temperature, while previous experiments were restricted to colder temperatures.
“The study of viscous hydrodynamic flow in highly pure electronic systems (like graphene) is important in condensed-matter physics because such behaviour is thought to play a role in strongly interacting quantum matter in general,” Ku tells Physics World. “Our work opens up the exciting possibility of exploring phenomena like (hitherto unobserved) electronic turbulence, which is thought to play a role in the unusual electronic properties of high-temperature superconductors, for example.”
The study, which is detailed in Nature, also shows that magnetic field imaging with NV centres can reveal novel transport phenomena. This means that NV defects could be used to study other high-quality materials that may display electron hydrodynamics, as well as other types of exotic electronic transport behaviour (such as topological superconductivity). Understanding viscous electronic flow may even allow researchers to exploit this behaviour in new microelectronics devices made from graphene and related materials.