Shear thickening is a property of fluids that causes them to stiffen when they undergo a shearing force. This can be a problem in industrial processes such as pouring cement, but the property is increasingly being used to create innovative protective equipment such as body armour.
In a home experiment, slowly stirring a solution of cornflour in water has no effect, but stirring it with more force causes the mixture to harden. Once the force is removed, the material returns to its liquid-like state.
‘If we replace the corn starch with high strength ceramic particles, we can create many new and interesting materials,’ says Norman Wagner, professor of chemical engineering at the University of Delaware. ‘I’ve spent a lot of time trying to work out how to overcome shear thickening. Now, I’m focusing my attention on how to put it to good use.’
When successfully dispersed, these particles can increase the viscosity – or resistance to flow – and produce a highly non-Newtonian fluid. This happens at a critical shear stress. Below this, the material continues to act as a liquid – and, in fact, will often exhibit shear thinning up until the critical point.
‘When you hit the critical shear stress, it becomes almost like a solid,’ says Wagner.
The nature of the particles themselves is important in controlling the properties of the shear thickening fluid (STF) – such as the rate response or speed at which it ‘reacts’ to the force. Attributes like particle size, shape and hardness can have a huge effect on these properties. For instance, using anisotropic particles – which are long and thin – will be more effective than spherical particles, because they have greater interaction with one another and so are more likely to clump together at a molecular level when put under stress. These effects can also be magnified by replacing spherical particles with cube-like particles, for instance, or by choosing hard or surface-coated particles.
Everyday examples where shear thickening could cause problems include squeezing toothpaste out of a tube, or pouring concrete. In these cases – and many others – the problems have largely been overcome because people like Wagner have been set loose on solving them. Other industrial problem areas include spray painting and semiconductor production.
However, a number of applications have emerged in which shear thickening can be an advantage. The most well-known example is its potential role as the basis of lightweight body armour. Current technology tends to be based on Kevlar, which – though light – is relatively bulky. Several research teams, including Wagner’s, are looking into how STFs might be used to enhance this type of armour, in order to make it more lightweight and effective.
It is tempting – but misleading – to think of a thin liquid ‘layer’ within this type of armour. Instead, the STF takes the form of a ‘gel’ that is incorporated into the matrix of the Kevlar. This is usually done by soaking the Kevlar sheets in a diluted form of the STF – after which they are dried in order to drive off the solvent. These Kevlar sheets are now slightly heavier than before – but far more resistant to something like a bullet strike.
STFs are suspensions of solid particles within a fluid. Under the impact of a force, the particles clump into ‘hydroclusters’ – causing the material to stiffen like a solid. In a structure like this body armour, the ‘stiffened’ form of the STF helps to spread the effect of the force – and prevent injury.
Wagner says that armour is not just for the military – and there is interest in developing multi-threat armour for ‘first responders’ such as firefighters and police.
‘It needs to be effective against bullets, but also against shrapnel and knives,’ he says.
Wagner’s spin-off company, STF Technologies, has developed not just materials for body armour, but also for a puncture-resistant surgical glove – in which a thin textile glove is infused with an STF, creating a flexible glove that can withstand puncturing by needlesticks.
However, the uses of STFs could extend way beyond the battlefield and operating room – and even into space where conditions are potentially far more demanding than here on Earth.
‘A bullet travels at hundreds of metres per second but a micrometeorite is much faster,’ Wagner says. ‘It has so much energy that it can vaporise the outer layer of a space suit, and we need to protect against that.’
With this aim in mind, Wagner has been testing samples of a Thermal Micrometeoroid Garment (TMG), which forms the outer protective layer for a space suit. Scientists at the Johnson Space Center have developed a device – a multi-stage gas gun in a large chamber – that fires mm-sized objects at speeds up to 10km/s, in an attempt to mimic the effect of micro-meteorites. The particle hits a sacrificial aluminium shield – the outer surface of the protective garment – which then vaporises, re-condenses, and moves forward as a ‘debris cloud’.
‘We’re engineering the absorber layers that are trying to stop that hot, moving combination of gas and particles,’ he says.
In one series of tests, where the researchers replaced a neoprene-coated nylon inner layer with STF-treated Kevlar, it prevented damage from a particle fired at 3.4km/s.
‘This STF-coated layer is also about half the weight and thickness of current nylon protective technology, which means we could improve protection while making the suit lighter and more flexible,’ he says.
The next experiments will take place later this year ‘in situ’ – outside the International Space Station (ISS), as part of ‘Misse’ (Materials ISS Experiment). The lay-ups will be exposed to the effects of low-earth orbit – which include extreme temperature and low pressure, as well as high levels of radiation and oxygen radicals. ‘We’ll then bring the samples back to Earth, to see if we can make materials that can survive not just low earth orbit, but potentially the Martian atmosphere,’ says Wagner.
The Mars’ research ties into Nasa’s Journey to Mars programme. Among the many areas of research – from new types of rocket propulsion to transfer spacecraft – there are concerns that the Martian landscape will be unbelievably harsh.
‘There is an official Martian simulator, and it uses very sharp rocks and volcanic dirt – which would be very abrasive,’ Wagner says. The goal here is to increase the longevity of the suit. ‘Current suits last for a few hundred hours, but on Mars they would need to last for much longer,’ he says.
The anti-abrasion properties of the suit are already being tested, as it has current relevance: while a micrometeorite can ‘vaporise’ a space suit’s protective layer, it can do the same to a metal hand rail, for instance – turning a smooth surface into a jagged one.
The protective effect that helps to make effective – but light and flexible – body armour and space suits is also ideal for applications such as sports helmets. Wagner points to research from 2015, in which typical helmet foams were compared with those containing STFs. In some cases, the deflection of the foam with STF was around one-third that of the standard foam.
‘That’s a measure of the force that’s being transferred to the head,’ says Wagner.
The team has looked at other types of sports protection – including football shin pads, and various protective gear for sports such as hockey and ice skating. And also on another sports application that looks at the problem of concussion in a different way. Other than a direct blow to the head, another common injury in American Football is whiplash. This is caused by a player being slammed to the ground, and their head being bent back on impact. Players are trained to stiffen their neck muscles in this kind of situation, but do not always respond quickly enough.
‘Even if you’re wearing a helmet, this can cause concussion,’ says Wagner.
In collaboration with Eric Wetzel of the US Army Research Laboratory – who is also the co-developer of the original body armour work – Wagner has been working on a special ‘tether’ that connects the helmet to the torso.
Just as STFs can improve the puncture resistance of armour, so they can modify the properties of rubber. In normal circumstances, the tether - or ‘dynamic ligament’ - will stretch just like a conventional piece of rubber. However, when suddenly stressed, it will stiffen up – and prevent the head from jerking backwards. (The effect is similar to that of a car seat belt – which is easily pulled into position at slow speeds, but remains fixed if jerked suddenly – as it would in an accident.)
‘The tether has 100 times more resistance at high speed,’ says Wagner.
The tether could also be used by paratroopers, he says, as they can also experience this type of injury.
In similar fashion, researchers in Wagner’s team have developed a rate responsive STF that has improved the design of a prosthetic leg. He says that creating a synthetic Achilles tendon – which responds in a precise way – will allow the development of a prosthetic leg that can be used in a wide variety of activities.
‘Often, an amputee will need a different leg for different activities – such as walking, hiking and running,’ he says. ‘The important thing is that this works without the use of robotics or batteries. People need advanced materials that respond to the environment – without the need for external control.’
Building these types of STF-based products relies on being able to predict shear thickening behaviour. This is particularly important in industrial applications – where a sudden ‘seizing up’ of flow in a pipe, for example, could be critical. Now, researchers from Cornell University in the US – in collaboration with researchers at the universities of Edinburgh and Cambridge in the UK – have devised a method of ‘tuning’ shear thickening behaviour, by modifying it during flow.
By applying oscillations perpendicular to the direction of flow, the researchers say they can break up the clusters that cause shear thickening and overcome the effect.
The high frequency, low amplitude oscillations were applied by using a motor to drive an oscillating shaft, dipped into the fluid. In a second experiment, they suspended the rotating gear of a motor into a cornstarch solution – which immediately thickened and caused the motor to stall. When a speaker was turned on – to apply a high frequency, low amplitude signal – the fluid thinned enough to allow the motor to turn again.
‘Overall, the ability of in situ manipulation of shear thickening paves a route towards creating materials whose mechanical properties can be controlled, write the researchers, led by Itai Cohen of Cornell University, in a paper in 2016 in Proceedings of the National Academy of Sciences (doi/10.1073/pnas.1608348113).
The method could be used to prevent blockages during paste extrusion, improve the flow of carbon black in energy storage devices, or improve 3D printing methods, the researchers suggested.
Meanwhile, scientists at the US National Institute of Standards and Technology (NIST) have developed a new model to explain exactly how STFs work – and it combines the results of two competing theories. The researchers say this improved understanding will allow better control of STFs – whether by avoiding or harnessing the effect.
There are currently two conflicting explanations for shear thickening behaviour: one says that contact friction between particles is the dominant mechanism; the second ‘hydrodynamic’ model contends that shear force cause particles to form into ‘hydroclusters’.
‘There is mounting evidence that contact friction plays a dominant role in colloidal shear thickening. However, this is controversial because of contrary evidence,’ write John Royer of NIST and colleagues, in a 2016 paper in Physical Review Letters (doi/10.1103/PhysRevLett.116.188301). The scientists analysed the flow behaviour of a suspension of silica beads, applying a range of stresses from small to large. At low particle concentration, measurements were consistent with the hydrodynamic model. Then, as stress was increased, the model seems to break down. The competing model requires materials to expand once stress passes a threshold – and there is currently no experimental evidence for that.
However, the NIST researchers found that both theories are correct – as there is a smooth ‘transition’ from a hydrodynamics-dominated to a friction-dominated behaviour as shear thickening becomes more pronounced. ‘This transition demonstrates that shear thickening is driven primarily by frictional contacts, with hydrodynamic forces playing a supporting role at lower concentrations of particles, when mixtures are less dense,’ said Royer.
It is hoped this improved understanding of the mechanism of shear thickening should help to tame its curious effects – whether by overcoming them to improve the efficiency of industrial processes, or harnessing them for innovative new applications such as body armour.