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From left, Itai Cohen, professor of physics, Ph.D. student Prateek Sehgal and Brian Kirby, the Meinig Family Professor of Engineering in the Sibley School of Mechanical and Aerospace Engineering, use acoustic energy to control the viscosity of shear-thickening materials, which are a class of materials that flow like liquid but solidify when squeezed or sheared quickly.

Acoustic energy harnessed to soften shear-thickening fluids

You won’t be able to hear it, or even see it yet, but Cornell researchers are using ultrasonic waves to manipulate the viscosity of shear-thickening materials, turning solids to slush – and back again.

Their study, “Using Acoustic Perturbations to Dynamically Tune Shear Thickening in Colloidal Suspensions,” was published Sept. 17 in Physical Review Letters.

Doctoral student Prateek Sehgal manipulates the viscosity of shear-thickening materials by using an acoustic transducer – called a piezo – that generates ultrasonic waves.

Shear-thickening fluids are a class of materials that flow like liquid but solidify when squeezed or sheared quickly, such as quicksand and Oobleck, the children’s play slime. Technical applications for the material range from soft body armor and astronaut suits to 3D printing metals and ceramics.

But the shear-thickening process can be uncooperative: The more you manipulate the material, the more it solidifies, which in the case of 3D printing and the manufacture of concrete can lead to gunked-up nozzles and jammed hoppers.

Itai Cohen, professor of physics in the College of Arts and Sciences and the paper’s co-senior author, previously found a way to manipulate – or “tune” – the material by breaking apart the rigid structures or force chains formed by the particles in these suspensions through perpendicular oscillation. But that method proved to be impractical. It isn’t easy, after all, to shake and twist a factory pipe.

So Cohen and Ph.D. student Meera Ramaswamy partnered with Brian Kirby, the Meinig Family Professor of Engineering in the Sibley School of Mechanical and Aerospace Engineering, and Ph.D. student Prateek Sehgal, who have been using acoustic transducers to manipulate micro- and nanoscale particles in Kirby’s lab.

“We started by solving a biological problem, which was how do we use acoustics to separate out different nanoparticles of different sizes?” Kirby said. “This [research] was closely related. Rather than having a very sparse suspension of tiny little particles and trying to move them to a specific spot, now you have a very dense suspension of many particles, and you’re trying to change its material properties.”

Sehgal developed a simple but effective device that consists of a bottom plate with an acoustic transducer – called a piezo – that generates ultrasonic waves.

“When you excite that piezo at a specific frequency and a specific voltage, it emanates the acoustic waves through the bottom plate to the suspension. These acoustic disturbances break the force chains responsible for shear-thickening,” said Sehgal, co-lead author of the paper with Ramaswamy.

“The disturbances you’re inducing are actually really, really tiny, so it doesn’t take much to break the contact forces between the micro-particles,” Cohen said. “This is the key insight that allowed us to think about applying these kinds of perturbations and getting it to work. Basically, any geometry where you have a flow that’s thickened, you can now just slap a piezo on and de-thicken that region. This strategy just opens up the applicability to a much broader range of applications.”

The researchers developed the approach by manipulating particles in substances up to 1.3 mm thick, but because ultrasound waves can propagate long distances in material, Kirby anticipates it being used on pipes as wide as a foot. Potential applications include food processing, particularly for materials that have particulate suspensions like pastes, the manufacture of concrete, as well as the 3D printing of ceramics and metals.

“Typically, the nozzles in 3D printers tend to be really small, and they often get jammed because of the [materials] that are flowing through them,” Ramaswamy said. “You want to really de-thicken or reduce the viscosity of that suspension and unjam it so that it can flow more easily.”

The use of acoustic energy is also a valuable scientific tool for researchers who are studying a material’s thickening behavior and system dynamics. Typically, to study thickening, one needs to start with a relaxed suspension and ramp up the flows. This process, however, can take a long time.

“If I had to shear my material in a rheometer (a device that measures flow properties), I’d have to stop it, let it settle down and then do it again. It would take forever,” Cohen said. “The acoustic waves allow scientists to relax the suspension much more rapidly. Now it’s a much shorter time scale to reset the material properties to their prethickened state, so we can also study the thickening process more efficiently. That in itself is an interesting scientific application.”

The research was supported by the National Science Foundation.

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