Doctoral student Prateek Sehgal manipulates the viscosity of shear-thickening materials by using an acoustic transducer – called a piezo – that generates ultrasonic waves. Photo: Jason Koski/Cornell University.Researchers at Cornell University are using ultrasonic waves to manipulate the viscosity of shear-thickening materials, turning solids to slush – and back again. The study is reported in a paper in Physical Review Letters.
Shear-thickening fluids are a class of materials that flow like liquid but solidify when squeezed or sheared quickly. Examples include quicksand and Oobleck, the children's play slime. Technical applications for these materials range from soft body armor and astronaut suits to the 3D printing of 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 and the paper's co-senior author, previously found a way to manipulate – or ‘tune’ – shear-thickening fluids 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.
Cohen and PhD student Meera Ramaswamy partnered with Brian Kirby, professor of engineering, and PhD student Prateek Sehgal, who have been using acoustic transducers to manipulate micro- and nanoscale particles in Kirby's lab. 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," explained 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 1.3mm thick, but because ultrasound waves can propagate over long distances in a material, Kirby anticipates it being used on pipes as wide as a foot. Potential applications include food processing, particularly for materials that comprise particulate suspensions, like pastes, and concrete manufacture, as well as the 3D printing of ceramics and metals.
The use of acoustic energy could also provide a valuable scientific tool for researchers who are studying a material's thickening behavior and system dynamics.
This story is adapted from material from Cornell University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.