Oobleck-like Fluids Contributes to Smart Material Design

Blue non-newtonian fluid rippling
UChicago research on the science behind non-Newtonian fluids – which change viscosity under under stress among other properties – could lead to applications ranging from clump-free paint to wearable protective gear. (Image copyright istockphoto.com)

Oobleck-like Fluids Contributes to Smart Material Design: If you mix cornstarch and water in the right proportions, you get something that seems not-quite-liquid but also not-quite-solid. Oobleck flows and settles like a liquid when untouched, but stiffens when you try to pick it up or stir it with a spoon. The properties of oobleck and other non-Newtonian fluids — including Silly Putty, quicksand, paint and yogurt — change under stress or pressure and scientists have long struggled to prove exactly why.

Now, researchers at the University of Chicago’s Pritzker School of Molecular Engineering (PME) have used piezoelectric nanoparticles, which themselves change in response to pressure, to investigate the fundamental physics of non-Newtonian fluids. The team discovered a key role for friction between particles in causing the materials to flip from a fluid to a more solid structure.

“This not only answers long-standing basic questions about the physical origins of these materials, but opens up doors for the design of new non-Newtonian fluids with practical applications,” said Stuart Rowan, the Barry L. MacLean Professor of Molecular Engineering and co-senior author of the paper, published in Proceedings of the National Academy of Sciences.

Among those potential applications: paint that doesn’t clump, liquids that harden into a mold when shaken, and wearable protective gear that stiffens when hit.

“This…opens up doors for the design of new non-Newtonian fluids with practical applications.”

Prof. Stuart Rowan

Piezoelectric probes

Oobleck-like Fluids Contributes to Smart Material Design: A hallmark of non-Newtonian fluids is that their viscosity — how thick they are — changes dramatically when the materials are under stress. For some materials, this means thinning with stress. Shaking a ketchup bottle can make the condiment drastically more pourable; yogurt, mayonnaise and toothpaste maintain their shape in a container yet become more liquid-like upon use.

But other materials like oobleck, which is a concentrated particle suspension, behave just the opposite: it can feel solid while being manipulated yet collapse into a puddle when placed down.

Scientists have formulated hypotheses about why concentrated particle suspensions change when sheared — being exposed to multiple forces acting in different directions. These hypotheses mostly relate to how the molecules and particles that make up the materials can interact with each other in different ways under different conditions — but each hypothesis is hard to prove.

“To understand these concentrated particle suspensions, we want to be able to look at the nanoscale structure, but the particles are so incredibly crowded together that imaging these structures is very hard,” explained postdoctoral scholar Hojin Kim, the first author of the new paper.