As a dense suspension of piezoelectric nanoparticles shear-thickens due to a transition from frictionless (gray) to frictional (red) particle–particle interactions, friction-induced piezoelectricity in the contacting particles generates electric charge, which in turn increases the conductance of the surrounding fluid. Image courtesy of Kim et al.If you mix cornstarch and water in the right proportions, you get something that seems not-quite-liquid but also not-quite-solid. This oobleck, as it’s called, 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 understand 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. This allowed them to uncover 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, professor of molecular engineering and co-senior author of a paper on this work in the Proceedings of the National Academy of Sciences. These potential applications include paint that doesn’t clump, liquids that harden into a mold when shaken and wearable protective gear that stiffens when hit.
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 – 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 paper.
To overcome this challenge, Kim collaborated with Rowan, Aaron Esser-Kahn, also a professor in the PME and an expert in piezochemistry, and Heinrich Jaeger, a professor of physics. The team developed a technique for measuring the change in electrical conductance of a particle suspension resulting from the shear force exerted upon it. Then, they suspended piezoelectric nanoparticles in a liquid at such a concentration that the suspension exhibited non-Newtonian properties in the same way as oobleck.
The researchers applied shear force to the top and bottom of the suspension and simultaneously measured the resulting changes to both viscosity and electrical conductance. That let them determine how the particles were interacting as they changed from a more liquid-like to a more solid-like material.
“We found that friction between particles was critical to this transition,” said Kim. “In this concentrated particle solution, there is a tipping point when the friction reaches a certain level and the viscosity abruptly increases.”
Understanding the physical forces at play in a concentrated particle solution is one step toward being able to design new non-Newtonian fluids in the lab. One day, these engineered materials could have customized properties that let scientists control their viscosity through stress. In some instances, this could translate to less clumping and clogging of liquids like paint and concrete. In other cases, it might mean a purposeful hardening of materials when desired.
“For any application, we hope we can eventually determine the ideal combination of solvents and particles and shear conditions to get the properties we want,” said Kim. “This paper might seem like very fundamental research but, in reality, non-Newtonian fluids are everywhere and so this has a lot of applications.”
For now, the researchers are planning to take advantage of the stress-induced piezoelectric activity of their nanoparticle suspensions to design new adaptive and responsive materials that, for example, become stiffer under mechanical force.
This story is adapted from material from the University of Chicago, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.