University of Chicago Researches Non-Newtonian Fluids

Researchers at the University of Chicago’s Pritzker School of Molecular Engineering (PME) has investigated the fundamental physics of non-Newtonian fluids using piezoelectric nanoparticles. The team discovered the role friction between particles play in causing materials to flip from a fluid to more solid structure. 
 
Findings were published in Proceedings of the National Academy of Sciences. The properties of oobleck and other non-Newtonian fluids change under pressure or stress, but until now scientists have struggled to prove why. 
 
“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 professor and co-senior author of the paper, Barry L. MacLean. 
 
Potential applications from this research includes paint that does not clump, wearable protective gear that stiffens when hit, and liquids that harden into a mold when shaken.
 
Non-Newtonian fluids are known for their viscosity and its changes under stress. For example, shaking a ketchup bottle to make it more pourable. Other materials, like oobleck however, are a concentrated particle suspension that behave reversed; feeling solid while manipulated and collapse when placed down. 
 
To further understand this, scientists have hypothesized how the particles and the molecules that make up materials can interact with each other in different conditions.
 
“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,” said Hojin Kim, the first author of the paper.
 
A team of researchers developed a technique to measure the change in electrical conductance based on the shear force exerted on it. The nanoparticle was then suspended in a liquid in a particular concentration to exhibit non-Newtonian properties similar to oobleck. 
 
Force was applied to both the top and bottom of the liquid and simultaneously measured the resulting changed to the viscosity and the electrical signals. This allowed the team to determine how the particles were interacting as they changed from a more liquid to a more solid-like material.
 
Researchers hope that this will one day lead to engineered materials that could have customized properties, allowing scientists to control viscosity through stress. 
 
This could lead to less clumping and clogging of liquids like concrete and paint, and also lead to more 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 not seem like very fundamental research but in reality, non-Newtonian fluids are everywhere and so this has a lot of applications.”
 
Currently researchers plan to take advantage of the stress-induced piezoelectric activity of their nanoparticle suspensions in order to develop a new adaptive and responsive material that could become stiffer under mechanical force.