Scientists at Cornell have captured the microscopic origin of fluid thinning and thickening behaviors using three-dimensional imaging techniques. They observed the dance of the micron-sized suspended particles in space and in real time. They have explained about the properties of cornstarch-water mixture that prevents sinking and paint, which is thin enough to be spread with a brush, but thick enough to stay on a wall.
The study links the change that occurs in liquid viscosity under shear to the direct imaging of motions of the particles. The research team was led by associate professor of physics at Cornell, Itai Cohen. The study aimed to find out the microscopic origin of non-Newtonian fluids, whose viscosities vary according to the speed of shearing. Toothpaste, which is in a solid form inside a tube flows out like a liquid when sheared i.e. when the tube is squeezed.
A snapshot of the configuration of particles suspended in fluid. The colored spheres indicate the presence of hydroclusters, which form at high rates of shear.
The researchers utilized a sensitive force-measuring device and high-speed three-dimensional imaging and tracked the movements of the micron-sized particles that were suspended in fluids. They monitored the thinning and thickening behaviors that occurred under shear. They observed that when the motion of the particles induced thermally could not keep pace with their displacements, the fluids became less viscous.
When particles were forced past each other at high speeds they form clusters and the fluids became more viscous. This phenomenon explained why standing in a cornstarch-water mixture makes a person to sink, while running across does not. The Cornell observations challenge earlier theories about fluid viscosity changes.
Scientists at Cornell directly imaged the fluid layers and measured the viscosity of fluids. They found a comparable amount of delayering and layering and substantially different viscosity changes in the thickening and thinning regimes. They also proved that the main cause of changes in viscosity was not due to layering.
The Cornell findings will aid engineers and scientists in handling biological liquids such as blood and lymph and complex fluids such as pastes, detergents, and paints.