Down to Friction
Vigorously stirring a suspension of micrometer-sized particles, such as cornstarch in water, can make the flowing fluid so viscous it behaves like a solid. This effect, known as shear thickening, can be a significant problem in the manufacture of colloidal products, causing blockages and costly damage to equipment. Now, experiments are finally able to explain why shear thickening happens.
Researchers have debated the cause of shear thickening for decades. Stirring pushes the particles closer together, which could increase liquid drag because the liquid has to pass through smaller gaps. Alternatively, it could increase friction because the particles are forced into contact and have to rub past each other (see 18 November 2013 Viewpoint).
To disentangle the two possible mechanisms, the groups of Itai Cohen at Cornell University, New York, and Wilson Poon at the University of Edinburgh, UK, used a technique known as shear reversal. This involves stirring a fluid suspension in one direction until it thickens and then measuring the viscosity of the fluid as soon as it is stirred the opposite way. If increased friction between the particles causes shear thickening then the fluid should instantly flow more easily when the stirring direction is reversed and the particles are pulled out of contact with each other. But if drag forces are at play then the viscosity shouldn’t suddenly change when the stirring direction does. That’s because these forces are independent of the direction of fluid flow. In repeated experiments, the authors observed a steep drop in the suspension’s viscosity when they altered the stirring direction—a clear indication that contact forces cause shear thickening.
This research is published in Physical Review Letters.