A prototype system exhibiting a strong shear-thickening phenomenon is a 50/50 (by volume) mixture of cornstarch and cold water: the suspension appears like a milky liquid at rest but when stepped on briskly it turns semi-solid and can support a grown person’s weight. Similarly, when vibrated, the suspension spouts semi-solid tentacles or can sustain holes. For some fun videos of this, click on the image to the right or see below.
Over the last few years our lab has embarked on a systematic effort to understand the strong shear-thickening behavior observed in dense suspensions, whereby the shear stress jumps up almost discontinuously as a characteristic shear rate is approached. To this end we have been using rheometry to investigate a wide range of different particle types, shapes and particle-solvent combinations, including custom-molded “designer” particles in the 10-200 micron size range made by Joe DeSimone’s group at the University of North Carolina and Liquidia Technologies. A closely related effort focuses on dense suspensions whose solid-like behavior can be switched on and off with electric or magnetic fields. A new development in this area of “smart fluids” are E-field-activated suspensions whose yield stress in the on-state can exceed 100kPa as result of a giant electrorheological, or GER, effect. We developed a new synthesis for dense strontium-titanyl-oxalate-based GER suspensions that achieve stresses of 200kPa at E-fields of 5kV/mm.
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Suspensions are formed by dispersing solid particles in a liquid solvent. In contrast to colloids, the particles in suspensions are sufficiently large so that Brownian motion is not effective in randomizing their movement. Under applied shear, suspensions exhibit a range of behaviors that differ in characteristic ways from ordinary liquids. In particular for dense suspensions, in which the solids are highly concentrated, the behavior can become quite counter-intuitive: rather than exhibiting a constant viscosity when the shear rate is increased (as observed in ordinary liquids such as water), or perhaps becoming thinner (like many other fluids we are familiar with), in suspensions the viscosity can increase, in some cases to the point where the system behaves like a solid and cracks.
