rheopexi

Rheopexy is a time-dependent shear thickening behavior in some non-Newtonian fluids. In simpler terms, when a rheopectic fluid is subjected to constant shear stress, its viscosity increases over time. This is the opposite of thixotropy, where viscosity decreases with time under constant shear. The phenomenon is often observed in suspensions containing rod-shaped or plate-like particles.

The mechanism behind rheopexy involves the gradual organization and aggregation of these particles under sustained shear. Initially, the particles may be randomly oriented. As shear is applied, they begin to align and form weak structures or networks. This increased structural complexity impedes the flow of the fluid, leading to a rise in viscosity. The rate at which this structural buildup occurs determines the speed of the rheopectic effect.

Examples of fluids that can exhibit rheopectic behavior include certain suspensions of starch granules in water, printer's ink under specific conditions, and some types of gypsum pastes. The effect is sensitive to factors such as particle concentration, shape, and the magnitude of the applied shear stress. Understanding rheopexy is important in various industrial applications, including the formulation of paints, inks, and drilling fluids, where control over viscosity over time is crucial for proper application and performance.