Dilatant

Dilatant describes a material whose viscosity or stiffness increases with the rate of shear. In rheology, such materials are often called shear-thickening fluids. They contrast with shear-thinning substances, whose viscosity decreases under shear, and with Newtonian fluids, whose viscosity remains constant regardless of shear rate.

The mechanism is typical of dense suspensions of solid particles in a liquid. At low shear, particles slide past each other with lubrication; at higher shear, particle contacts become frictional and transient networks form, impeding flow and causing a sharp rise in viscosity. A common example is a cornstarch-and-water mixture. Dilatant behavior is exploited in some protective gear, where materials stiffen on impact; the exact response depends on particle size, concentration, and the suspending fluid’s properties.

In geotechnical contexts, dilatancy refers to volume increase in dense granular materials when sheared, such as sand. This dilation can lower pore pressure in saturated soils, increasing effective stress and strength, but may also contribute to instability if pore pressures cannot dissipate. The concept relates to the dilatancy angle, which quantifies the rate of volume change with shear.

Etymology: the word derives from Latin dilatare, to enlarge.