Skjærfortykende

Skjærfortykende is a term used in rheology to describe materials whose viscosity increases with increasing shear rate. It corresponds to the English term shear-thickening and to the concept of dilatancy. A skjærfortykende fluid becomes more resistant to flow as the rate of deformation (shear) grows.

In many concentrated suspensions of rigid particles in a liquid, viscosity is relatively low at small shear rates but rises markedly beyond a critical shear rate. This behavior can be continuous (CST), where viscosity increases gradually, or discontinuous (DST), where it rises abruptly and can lead to a temporary jam of the material. The specific response depends on particle concentration, size, shape, surface properties, and the suspending liquid.

Common examples include mixtures like cornstarch in water (often called oobleck) and other dense suspensions of solid particles. The mechanisms behind skjærfortykende behavior include hydrodynamic interactions that promote particle clustering under shear, increases in frictional contacts between particles at higher stresses, and the dilatant effect, where the material’s tendency to expand under shear contributes to the thickening.

Applications and implications include protective gear and impact-damping devices, where a skjærfortykende fluid can harden upon sudden impact. Characterization is typically performed with rheometers that measure viscosity as a function of shear rate or stress, producing flow curves that distinguish CST from DST. Etymology stems from skjær (shear) and fortykende (thickening).