shearthinning

Shearthinning, commonly written as shear thinning or described as pseudoplastic behavior, refers to fluids whose apparent viscosity decreases as the rate of shear increases. At rest or under low shear, such fluids are relatively thick, but they flow more readily when subjected to stirring, pumping, or spreading.

Mechanism often involves alignment and rearrangement of internal structures. In polymer solutions and suspensions, polymer chains or particles align with the flow, disentangle, or break down interactions, reducing flow resistance. This makes the fluid easier to deform at higher shear rates. Mathematical descriptions frequently use the power-law (Ostwald-de Waele) model, where viscosity decreases with shear rate according to eta = K*(gamma_dot)^(n-1) with n<1 for shear thinning; more complex models like Carreau or Herschel-Bulkley can accommodate yield stress fluids as well.

Occurrences and applications: Shear-thinning behavior is common in many materials, including polymer melts, paints, ketchup, mayonnaise, whipped creams, blood, and various cosmetics and foods. In processing and consumer use, it facilitates pumping, coating, and spraying, while viscosity recovers when shear is removed, aiding stability and handling.

Relation to other terms: Shear thinning is distinct from thixotropy, which describes a time-dependent decrease in viscosity under constant shear and gradual recovery when shear ceases. It is also the opposite of shear thickening, where viscosity increases with shear rate.