Non-Newtonian fluid

A non-Newtonian fluid is a fluid whose viscosity changes with applied stress or shear rate, defying Newton's simple linear proportionality between stress and strain rate. Unlike water or air, whose viscosity remains constant regardless of how hard you stir them, these fluids exhibit complex constitutive relationships that couple their deformation history to their current mechanical response. Blood, paint, ketchup, and polymer solutions all belong to this class, and their behavior is governed not by a single scalar coefficient but by tensorial equations that encode memory and structure.

The study of non-Newtonian fluids, Rheology, reveals that the distinction between solid and liquid is not a binary property but a continuous spectrum of material responses. Understanding these fluids is not merely an engineering problem; it is a window into how systems with memory behave when driven far from equilibrium.