High Quality Content by WIKIPEDIA articles! In physics, Washburn's equation describes capillary flow in porous materials. The equation is derived for capillary flow in a cylindrical tube in the absence of a gravitational field, but according to physicist Len Fisher can be extremely accurate for more complex materials including biscuits (see dunk (biscuit)). Following National biscuit dunking day, some newspaper articles quoted the equation as Fisher's equation. In his paper from 1921 Washburn applies Poiseuille's law for fluid motion in a circular tube.