In physics, the Washburn's equation describes capillary flow in a bundle of parallel cylindrical tubes; it is extended with some issues also to imbibition into porous materials. The equation is named after Edward Wight Washburn; also known as Lucas–Washburn equation, considering that Richard Lucas wrote a similar paper three years earlier, or the Bell-Cameron-Lucas-Washburn equation, considering J.M. Bell and F.K. Cameron's discovery of the form of the equation fifteen years earlier.
In case of a fully wettable capillary, it is
where is the time for a liquid of dynamic viscosity and surface tension to penetrate a distance into the capillary whose pore diameter is . In case of a porous materials many issues have been raised both about the physical meaning of the calculated pore diameter  and the real possibility to use this equation for the calculation of the contact angle of the solid.
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. Inserting the expression for the differential volume in terms of the length of fluid in the tube , one obtains
where is the sum over the participating pressures, such as the atmospheric pressure , the hydrostatic pressure and the equivalent pressure due to capillary forces . is the viscosity of the liquid, and is the coefficient of slip, which is assumed to be 0 for wetting materials. is the radius of the capillary. The pressures in turn can be written as
where is the density of the liquid and its surface tension. is the angle of the tube with respect to the horizontal axis. is the contact angle of the liquid on the capillary material. Substituting these expressions leads to the first-order differential equation for the distance the fluid penetrates into the tube :
The Washburn constant may be included in Washburn's equation.
It is calculated as follows:
- Edward W. Washburn (1921). "The Dynamics of Capillary Flow". Physical Review 17 (3): 273. Bibcode:1921PhRv...17..273W. doi:10.1103/PhysRev.17.273.
- Lucas, R. (1918). "Ueber das Zeitgesetz des Kapillaren Aufstiegs von Flussigkeiten". Kolloid Z. 23: 15.
- Bell, J.M. and Cameron, F.K. (1906). "The flow of liquids through capillary spaces". J. Phys. Chem. 10: 658–674.
- Dullien, F. A. L. (1979). Porous Media: Fluid Transport and Pore Structure. New York: Academic Press. ISBN 0-12-223650-5.
- Marco, Brugnara; Claudio, Della Volpe; Stefano, Siboni (2006). "Wettability of porous materials. II. Can we obtain the contact angle from the Washburn equation?". In Mittal, K. L. Contact Angle, Wettability and Adhesion. Mass. VSP.
- Micromeritics, "Autopore IV User Manual", September (2000). Section B, Appendix D: Data Reduction, page D-1. (Note that the addition of 1N/m2 is not given in this reference, merely implied)
- Micromeritics, "A new method of interpolation and smooth curve fitting based on local procedures", Journal of the Association of Computing Machinery (1970). Volume 17(4), pp.589-602.