In physics, 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 in 1906.
The simplified Washburn's equation 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 . The contact angle between liquid and solid 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 is sufficiently accurate in many cases when the capillary force is still significantly greater than the gravitational force.
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:
In the derivation of Washburn's equation, the inertia of the liquid is ignored as negligible. This is apparent in the dependence of length to the square root of time, , which gives an arbitrarily large velocity dL/dt for small values of t. An improved version of Washburn's equation, called Bosanquet equation, takes the inertia of the liquid into account.
where the surface tension-to-viscosity ratio represents the speed of ink penetration into the substrate. In reality, the evaporation of solvents limits the extent of liquid penetration in a porous layer and thus, for the meaningful modelling of inkjet printing physics it is appropriate to utilise models which account for evaporation effects in limited capillary penetration.
According to physicist and igNobel prize winner Len Fisher, the Washburn equation can be extremely accurate for more complex materials including biscuits. Following an informal celebration called national biscuit dunking day, some newspaper articles quoted the equation as Fisher's equation.
Novel capillary pump
The flow behaviour in traditional capillary follows the Washburn's equation. Recently, novel capillary pumps with a constant pumping flow rate independent of the liquid viscosity  were developed, which have a significant advantage over the traditional capillary pump (of which the flow behaviour is Washburn behaviour, namely the flow rate is not constant). These new concepts of capillary pump are of great potential to improve the performance of lateral flow test.
- 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. doi:10.1007/bf01461107.
- Bell, J.M. & Cameron, F.K. (1906). "The flow of liquids through capillary spaces". J. Phys. Chem. 10: 658–674. doi:10.1021/j150080a005.
- 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.
- Joachim Schoelkopf , Patrick A. C. Gane, Cathy J. Ridgway, OMYA AG, Oftringen, Switzerland and G. Peter Matthews, "Influence of Inertia on Liquid Absorption into Paper Coating Structures", University of Plymouth, UK
- Oliver, J. F. (1982). "Wetting and Penetration of Paper Surfaces". 200: 435–453. doi:10.1021/bk-1982-0200.ch022. ISSN 1947-5918.
- Leelajariyakul, S.; Noguchi, H.; Kiatkamjornwong, S. (2008). "Surface-modified and micro-encapsulated pigmented inks for ink jet printing on textile fabrics". Progress in Organic Coatings. 62 (2): 145–161. doi:10.1016/j.porgcoat.2007.10.005. ISSN 0300-9440.
- "The 1999 Ig Nobel Prize Ceremony". improbable.com. Improbable Research. Retrieved 2015-10-07.
Len Fisher, discoverer of the optimal way to dunk a biscuit.
- Barb, Natalie (25 November 1998). "No more flunking on dunking". bbc.co.uk. BBC News. Retrieved 2015-10-07.
- Fisher, Len (11 February 1999). "Physics takes the biscuit". Nature. 397: 469. Bibcode:1999Natur.397..469F. doi:10.1038/17203. Retrieved 2015-10-07.
Washburn will be turning in his grave to learn that the media have renamed his work the "Fisher equation".
- Weijin Guo; Jonas Hansson; Wouter van der Wijngaart (2016). "Viscosity Independent Paper Microfluidic Imbibition" (PDF). MicroTAS 2016, Dublin, Ireland.
- Weijin Guo; Jonas Hansson; Wouter van der Wijngaart (2016). "Capillary Pumping Independent of Liquid Sample Viscosity". Langmuir. 32 (48): 12650–12655. doi:10.1021/acs.langmuir.6b03488.