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## Washburn's equationIn physics, ## Additional recommended knowledgeIt is where 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 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 where is the sum over the participating pressures, such as the atmospheric pressure *P*_{h}=*h**g*ρ −*l**g*ρsinψ
where ρ is the density of the liquid and γ its surface tension. ψ is the angle of the tube with respect to the horizontal axis. φ 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 solutions of this differential equation are also discussed in this paper. |

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Washburn's_equation". A list of authors is available in Wikipedia. |