The rate of capillary penetration and the applicability of the washburn equation

A rigorous formulation is presented for the rate of capillary penetration by a liquid, and the resultant differential equation is solved numerically. The computed results explain an apparent anomaly posed by the Washburn equation. It is shown that the Washburn equation would be appropriate for the interpretation of capillary penetration rates for most systems of practical interest. Some printing operations represent a potential exception to this rule.