EXPERIMENTAL-STUDY OF THE WASHBURN EQUATION FOR LIQUID FLOW IN VERY FINE CAPILLARIES

The Washburn equation describes the flow of a liquid under its own capillary force in a horizontal tube. The equation has been tested and shown to be adequate in the range of capillary radii from 3 to 400 μm, but tests for smaller capillaries have indicated anomalously high flow rates. This communication reports tests of the Washburn equation for capillary radii down to about 0.1 μm for the flow of water in glass capillaries and 0.20 μm for cyclohexane in glass capillaries. No evidence was found for systematic deviations from the Washburn equation by cyclohexane, or for water flowing in capillaries with radii above 0.3 μm. For water flowing in narrower capillaries there is an abrupt decrease in flow rates and bubbles are observed in the flowing liquid. The presence of the bubbles accounts for the decreased flow rates because of the Jamin effect (caused by the difference between advancing and receding contact angles). © 1979.