Critical evaluation of upward wicking tests

Abstract

It has been and still is common practice to use in-plane wicking measurements to evaluate the absorbing power or liquid transport capabilities of fibrous sheet materials [1, 2, 3, 4]. Most versions of test methods used for this purpose start out by dipping one end of a sheet into liquid and monitoring the subsequent upward movement into the sheet, either by following the position of the liquid front or by gravimetric or volumetric changes. If the upward distance traveled by the liquid becomes long enough (in many cases, more than a few centimeters), there can be a noticeable effect of gravity on the flow rate. The ultimate effect, of course, will be when the liquid stops moving completely. At that point, the capillary pressure that raises the liquid is balanced by the effect of gravity, that is, by the weight of raised liquid. Most testing procedures that are based on upward wicking either ignore the effect of gravity or implicitly assume that it will have a similar proportional influence on all materials. This report will demonstrate that such procedures can lead to misleading results and conclusions.