Liquid Transport Through Fabrics; Wetting and Steady-State Flow

Abstract

When a liquid front is forced upward through a fabric at a constant rate, the pressure measured in the liquid generally goes through first a negative and then a positive change during the interval required to wet through the fabric. Such a re sponse is unexpected if, as is customary, the fabric is considered as an array of cylindrical capillaries with axes in the direc tion of flow. With cylindrical capillaries the pressure could change in such a manner only if there were a change in the sur face energy of the capillary walls. In this paper the observed pressure changes are explained on the basis that the interfiber capillary channels are actually hourglass-shaped, so that the inclination of the fiber walls at the contact line, the meniscus shape, and thus the capillary pressure all change as the liquid front passes through. This analysis applies strictly to systems in equilibrium; for a moving liquid front, an increase in liquid flow velocity could be responsible for an increase in the height of the positive pressure peak (a measure of the resistance of the fabric to being wetted through). Experimental results on a group of fabrics support this prediction.