Imbibition of water into a cellulose foam: the kinetics

Abstract

Cellulose foams are representative of many porous engineering solids that can absorb a large quantity of fluid such as water. Experiments are reported to give insight into water rise in cellulose foams and the underlying mechanisms. The water rise characteristic of water height h versus time t displays a distinct knee on a log-log plot; this knee separates an initial regime where h scales as t1/2 from a subsequent regime where h scales as t1/4. The rate of water rise below the knee is consistent with the Washburn law of water rise in a single dominant capillary, and the knee in the h(t) curve suggests that the Jurin height of this large capillary has been attained. Water rise in the foam above the knee of the h(t) curve is interpreted as water rise in a population of small capillaries with a wide range of radius that feed off the dominant capillary. A series of critical experiments support this interpretation, including water rise in inclined columns, and water rise from a limited reservoir of water. A simple analytical model is used to provide a physical explanation for the observations. Additionally, X-ray computer tomography is used to deduce the probability density function of the small capillaries. The experimental findings are in support of the hypothesis that water rise in the cellulose foam is driven by capillary action and not by diffusion.