Water rise in a cellulose foam: By capillary or diffusional flow?

Critical experiments and predictive models reveal that water rise through a cellulose foam is initially by capillary rise, followed by non-linear diffusion in the presence of trapping sites. Classical ideas on capillary rise are supported by observations that the Washburn law is obeyed up to the Jurin height. However, water rise continues beyond the Jurin height, and this subsequent phase is diffusion-controlled according to the following evidence: the nature of the quantitative dependence of water rise upon time, the insensitivity of water rise to the direction of gravity, and the fact that the water front continues to rise in the foam after the water reservoir has been removed. Water diffusion occurs through the cellulose fibre network, along with trapping/de-trapping at molecular sites. The diffusion equations are solved numerically, and, upon comparing the predictions with the observed response, values are obtained for the diffusion constant and for the ratio of trap density to lattice density. The diffusion model explains why the drying of a damp foam is a slow process: the emptying of filled traps requires diffusion through an adjacent lattice of low water content. (C) 2018 The Authors. Published by Elsevier Ltd.