A SHARP WET FRONT ANALYSIS OF CAPILLARY ABSORPTION INTO N-LAYER COMPOSITE

New analytical results are found for the capillary absorption of a liq uid into a porous (sorptive) composite solid composed of n parallel la yers of arbitrary thickness and material properties. The analysis is b ased on the 'sharp wet front' model (also known as the the Green-Ampt model) and the results are exact for that model. A contact impedance a cting at the interface between lavers is included. The physical approx imations of the model are discussed. The cumulative capillary absorpti on as a function of time is expressed in terms of the sorptivity, perm eability and porosity of the materials of the individual layers. Exper imental data obtained on composite bars of gypsum plaster and plaster/ sand materials agree well with model predictions, although they test t he model over only a limited range of parameter values. For these comp osite specimens, there is no evidence of a hydraulic contact impedance at the interface between layers. An asymptotic expression is given fo r the effective sorptivity of an alternating composite ABAB....