Experimental characterization of hydration and pinning in bentonite clay, a swelling, heterogeneous, porous medium

Abstract

This paper reports experimental results regarding the scaling properties of wetting fronts during the hydration of repacked bentonite clay with a wide range of pore sizes (over five decades). The process of hydration in this swelling, heterogeneous, porous medium was analyzed using a Hele-Shaw hydration cell. In highly disordered media, anomalous diffusion of the hydration interface and the effect of capillary forces are best understood using a volume filling structure model. This concept of porous media also allows the changes in the position of the mean position of the interface and the water volume intake over time to be explained. The change in the mean interface position follows the expression < h(x,t)> proportional to t(y) with gamma being time dependent until pinning occurs. This is the first report of the pinning of a water interface in a natural porous medium as a consequence of the medium heterogeneity and capillary force disorder produced by the randomness of its pore sizes. The hydration process produced interfaces with a characteristic roughness described by an exponent of 0.63. The results obtained from these experiments reveal a phenomenology unexplained by current theoretical models. The process cannot be classified into any known universality class of dynamics.