Morphology, propagation dynamics and scaling characteristics of drying fronts in porous media

Effects of surface wettability on the dynamics and morphology of evaporation-induced drying fronts (DF) receding into hydrophilic (HI) or hydrophobic (HO) sands were studied using highly-resolved neutron radiography. A more pronounced pinning-depinning motion (larger steps and longer waiting times) was observed for the DF in HI sand relative to HO, due to weaker capillary pinning forces in the latter. For length scales l smaller than a correlation length , quenched disorder dominates DF roughness expressed by a roughness exponent α = 0.77, whereas for l > ζ the fluctuations induced by the DF motion annealed the disorder and result in α = 0.44. Although wettability suppressed evaporation rates in HO relative to HI sand, differences in the evaporation dynamics and pinning behavior did not alter the DF morphology indicators-the fractal dimension and roughness exponent-which remained similar for the HI and HO sands. Copyright 2012 by the American Geophysical Union.