A capillary microstructure of the wetting front

This article reports the experimental results of a study of the wetting-front microscale structure formed only by capillary forces in homogeneous and random etched glass capillary models. In the homogeneous model, water propagates through the capillary system, evenly filling the capillaries across the direction of flow. Air is trapped by the pinch-off mechanism inside the pore bodies in the form of individual bubbles. The experiments specified three consecutive steps of the pinch-off mechanism, ‘film flow’, ‘snap-off’, and ‘interface movement’. In the random model, both the bypass and pinch-off, forming ‘bypass/cut-off’ mechanism, create residual air structure. Bypass traps air inside large capillary-pore aggregates which are bounded by small-diameter capillaries in where pinch-off traps air in the adjacent pores. An analysis of the residual air distribution versus depth below the surface in the homogeneous and random micromodels made it possible to identify three successive zones, namely a transition zone, a transmission zone, and a wetting-and-front zone. In the transition zone, the residual air content increases with depth from zero to the constant value in the transmission zone where it remains practically constant. The capillary processes within the wetting-and-front combined zone govern air replacement with wetting and formation of the transmission zone.