Effects of contact angle on fingered flow during non-ponding infiltration into dry sand layers

Gravity-driven fingered flow may readily occur in dry sand layers with small capillary force at the same order of magnitude as gravitational force. Capillary force is related to the contact angle. The present study examined the effects of contact angle on fingered flow development, finger size and fingertip velocity during non-ponding infiltration into dry sand layers. For contact angles of 48-87 degrees, an induction zone always formed in the sand layer, and only one finger developed from the induction zone. Fingertip width reached a minimum value when the contact angle was approximately 70 degrees, and the width and velocity of fingertips were negatively correlated. This finding suggests that a contact angle of approximately 70 degrees has a physical significance for fingered flow to determine fingertip velocity and width. In addition, when the contact angle was 91 degrees, the supplied water did not spontaneously penetrate the sand surface; instead, water drops built up on the surface and two fingers directly formed from the surface. This indicates that 90 degrees is indeed the critical contact angle for water repellency and affects fingered flow development during non-ponding infiltration.