Front dynamics and fingering of a driven contact line

Abstract

Using photographic imaging and dye tracking experiments, we scrutinize the dynamics of a contact line when a finite volume of partially wetting fluid is driven by gravity to spread over a slightly inclined dry plane. Unlike spreading mechanisms driven by molecular forces, gravity-driven spreading over a dry plane is shown to possess a characteristic interfacial ‘nose’ that overhangs the contact line when the film thickness is in excess of the capillary length. A unique recirculating vortex exists within the nose front which spreads at speeds corresponding to capillary numbers in excess of 10⁻². Our experiments show that fingering from a gravity-driven straight front occurs when the above nose configuration cannot be sustained across the entire front as the film thins and the apparent contact angle θ reaches π/2. The fingers retain the nose configuration while the remaining segments of the front evolve into a wedge configuration and stop abruptly due to their large resistance to fluid flow. This fingering mechanism is insensitive to fluid wettability, noise or surface heterogeneity. Via matched asymptotics, we obtain accurate estimates of fingering position and speed at θ=π/2 that are in good agreement with measured values. This new mechanism is distinct from other instability and sensitivity fingering mechanisms and can be in play whenever θ of the straight front approaches π/2 from above as the film thins.