CAPILLARY DROPLETS EMBEDDED IN A LUBRICATING FILM: ENHANCING ROCK-LIQUID SOLUTE TRANSFER

Fractures in the vadose zone can significantly increase the complexity of mass transport to aquifers. For many years fractures have been investigated for their diverse roles within the vadose zone, such as barriers, enhanced transport paths or collectors of evaporates during dry events. Here we present an aspect of liquid behavior not previously reported that may enhance the transfer of solutes across the matrix-fracture interface during unsaturated transport. This fluid feature is an embedded capillary droplet that evolves when growing waves on free-surface films contact the opposite fracture wall. Once formed, capillary droplets persist, as a separate fluid element, isolated from the surrounding wavy film. Capillary droplets move down gradient at intermediate speeds relative to films and waves, and leave behind in their path a dry continuous patch.

As film passage dilutes solutes along the matrix interface, solute transfer will be driven by diffusion between film and matrix. Waves on the gas-liquid interface can in turn increase exchange by small advective components. However, capillary droplets provide a much greater exchange potential than either flat or wavy films. The increased pressure gradient provided by capillarity in the droplet creates a scenario where they become collectors of liquid and solute. Because of the increased rate of chemical leaching, a potential application of embedded capillary droplets is in either contaminant remediation or in mineral extraction. This is especially valuable when increasing the solute exchange is more important than increasing the volume of fluid transported. For example, in heap-leach methods for gold extraction, an aqueous cyanide solution is sprinkled or dripped onto a heap of ore rock. The solution leaches the gold from the ore as it percolates through the heap. Because the process of dissolution of ore occurs by diffusion of the cyanide solution into the permeable rock, the formation of embedded capillary droplets could play an important role in suctioning dissolved components from the rock matrix.