A high-speed video analysis of advancing and receding droplet interfaces in simulated rock fractures

While the hydrologic community is generally in agreement that fluid, including contaminants, can travel through fractures in the vadose zone even when the matrix is unsaturated, there is still much to understand about the physics of fluid movement in these fractures. Laboratory investigations have identified three modes of flow using parallel plate geometry: free surface films, droplets (liquid bridges) and rivulets, each mode having different properties (i.e. speed, extent of contact with the substrate, etc). This study examines the role of a film left behind by the receding interface of a droplet; this investigation is relevant to concerns such as determining how much fluid can be absorbed by the rock matrix. Droplet flow through unsaturated rock fractures was modeled experimentally using parallel, sandblasted glass plates and a high-speed camera. Initial thickness of film left behind a droplet moving at terminal velocity was measured as a function of aperture. Rate of volume decrease of the droplet was also recorded. Droplets in larger apertures generally left thicker films, however Poiseuille flow dictates that thicker films move down a surface at a faster average speed. These effects compete to govern how much fluid is left trailing the droplet.