The infiltration of water into unsaturated geologic media is an immiscible displacement process that is unstable with respect to gravity and can thus lead to the formation of gravity-driven fingers. Where the geologic media (e.g., rock, soil) is fractured, gravity-driven fingers within the fractures may lead to extremely rapid vertical migration of waterborne contaminants. We designed analog fractures to facilitate the competition between viscous, gravity, and capillary forces that is expected to control finger behavior, then conducted an extended experimental investigation to observe and measure finger behavior. Results show that the spatially variant two-dimensional nature of fracture geometry leads to different behavior than is reported for the related problem of gravity-driven fingers in porous media. Observations of finger behavior are presented, along with a simple scale analysis used to relate the key measures of finger velocity, finger width, and fingertip length. We also present a series of illustrative experiments designed to guide future research.

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