Experiments on Salt Fingers in a Hele Shaw Cell

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Science  03 Jan 1986:
Vol. 231, Issue 4733, pp. 39-41
DOI: 10.1126/science.231.4733.39


"Salt fingers" are generated when a slowly diffusing solute overlies a more rapidly diffusing solute in a gravitationally stable fluid. The phenomenon is important in many areas of science; for example, it affects the temperature and salinity structure in the upper half of the ocean, the production of crystals when liquid alloys solidify, and the distribution of properties in magma chambers below the earth's crust. The evolution of a salt finger from initial instability to a fully developed feature is difficult to follow in a fluid because of the close-packed array of cells that is formed. However, when the fluid is contained in a Hele Shaw cell, individual fingers occupy the entire gap width, thereby enabling the experimenter to monitor and record the evolution much more easily. The instability of an array of wide fingers to disturbances that lead to (preferred) smaller scales is documented in a sequence of photographs, and a qualitative, physically consistent argument is offered to explain the instability.