Salt fingers


To demonstrate the growth and evolution of salt fingers

What Happens:

A tank of water is stratified with a surface layer of warmer water containing a few crystals of flourescein dye. The tank is viewed from the side, and illuminated with a vertical slit of projector light coming through the right side of the tank. The video is shot with a factor of ten timelapse. Green fingers of flourescein colored water form and fall through the clear water.

Physics of the Phenomenon:

This is an example of a double-diffusive instability (Turner, 1973) called salt fingers. In this case, the salt is a flourescein salt, which makes the water "heavy on top" in the salt concentration, but the stratification is kept gravitationally stable by the "warm on top" temperature gradient. The key to the instability is the fact that heat diffuses much more rapidly than salt (hence the term double-diffusion). A downward moving finger of warm saline water (see diagram) cools off via molecular diffusion of heat, and therefore becomes more dense. This provides the downward buoyancy force that drives the finger. Similarly, an upward-moving finger gains heat from the surrounding fingers, becomes lighter, and rises.

The net effect is a vertical exchange of water containing the salt, and hence a downwards salt flux. The heat flux is also downward, but is much smaller since most of the heat diffuses out sideways to adjacent fingers. The combined heat and salt fluxes yield a "density" flux that is downwards. Hence the top layer of water actually becomes less dense over time, and the lower layer becomes more dense. In terms of eddy diffusivities, the effective salt and heat diffusivities are positive (i.e., downgradient), but the density diffusivity is negative -- an upgradient flux!

References: Turner, J.S., Buoyancy Effects in Fluids, Cambridge University Press, Cambridge, U.K.368 pp.,1973.


Movie and text - Barry Ruddick
Digitization of movie - Dave Hebert

Load and run salt fingers movie