Salt Fingers


Molecular diffusion, which acts to smooth out gradients in the concentration of the diffusing quantity, is usually thought of as having a stabilizing effect. However, the fact that heat diffuses about one hundred times faster than salt in water allows diffusion to be the cause of a curious type of instability called salt fingers.

Consider the scenario where warm and salty water overlies cool and fresh water. Now suppose a small parcel of fresh water is displaced upward into the salty water. Since the heat diffuses so much faster than the salt, this parcel soon becomes warm while remaining fresh. Thus, it is buoyant relative to the water around it, and it rises even further. That is, the system is unstable to vertical motions. (Note that a similar argument holds for a salty parcel being displaced downwards.) In practice, many of these parcels tend to go unstable at the same time, producing long, finger-like structures. Hence, the name "salt fingers".

Experimental Set-up:

A large beaker (1 litre capacity) was filled to approximately 3/4 capacity with cool (10 C) tap water. Half a gram of salt was dissolved in 200 ml of hot (60 - 75 C) water, and a small amount of dye was added. The hot solution was then carefully poured onto the surface of the cool water in the beaker. Mixing as a result of the pouring action was minimized by pouring into a small floating container with a sponge bottom which allowed the hot solution to seep out onto the surface of the cool water. After a short period of time, salt fingers appeared spontaneously.


See below for images and/or movies of salt fingers captured during the experiment. It was observed that the instability is very sensitive to the amount of salt in the upper layer (i.e. not very much salt is required to make the instability appear, and too much will cause the instability to occur immediately). Also, it was noted that the onset of instability is most easily seen when the water is motionless, and for this the use of the floating sponge-bottomed container was crucial.
Salt Fingering Experiment:
A QuickTime movie of this experiment is available.
... or try an mpeg movie

Experiment performed and written up by:
Paul F. Choboter, Sept. 98,
Thanks go out to Adarsh Mehta for her help in conducting the experiment.
Page constructed by:
Bruce R. Sutherland, Apr. 98,