Sizes and shapes of salt fingers near the marginal state

Experiments were carried out to investigate the onset, size and shape of fingers in a salt–sugar two-component system. The results on the critical Rayleigh number ratio for the marginal state agree well with the prediction of the linear stability theory of Stern (1960). The theory also predicts that the fingers at the marginal state should be as wide as the layer thickness. In our experiments, in a layer of approximately 3·5 cm deep, the fingers, when they appeared, were always long and narrow, with typical lateral dimensions of approximately 1 mm.