Salt - Fingering Theory

Salt - Fingers Fluxues     |     Salt - Fingering in Shear

Salt-fingering is a form of double-diffusive instability that arises when a fluid is density stratified by 2 components with differing molecular diffusivities. In seawater, temperature diffuses 100 times more rapidly than salt. In parts of the subtropical ocean and elsewhere where warmer saltier water lies over cooler fresher water, displacement of a O(10 cm) size blob of water upward will cause it to warm up while staying fresh. Thus, it becomes lighter than the surrounding fluid and continues to rise. Likewise, a blob of water displaced downward will cool off but remain salty, becoming heavier than the surrounding water and continue to sink. This instability takes the form of up- and downgoing columns of fluid (Fig 1). This phenomena is also thought to occur in the atmospheres of stars and gas giants, in liquid metal alloys and in liquid rock. You can create a demonstration at home using sugar water over salt water.

Fig. 1: Salt finger images in the lab and in numerical simulations.

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