Unstirred coffee gets cold before sugar is diffused to the top. This means heat and sugar diffuse at different rates. Instability is possible even in a system with negative density gradient if the density is controlled by two components that diffuse at different rates. Density gradient in a fluid leads to convection. Double Diffusive Convection is a fluid dynamic phenomena in which convection in fluids is driven by two different density gradients having different rate of diffusion. Two forms of double diffusive instability are:
1. Salt fingers, and
2. Double diffusive convection
Requirements for Double diffusive convection:
1. Fluid should have two or more components with different molecular diffusivities (for example: salinity and temperature)
2. These components make opposing contributions to the vertical density gradient.
Diffusive regime vs finger regime:
Diffusive convection occurs when the fast diffusing component has an unstable distribution. As an example we can think of a system with cold fresh water on top of warm salty water. Here, the fast diffusing component (heat) has an unstable distribution. Finger regime occurs when slow diffusing component has an unstable distribution. For example: warm salty water on top of cold fresh water.