In this paper, we explain how the mixing of two fluids can be enhanced
by hydrodynamic instabilities when the mixing fluids have different
viscosities.
For flows at high velocities, turbulence creates chaotic flow
conditions that get a volume of fluid mixed very quickly (which is why
we stir our coffee to get the sugar dissolved quickly). At low
velocities and in small geometries, however, the flow is laminar and,
typically, mixing occurs very slowly. This is important in nature, for
instance, in flows through porous media, because biological activity
and chemical reactions are limited by how fast the fluids come into
contact. It is also important in engineering applications, especially
in the context of microfluidics, where it is difficult to get the
reactants to mix quickly. Many methods have been proposed to achieve
fast mixing in small devices, but all the strategies explored so far
assume that the fluids to be mixed have the same viscosity. In this
paper, we explain how mixing efficiency can be enhanced when the
fluids are of different viscosities. In that case, the flow is
unstable (through a phenomenon called 'viscous fingering'), which
creates disorder in the flow and leads to faster mixing.
