Endre Joachim Mossige : Evaporation driven Rayleigh-Taylor instabilities in aqueous polymer solutions
It is well known that solvent evaporation in evaporating polymer solutions can lead to thermal or Marangoni-type instabilities. Here we demonstrate that another scenario is possible, namely that these solutions can become Rayleigh-Taylor unstable due to the build-up of a dense interfacial layer at the air-liquid interface. We observe the onset of the instability as descending plumes of the interfacial material and use shadowgraph imaging techniques to monitor the onset times of the pluming, tp, at different polymer concentrations, c0. In diluted solutions, we observe two limiting behaviors for tp which depend on the polymer diffusivity. For high diffusivity polymers (low molecular weight) the pluming time scales as (1/c0)^2/3 while in the absence of diffusion (high molecular weight polymers), the pluming time scales as 1/c0. Above a critical concentration, viscosity (nu) delays the growth of the instability which causes tp instead to scale as (nu/c0)^2/3. These scaling results are not restricted to polymer solutions or evaporation driven instabilities, but are transferable to other binary systems undergoing gravity driven instabilities.