Evaporation driven Rayleigh-Taylor instability in aqueous polymer solutions

The Rayleigh-Taylor (RT) instability can occur when a heavy fluid rests on top of lighter fluid and is often observed as plumes of the heavier fluid descending through the lighter fluid. This instability has been well described for immiscible and miscible systems, however few studies have focused on polymer solutions, which are important for many applications. Here we study the RT instability in aqueous polymer solutions containing dextran and polyethylene glycol.  By means of optical measurements, we show that unstratified aqueous polymer solutions can spontaneously exhibit the RT instability when exposed to air. We further show that the instability is driven by evaporation of water, which results in the creation of a dense, polymer-rich layer above the bulk fluid.  Hence, unlike in previously reported RT instabilities, this evaporation driven instability is sustained continuously as long as evaporation is present. By varying the diffusivity, viscosity and density of our polymer solutions, we further characterize the onset and evolution of the spontaneous RT instability.