(429e) Large Scale Convection During Phase Separation of Liquid Mixtures

In the second half of 1990’s, Reul Shinnar and co-workers showed that when a liquid mixture is deeply quenched below its critical temperature, it phase separates very rapidly, even in the presence of surface acting agents. On the other hand, when the same mixture is agitated isothermally while in its two-phase state, its settling time greatly increases if surfactants are added. After monitoring the separation process microscopically through a series of direct visualizations, it was concluded that this effect was due to a bulk motion of the fluid mixture arising during phase transition. This was confirmed in 2005, when Ruel Shinnar and coworkers showed that complete phase segregation may occur on a 10 cm scale within a few seconds, even in the absence of buoyancy. In fact, quenching a nearly density-matched hexadecane-acetone liquid mixture below its critical temperature in a 20-cm-long condenser tube with a 1 cm diameter, they observed a rapid axial migration of the acetone-rich drops towards the warmer regions of the condenser.  As the typical 1 cm/s drop speed (with temperature gradients of about 0.25 K/cm) is far larger than that due to thermocapillary migration, a non-classical explanation had to be found, and so the phenomenon was simulated using the diffuse interface model. In the end, it appeared that the observed unidirectional, large-scale convection is induced by the Korteweg forces resulting from the non-equilibrium condition of  the mixture as it phase separates.