Subcritical convection in colloids and liquid binary mixtures with positive thermal diffusion

Abstract

The paper analyzes convection of molecular binary mixtures and colloids with positive thermal diffusion that occurs in the case of heating from below and below the equilibrium stability threshold for a homogeneous liquid. The convection is pulsed in nature and is realized in the form of periodic bursts. Positive thermal diffusion displaces the heavy component into the upper cold layers of the liquid. There, the heavy component accumulates in the boundary layer, forming a growing unstable density stratification. When the inhomogeneity exceeds the critical value for a given geometry, a convective surge occurs, and the heavy component moves down to a stable position. The study has found the critical thickness of the boundary layer, the critical concentration difference at the boundaries of the cavity, and the time at which instability occurs. Based on the analysis of the problem of the concentration profile dynamics under the influence of gravitational sedimentation and thermal diffusion, experimental facts are explained – periodic convective bursts of the binary mixture in channels and in the Hele-Shaw cell below the critical Rayleigh number. The described mechanism probably explains the convection observed by other experimenters in a horizontal flat layer of magnetic fluids below the critical Rayleigh number at Ra < 1708.