Ubiquitous transient stagnant domain formation during thermal convection in a well-mixed two component fluid with large viscosity difference

The formation of a transient stagnant domain in the presence of thermal convection was previously reported near the sol-gel transition temperature of a gelatin solution. The transient stagnant domain is observed near a critical Rayleigh number where a "roll" pattern is usually stable. It is important to understand the origin of the transient stagnant domain formation since it induces a large deformation of convection patterns; the nature of the formation of the transient stagnant domain remains unclear. Here, we observe thermal convection using several different fluids and find that stagnant domain formation is ubiquitous in two component mixtures. In addition, we find that difference in viscosity between the two components is crucial for transient stagnant domain formation, more so than the concentration gradient induced by the temperature gradient.