Experimental investigation of the transition of Marangoni convection around a stationary gas bubble towards turbulent flow

Thermocapillary-driven convection around a gas bubble under a horizontal heated wall is experimentally investigated under gravitational conditions. This physical problem is mainly characterized by a dimensionless number that represents the ratio of convective heat flow induced by capillary convection to the heat transfer due to conduction, which is termed Marangoni number (Mg). In this work, in order to reach very high values of Marangoni numbers, a more concise and sophisticated experimental setup was used, where the experiments were mainly pursued inside a pressure chamber. The experimental work was launched using a PIV technique and shadowgraphy. Also, temperature measurements at different locations in the matrix fluid around the air bubble were conducted to determine flow at different boundary conditions. This study reveals, that under appropriate conditions, a surface tension driven (thermocapillary) flow can become turbulent. As the Marangoni number increases, thermocapillary bubble convection traverses several nonperiodic modes until it reaches the turbulent state. We provide empirical equations to express the guideline borders between the nonperiodic flow mode regimes.