Effect of interfacial heat transfer on basic flow and instability in a high-Prandtl-number thermocapillary liquid bridge

The effects of interfacial heat transfer on the basic flow and instability of thermocapillary convection in high-Prandtl-number (Pr) liquid bridges are investigated experimentally and computationally. Liquid bridges of silicone oil (Pr = 28) are formed in a gap between the lower cooled rod and the upper heated rod, both with a diameter of 5 mm, where the rods are surrounded by a cylindrical enclosure made from an acrylic block. The instability data are collected experimentally for AR = 0.30-0.50 and a wide range of (TC-Ta), where AR is the aspect ratio (= height/diameter) of the liquid bridge, TC is the cooled rod temperature, and Ta is the ambient gas temperature. The data indicate the appreciable effect of interfacial heat transfer on the instability and basic flow pattern of thermocapillary convection in liquid bridges. Each instability curve for AR = 0.35-0.50 shows a local peak of the critical temperature difference and the oscillation frequency at a certain (TC-Ta), where such a peak is associated with the transition of the azimuthal oscillation mode. The heat transfer ratio QLB/QHR is evaluated from the numerical simulation to discuss the effect of interfacial heat transfer, where QLB and QHR are the heat transfer rates at the liquid bridge surface and at the heated rod surface, respectively. It is found that the instability conditions for different AR values are well correlated with QLB/QHR. This correlation is consistent with the effect of QLB/QHR on the basic flow and temperature fields, both in the liquid bridge and in the ambient gas. The resultant change in the basic flow and temperature field inside the liquid bridge leads to the change in the onset conditions of oscillatory thermocapillary convection.