Flow transition and hydrothermal wave instability of thermocapillary-driven flow in a free rectangular liquid film

Thermocapillary-driven flow in a free liquid film, which has two gas-liquid interfaces, is experimentally investigated. Silicone oil of 5 cSt is employed as the test fluid. Two-dimensional basic flow known as 'double-layered flow' after Ueno and Torii (2010) is realized under small-enough Marangoni number Ma, the non-dimensional number to describe the intensity of thermocapillary effect, under the geometry considered in the present study. The flow exhibits a transition from the two-dimensional steady flow state to the three-dimensional oscillatory state when the intensity of the imposed thermocapillary effect along the free surfaces exceeds the threshold. In this oscillatory regime, two types of hydrothermal wave instabilities are observed: the traveling-wave flow and the standing-wave flow. We especially focus on the traveling-wave instability and compare it with a hydrothermal wave in a liquid layer with a single free surface investigated by other researchers.