Internal flow structure and dynamic free-surface deformation of oscillatory thermocapillary convection in a high-Prandtl-number liquid bridge

This paper presents changes in the internal flow structure and dynamic free-surface deformation of thermocapillary convection in a high-Prandtl-number (Pr = 28) liquid bridge associated with a discontinuous change in the oscillation frequency (i.e., frequency skip). After transition to the oscillatory state, the oscillation frequency of the flow and temperature fields often increases monotonically with further increases in the driving force for convection, but a sudden drop in oscillation frequency under certain conditions is observed when the oscillation mode is rotating-wave type with an azimuthal mode number of m = 2. To understand such a phenomenon, which is sometimes called a frequency skip, the velocity field inside a liquid bridge and dynamic deformation of the free surface are measured with three-dimensional, two-component particle image velocimetry (3D-2C PIV) and a micro-imaging displacement meter (MIDM), respectively. Additionally, the temperature fluctuations on a free surface are measured with an infrared camera, and the phase relationships among the velocity field, dynamic free-surface deformation, and temperature fluctuation are discussed. These measurements indicate a change in flow structure with no distinct difference in oscillation mode before and after the frequency skip. The flow structure twisted azimuthally before the frequency skip, but this twisted structure was suppressed after the frequency skip. The strength of the in-plane velocity and the amount of free-surface deformation also changed dramatically as a result of the change in flow structure. The present study suggests the presence of an instability mode that has not been reported previously. [GRAPHICS]