Simultaneous measurement of 3D velocity and 2D temperature fields for unsteady thermocapillary convection in thin liquid films

Thermocapillary convection driven by surface-tension gradients is an interfacial transport mechanism governing flow phenomena across a wide range of length scales, from thin liquid films to molten material processing. In crystal growth technologies, high-purity bulk single crystals are typically fabricated using the Czochralski and floating-zone methods, both of which can be adversely affected by hydrothermal waves (HTWs) arising from thermocapillary instabilities. The development of a digital twin for HTW dynamics can enable flow control and optimize crystal growth by minimizing defects. Such a framework requires three-dimensional (3D) velocity information; however, the full 3D velocity structure of HTWs has not yet been experimentally captured. The present study addresses this gap by acquiring 3D velocity fields of a fully developed HTW in a thin liquid film using tomographic stereo particle image velocimetry (TSPIV), combined with simultaneous infrared (IR) measurements of the free-surface temperature. Experiments were conducted in a rectangular container filled with silicone oil (Prandtl number 16.1), where characteristic HTW features—including roll propagation angle, roll frequency, phase velocity, and spatial wavenumber—were identified. The values obtained from TSPIV measurements show quantitative agreement with those reported in previous studies. In addition, the combined TSPIV and IR measurements captured source and suction flow structures near the free surface, revealing correspondence between temperature patterns and flow organization. These results demonstrate the feasibility of utilizing IR-based free-surface temperature data as a key component in constructing a digital twin of HTW-driven thermocapillary convection and provide experimental insights into three-dimensional flow structures relevant to high-purity crystal growth.