Experimental study of coherent structures of finite-size particles in thermocapillary liquid bridges

The motion of finite-size particles in thermocapillary flow is investigated experimentally. Particular attention is paid to the unique phenomenon known as a particle accumulation structure (PAS), where particles gather in a solidlike structure in a half-zone liquid bridge. We study the correlation between the PAS and the flow field due to the hydrothermal wave instability by measuring the surface temperature and dynamic surface deformation. An image processing method is developed in order to verify the physical model of the particle behavior. The structures of the particle depletion zone, the PAS, and the toroidal core are observed clearly using long-term averaged images in the rotating frame of reference. The results of precise experiments reveal that the correlation between the PAS and the temperature field over the free surface is strongly dependent on the test fluid. Reconstruction of the PASs is achieved by applying particle tracking velocimetry. Time-series analysis of the particle position indicate that the ratio of the hydrothermal wave frequency to the turnover frequency of the particle is independent of the intensity of the thermocapillary-driven convection.