Experimental study on behaviors of low-Stokes number particles in weakly chaotic structures induced by thermocapillary effect within a closed system with a free surface

We focus on the behaviors of the low-Stokes-number particles in a thermocapillary-induced convection within a closed geometry with a free surface, known as a half-zone liquid bridge. The motions of the spherical particles suspended by the bridge of a high-Prandtl-number liquid (Pr=28.6) are tracked via two-dimensional and three-dimensional particle tracking velocimetry. Special attention is paid to the behaviors of the suspended particles in the flow regime in which coherent structures by the particles are realized; so-called particle accumulation structures (PASs). Through the experiments, we succeed in reconstructing three-dimensional trajectories of particles that form PASs, the toroidal core, and a structure wrapping the toroidal core. We illustrate the spatial and temporal correlations between the particles forming such three different coherent structures and corresponding regular flow structures; so-called Kolmogorov-Arnold-Moser (KAM) tori. It is found that the particles on the toroidal core exhibit helical motion through extracting the paths of the particles in azimuthal-axial planes with a small depth in the radial direction, which qualitatively corresponds to a Poincaré section.