A model for particle accumulation of density-matched tracers in thermocapillary flows

We numerically investigate the motion of small density-matched particles in thermocapillary driven flow in a cylindrical liquid bridge under zero gravity. A model is proposed that explains the recently reported complete demixing into a \emph{Particle Accumulation Structure} (PAS) without consideration of inertial effects. We explain the paradox that optimum demixing efficiency is obtained for density-matched particles, in agreement with experiments. The PAS phenomenon depends on the flow topology in the vicinity of a certain closed streamline of the three-dimensional flow. The accumulation can be explained by a repeated particle--free-surface collision process which transfers particles among different streamlines until a stable limit cycle is reached. The given model illustrates how a closed streamline can mimic an attractor in an incompressible flow.