Collective dynamics and conical structure of particles levitated by thermophoretic force*

We study the dynamics of two to many particles levitating in a thermophoretic trap. Establishing the potential against gravity, we confined many 10 micron-scale polyethylene and ceramic particles in a thermal cell at air pressures of 1-10 Torr. We find that for more than 100 particles, the particles form an irrotational vortex line despite the air being far from the Rayleigh-Bénard convection regime. We find that the circulation of the vortices increases when more particles are levitated, which suggests that the dynamics are collective in nature. Furthermore, we find that the particles in these vortices settle into a "hollow cone" structure. We present a discussion of possible causes for these novel unexplained phenomena. A better understanding of these phenomena will not only improve our understanding of thermophoretic dynamics, but also offer a new path to simulate the aggregation dynamics of particles in micrograviation environments.