New optical manipulation method for metallic Janus particles without feedback

Abstract

Active matter exhibits different non-equilibrium behaviors when confined either via imposed topological boundaries or external fields. Elucidating the dynamics of self-propelling particles is critical to understanding the emergence of these non-equilibrium behaviors. Here we report on our experimental investigations of metallic Janus particle in an optical point trap. We observed Rosette-like trajectories of the particle especially at high laser power. This is very different dynamics compared to having passive probes under static or intermittent harmonic traps as hypothesized under some conditions. Moreover, particle orientation becomes bimodal depending when the particle moves toward or away from the trap. A Langevin formulation with a spatial dependent particle speed proportional to the gradient of the beam profile under a harmonic trap captures well the experimental data. We then used this type of dynamics to guide metallic Janus particles in a multi-point trap system. Successful guidance is observed at different geometries even leading to novel oscillator-type dynamics never reported before in the particle level without the use of a feedback mechanism. For the case of two Janus particles in the trap, the mean field interaction potential was extracted via inversion of the g(r). The potential was found to be repulsive and long range with interaction length scale controlled by the laser power.