Harnessing thermal fluctuations for purposeful activities: The manipulation of single micro-swimmers by adaptive photon nudging

A simple scheme is presented for remotely maneuvering individual microscopic swimmers by means of on-demand photo-induced actuation, where a laser gently and intermittently pushed the swimmer along its body axis (photon nudging) through a combination of radiation-pressure force and photophoretic pull. The proposed strategy utilized rotational random walks to reorient the micro-swimmer and turned on its propulsion only when the swimmer was aligned with the target location (adaptive control). A Langevin-type equation of motion was formulated, integrating these two ideas to describe the dynamics of the stochastically controlled swimmer. The strategy was examined using computer simulations and illustrated in a proof-of-principle experiment steering a gold-coated Janus micro-sphere moving in three dimensions. The physical parameters relevant to the two actuating forces under the experimental conditions were investigated theoretically and experimentally, revealing that a ∼7 K temperature differential on the micro-swimmer surface could generate a propelling photophoretic strength of ∼0.1 pN. The controllability and positioning error were discussed using both experimental data and Langevin dynamics simulations, where the latter was further used to identify two key unitless control parameters for manipulation accuracy and efficiency; they were the number of random-walk turns the swimmer experienced on the experimental timescale (the revolution number) and the photon-nudge distance within the rotational diffusion time (the propulsion number). A comparison of simulation and experiment indicated that a near-optimal micron-precision motion control was achieved.