Propulsion Mechanisms of Light-Driven Plasmonic Colloidal Micromotors

Colloidal micromotors are important candidates for a wide spectrum of applications, ranging from medicine to environmental remediation. Thus far, the propulsion force determination has been obtained from the colloidal motor motion speed and surrounding viscosity via the Stokes drag. Herein, a precise force measurement method and detailed analysis of the fundamental propulsion mechanisms of colloidal Janus micromotors propelled by thermophoretic and steam bubble force vectors, revealing findings uninvestigated to date, are presented. Optical tweezers provide fast and high-precision force measurements in all three orthogonal dimensions simultaneously. Colloidal Janus micromotors are compared with isotropic hot Brownian reference microparticles, which have no defined force vector that propels them perpendicular to the direction of the laser beam. Janus micromotors display a defined laser power intensity-dependent thermophoretic propulsion, as well as bubble force-based propulsion, after surpassing the threshold value for the water boiling. The steam bubble propulsion force vector and the thermophorethic force vectors sum up for the Janus micromotor propulsion direction. On the contrary, the bubble force counteracts photophoretic force in propagation direction of light. Moreover, the thermal-based reduction of viscosity around the Janus colloidal motor contributes significantly to its speed and guidance abilities.