Orbiting microparticle dimer with symmetry-breaking on photothermal Marangoni flow

Abstract

Light-fueled microparticle rotors, capable of continuous rotation under a static light source, hold significant potential for applications in optically driven micromachines, microfluidics, particle transport, and soft-matter nonlinear optics. However, achieving high-speed and directional rotation of microparticles using a static low-power-density light source remains a challenge. We propose asymmetric Marangoni flow to drive the rapid rotation of microparticles, and demonstrate that a dimeric active microparticle (DAP) exhibits high-speed directional rotation in a low-power-density annular optical trap at the water–air interface. The driving force arises from the asymmetric Marangoni flow induced by the non-uniform laser heating of the DAP. The average linear velocity of the DAP rotation is regulated by the laser power. Furthermore, the enhanced asymmetry results in larger rotation speed, which is experimentally corroborated by polystyrene sphere with larger diameter. The rotation speed of the particle depends on the competition between the increase in the viscous drag force in Marangoni flow and the increase in the viscous drag force in still water. Finally, the asymmetric Marangoni flow is successfully utilized to drive the rotation of trimeric active microparticles and cell-carrying DAPs. This technology, characterized by its low power density and small temperature rise, demonstrates promising potential for applications in active matter, microscale robotics, and drug/cell delivery microsystems.