Optothermal Revolution: Colloids in an Optical Ring Trap

Directional motion is commonly observed in various living active systems, such as bacterial colonies moving through confined environments. In these systems, the dynamics arise from the collective effects of mutual interactions between individual elements, as well as their interactions with obstacles or boundaries. In this study, we turn our focus to an artificial system and experimentally investigate the emergence of directional revolution in dimer and trimer structures composed of colloidal particles in ring-shaped optical illumination. In this case, the movement of these colloidal structures is exclusively facilitated by optothermal interactions─without any direct mechanical force applied from external optical field. Depending on the optical absorption properties of the colloidal particles, these optothermal interactions can exhibit both attractive and repulsive characteristics. The attractive interactions provide the necessary driving force that propels the motion, while the repulsive interactions serve to control the structural parameters of the system. The arrangement and interaction of the colloidal particles within these dimer and trimer structures fuel the controlled, directional revolution, with the optical gradient force acting as a confining factor, guiding the movement along a specific path. Notably, the dynamics of these systems can be tuned by altering the intensity of the optical field. This study can be useful as a model for understanding insights into biological systems where group dynamics and environmental interactions are key to coordinated movement.