Emergence of Directional Rotation in an Optothermally Activated Colloidal System

We experimentally demonstrate the emergence of directional rotation in thermally active-passive colloidal structures under optical confinement. The observed handedness of the rotation of the structure can be controlled by changing the relative positions of the constituent colloids. We show that the angular velocity of rotation is sensitive to the intensity of the incident optical fields and the size of the constituent colloidal entities. The emergence of rotational dynamics can be understood in the context of the asymmetric temperature distribution in the system and the relative location of the active colloid, which creates a local imbalance of optothermal torques in the confined system. Our work demonstrates how localized optothermal fields lead to directional rotational dynamics without explicitly utilizing the spin or orbital angular momentum of light. We envisage that our results will have implications in realizing Brownian engines and can directly relate to rotational dynamics in biological and ecological systems.