Optothermally Driven Colloidal Matter: Activity and Guided Motion

Abstract

Light carries energy, linear momentum, and angular momentum, which can be directly transferred to microscopic and nanoscopic objects. This transfer allows precise control and manipulation of their motion by suppressing inherent Brownian dynamics. However, optical fields often produce temperature fields as a byproduct. These thermal fields, along with fluid-mediated effects like thermo-osmosis, thermophoresis, and convection, significantly influence the dynamics of colloidal systems at micro- and nanoscales. In this thesis, we investigate the optothermal interaction-mediated dynamics of colloidal systems at the microscale, using experimental and numerical approaches to explore the effects of optical and optothermal interactions on the dynamics of colloidal particles. We first examined the dynamics under large-area optical fields. Such beams elevate the temperature of colloidal particles based on their individual absorption properties. These temperature differences induce attractive thermo-osmotic interactions between particles, leading to the formation of self-assembled colloidal structures. Depending on the colloidal species and their arrangements, these interactions can be reciprocal or non-reciprocal. In cases of non-reciprocal interactions, imbalances in net force and torque arise, resulting in dynamic behaviors such as propulsion and curling trajectories in the assembled structures. Next, we explored the behavior of optothermally self-assembled colloidal structures within confined environments created by focused or structured optical potential landscapes. In focused optical fields, gradient optical forces act as a confining potential, anchoring the colloidal system to the beam’s center, while optothermal (thermo-osmotic) interactions drive the formation of bound structures. These structures exhibit chiral rotational states, with their handedness adjustable by shifting the position of the focused beam. In contrast, structured optical fields, such as ring or line-shaped beams, define pathways that guide the motion of colloidal structures. At low optical intensities, the structures follow these paths in a bound manner, but at higher intensities, repulsive thermophoretic forces dominate, altering structures and their dynamics. This dynamic state can be tuned by adjusting the incident optical field intensity. The thesis concludes by summarizing the key experimental findings on optothermal interaction-mediated colloidal dynamics. We emphasize how optical fields can both confine and drive colloidal motion, with thermal effects playing a critical role in shaping these behaviors. Finally, we outline potential avenues for extending these results in future studies, offering insights into broader applications and refinements of optothermal control techniques.