Surfactants Control Optical Trapping near a Glass Wall

Beyond their original capability to grab and hold tiny objects, optical tweezers have emerged as a powerful tool to investigate fundamental physics at microscopic scales. A precise characterization of the optical trap is one of the key requirements in such applications. A typical trapping system often involves a colloidal particle, stabilized in a fluid as an optical probe. Surfactants are commonly added to provide colloidal stability, but their incidental effects on the tweezer-particle interactions have been overlooked despite their prevalent use. Here, we study the interplay among the tweezer, the particle, and the surfactants adsorbed on the interfaces, including a nearby glass wall. In trapped particles' motion analysis, we find that the surfactants can affect the motion of the particle through the interactions between them. We discuss the effect of the surfactants' assembly structures on the particles' statistical behaviors. In particular, we investigate the thermal effect on the particle surroundings induced by the optically heated particle by analyzing the difference between metallic and dielectric probes. Our results explain how, under nanoscale confinement, the adsorbed surfactants can affect the particle behavior in an optical trap and propose a possible strategy of using an optically heated particle for localized surface modulation.