Observation of optothermal trapping of nanoparticles near liquid-solid interfaces

When a laser is focused into a liquid medium with a high absorption coefficient at the laser wavelength, localized heating occurs due to the absorption of light. The resulting temperature distribution induces various thermally-driven fluid phenomena, such as thermal convection, thermo-osmotic flow, and the thermophoresis of dispersed particles. The trapping of dispersed nano- or microparticles at the heating region due to these thermally-driven fluid phenomena is known as optothermal trapping. Compared to conventional optical tweezers that use optical forces acting on the particles, optothermal trapping offers the advantage of a wider trapping area because the temperature variation and fluid flows occur at larger scale than laser focus. However, optothermal trapping involves multiple competitive effects, that is, thermophoresis, thermo-osmotic flows, and thermal convection, making it difficult to isolate the contribution of each mechanism. In this study, we propose an experimental method based on the optically-trapped particle tracking velocimetry, which has been proposed by our group recently, to analyze thermally-induced near-wall particle motion with a fixed distance between the particle and the wall surface. This method enables the investigation of the thermo-osmosis effect, which is considered significant only near the wall surface. The results show clear optothermal trapping behavior of the particles along thermo-osmotic slip flows. Moreover, the magnitude of the flow shows a nonnegligible degradation over time.