Flow-Induced Transport via Optical Heating of a Single Gold Nanoparticle

Abstract

Optothermal trapping has gained increasing popularity in manipulation such as selecting, guiding, and positioning submicron objects because of a few mW laser power much lower than that required for optical trapping. Optothermal trapping uses thermal-gradient-induced phoretic motions, but the underlying physics of driving force has not been fully understood. In this study, we performed optothermal trapping of 500 nm-diameter colloidal silica via a continuous laser illumination of a single gold nanoparticle from the bottom in a closed chamber. Under illumination, the tracer particles were attracted to the gold nanoparticle and trapped. Notably, the direction of migrating particles was always to hot gold nanoparticles regardless of the configuration of gold nanoparticles placed at two opposite sides of the chamber, on the bottom surface of an upper substrate (ceiling) or on the top surface of a lower substrate (floor). The previous interpretation based on thermal convective flow from the bottom to the top and circulating inside the chamber was only applicable to floor configuration and failed to explain our observation for the ceiling. Instead, temperature-induced Marangoni effect at the water/superheated water interface is likely to play a role. This study promoted a better understanding of the driving mechanism in optothermal trapping. Moreover, as an application of the single-particle platform, we showed the photothermal phase separation-induced microdroplet formation of thermoresponsive polymers and the coating of non-thermoresponsive polymers on nanoparticles.

Tailored optical propulsion forces for controlled transport of resonant gold nanoparticles and associated thermal convective fluid flows

“…The speed of the tracer NP and its increase in the proximity of the G-NP, as well as the tracer trajectory always pointing towards the hot G-NP, support our hypothesis. Note that the thermophoretic force cannot explain the observed motion of tracers towards the heat source since its direction in the considered water solution should be from the hot to cold region (positive Soret coefficient) 29 . We have observed that the trajectories followed by the tracers initially located at the same distance from the heat source (~6 μm) have different shapes depending on the constant or variable speed of the G-NP.…”

“…Indeed, the high temperature of the G-NP and therefore of the surrounding medium can produce different optofluidics effects. Taking into account the experimental conditions (trapping near the ceiling substrate) and the estimated temperature of the G-NP (~500 K) and following the reasoning as well as simulation results of a recent work 29 (where the optothermal flows generated by a single 100 nm gold NP fixed on a ceiling substrate were considered), we suppose that the flow originates from the temperature-induced Marangoni effect at the liquid water/superheated water interface due to nanobubble formation. The speed of the tracer NP and its increase in the proximity of the G-NP, as well as the tracer trajectory always pointing towards the hot G-NP, support our hypothesis.…”

“…Thermal optofluidics relies on the optothermal control of fluid motion, which can be achieved by using resonant plasmonic structures. Different physical phenomena are involved in the generation of such fluid flows 18 , 19 , 21 , 29 . A deeper understanding of the rather complex thermal mechanisms providing fluid motion at the microscale would allow control of the direction, velocity and extension of the fluid flows and, in turn, of the optothermal transport of colloidal particles.…”

“…It has been shown that an increase in the local temperature of a metal NP (or an NP assembly) is responsible for the appearance of convective fluid flow. Its configuration depends, in particular, on the position of the NP (usually attached on a floor or ceiling substrate) in the chamber and the width of the latter 19 , 21 , 29 . By illuminating periodic arrays of closely spaced plasmonic nanostructures (lithographically fixed on a floor glass substrate), the collective heating produces a short-range fluid convection motion with a speed of ∼1 µm/s, which can be increased up to ∼10 µm/s by using an optically absorptive substrate 30 .…”

“…One of the parameters playing an important role in the fluid dynamics is the temperature of the local heater. A significant local increase in the temperature, in the range of Δ T ∼ 70 −250 K, produces the temperature-induced Marangoni effect at the liquid water/superheated water interface due to nanobubble formation, which allows a fluid flow speed of 15–30 µm/s to be reached by optically heating a single gold sphere with a diameter of 100 nm 29 . For temperatures above the microbubble formation threshold ( T ∼ 580 ± 20 K) 31 , 32 , the so-called temperature-induced Marangoni effect 33 provides strong long-range convection flow.…”