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

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.