How thermophoresis deepens the trapping potential of optical tweezers: A quantitative analysis using Brownian dynamics simulations

Optothermal manipulations, which combine optical tweezers with thermal effects, have recently attracted significant interest. In this study, we developed a Brownian dynamics simulation (BDS) model incorporating optical gradient forces, optical dissipative forces, and thermophoretic forces to evaluate optothermal trapping of polystyrene nanoparticles in water containing polyethylene glycol 6000 (PEG 6000), induced by a focused near-infrared laser beam. The addition of PEG 6000 to water reverses the transport direction, causing thermophoresis from cold to hot and helping to trap particles at the laser focus. In our simulations, we focused on the trapping behavior of nanoparticles under two laser wavelengths: 1064 and 1560 nm. Their markedly different absorption in water alters the balance between optical and thermophoretic forces. At the laser wavelength of 1560 nm, the high absorption coefficient of water prevents the use of high laser intensity. As a result, thermophoresis led to loose nanoparticle accumulation around the laser spot rather than tight optical trapping. In contrast, at 1064 nm, the use of high laser intensity generated a deep optical trapping potential. Combined with moderate thermophoretic assistance—driven by a temperature increase of several Kelvin—this resulted in a markedly higher trapping efficiency for nanoparticles. Thus, our BDS model enables the quantitative separation and evaluation of optical and thermal forces in optothermal manipulation and is useful for designing manipulation behaviors ranging from loose accumulation to tighter confinement. The BDS script is freely available in the supplementary material.