Nanobubble-driven superfast diffusion dynamics of Brownian particles

TL;DR

This study reveals that photon-activated gold nanoparticles exhibit superfast diffusion driven by nanobubbles, with diffusion rates vastly exceeding typical Brownian motion, offering insights for designing light-powered micro/nanomotors.

Contribution

The paper uncovers a nanobubble-induced propulsion mechanism causing superfast diffusion in light-activated nanoparticles, combining experimental 4D-EM imaging with physical modeling.

Findings

Diffusion constant increases with laser fluence following a power-law.

Nanobubbles induce strong random forces leading to superfast diffusion.

Nanoparticles exhibit ballistic motion at nanosecond timescales.

Abstract

Dynamics of active or self-propulsive Brownian particles in nonequilibrium status, has recently attracted great interest in many fields including biological entities and artificial micro/nanoscopic motors6. Understanding of their dynamics can provide insight into the statistical properties of biological and physical systems far from equilibrium. Generally, active Brownian particles can involve either translational or rotational motion. Here, we report the translational dynamics of photon-activated gold nanoparticles (NPs) in liquid cell imaged by four-dimensional electron microscopy (4D-EM). Under excitation of femtosecond (fs)-laser pulses, we observed that those Brownian NPs exhibit a superfast diffusive behavior with a diffusion constant four to five orders of magnitude greater than that in absence of laser excitation. The measured diffusion constant was found to follow a power-law dependence on the fs-laser fluence. Such superfast diffusion is induced by strong random driving forces arising from rapid nucleation, expansion and collapse of photoinduced nanobubbles (NBs) near the NP surface. In contrast, the motion of the NPs exhibit superfast ballistic translation at a short time scale down to nanoseconds (ns). Combining with physical model simulation, this study reveals a NB-propulsion mechanism for the self-propulsive motion, providing physical insights for better design of light-activated artificial micro/nanomotors.