Direct Visualization of Barrier Crossing Dynamics in a Driven Optical Matter System

TL;DR

This study uses optical microscopy to directly visualize barrier crossing trajectories of nanoparticle pairs, revealing a two-step mechanism and showing that crossing rates depend on encounter frequency rather than drive force.

Contribution

It demonstrates the first direct visualization of barrier crossing dynamics in a driven optical matter system, providing detailed trajectory analysis and insights into the crossing mechanism.

Findings

Total event rate increases with driving force due to more encounters.

Barrier crossing rate is independent of drive force, driven by thermal fluctuations.

Method can be extended to more complex systems with anisotropic particles.

Abstract

A major impediment to a more complete understanding of barrier crossing and other single-molecule processes is the inability to directly visualize the trajectories and dynamics of atoms and molecules in reactions. Rather, the kinetics are inferred from ensemble measurements or the position of a transducer (e.g. an AFM cantilever) as a surrogate variable. Direct visualization is highly desirable. Here, we achieve the direct measurement of barrier crossing trajectories by using optical microscopy to observe position and orientation changes of pairs of Ag nanoparticles in an optical ring trap, i.e. passing events. A two-step mechanism similar to a bimolecular exchange reaction is revealed by analysis that combines detailed knowledge of each trajectory, a statistically significant number of repetitions of the passing events, and the driving force-dependence of the process. We find that while the total event rate increases with driving force, this increase is only due to increased rate of encounters. There is no drive force-dependence on the rate of barrier crossing because the key motion for the process involves a radial motion of one particle as a thermal (random) fluctuation allowing the other to pass. This simple experiment can readily be extended to study more complex barrier crossing processes by replacing the spherical metal nanoparticles with anisotropic ones or through more intricate optical trapping potentials.