Enhanced dispersion in an oscillating array of harmonic traps

TL;DR

This study combines experiment, theory, and simulation to reveal how oscillating harmonic traps can significantly enhance colloidal particle dispersion through a novel 'slingshot' mechanism, especially at certain frequencies and trap stiffnesses.

Contribution

It introduces a new understanding of colloidal dispersion in oscillating traps, including a theoretical model and experimental validation of the 'slingshot' mechanism for enhanced diffusion.

Findings

Maximum dispersion occurs at a critical frequency with stiff traps.

Particles diffuse faster along the oscillation direction.

Theoretical and simulation results agree well with experiments.

Abstract

Experiment, theory, and simulation are employed to understand the dispersion of colloidal particles in a periodic array of oscillating harmonic traps generated by optical tweezers. In the presence of trap oscillation, a non-monotonic and anisotropic dispersion is observed. Surprisingly, the stiffest traps produce the largest dispersion at a critical frequency, and the particles diffuse significantly faster in the direction of oscillation than those undergoing passive Stokes-Einstein-Sutherland diffusion. Theoretical predictions for the effective diffusivity of the particles as a function of trap stiffness and oscillation frequency are developed using generalized Taylor dispersion theory and Brownian dynamics simulations. Both theory and simulation demonstrate excellent agreement with the experiments, and reveal a new ``slingshot'' mechanism that predicts a significant enhancement of colloidal diffusion in dynamic external fields.