Transition path time over a barrier of a colloidal particle in a viscoelastic bath

TL;DR

This study experimentally investigates the transition path times of colloidal particles crossing energy barriers in viscoelastic fluids, revealing non-trivial dependencies and effective viscosities that align with linear microrheology measurements.

Contribution

It provides the first experimental validation of theoretical predictions for transition path times in viscoelastic baths using optical tweezers.

Findings

Transition path times depend non-trivially on barrier curvature.

Mean transition times are reduced in viscoelastic fluids compared to Newtonian fluids.

Effective viscosity from transition times matches microrheology measurements.

Abstract

We experimentally study the statistics of the transition path time taken by a submicron bead to successfully traverse an energy barrier created by two optical tweezers in two prototypical viscoelastic fluids, namely, aqueous polymer and micellar solutions. We find a very good agreement between our experimental distributions and a theoretical expression derived from the generalized Langevin equation for the particle motion. Our results reveal that the mean transition path time measured in such viscoelastic fluids have a non-trivial dependence on the barrier curvature and they can be significantly reduced when compared with those determined in Newtonian fluids of the same zero-shear viscosity. We verify that the decrease of the mean transition path time can be described in terms of an effective viscosity that quantitatively coincides with that measured by linear microrheology at a frequency determined by the reactive mode that gives rise to the unstable motion over the barrier. Therefore, our results uncover the linear response of the particle during its thermally activated escape from a metastable state even when taking place in a non-Markovian bath.