Thermally activated transitions of a Brownian particle in a viscoelastic fluid environment
Thermally activated transitions are ubiquitous in nature, such as in chemical reactions, protein folding, and drug binding, to name but a few. Although this phenomenon is nowadays well understood when occurring in perfectly viscous environments, very little is known about thermally-activated transitions in viscoelastic fluids. This kind of non-Newtonian materials with intricate flow properties, e.g., polymer solutions, micellar fluids, and colloidal suspensions, are widespread in many soft matter systems of greatest relevance to both fundamental and applied sciences. In this work, we investigate the motion of a micron-sized bead embedded in a micellar viscoelastic fluid transiting between the wells of a bistable optical potential. The precise characterization of both the potential and the fluid viscoelasticity enables a neat comparison between our experimental results and a theoretical model based on the generalized Langevin equation.
Our findings reveal an amplification of the transition rates, as compared o those in a Newtonian fluid, which stems from the interplay between the slow relaxation of fluid and the nonequilibrium particle dynamics around the potential barrier. Our analysis provides a general understanding of the effect of viscoelastic memory friction on the escape of a Brownian particle from a metastable state, which is a prerequisite for further investigations on transport phenomena in more complex energy landscapes and environments, as those commonly encountered in many biological processes and mesoscopic controlled delivery systems