Experimental evidence of symmetry breaking of transition-path times

Abstract

While thermal rates of state transitions in classical systems have been studied for almost a century, associated transition-path times have only recently received attention. Uphill and downhill transition paths between states at different free energies should be statistically indistinguishable. Here, we systematically investigate transition-path-time symmetry and report evidence of its breakdown on the molecular- and meso-scale out of equilibrium. In automated Brownian dynamics experiments, we establish first-passage-time symmetries of colloids driven by femtoNewton forces in holographically-created optical landscapes confined within microchannels. Conversely, we show that transitions which couple in a path-dependent manner to fluctuating forces exhibit asymmetry. We reproduce this asymmetry in folding transitions of DNA-hairpins driven out of equilibrium and suggest a topological mechanism of symmetry breakdown. Our results are relevant to measurements that capture a single coordinate in a multidimensional free energy landscape, as encountered in electrophysiology and single-molecule fluorescence experiments.

Faster Uphill Relaxation in Thermodynamically Equidistant Temperature Quenches

“…It is meanwhile possible to probe the transient, nonequilibrium dynamics of colloids and single molecules, e.g., by temperature-modulated particle tracking [4] and timemodulated [44], temperature-modulated [45], temperaturejump [46], and holographic [47] optical tweezers, as well as optical pushing [48]. These experiments allow for systematic investigations of the dependence of relaxation on the direction of the displacement from equilibrium, which is the central question of the present Letter.…”

“…1) remains elusive. Moreover, as a result of the projection to a lower-dimensional subspace, it is expected that observables in many experiments, in particular those tracking individual particles [4] and single molecules [46,47], relax in a manner that is not Markovian [8].…”

Second Law of Thermodynamics at Stopping Times

“…Using numerical simulations we illustrate how an experiment can be constructed for which W (T ) < 0 < ∆f (T ) , such that the mesoscopic system exerts force on its surroundings while the average free energy difference ∆f (T ) is positive. We consider the example of a stretched polymer described by model (19) with free energy (20). show that it is possible to extract work from a mesoscopic system even when the free energy difference is positive by stopping at a cleverly chosen moment (the blue circles denote the average free energy difference).…”

Direct detection of molecular intermediates from first-passage times

“…In contrast, mesoscale colloidal model systems represent an ideal test bed for understanding and developing these proposed fundamental connections. Here it is possible to manipulate and control the free energy landscape [22,23,24,25,26,27] whilst fully resolving the dynamics with no unknown or hidden degrees of freedom.…”

“…to allow for escape of the particle from the associated potential minima in observable periods of time. Optical tweezers are also used to automate the data acquisition process [25]. For each first-passage time measurement, a colloidal particle is trapped within the bulk, moved to the centre of the channel, i.e.…”