Non-equilibrium sampling in force spectroscopy of soft systems.

Force spectroscopy of soft biological systems are notoriously difficult, primarily because these systems are so soft that measurement perturbs the system and drives it out of its natural or equilibrium state that we want to characterise.  Consequently, the measured force of these soft systems depends sensitively upon the initial condition (because thermal fluctuations in the equilibrated soft system are dominant) and upon the rate of measurement (because the relaxation rate can be slower than that of the measurement).  In  2012 we demonstrated a new theory-based method to extract equilibrium averages from force experiments on soft matter that is driven out of equilibrium by the measurement.  Effectively, we found that nature biases such measurements:  knowing what that bias is, measuring it, and then extracting it from the ensemble of non-equilibrium measurements leads to an equilibrium average - an average that does not depend upon the measurement protocol and characterises the soft system itself.

We have demonstrated this method on two soft systems:  (1) an optically trapped colloid particle (experiments), and (2) a simulated rotaxane molecule that responds to forces like a sacled-down version of an automobile shock adsorber.