Viscoelasticity of conformational transitions in single biopolymers: A simple microscopic model for internal viscous friction

I'll first talk about the experimental motivation for this work, which are single molecule measurements of polymers that have internal (conformational) transitions. The experiments measure the fluctuations of single polymers held at high stretch in an Atomic Force Microscope (AFM), which via the fluctuation-dissipation theorem allows an indirect calculation of their viscoelastic properties at different controlled forces. The results show a minimum in both the elastic and viscous friction constant at a critical force; the minimum in the elastic constant is readily explained using eqm statistical mechanics, whilst the cause of the minimum in the friction constant is not so clear. By calculating the effective linear response function of a discrete bistable landscape, representing the internal transitions of each monomer, we find an expression for the effective (differential) friction constant of this landscape, which explains the observed behaviour. I will then present a more intuitive (hand-wavy) calculation of the same friction constant, that suggests that this 'internal' friction is fundamentally related to the short-time response of the system.