Ergodicity breaking and conformational hysteresis in the dynamics of a polymer tethered at a surface stagnation point

摘要

We study the dynamics of long chain polymer molecules tethered to a plane wall and subjected to a stagnation point flow.Using a combination of theory and numerical techniques,including Brownian dynamics(BD),we demonstrate that a chain conformation hysteresis exists even for freely draining(FD)chains.Hydrodynamic interactions(HI)between the polymer and the wall are included in the BD simulations.We find qualitative agreement between the FD and HI simulations,with both exhibiting simultaneous coiled and stretched states for a wide range of fixed flow strengths.The range of state coexistence is understood by considering an equivalent projected equilibrium problem of a two state reaction.Using this formalism,we construct Kramers rate theory(from the inverse mean first passage time for a Markov process)for the hopping transition from coil to stretch and stretch to coil.The activation energy for this rate is found to scale proportionally to chain length or Kuhn step number.Thus,in the limit of infinite chain size the hopping rates at a fixed value of the suitably defined Deborah number approach zero and the states are "frozen." We present the results that demonstrate this "ergodicity breaking."