Multiple free energy minima in systems of confined tethered polymers—toward soft nanomechanical bistable elements

The conformations of a flexible polymer chain end-tethered within a confined geometry can exhibit multiple states separated by a substantial free energy barrier—bistability. It may be possible to exploit this feature to build soft nanomechanical elements actuated by an electric or flow field. The present work uses Brownian dynamics simulations of a coarse-grained model of DNA to examine the characteristics of two types of systems, denoted entropically bistable and competitively bistable. An example of an entropically bistable system is a polymer tethered at a short pore connecting two large open regions. Bistability in this case arises because of the presence of a pair of entropically favorable states, where all the polymer segments are in one or the other open region. An example of a system that may display competitive bistability is a polyelectrolyte chain end-tethered at the mouth of a long pore that opens into a large region. In the absence of an electric field the bulk of the chain resides in the open region. If a sufficiently large field is applied parallel to the pore, the chain resides entirely in the pore. At intermediate field strength, however, simulations predict that the competition between conformational entropy and electrostatic energy can lead to a pair of free energy minima, one where the chain is mostly in the pore and the other where the chain is mostly in the open space. For both of these cases, computation of the potential of mean force yields predictions of free energy barrier heights larger than thermal energy.