Flow responsive nano-channel coated with semiflexible polymer brushes

KEVIN SPEYER; CLAUDIO PASTORINO

Workshop; Stochastic dynamics: models and applications; 2018

A system composed of a polymer brush bilayer is filled with liquid, and studied under equilibrium and flow conditions with Molecular Dynamics simulations. The polymers are end-grafted to one of the apposingplanar walls, which form an slit channel. The distance between walls is set in such a way that polymers from opposing walls, can interact among each another. In poor solvent conditions, the polymers get incontact, forming bundles across the channel. The liquid solvent is simulated with Lennard-Jones particles that fill the slit-like channel. Various parameters, such as distance between walls, quality of the solvent andpolymer bending rigidity are varied to investigate the relation between the flow and the external force. A non-linear relation between external force and fluid velocity is observed for a particular set of parameters.This non-linear response is linked with a morphological change in the polymer brushes. Under equilibrium conditions, the polymers grafted to apposing walls touch each other, forming bundles across the channel.Applying an external force to the system induces a flow of liquid in the channel. For large enough forces, the bundles are broken as the chains lean in the direction of the flow. This morphological alteration of thepolymer configurations translates in a sudden change in the flow, as the external force increases. This non-linear behavior contrasts with the expected linear relation for a simple liquid in a macroscopic channel.This sudden increase in the liquid velocity as a function of the external force resembles the response of the current flowing through a diode, as a function of the potential difference applied. The dependence of theforce threshold at which the morphological changes happen in the polymer brush are explored as a function of grafting density.This finding can lead to new flow control techniques in experimental micro and nano-fluidic devices. To the best of our knowledge, gating with polymer brushes was studied and applied by an external stimuli,like pH, solvent quality or temperature. In the studied system however, the pressure gradient of the flow induces morphological changes in the channel, modifying the response of the flow to the force.