Cyclic motion of grafted polymers under liquid flow: inversion of transport and possible mechanism of molecular separation

CLAUDIO PASTORINO; MARCUS MÜLLER

Halle

Workshop; Theory and Computer Simulation of Polymers: New Developments, 4th International Workshop; 2015

We studied by molecular simulation the  dynamics of a polymeric liquid underflow, confined between brush-coated substrates. The brush serves as a model ofa soft, deformable and elastic medium, whose molecular degrees of freedom playa role in the liquid behavior.  The temperature was held constant with aDissipative Particle Dynamics thermostatthat keeps the hydrodynamic correlations in the system.  Moving the confiningwalls at constant velocity, or applying an external volume force, Couette andPoiseuille flows were imposed to the system and  the velocity and densityprofiles were obtained from the simulations, as a function of shear rate orexternal force.  We found  a cyclic dynamics of the grafted chains, which was observed experimentally in  fluorescence  studies of single DNA molecules subjected to liquid flow. We measured the angular distribution functions of the end-to-end vector with the confining wall, which are the landmark of this cyclic behavior.   This non-trivial motion produces in turn, a local inversion of the total flow velocity  in the vicinity of the brush-melt interface.By confining with an external potential "tracer" particlesin the region of the interface or secondary chains physisorbed in the wall,we observe  inverse  mass flow of those molecules, for a certain sets of the interface parameters. This finding, independent of the flow type, provides evidence that the cyclic motion of the brush, could serve as a mechanism of molecule separation in microfluidic devices.