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

CLAUDIO PASTORINO; MARCUS MÜLLER

Buenos Aires

Workshop; Dynamics in soft and hard condensed matter; 2015

Departamento de Físico - FCEyN - UBA

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 a Dissipative 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 wasobserved experimentally in  fluorescence  studies of single DNA moleculessubjected to liquid flow. We measured the angular distribution functions of theend-to-end vector with the confining wall, which are the landmark of thiscyclic behavior.   This non-trivial motion produces in turn, a local inversionof the total flow velocity  in the vicinity of the brush-melt interface. By confining with an external potential "tracer" particles in 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 interfaceparameters. This finding, independent of the flow type, provides evidence that the cyclic motionof the brush, could serve as a mechanism of molecule separation in microfluidic devices.