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

PASTORINO, C.; MÜLLER, M.

Ouro Preto, Brasil

Conferencia; Conference On Computational Physics; 2008

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.  Brush and polymer melt are typically composedof 32-bead chains of identical chemical identity and described withcoarse-grained potentials that retain the main properties of the chains. Thetemperature 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 tumbling 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. Weanalyzed also to which extent this behavior is maintained using liquids ofshorter chain lengths, that penetrate deeper in the brush layer.  Finally,  byconfining with an external potential "tracer" particles in the region of theinterface, we could invert completely the mass flow of those particles. Thisfinding, independent of the flow type, provides evidence that the cyclic motionof the brush, could serve as a mechanism of molecule separation in microfluidicdevices.