Cross stream chain migration in nanofluidic channels: Effects of chain length, channel height, and chain concentration

Abstract

We use molecular dynamics simulations to study the shear flow of a polymer solution in a nanochannel by using an explicit, atomistic model of the solvent. The length scales representing the chain size, channel size, and the molecular scale structure in these nanochannels are comparable. The diffusion and hydrodynamic interactions in the system are governed by the intermolecular interactions in the explicit solvent model that is used in the simulations. We study the cross stream migration of flexible polymer chains in a solution that is subjected to a planar Couette flow in a nanochannel. We present a detailed study of the effects of chain length, channel size, and solution concentration on the cross stream chain migration process. Our results show that when a dilute solution containing a longer and a shorter chain is subjected to shear flow, the longer chains that are stretched by the flow migrate away from the channel walls, while the shorter chains that do not stretch also do not exhibit this migration behavior. The thickness of the chain depletion layer at the channel surface resulting from cross stream migration is found to increase with an increase in the channel height. On the other hand, this degree of migration away from the channel walls is found to decrease with an increase in the solution concentration. In solutions with concentrations comparable to or greater than the overlap concentration, the depletion layer thickness in shear flow is found to be comparable or slightly smaller than that observed in the absence of flow.

Reduction of the effective shear viscosity in polymer solutions due to crossflow migration in microchannels: Effective viscosity models based on DPD simulations

“…The drift away from the walls increases with Wi, and is balanced by back-diffusion when the concentration builds up in the in the core volume of channel. This is in line with earlier findings where increased concentration of polymer reduces the depletion layer thickness [22].…”

“…This type of migration increases for increasing polymer/dumbbell length, and vanishes in very narrow channels due to a cancellation between the two walls [8]. Molecular dynamics simulations in Couette flow show that the depletion layer grows with increased W i as predicted for hydrodynamic migration [22], and the same behavior was found in Poseuille flow with DPD simulations [20]. With increasing depletion layer thickness, we expect that the shear thinning effect is amplified, giving a steeper slope of the effective viscosity curve as function of W i.…”

“…The depletion layer thickness δ increases with channel width h in Couette flow [22] and in Poseuille flow [19], with an apparent asymptotic behavior (with the limit δ ≃ R g ). Near-wall depletion implies a concentration build-up in the core flow that is larger for narrower channels, implying a selfregulating effect from gradient diffusion back towards the walls.…”

“…Confined polymers between flat walls without flow offer analytic treatments [9,13,14], as does simplified dumbbell models in shear flow [15]. For more realistic cases of migration with long polymer chains, there are a number of empirical results from experiments [7,10], DPD simulations [16,17,18,19,20] and Brownian and molecular dynamics simulations [6,21,22] that qualitatively describe the depletion layer behavior within the given parameter range. Ready-to-apply models for the depletion layer thickness δ as function of the Weissenberg number W i, the channel width h, ion contents/salinity, and adsorption properties are still lacking as far as we are aware.…”

Effect of solvent quality on Poiseuille flow of polymer solutions in microchannels: A dissipative particle dynamics study

“…Computational simulation is a powerful tool for studying the flows of polymer solutions in microchannels. Various numerical simulation methods were used to study Poiseuille flow of confined polymer solutions, such as Brownian dynamics, lattice‐Boltzmann methods, molecular dynamics, and dissipative particle dynamics . Jendrejack et al .…”