Effects of polymer adsorption on the effective viscosity in microchannel flows: phenomenological slip layer model from molecular simulations

Abstract

Molecular simulations (Dissipative Particle Dynamics -DPD) were used to quantify the effect of polymer adsorption on the effective shear viscosity of a semi-dilute polymer solution in microchannel Poseuille flow. It is well known that polymer depletion layers develop adjacent to solid walls due to hydrodynamic forces, causing an apparent wall slip and reduced effective viscosity (increased total flow rate). We found that depletion layers also developed in the presence of hydrodynamically rough adsorbed layers on the wall. Polymer-polymer (steric) repulsion between flowing and adsorbed polymer expanded the depletion layer compared to no-adsorption cases, and the effective viscosity was reduced further. Desorption occurred for higher shear rates, reducing the repulsion effect and shrinking the depletion layers. A phenomenological algebraic model for the depletion layer thickness, including a shear modified adsorption isotherm, was developed based on the simulation data. The depletion layer model can be used together with the effective viscosity model we developed earlier.

CO2 Viscosification by Functional Molecules from Mesoscale Simulations

“…38 In particular, Figure S3 in the Supporting Information shows that even though the particle concentration profile displays structuring near the walls, as is always the case with molecules near surfaces, the velocity profile remains linear. The slip length is negligible, as is also the depletion layer 31 near the walls. One needs to keep the slip velocity and depletion layer small, since they tend to artificially reduce the viscosity; 31 the method we use achieves both.…”

“…This procedure minimizes the slip length and avoids artificial adsorption of the copolymers. The CO 2 beads that are within a distance z ≤ 0.15 r C of each wall are assigned zero velocity along the z -direction, to avoid slip. − Such setup gives rise to a linear velocity gradient along the z -direction, once the stationary state is reached, as shown in Figure b. Periodic boundary conditions are applied in the x - and y -directions, but not along the z -axis, since the walls are placed perpendicularly to that direction.…”

“…The slip length is negligible, as is also the depletion layer 31 near the walls. One needs to keep the slip velocity and depletion layer small, since they tend to artificially reduce the viscosity; 31 the method we use achieves both. Additional details are provided in the Supporting Information.…”

The viscosity of polyelectrolyte solutions and its dependence on their persistence length, concentration and solvent quality

“…Zero velocity along the z -direction is applied to the solvent particles whose distance from either wall is z ≤ 0.15 r c , to avoid slip. 50 This restriction is applied only to solvent beads, since imposing to PE chains would graft them to the surfaces. In Fig.…”

Modeling of Branched Thickening Polymers under Poiseuille Flow Gives Clues as to How to Increase a Solvent’s Viscosity

“…Poiseuille flow is obtained in this work when a constant external force F p = 0.02 (in reduced units) is applied along the x -direction to all beads in the system. The beads that are within a distance z ≤ 0.15 r C of each wall are assigned zero velocity along the z -direction, to avoid slip. − Since the walls are placed perpendicularly to the z -axis, periodic boundary conditions are applied only in the x - and y -directions. When the stationary state is reached, this setup produces a parabolic velocity profile along the z -direction, as is shown in Figure b.…”