Molecular transport and flow past hard and soft surfaces: computer simulation of model systems

Abstract

Abstract. The properties of polymer liquids on hard and soft substrates are investigated by molecular dynamics simulation of a coarse-grained bead-spring model and dynamic single-chain-in-mean-field (SCMF) simulations of a soft, coarse-grained polymer model. Hard, corrugated substrates are modelled by an FCC LennardJones solid while polymer brushes are investigated as a prototypical example of a soft, deformable surface. From the molecular simulation we extract the coarse-grained parameters that characterise the equilibrium and flow properties of the liquid in contact with the substrate: the surface and interface tensions, and the parameters of the hydrodynamic boundary condition. The so-determined parameters enter a continuum description like the Stokes equation or the lubrication approximation.At high temperatures the Navier slip condition provides an appropriate description of the flow past hard, corrugated surfaces. The position, x b , where the hydrodynamic boundary condition is to be enforced, agrees with the location of the liquid-solid interface and the slip length can be consistently identified by comparing planar shear flow and parabolic, pressure-driven flow. If the surface become strongly attractive or the surface is coated with a brush, the Navier slip condition will fail to consistently describe the flow at the boundary. This failure can be traced back to a boundary layer with an effective, higher viscosity.The solvent flow past a polymer brush induces a cyclic, tumbling motion of the tethered chain molecules. The collective motion gives rise to an inversion of the flow in the vicinity of the grafting surfaces and leads to strong, non-Gaussian fluctuations of the molecular orientations in the flow. Both, molecular dynamics as well as dynamic SCMF simulations, provide evidence that the flow past a polymer brush cannot be described by Brinkmann's equation.The hydrodynamic boundary condition is an important parameter for predicting the motion of polymer droplets on a surface under the influence of an external force. The steady state velocity is dictated by a balance between the power that is provided by the external force and the dissipation. If there is slippage at the liquid-solid interface, the friction at the solid-liquid interface and the viscous dissipation of the flow inside the drop will be the dominant dissipation mechanisms; dissipation at the three-phase contact line appears to be less important on a hard surface.On a soft, deformable substrate like a polymer brush, we observe a lifting up ofFlow past hard and soft surfaces 2 the three-phase contact line. Controlling the grafting density and the incompatibility between the brush and the polymer liquid we can independently tune the softness of the surface and the contact angle and thereby identify the parameters to maximise the deformation at the three-phase contact.

Brushes of semiflexible polymers in equilibrium and under flow in a super-hydrophobic regime

“…For polymer-liquid interactions the parameter was set to ε pl = 1/3ε, representing the chemical incompatibility between species. Rough surfaces with similar interaction potential and ε pl parameter have been studied previously [46][47][48][49], resulting in highly hydrophobic substrates and yielding to contact angles θ ∼ 120 o for droplets deposited over that surfaces. The rigidity of the polymers is imposed by a harmonic potential…”

Correlation between surface topography and slippage: a Molecular Dynamics study

“…The flow of simple liquids over flat substrates is often described by the no-slip boundary condition, δ = 0 36 , and the hydrodynamic position, z h , often coincides with the location of the sharp solid-liquid interface, as intuitively expected. Finite slip has been observed in complex liquids 24,34,[37][38][39] or at specific substrates: superhydrophobic 4,13,14,[40][41][42] and chemically patterned ones 43,44 .…”

Surface wave excitations and backflow effect over dense polymer brushes

“…We implement a Dissipative Particle Dynamics (DPD) code [18][19][20][21] with explicit solvent and numerically analyse the dynamics of a linear flexible homo-disperse polymer brush subdued to a simple liquid parabolic flow in a slit-pore geometry. The coarse-grained DPD procedure applies to both solvent molecules and polymer monomers, offering the pos-sibility to (i) reproduce hydrodynamic interactions while retaining a detailed view of the brush dynamics on the scale of the coarse-grained monomers; (ii) access both the polymer dynamics, influenced by the imposed flow, and the flow field, perturbed by the presence and motion of the brush.Recent studies of polymer brushes under flow [22][23][24] have highlighted an unexpected behavior in the velocity profile in the vicinity of the brush surface. These studies have reported that the velocity field reverses on increasing the flow field and have tentatively associated such result to the peculiar dynamics of the single polymer undergoing a cyclic motion of stretching, elongation and recoiling.…”

“…Recent studies of polymer brushes under flow [22][23][24] have highlighted an unexpected behavior in the velocity profile in the vicinity of the brush surface. These studies have reported that the velocity field reverses on increasing the flow field and have tentatively associated such result to the peculiar dynamics of the single polymer undergoing a cyclic motion of stretching, elongation and recoiling.…”