By means of molecular dynamics simulations and scaling theory we study the response of opposing polymer brushes to constant shear motion under good solvent conditions. Model systems that contain explicit solvent molecules (Lennard-Jones dimers) are compared to solvent-free systems while varying of the distance between the grafted layers and their molecular parameters, chain length and grafting density. Our study reveals a power-law dependence of macroscopic transport properties on the Weissenberg number, W, beyond linear response. For instance, we find that the kinetic friction constant scales as mu approximately W(0.57) for large values of W. We develop a scaling theory that describes our data and previous numerical data including recent experiments.

Molecular dynamics simulations of a short-chain polymer melt between two brush-covered surfaces under shear have been performed. The end-grafted polymers which constitute the brush have the same chemical properties as the free chains in the melt and provide a soft deformable substrate. Polymer chains are described by a coarse-grained bead-spring model with Lennard-Jones interactions between the beads and a FENE potential between nearest neighbors along the backbone of the chains. The grafting density of the brush layer offers a way of controlling the behavior of the surface without altering the molecular interactions. We perform equilibrium and non-equilibrium Molecular Dynamics simulations at constant temperature and volume using the Dissipative Particle Dynamics thermostat. The equilibrium density profiles and the behavior under shear are studied as well as the interdigitation of the melt into the brush, the orientation on different length scales (bond vectors, radius of gyration, and end-to-end vector) of free and grafted chains, and velocity profiles. The viscosity and slippage at the interface are calculated as functions of grafting density and shear velocity.

In this work we compare and characterize the behavior of Langevin and Dissipative Particle Dynamics (DPD) thermostats in a broad range of non-equilibrium simulations of polymeric systems. Polymer brushes in relative sliding motion, polymeric liquids in Poiseuille and Couette flows, and brush-melt interfaces are used as model systems to analyze the efficiency and limitations of different Langevin and DPD thermostat implementations. Widely used coarse-grained bead-spring models under good and poor solvent conditions are employed to assess the effects of the thermostats. We considered equilibrium, transient, and steady state examples for testing the ability of the thermostats to maintain constant temperature and to reproduce the underlying physical phenomena in non-equilibrium situations. The common practice of switching-off the Langevin thermostat in the flow direction is also critically revisited. The efficiency of different weight functions for the DPD thermostat is quantitatively analyzed as a function of the solvent quality and the non-equilibrium situation.

The equilibrium and flow properties of a polymer liquid confined in a brush-coated channel are studied by molecular dynamics simulations using a dissipative particle dynamics (DPD) thermostat. We focus on the regime of high-grafting density, where the brush progressively becomes a stiff and smooth, soft surface and layering of the polymer melt at the brush-melt interface is observed. We use the Gibbs criterion to localize the brush-melt interface and analyze its equilibrium fluctuation in terms of a capillary wave Hamiltonian augmented by an elastic term that accounts for the deformability of the brush. Poiseuille and Couette flows are investigated, and the slip length and location of the hydrodynamic boundary are computed. In the high-grafting regime, the brush roughness decreases and slippage is observed. The results are compared to the effective channel width, which is defined via the integrated flow rate for Poiseuille flow. Evidence of local changes of the near-surface viscosity is provided, and the consistency of the Navier slip boundary condition is investigated.

Using molecular simulations, we study the properties of a polymer brush in contact with an explicit solvent under Couette and Poiseuille flow. The solvent is comprised of chemically identical chains. We present evidence that individual, unentangled chains in the dense brush exhibit cyclic, tumbling motion and non-Gaussian fluctuations of the molecular orientations similar to the behaviour of isolated tethered chains in shear flow. The collective molecular motion gives rise to an inversion of hydrodynamic flow direction in the vicinity of the brush-coated surface. Utilising Couette and Poiseuille flow, we investigate to what extend the effect of a brush-coated surface can be described by a Navier slip condition.

We studied the phase behavior of various ternary bilayer mixtures composed of cholesterol, an unsaturated lipid, and a fully saturated lipid, by means of molecular dynamics simulations of the MARTINI coarse grain model. We aimed at comparing lateral organization and local properties of bilayers containing phosphatidylcholine (PC) lipids, either with two unsaturated tails (symmetric), or one unsaturated and one saturated tail (asymmetric), as the low-melting component of the mixture. The number of unsaturations per chain was systematically varied in both classes of unsaturated lipids, to account for its consequences in segregation. In the asymmetric unsaturated PCs, the saturated tail was kept identical to the hydrophobic chains of the fully saturated lipid component. Membranes with a symmetric or an asymmetric unsaturated lipid, with the same kind of unsaturated chain, show different phase behavior. Symmetric polyunsaturated PCs set the separation in two phases. Instead, the asymmetric polyunsaturated lipids induced nonideal mixing of components in single-phase bilayers. A significative drop of temperature, within the accessible temperature range, enhances the segregation in mixtures with the more unsaturated asymmetric PC, but still within a single phase. This different phase behavior between membranes with symmetric and asymmetric unsaturated PCs is also observed for lipids with the same total number of unsaturations. On the other hand, the degree of unsaturation per se enhances the segregation, by increasing the composition fluctuations in single-phase membranes with asymmetric PC lipids, and raising the line tension in the two-phase bilayer mixtures with symmetric polyunsaturated PCs. Dynamic clusters of unsaturated asymmetric lipids can be identified. The clusters show no correlation between leaflets, as observed for the phase domains in mixtures with the symmetric polyunsaturated PCs. Interestingly, we found that asymmetric PC lipids have a preferential orientation such that their saturated tails increase their density toward the periphery of the clusters, facing regions enriched in the fully saturated lipids and cholesterol. The degree of unsaturation increases the cluster size and also enhances the anisotropy of the orientation. The surface density of cholesterol follows a gradient that favors its interaction with the saturated tails. Such gradients in composition lead to gradients in order parameters, such as the conformational order and the area of the tails, which increases away from the unsaturated lipid clusters. We compared, in addition, differences in hydrophobic length mismatch between acyl chains of the low-melting and high-melting components, in mixtures containing either symmetric or asymmetric unsaturated lipids.

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.

We undertake the investigation of sheared polymer chains grafted on flat surfaces to model liposomes covered with polyethylene glycol brushes as a case study for the mechanisms of efficient drug delivery in biologically relevant situations, for example, as carriers for topical treatments of illnesses in the human vasculature. For these applications, specific rheological properties are required, such as low viscosity at high shear rate to improve the transport of the liposomes. Therefore, extensive non-equilibrium, coarse -grained dissipative particle dynamics simulations of polymer brushes of various lengths and shear rates are performed to obtain the average viscosity and the friction coefficient of the system as functions of the shear rate and polymerization degree under theta -solvent conditions, and find that the brushes experience considerable shear thinning at large shear rates. The viscosity () and † Corresponding author. Electronic mail: agama@alumni.stanford.edu 2 the friction coefficient () are shown to obey the scaling laws ~̇− 0.31 , and ~̇0 .69 at high shear rate () in theta solvent, irrespective of the brushes degree of polymerization.These results confirm recent scaling predictions and reproduce very well trends in measurements of the viscosity at high (̇) of red blood cells in a liposome containing medium.