Wall-spring thermostat: An approach for controlling the dynamics of highly coarse-grained model fluids at surfaces

In order to address the hierarchy of time and length scales from the atomistic constituents to the scales of many nanometers, highly coarse-grained models of soft-matter fluids, such as e.g., polymers, have proven predictive and computationally efficient.

The rheological properties of a confined fluid depend on the interaction between solid surface and the polymer fluid. In highly coarse-grained models, where one coarse-grained segment represents multiple monomeric repeat units, the solid surface appears smooth on the scale of a segment. Thus, special simulation techniques are required to control the single-chain dynamics and friction at the solid-fluid contact. We devise a simulation strategy -- the wall-spring thermostat -- where transient bonds are formed between the solid surface and the highly coarse-grained segments, based on a grand-canonical Monte-Carlo algorithm. These transient bonds mimic strong, specific interactions of the segments with the solid. The attraction, induced by the transient bonds, can be compensated by a permanent, analytically known potential such that static properties are identical to those of a system without wall-springs. The single-chain and collective dynamics at the surface can be tailored by the areal density of transient wall-springs and their lifetime.

Wall-spring thermostat allows us to capture dynamic heterogeneities at surfaces, such as those quantified by the non-Gaussian behavior of the van-Hove self-correlation of polybutadiene at silica surfaces, obtained by atomistic simulations. The parameterized highly coarse-grained model enables us to explore the dynamics of polymers at solid surfaces for a wide range of molecular weights. We study the Navier-slip boundary condition under liquid flow and demonstrate that both, the slip length and the position of the hydrodynamic boundary, increase like the polymer's end-to-end distance. Since both lengths are approximately equal, the velocity profile vanishes close to the narrow interface between polymer melt and solid.