Effect of molecular weight on the capillary absorption of polymer droplets.

We study capillary absorption of small polymer droplets into nonwettable capillaries using coarse-grained molecular dynamics simulations and a simple analytical model. Studies of droplets of simple fluids have revealed that the capillary process depends on the ratio of tube-to-droplet radii [Willmott Faraday Discuss., 2010, 146, 233; Marmur J. Colloid Interface Sci. 1988, 122, 209]. Here we consider the absorption of droplets of polymers and study the effect of polymer chain length on the capillary absorption process. Our simulations reveal that for droplets of the same size (radius), the critical tube radius, below which there is no absorption, increases with the length of the polymer chains that constitute the droplets. We propose a model to explain this effect, which incorporates an entropic penalty for polymer confinement and find that this model agrees quantitatively with the simulations. We also find that the absorption dynamics is sensitive to the polymer chain length. In some cases during the capillary uptake transient partial absorption states, where the droplet is partially in and partially out of the tube, were observed. Such dynamics cannot be explained by a generalized Lucas-Washburn approach.

Molecular dynamics simulations of polymeric fluids in narrow channels: methods to enhance mixing.

Mixing of shear thinning polymeric fluids in long channels with patterned boundary conditions is studied through molecular dynamics simulations. Patterned wettability was shown to induce spatially varying slip lengths at the channel walls which in turn induce mixing in the fluid. To quantify the amount of mixing for different wave lengths of patterns, transverse velocity profiles were evaluated. The transverse velocity profiles from the molecular dynamics simulations were then compared with predictions from continuum modeling and good quantitative agreement was found. Offsetting the pattern was shown to produce better mixing in the center of the channel. Transverse flow is found to increase when the radius of gyration of the chains is smaller than the pattern length. We also implement an oscillating (time dependent) body force and find that the transverse flow increases significantly. However, we do not find an increase in transverse flow with frequency of the oscillation as predicted from continuum modeling and we postulate reasons for this behavior.