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Non equilibrium coarse grained, dissipative particle dynamics simulations ofcomplex fluids, made up of polymer brushes tethered to planar surfaces immersedin a solvent yield non monotonic behavior of the friction coefficient as afunction of the polymer grating density on the substrates, \Gamma, while theviscosity shows a monotonically increasing dependence on \Gamma. This effect isshown to be independent of the degree of polymerization, N, and the size of thesystem. It arises from the composition and the structure of the first particlelayer adjacent to each surface that results from the confinement of the fluid.Whenever such layers are made up of as close a proportion of polymer beads tosolvent particles as there are in the fluid, the friction coefficient shows aminimum, while for disparate proportions the friction coefficient grows. At themushroom to brush transition (MBT) the viscosity scales with an exponent thatdepends on the characteristic exponent of the MBT (6/5) and the solvent qualityexponent (\nu = 0.5, for theta solvent), but it is independent of thepolymerization degree (N). On the other hand, the friction coefficient at theMBT scales as {\mu}~N^(6/5), while the grafting density at the MBT scales as{\Gamma}~ N^(-6/5) when friction is minimal, in agreement with previous scalingtheories. We argue these aspects are the result of cooperative phenomena thathave important implications for the understanding of biological brushes and thedesign of microfluidics devices, among other applications of current academicand industrial interest.

Scaling features of the tribology of polymer brushes of increasing grafting density around the mushroom to brush transition