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Effect of Electrostatic Interactions on Lubrication in Brush Systems

Andrey Dobrynin Institute of Materials Science and Department of Physics, University of Connecticut

Interactions between tethered layers composed of aggrecan (charged bottle-brush) macromolecules are responsible for molecular origin of the cartilage biomechanical behavior. To elucidate the role of the electrostatic forces in interaction between bottle-brush layers we perform molecular dynamics simulations of charged and neutral bottle-brush macromolecules tethered to substrates. We study lubricating properties of neutral and charged bottle-brush coatings as a function of the compression and shear stresses, and brush grafting density. Simulations show that in charged bottle-brush systems under shear there is a layer with excess of counterions located in the middle between brush bearing surfaces. The main deformation mode of the charged bottle-brush layers is associated with the backbone deformation resulting in the backbone deformation ratio a and shear viscosity h being universal functions of the Weissenberg number. In the case of neutral bottle-brush systems in addition to the backbone deformation there is also side chain deformation. The coupling between backbone and side chain deformation violates universality in deformation ratio a dependence on the Weissenberg number and results in scaling exponents varying with compression stress and brush grafting density. Existence of different length scales controlling deformation of neutral bottle–brushes manifests itself in shear viscosity, h, dependence on the shear rate, . Shear viscosity, h, as a function of the shear rate, , has two plateaus and two shear thinning regimes. For both systems the value of the friction coefficient increases with increasing the shear rate. The values of the friction coefficient for charged bottle-brush systems are about ten times smaller than corresponding values for neutral systems at the same shear rate