Impact of molecular structure on the lubricant squeeze-out between curved surfaces with long range elasticity

摘要

The properties of butane (C_4H_(10)) lubricants confined between two approaching solids are investigated by a model that accounts for the curvature and elastic properties of the solid surfaces.We consider the linear n-butane and the branched isobutane.For the linear molecule,well defined molecular layers develop in the lubricant film when the width is of the order of a few atomic diameters.The branched isobutane forms more disordered structures which permit it to stay liquidlike at smaller surface separations.During squeezing the solvation forces show oscillations corresponding to the width of a molecule.At low speeds (<0.1 m/s) the last layers of isobutane are squeezed out before those of n-butane.Since the (interfacial) squeezing velocity in most practical applications is very low when the lubricant layer has molecular thickness,one expects n-butane to be a better boundary lubricant than isobutane.With n-butane possessing a slightly lower viscosity at high pressures,our result refutes the view that squeeze-out should be harder for higher viscosities;on the other hand our results are consistent with wear experiments in which n-butane were shown to protect steel surfaces better than isobutane.