Viscoelastic drop spreading: Cox-Voinov theory with normal stress effects

The dynamics of slowly spreading drops is dictated by the contact line motion. In Newtonian fluids, the classical Cox-Voinov theory links the macroscopic contact angle to the microscopic contact angle and the contact line velocity. Here, we investigate the effects of viscoelastic normal stresses on wetting dynamics. We first analytically derive an asymptotic expression for the radius of a spreading drop, and find the existence of two qualitatively different regimes. For weak viscoelasticity, the contact line dynamics follows a modified Cox-Voinov theory, where the microscopic contact angle is now replaced by an apparent microscopic angle dependent on the magnitude of viscoelasticity. By contrast, at larger values of viscoelasticity, the wetting dynamics, although affected by viscoelasticity, is independent of the microscopic properties, as had been previously anticipated in the case of complete wetting. We then discuss the intricate differences between spreading and retraction dynamics in the presence of viscoelasticity.