Spreading kinetics of shear-thinning fluids in wetting and dewetting modes

The spreading dynamics of non-Newtonian fluids, in wetting and dewetting mo des, plays a key role in numerous applications in particular in coating, ad hesive bonding, and printing. The very common case of the shear-thinning be havior has been considered in this study. The wetting dynamics has been stu died by depositing sessile drops on glass slides. The dewetting kinetics ha s been evaluated by measuring the rate of growth of dry zones nucleated in an unstable liquid film formed on Teflon-coated glass slides. The spreading kinetics of a liquid on a rigid substrate is governed by viscous dissipati on in the liquid, the capillary driving force being compensated for by the braking force resulting from viscous shearing in the liquid. In the case wh ere the liquid is not Newtonian but shear-thinning or pseudoplastic, a devi ation from the classical hydrodynamic theory (Newtonian behavior) for wetti ng is obviously observed, in particular a slower wetting kinetics correspon ding to an apparent increase of the liquid viscosity as the spreading speed decreases. The shape, slightly nonspherical, of shear-thinning drops havin g a size smaller than the capillary length, is also simply interpreted, obs erving that the actual viscosity increases from the edge to the center of d rops during wetting, near the solid surface. In the dewetting mode no drast ic changes are observed when compared with the general behavior of Newtonia n liquids. The rate of growth of dry zones nucleated in an unstable liquid film stays constant, as for Newtonian liquids, at: least at the early stage s of the growth of dry patches. The proposed adaptation of the hydrodynamic theory is supported by several experimental results concerning the kinetic s of spreading in the wetting and dewetting modes. A good agreement is obse rved between the proposed theory and the results.