Wetting and dewetting kinetics of shear-thinning liquids

The spreading of liquids on solid surfaces is of considerable interest and importance in many fields of industrial activity. The dynamic aspects of spreading are particularly relevant in several practical applications, which involve adhesion phenomena, such as coating, adhesive bonding, printing and composite manufacturing. The spreading of Newtonian liquids has been largely described in the literature from the hydrodynamic theory. The essence of this theory will be reviewed and illustrated with examples of Polydimethylsiloxane (PDMS) oil spreading. We have recently extended the hydrodynamic theory of liquid spreading to non-Newtonian fluids (PDMS filled with silica particles, typographic inks), in wetting and dewetting modes. The frequently encountered case of shear-thinning behavior will be considered in this study. The spreading kinetics of a liquid drop on a rigid substrate is controlled by viscous dissipation in the liquid, the capillary driving force being compensated by the braking force resulting from viscous shearing in the liquid. In the case where the liquid is non-Newtonian but shear-thinning, a deviation from the classical hydrodynamic theory for wetting is observed. The slightly non-spherical shape of shear-thinning liquid drops having a size smaller than the capillary length can also simply be interpreted by calculating that the actual viscosity increases from the edge to the center of the drops within the wedge shape formed during wetting in the vicinity of the advancing liquid front. No drastic changes are observed in the dewetting mode for non-Newtonian liquids as compared with the general behavior of Newtonian liquids. The rate of growth of dry zones nucleated in an unstable liquid film stays constant, as for Newtonian liquids, at least in the early stages of the growth of dry patches.