Viscoelastic effects in the spreading of liquids

THE Spreading of liquids on solid substrates is important in many natural and industrial processes, including eye irrigation, adhesive bonding, tertiary oil recovery and the application of insecticides, paints and inks. A liquid drop placed on a flat, smooth, horizontal solid spreads as excess capillary potential energy is dissipated during the motion, If the solid is rigid, energy is dissipated entirely by viscous flow within the liquid(1). For a nonrigid solid, however, a local microscopic deformation, or 'wetting ridge'(2,3), forms near the wetting front, and its motion also induces viscoelastic dissipation(4-6). Here we report direct evidence of the wetting ridge obtained using scanning interferometric microscopy. Our experiments demonstrate, for liquids and solids with properties representative of practically important substances, that the mesoscopic surface deformation can significantly affect macroscopic phenomena. For instance, the formation of a nanoscale wetting ridge can increase the spreading time of a drop by considerably more than an order of magnitude, with potentially important consequences in technological and natural settings.