The wetting of solid surfaces by simple liquids is generally treated as being the result of physico-chemical interactions between an infinitely rigid material, e.g. a metal, and an infinitely deformable (at constant volume) fluid, viz. the liquid. Young’s equation is assumed to apply at the solid/liquid/vapour triple line when the system is at (quasi-) equilibrium: The term ‘quasi’ is employed since, if the contact angle deviates from equilibrium, by an incremental angle dq, depending on its sign, a spreading or shrinking force results. This implicitly assumes that only tensions parallel to the solid surface need equilibration, due to the infinite solid rigidity. In fact, the component of liquid surface tension, g, perpendicular to the solid, gsinq0, induces surface strain of order g/G, where G is the elastic shear modulus of the solid. Under most conditions, this effect is negligible and Young’s equation is an extremely good approximation. However, if the solid is very soft, or the liquid is of very high surface tension, g/G may be significant, leading to modifications both in the statics and dynamics of wetting. This may occur for organic liquids on soft tissues: for example hydrogels, or for liquids of high surface tension, such as molten metals, on less soft solids.
The consequences are intriguing. From a statics point of view, analysis of the triple line region, when g/G is not negligible, leads to interesting results. Using the conventional definition of contact angle (measured between the tangents to the LV and SL interfaces, when the solid is undeformed) leads to the finding that solid strain is of no importance if the phases remain distinct, and if the drop is sufficiently large. However, for small drops, effects of Laplace’s pressure become significant, leading indirectly to reduced equilibrium contact angle. It is as if the system tended towards that of a liquid lens floating on a second, immiscible liquid. Under dynamic conditions, the effect can be even more important. The spreading (shrinking) force becomes balanced dynamically by viscous dissipation within the liquid under ‘classic’ conditions. In the present context, a region of solid strain must accompany the triple line during its motion, and this may lead to further energy dissipation, either by viscoelasticity or by plasticity. The overall result is that the (de)wetting is slowed down by solid properties. In fact, the liquid viscosity may become a secondary influence! These various effects may have important consequences for high temperature (de)wetting, in particular when molten metals are considered.