Unlike liquids, for crystalline solids the surface tension is known to be different from the surface energy. However, the same cannot be said conclusively for amorphous materials like soft cross-linked elastomers. To resolve this issue we have introduced here a direct method for measuring solid-liquid interfacial tension by using the curved surface of a solid. In essence, we have used the inner surface of tiny cylindrical channels embedded inside a soft elastomeric film for sensing the effect of the interfacial tension. When a liquid is inserted into the channel, because of wetting-induced alteration in interfacial tension, its thin wall deflects considerably; the deflection is measured with an optical profilometer and analyzed using the Föppl-von Kármán equation. We have used several liquids and cross-linked poly(dimethylsiloxane) as the solid to show that the estimated values of the solid-liquid interfacial tension matches with the corresponding solid-liquid interfacial energy reasonably well.soft solid | surface tension | surface energy | bulging | wetting S urface energy of a material is the energy required to create a unit area of new surface by the process of division, whereas surface tension is the isotropic surface stress associated with its deformation. For liquid, these two quantities are numerically equal because, when a liquid surface is deformed, the separation distances between molecules at the surface do not necessarily alter as molecules can move from the bulk to a deforming surface. It is generally not so for a solid, as it has been argued (1-3) that a solid surface consists of a constant number of atoms, so the work done to alter the separation distance between atoms at the surface is expected to depend on this distance itself. As a result, work of deformation is not necessarily the same as the thermodynamic work required to create a new surface. It is not clear, however, if this picture is true for all kinds of solids. For crystal surfaces the question is somewhat resolved as it has been shown both experimentally and theoretically (2, 4) that surface free energy is a function of area itself and therefore the surface tension is expected to be different from surface energy. However, for amorphous materials, like polymeric solids and cross-linked elastomers, the issue remains unresolved because, for these materials, the molecules can have local mobility which allows them to show liquid-like behavior, e.g., surface reconstruction in response to external cues (5, 6). It has not been possible, however, to state anything conclusively because for most solids surface tension has not been measured accurately. The difficulty arises because, when a solid deforms due to the application of external forces, the internal stresses at the bulk of it far exceed that at the surface (7), which prevents the estimation of surface tension. Even when the sole effect of surface tension is considered, for most solids it remains almost immeasurable as the deformation due to surface tension remains significantly small.In ...
