Interfaces between different phases of matter surround us, and since the days of van der Waals have been known to provide key insights into the intricate workings of the atomic world. Indeed, surface tension itself and the... [ view full abstract ]
Interfaces between different phases of matter surround us, and since the days of van der Waals have been known to provide key insights into the intricate workings of the atomic world. Indeed, surface tension itself and the sharpness of the liquid-gas interface point directly to an underlying world governed by intermolecular forces. Wetting is the intrusion of a third phase into the interface between two different coexisting phases of matter and the concomitant ability of the interface to make contact with the third phase. A classic example of this is the adsorption at a wall-gas interface, completely wet by liquid. As was noted by Derjaguin in the 1940s, the thickness of the adsorbed liquid layer as a function of the partial pressure satisfies a ``1/3'' power-law, which is determined directly by the long-ranged decay of dispersion intermolecular forces. At a planar, homogeneous solid-gas interface the prewetting transition is first-order and refers to the jump from a thin to a thick layer of liquid adsorbed at the wall, as the pressure is increased towards saturation. Technically, prewetting is associated with a first-order wetting transition and refers to a line of such thin-thick transitions occurring near saturation pressure and above the wetting temperature. Sculpting or patterning a surface vastly increases the zoo of phase transitions possible for solid-fluid interfaces.
For example, it was recently shown that a step on a planar substrate can nucleate the thick prewetting film at a lower pressure and cause it to continuously spread out across the surface as the prewetting line is approached. The surface patterning turns a first-order thin-thick prewetting transition into a continuous one, but occurring parallel to the substrate. Therefore, continuous prewetting is analogous to Derjaguin's complete wetting transition, but now occurring in "flatland" - since the surface is effectively two-dimensional. This also means that the Derjaguins ``1/3'' power-law for ``complete prewetting'' becomes ``1/4''. A simple example of chemical patterning is when a stripe of a different material is inserted into a planar substrate. Such stripe may induce a first-order ``unbending'' transition, where the local adsorption near the stripe jumps between two microscopic values. Unbending also occurs on homogeneous but non-planar substrates and is closely related to the change from Wenzel to Cassie-Baxter states on rough surfaces.
In this talk, we consider a case where both the outer wall and the stripe phases exhibit first-order wetting transitions, with the wetting temperature of the stripe being lower. We show that the interplay of continuous prewetting and unbending considerably enriches the wetting phase diagram and leads to the appearance of a new exciting interfacial phase transition. Using microscopic density functional theory, we show that, for thin stripes, the lines of prewetting and unbending may merge, leading to a new two-dimensional-like wetting transition occurring along the walls. The influence of intermolecular forces and interfacial fluctuations on this phase transition and at complete prewetting are considered in detail. Our results have potential ramifications for the design of superhydrophobic surfaces and controlled micro-/nanofluidics.
