The Elusive Liquid-Solid Interface

Experimental studies of the liquid-solid interface are complicated because the interface is embedded and involves only a small number of atoms relative to the bulk liquid and solid.In his Perspective, Johnson highlights recent advances toward characterizing the interface.He highlights the report by Donnelly et al., who have used transmission electron microscopy (TEM) to study xenon inclusions in aluminum.The report provides evidence for progressive layering in the liquid near the interface, demonstrating the power of well-designed TEM studies to elucidate such interfaces.In macroscopic terms, the boundary between a liquid in contact with a solid surface is described by an interface energy that determines how the liquid wets or spreads out on the surface.If the interface energy is high, wetting is limited; if the interface energy is low, there is nearly complete wetting; in between, one talks about partial wetting.On a microscopic level, the interface is a two-dimensional surface with macroscopic dimensions along the contact surface, while perpendicular to the surface, it is often considered as a discontinuity with infinitesimal thickness.During the last couple of decades, however, it has become clear that the structure of the liquid-solid interface is more complex than anticipated.Experiments and computer simulations have shown that a degree of ordering progresses in layers from the solid surface over a few atomic distances into the liquid.In this issue, Donnelly et al. (1) beautifully demonstrate this phenomenon for liquid xenon in contact with aluminum crystal facets.They report high-resolution transmission electron microscopy (TEM) of aluminum samples containing liquid xenon inclusions within faceted aluminum cavities.By viewing the samples with the aluminum facets in an edge-on orientation, they were able to look along the interface and obtain direct evidence for ordering parallel to the interface in the liquid xenon in the three layers nearest to the aluminum surface.The authors use ion implantation to produce aluminum samples containing xenon inclusions a few nanometers in diameter.They exploit the property that a small particlefree or embeddedis confined under a pressure P = 2γ/r, where r is the radius of the particle and γ is the surface energy for a free particle or the interface energy for an embedded particle (2).The interface energy between xenon and aluminum is practically equal to the surface energy of aluminum of 1.2 Jm 2 (3).For inclusions with r < 2 nm, it follows that P is in the Gigapascal range, high enough for xenon to be solid at room temperature.Somewhat larger inclusions will be under lower pressure and contain liquid xenon.Solid noble gas inclusions in aluminum were first discovered by conventional TEM around 1984 (4, 5).Recently, Furuya and co-workers (6, 7) have produced