**Peter Yatsyshin, Andy Parry and Serafim Kalliadasis explain their latest research on phase transitions for fluids in confinement. You can read the full paper P Yatsyshin et al 2016 J. Phys.: Condens. Matter 28 275001 now.**

Interfaces between the different phases of matter surround us, and since the days of van der Waals have been known to provide key insights into the workings of the atomic world. Indeed, the surface tension itself and the sharpness of the liquid-gas interface point directly to an underlying world governed by molecular forces. 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, O(1/r^{6}), decay of dispersion forces present in most molecular fluids.

Patterning a surface vastly increases the zoo of phase transitions possible for solid-fluid interfaces. In our paper we consider the effect that surface geometry or heterogeneity has on the prewetting transition. 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. Using microscopic Density Functional Theory, we show

that by patterning the surface, by for example sandwiching two materials together, one 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. This is analogous to Derjaguin’s complete wetting transition but is now occurring in ‘flatland’ – since the surface is effectively two dimensional. This means that while complete wetting is characterised by Derjaguins “1/3” power-law, the exponent for complete prewetting is “1/4”. We also show that similar phenomena occur in capillaries and wedges. In essence, the surface patterning has turned a first-order thin-thick transition into a continuous one but occurring parallel to the substrate – a feature which may well have ramifications for the design of lab-on-a-chip devices and controlled micro-/nanofluidics.

**About the authors**

Peter Yatsyshin started his scientific career as a solid state physicist working on photoionisation of negative ions, but moved to statistical mechanics of classical fluids by completing his PhD at the Department of Chemical Engineering, Imperial College, under the supervision of Serafim Kalliadasis. His is currently a post-doctoral research associate at Imperial College working with Serafim Kalliadasis on various aspects of classical fluids including investigations of surface phase transitions on patterned and sculpted substrates, dynamics of phase separation in colloidal systems and computational aspects of classical density functional theory, where he is actively developing a numerical toolbox for multiscale modelling of soft-matter systems.

Andrew Parry is Professor of Mathematical Physics at the Department of Mathematics, Imperial College. He first joined Imperial in 1992 as Lecturer where he also held his EPSRC Advanced Fellowship on “Fundamental Statistical Mechanics of Inhomogeneous Fluids”. He was promoted to Reader in 1998 and Professor in 2001. He has in addition held visiting positions in Germany (Wuppertal) and in Spain (Madrid). His research is in the equilibrium statistical mechanics of interfacial phase transitions including the analysis of novel phase transitions induced by confinement and substrate geometry, fluctuation effects arising from interfacial wandering and the derivation of interfacial Hamiltonians from microscopic models. Prof. Parry is a Fellow of the Institute of Physics.

Serafim Kalliadasis was appointed to Reader in Fluid Mechanics at the Chemical Engineering Department of Imperial College in 2004 and was promoted to Professor in Engineering Science and Applied Mathematics at the same Department in 2010. He has held several visiting positions including France (Laboratoire FAST), USA (Stanford) and Spain (Universidad Complutence de Madrid). His research sits at the interface between applied and computational mathematics, complex systems and chemical engineering. Prof. Kalliadasis is a Fellow of the American Physical Society and of the Institution of Chemical Engineers (UK).

This work is licensed under a Creative Commons Attribution 3.0 Unported License. Images created and provided by the authors specifically for this post. Copyright P Yatsyshin *et al* 2016. All rights reserved.

Categories: Journal of Physics: Condensed Matter