Triple-point wetting of Ne on solid CO2

Aspects of prewetting at nonplanar surfaces

We employ Monte Carlo simulations in the grand canonical ensemble (GCEMC) to investigate the impact of nonplanarity of a solid substrate on the locus of the prewetting phase transition. The substrate is modelled as a periodic sequence of furrows of depth D and periodicity sx in the x direction; the furrows are infinitely long in the y direction. Our results indicate that a necessary prerequisite for a prewetting transition is the formation of a(n approximately) planar interface between molecularly thin films and an adjacent (bulk) gas. Thus, in general the prewetting transition is shifted to larger chemical potentials because the formation of a planar film-gas interface is more difficult next to a nonplanar compared with a planar solid surface. However, this shift turns out to be nonmonotonic depending on D on account of subtle packing effects manifested in the deviation of the local density Deltarho(x,Deltaz;D) at the nonplanar solid surface from that at a planar substrate. If D becomes sufficiently large prewetting as a discontinuous phase transition is suppressed because inside the furrow a highly ordered film forms that prevents a planar film-gas interface from forming.

Triple-point wetting of van der Waals films on self-affine and mound rough surfaces

The influence of random self-affine and mound substrate roughness on the wetting scenario of adsorbed van der Waals films is investigated as a function of characteristic roughness parameters. The roughness influence, which leads to triple-point wetting, is calculated by the bending free energy penalty of a solid film picking up the substrate morphology. For self-affine roughness, an increment of the roughness exponent H and/or a decrement of the roughness ratio w/xi (with w being the rms roughness amplitude and xi the in-plane correlation length) leads to a noticeable increment of the thickness of adsorbed solid films. Similarly for mound roughness the thickness dependence of the solid wetting layer on the average mound separation lambda and system correlation length zeta follow the general scenario that smoother substrates (w/zeta<<1 and/or w/lambda<<1) lead to thicker solid films. Nevertheless, in this case the thickness increment is a highly nonmonotonic function of zeta and lambda for lambda<or=zeta.

Triple-point wetting on rough substrates

The influence of substrate roughness on the wetting scenario of adsorbed van der Waals films is investigated by theory and experiment. Calculating the bending free energy penalty of a solid sheet picking up the substrate roughness, we show that a finite roughness always leads to triple-point wetting reducing the widths of the adsorbed solid films considerably as compared to that of smooth substrates. Testing the theory against our experimental data for molecular hydrogen adsorbed on gold, we find quantitative agreement.