Partially Wetting Thin Liquid Films: Structure and Dynamics Studied with Coherent X Rays

We studied the surface structure of thin liquid films vapor deposited on solid substrates in a partial wetting situation by means of coherent x-ray scattering. No dynamics has been observed showing the absence of capillary waves on liquid films partially wetting a substrate. Instead an exponential form of the height-height correlation function has been found pointing toward a solidlike behavior of the thin liquid films at large length scales. The exact surface structure and degree of replication with the substrate depend on the deposition rate of the molecules.

Figure 1

$S(q)$ from vapor deposited thin liquid films adsorbed on silicon and glass substrates. The film thicknesses are from top to bottom 600, 500, 400, 300, 100, and 60 Å. The 300 and 400 Å thick films correspond to cyclohexane films adsorbed on glass substrate, while all other data correspond to hexane films on silicon substrates. The 60 Å film shows a $S(q)\propto q^{-1}$ behavior. All thicker films can be described by $S(q)\propto q^{-3}$.

Figure 2

Rocking scans of thin liquid vapor deposited hexane films on silicon substrates ($q_z=5.8\times {10}^{-2}{\AA }^{-1}$). (Top) slowly grown films; from bottom to top: monolayer coverage, 30, 60, 100, and 140 Å thick films. (Bottom) rapidly grown films: (a) 30 Å thick film (b) repeated scan, the arrow indicates rapid condensation, (c) 150 Å thick film.

Figure 3

CCD images from the scattering of a 400 Å thick hexane film adsorbed on a silicon substrate.

Figure 4

Speckle signal from a 400 Å thick hexane film on silicon and from the empty substrate as obtained by a cut through the CCD image at $q_y=0{\AA }^{-1}$. The inset shows the speckle structure in more detail.