Wetting on Solid Surfaces

People working on the project:

Ralf K. Heilmann (Post-Doctoral Fellow)
Peter S. Pershan (Principal Investigator)

Also see Phys. Rev. B 63, 205405 (2001):
Quenching of Capillary Waves in Composite Wetting Films from a Binary Vapor:
An X-Ray Reflectivity Study

Outline of the project:

The physical properties of thin fluid films are important for processes such as lubrication, gas purification and storage, heterogeneous catalysis, and refrigeration, as well as for others that are basic to the painting, adhesion, coating, pharmacology, and other industries. We use x-ray reflectivity to characterize the microscopic structure of various thin fluid layers.

Experimental

We study films adsorbed onto solid substrates from vapor. A pool of liquid inside an isothermal chamber at a temperature T is allowed to equilibrate with its own vapor. A flat substrate suspended at some height h relative to the bulk liquid surface is held at some temperature T+dT. For temperatures above some wetting temperature Tw characteristic for the substrate/liquid combination a wetting film will form on the substrate through condensation from the vapor. In equilibrium the thickness of the wetting film will be controlled mainly through three potentials: i) the attractive van der Waals potential, favoring a macroscopically thick film, ii) the chemical potential difference between liquid at the elevated substrate temperature and bulk liquid/vapor at temperature T, and iii) the gravitational potential due to the height difference h.

Controlling the temperature offset dT we can continuously vary the film thickness in the approximate range from 10 to 200 A. Specular x-ray reflectivity gives us information on the average electron density profiles normal to the film surface (film thickness, density, interfacial roughnesses), while off-specular diffuse x-ray scattering is sensitive to lateral density correlations. From the thickness versus dT behavior we can learn about the interaction potential between substrate and film on an Angstrom length scale.

Wetting from a Binary Vapor

In spite of the extensive theoretical and experimental research that has been done on the critical properties of binary fluids and, separately, on critical adsorption and wetting phenomena, there are relatively few measurements of the atomic scale composition distribution of the components of a binary fluid mixture within the thickness of either a thin fluid film adsorbed on solid or liquid substrates, or across the surface profile at other interfaces.

Adsorption from a binary vapor is more complicated than from a one component vapor, since all interaction potentials are now concentration dependent. A wide range of wetting scenarios and density profiles can be imagined. Some obvious examples are:

Only component A wets the substrate. But when the wetting film gets thicker component B will be incorporated into the film as well.
Both components wet the substrate. Component A is preferred at the substrate, but component B has a lower surface tension at the liquid/vapor interface. Is there phase separation within a thin wetting film? At which temperatures and film thicknesses? If there is phase separation, does confinement affect the roughness of the A-rich/B-rich interface?
If component A is preferred both at the solid/liquid and liquid/vapor interface, can we get an A-rich/B-rich/A-rich profile?
Combinations of the above scenarios are possible if A and B have different wetting or dewetting temperatures.

In all these scenarios it is interesting to compare density profiles of thin wetting films with the corresponding bulk phase diagram of the mixture. Of course it is also very interesting to see what, if any, effects the constrained geometry of the wetting film and the interactions with the substrate have on bulk critical phenomena (phase separation above/below critical solution temperature, liquid/vapor critical temperature). Confinement might even give rise to new critical phenomena (e.g. interface localization transition).

The System Methylcyclohexane/Perfluoromethylcyclohexane

See Phys. Rev. B article above.

This work is supported at Harvard by the NSF Grant No. NSF-DMR-98-72817. Measurements at the NSLS are supported by DOE grant DE-AC02-76CH00016

Group publications relevant to the project:

I.M. Tidswell, T.A. Rabedeau, P.S. Pershan, J.P. Folkers, M.V. Baker and G.M. Whitesides
"Wetting films on chemically modified surfaces: An x-ray study.", Phys. Rev. B 44, 10869 (1991).

I.M. Tidswell, T.A. Rabedeau, P.S. Pershan and S.D. Kosowsky
"Complete wetting of a rough surface: An x-ray study", Phys. Rev. Lett. 66, 2108 (1991).

To X-Ray Group Page