Wetting on Solid Surfaces
People working on the project:
Also see Phys. Rev. B 63, 205405 (2001):
Quenching of Capillary Waves in Composite Wetting Films from a Binary
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.
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
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:
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
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.
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
"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).
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