Optical studies of monolayers

From 1984 through 1998 we studied properties of monolayers at the air-water interface using a variety of optical techniques. Much of the interest in monolayers stems from their current and potential technological applications and from their relevance to biophysical problems. In addition, monolayers provide good models for studying two-dimensional physics. For instance, monolayers display a rich variety of phases and phase transitions as a function of temperature, area and surface pressure. Using a heterodyne light scattering technique developed in our laboratory, we studied the effect that monolayers have on the propogation and damping of capillary waves. In particular, we have studied the way in which the monolayer morphology affects the capillary wave damping in phase coexistence regions. In addition, we built a Brewster angle microscope which allowed us to image the morphology of the monolayer in phase coexistence regions (see picture), as well as to gain information about the tilt structures of the molecules in condensed monolayer phases. We also investigated monolayers of a liquid crystal molecule. One of the interesting properties of these monolayers is that they form a two-dimensional smectic C phase -- i.e. a phase in which there is liquid-like positional order, but long range order in the orientation of the molecules. In this phase there are thermally induced fluctuations in the orientation of the molecules, which is visible as a "flicker" in the Brewster angle microscope pictures. We studied these fluctuations as well as the tilt and azimuthal orientation of the molecules within monolayer domains.
Light scattering from nonequilibrium liquid interfaces, at Tenth Symposium on Thermophysical Properties (Gaithersburg, MD), Wednesday, June 1, 1988
The asymmetry of the two Brillouin peaks of light scattered from capillary waves on a water-nitrogen interface subject to a temperature gradient has been observed using a Fourier transform heterodyne technique. The local oscillator is frequency-shifted by a few kHz to separate the Stokes and anti-Stokes components. Although the sign and the order of magnitude of the effect agree with linear theory, the magnitude of the experimental asymmetry is about one half of the one predicted by linear fluctuating hydrodynamics.
Molecular orientation in Langmuir monolayers studied by surface second harmonic generation, at APS Centennial Meeting 1999 (Atlanta, GA), Tuesday, March 23, 1999:
We used surface second harmonic generation (SHG) to study molecular orientation in a smectic-C liquid-crystal Langmuir monolayer at an air/water interface. The molecules under investigation are 4-octyl-4'-(3-carboxytrimenthyleneoxy) azobenzene (8AZ3). Because water is inversion symmetric, the SHG signal comes only from the surface. Furthermore, the magnitude of the surface SHG signal depends on the orientation of molecules, which makes SHG a sensitive probe of the phase of the monolayer. We measured the response of the SHG signal as we compressed the monolayer from a smectic-C phase to a more... Read more about Molecular orientation in Langmuir monolayers studied by surface second harmonic generation

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