Optical studies of monolayers at the air/water interface


Monolayers of amphiphilic molecules at the air/water interface, Langmuir monolayers, provide an experimentally accessible system for the study of physics in two dimensions. As a function of temperature and density, Langmuir monolayers display a series of phases and phase transitions; this thesis describes several optical experiments which clarify the macroscopic properties of some of these monolayer phases. In one experiment a combination of two techniques, laser light scattering and Brewster angle microscopy, is used to study the effect of inhomogeneous monolayers on the damping of capillary waves. It is shown that the monolayer contributes to the capillary wave damping when the size of a typical monolayer domain is equal to or larger than the capillary wavelength. Secondly, a new technique to measure the tilt angle of molecules from the surface normal is described. The validity of the technique is demonstrated by comparison to previously performed x-ray scattering experiments. The tilt angle of a monolayer of long-chain alcohol molecules is measured as a function of temperature and pressure near first- and second-order phase transitions from tilted to untilted monolayer phases. Finally, depolarized Brewster angle microscopy is used to study orientational fluctuations in a two-dimensional smectic-C liquid crystal. The results are in excellent agreement with theoretical predictions. In addition, the first measurements of orientational elasticity and viscosity in a liquid crystal system with variable density are presented.
Last updated on 07/24/2019