Dissertation Defense: Adam Schuman
Title: Brewster Angle Microscope Investigations of Two Dimensional Phase Transitions
Abstract: The liquid-liquid interface is investigated by microscopic and thermodynamic means to image and measure interfacial properties when a Gibbs monolayer at the interface undergoes a two-dimensional (2D) phase transition by varying the sample temperature. An in-house Brewster angle microscope (BAM) is constructed to visualize the interface during this transition while a quasi-elastic light scattering technique is used to determine the interfacial tension. Density modulated micro-separated phases (microphases) occur local to the phase transitions in the form of alternating regions of high and low density. BAM observations demonstrate the existence of microphases in 2D cluster, stripe, and mosaic morphologies for a buried interface of hexane over water with fluoro-alkanol surfactant dissolved in the bulk hexane. Theory of microphases often predict a single length scale for cluster and stripe phases as a result of the competition between an attractive and a repulsive interaction. The observation that two characteristic length scales are required to describe clusters whose diameter is much larger than the stripe period, combined with the solid nature of the clusters, suggests that a long-range elastic interaction is relevant. These results complement earlier x-ray measurements on the same system.