Development of an Ellipsometric Microscope for Measuring Liquid Surface Structure

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Law 9625867 The structure of droplets or the texture of films at liquid surfaces are indicative of the underlying surface interaction forces, line tension and surface elasticity. The understanding of surface interactions is important in many physical, chemical, and biological processes. For example, surface interactions control the phase diagram of Langmuir films which are frequently used as model systems for biological membranes, surface interactions also control the spreading properties of a liquid droplet which is important in the understanding of lubrication, similarly surface interactions influence the orientation of liquid crystal molecules which are used in many modern optical devices. There are therefore many scientific and technologically important reasons for studying the structure of droplets and the texture of films at liquid surfaces. With this award, we will develop an improved ellipsometric microscope to study liquid surface structure. It overcomes various technical difficulties associated with a previous ellipsometric microscope design which could only study thin films or droplets on solid surfaces. For isotropic films or droplets at liquid surfaces the improved ellipsometric microscope can determine the film thickness as a function of position with ~0.1 nm thickness resolution and ~1 um spatial resolution, while for anisotropic films of known thickness and composition at a liquid surface the ellipsometric microscope can determine the variation in film anisotropy with ~l um spatial resolution. This instrument will be used to study the nucleation and growth of wetting droplets as they coalesce to a uniform wetting film. The instrument will also be used to study topological defects in freely suspended liquid crystal films of known thickness. The unbinding of disclinations and dislocations is believed to play an important role in the 2-D melting process of freely suspended liquid crystal films. %%% As far as we are aware this will be th e first instrument of its kind which can determine the absolute tilt angle of anisotropic films with micrometer spatial resolution (provided the film thickness is known). This instrument should therefore prove of general use in many other situations of interest to the scientific and industrial community. For example, it could be used to study tilt angle transitions and defects in Langmuir films. It could also be used in monitoring the quality control of anisotropic films in an industrial or research setting, for example, it can quantify the uniformity of hightemperature superconducting films or Langmuir-Blodgett layers. ***