Scanning Ripplon Microscopy

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Langmuir film, a monolayr of molecules expanded on water surface, is important as a material for the LB film which has various industrial applications. The film shows phase transitions among two-dimensional gas, liquid and solid phases, and has attracted also a pure physical interest. In our previous studies we had established "Wide-band ripplon light scattering technique" for a non-contact measurement of ripplon, ie. high-frequency capillary waves thermally excited and propagating on water surface. This technique has been applied for investigating molecular dynamics in the Langmuir films ; local structure of the film, inhomogenuity and state of condensation have been revealed.The purpose of the present study is to develop "a ripplon microscope" by increasing spatial resolution of this light scattering technique and combining it with a surface scanning system. It would be useful for quantitative evaluation of the films by measuring the surface energy and the surface visco-elasticity, the values which sensitively reflect the state of monomolecular layr on water surface. It would provide us with a very effective means for structure analysis of Langmuir films. We also intended to clarify how the function of a LB film is determined by the structure and dynamics of the Langmuir film as its material.In this study we improved the signal processing and the optical system of the technique ; and constructed a surface scanning system. We succeeded in visualizing the surface structure with -10 mum spatial resolution and precision. We applied the new system for studying various effects of two-dimensional physics, including thermal expansion, and phase transitions. These results taught us an interesting fact two-dimensional substances also have critical phenomena as the more familiar three-dimensional ones, and their behavior is described within a framework of the mean field theory.