Convective Liquid Crystal Membranes

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This research project explores the unique properties of liquid crystals (LCs) as membrane barriers for electrical field- enhanced separation of fluid mixtures. Such materials are organic molecules which form partially ordered phases. Application of an electric field to the LC can influences spatial order within the crystal, the most dramatic effect being a variety of electrohydrodynamic (EHD) instabilities and anisotropy. In the presence of an electron this anisotropy produces current focus and space charge accumulation, which in turn leads to electric body forces and flow of the LC. With appropriate parameter selection this flow can enhance the charge accumulator, a form of positive feedback, thus producing the instability. Moreover, the result with a dynamic AC field is a series of bifurcations to chaotic turbulent flow as sub-micron dimensions. The EHD induced instability can be used to make structures exhibiting voltage controlled permeability. Permeation across the membrane barrier occurs by diffuse flux which when subjected to the electric force produces a large controllable increase in the permeability of the LC layer. For mixtures with different solubilities, the high permeability EHD state can become an effective and selective membrane for separation processes. In addition, because of the enhanced transport induced across the barrier, the technique can be used as a "permeability switch" for providing a rapid, clean signal to a sensing element for on-demand sampling and analysis of process streams.