Spreading kinetics at a molecular level
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Spreading kinetics at a molecular level Jean-Luc Buraud, Olivier Noel, Dominique Ausserre Université du Maine – LPEC –Molecular landscapes and biophotonics group, CNRS-UMR 6087, Le Mans – France ABSTRACT The spreading of a smectic nanodrop (8CB liquid crystal) on a solid surface was investigated by direct and real time imaging using the Surface Enhanced Ellipsometric Contrast (SEEC) microscopy [1-2]. The spreading ends with two molecular terraces (made of one monolayer and a bilayer). Two different behaviors were observed. In the first one the upper layer stays dense while shrinking. At the end of the process, the last molecules to disappear are located at the center of the initial disk. In the second one, nucleation and growth of holes is observed in the upper layer, in addition to shrinking. A model is proposed to describe the time evolution of the late stage structure. This model gives exact solutions of the kinetic equations, it covers strongly layered liquids such as smectic liquid crystals, it introduces the two dimensional Laplace pressure as an essential motor for spreading and it takes into account the liquid/gas transition in the surface layer that was consistently reported in experiments with 8CB. This model is in remarkable agreement with the experimental data and can explain the two observed behaviors [3]. INTRODUCTION Finite extended molecular films on solid substrates present considerable interest both in technological and scientific research. On the fundamental side, they are particularly interesting as they provide finite 2D or quasi-2D thermodynamic systems[4-8]. They can be obtained by the deposition of a nanodrop in air, which evolves towards a molecular film by wetting a solid surface. In practice, the molecules used to make surface patterns are non symmetric and present, at least at the solid liquid interface, some amphiphilic character. The most extensively studied system is the combination of 4-n-octyl-4’-cyanobiphenyl (8CB) liquid crystal with oxidized silicon wafers. It may be considered as the model system of the field. Experimental studies on this system were mainly conducted using ellipsometry[9], Atomic Force Microscopy (AFM) [10] or X-ray reflectivity [11]. This behaviour was observed in the whole range extending from the solid/smectic transition temperature, 21.5 °C, up to about the nematic/isotrope transition temperature, 40.5 °C [12]. Even when the free surface of a macroscopic 8CB drop presents a finite contact angle with the solid surface, the molecular structure of the edge is made up of a sequence of flat terraces with a decreasing thickness in the outwards direction, lying on a surface precursor film [10]. During the spreading process terraces are disappearing one after each other from the top, while the lower terraces enlarge. The first visible terrace counted from the solid is a single bilayer over the monolayer terrace, the second one is made of several coupled bilayers, the next ones are again single bilayers.
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EXPERIMENTAL AND MODEL Experimental details All the exper
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