Wetting of a Smooth Substrate by Crystal
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Wetting of a Smooth Substrate by Crystal
David J. Courtemanche and Frank van Swol Department of Chemical Engineering University of Illinois Urbana, Illinois, 61801
Abstract We report on a molecular dynamics (MD) study of the wetting state of a system of hard spheres near a smooth planar hard wall. A direct simulation at the melting point of a two-phase system between two walls develops all the way from complete wetting by fluid (cos(e) = 0) via partial wetting state to a final arrangement of complete wetting by crystal (cos(e) = 1). This implies that a hard sphere fluid spontaneously crystallizes at a smooth hard wall, contrary to existing beliefs. 1. Introduction The last decade has seen a large increase in activity in the traditional areas of wetting, adhesion and heterogeneous nucleation. To a large extend the renewed efforts were stimulated by the work of Cahn' who first argued the existence of wetting transition in fluid-fluid systems on the basis of an elegant scaling argument. Researchers have focused on the order of the wetting transition, looked for the prewetting line, investigated the role of the interaction ranges and the role of fluctuations. A comprehensive review has been published by Dietrich 2. Much of the focus up has been on fluid-fluid systems near substrates or in contact with a noncritical spectator phase. Experimental work on fluids near the sublimation line have primarily concentrated on other surface phase transitions such as surface reconstruction and roughening transitions. Studies of wetting phenomena are much smaller in number (see Dietrich 2 ). Most have focused on surface melting of single crystals and adsorbed crystalline layers which constitutes an example of the approach of complete wetting along the sublimation curve. However, a few studies have addressed wetting of He phases near the melting line 2, the topic of this paper. From a theoretical point of view this problem is far more complex due to the
Mat. Res. Soc. Symp. Proc. Vol. 237. ©1992 Materials Research Society
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difficulties of developing an accurate Density Functional Theory that captures the crystal and crystal-fluid interface. Similarly simulation of liquid-crystal coexistence is more challenging than two fluid phases. In this paper we will present the first thorough simulation of wetting near a substrate. We find that a smooth wall can be wetted by crystal, implying spontaneous hetereogeneous nucleation that is, prefreezing. This is consistent with observations of dense colloidal suspension nucleating near smooth container walls, but is contrary to speculations based on earlier simulation attempts 3,4,8.
2. Simulation Methods The MD simulations were performed on a system of N=21 00 hard spheres (see reference 5 for details of the MD). The initial calculations were started from
a perfect fcc crystal configuration with the (111) planes in the xy-plane. By partially stretching the crystal we induced melting in part of the system producing a final box of dimensions Lx = 5.5378185, Ly = 9.591777a and Lz= 40.234751
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