The interface between ultrathin deuterated poly(methyl methacrylate) films on polystyrene substrates is subject to a dispersive force driven instability consisting of thermally excited capillary waves. We have studied this example of liquid / liquid spinodal dewetting, using specular and off-specular neutron reflection measurements to characterize the length scale and growth rate of the instability of the buried, polymer interfaces. [S0031-9007(98)07842-9] PACS numbers: 68.15. + e, 61.12.Ha, The process by which an initially uniform film of a liquid on a nonwetting substrate breaks up into droplets is known as dewetting. Because of the importance of thin films and coatings in technology much experimental and theoretical work has been devoted to the study of the dynamics of dewetting. Much of this work has involved polymer films [1][2][3][4][5][6][7], both because of their technological importance and because their high viscosity means that experiments can be carried out on convenient time scales.The usual scenario for dewetting, whether for liquids on solids or liquids on liquids, involves the nucleation of holes. While this scenario for dewetting has been extensively studied, there is another possibility-that the initial stages of dewetting consist of the unstable growth of a capillary wave driven by dispersion forces across the film. This process, known as spinodal dewetting, has been analyzed theoretically and identified in physical systems such as metal films and polymer films on the basis of the characteristics of the final dewetted structure [1,8]. While there has been a suggestion, following analysis of the spatial correlations in dewetted structures of polymer thin films, that in some cases morphologies ascribed to spinodal dewetting in fact result from nucleation and growth of holes [9], very recent atomic force microscopy (AFM) measurements provide good evidence that, at least for ultrathin, very low molecular weight polystyrene on silicon, spinodal dewetting is indeed the mechanism in this case of liquid/solid dewetting [7].The dewetting of a polymer melt substrate by a polymer thin film is an example of liquid/liquid dewetting; this is a more complicated situation than the more commonly studied liquid/solid case, because both the polymer/polymer interface and the free film surface are deformable. Liquid/ liquid dewetting also presents an experimental challenge, since the interface of interest is buried, in contrast to the dewetting of a solid by a liquid film, where it is the liquid surface that is unstable.In this Letter, we characterize the earliest stages of dewetting of polystyrene (PS) by a thin poly(methyl methacrylate) (PMMA) film, and we show clear evidence that an instability of the buried, polymer/polymer, interface is a precursor to the dewetting process. The instability was monitored by the diffraction, at grazing incidence, of thermal neutrons from the growing capillary waves at the polymer/polymer interface. The characteristic wavelength of the fastest unstable mode and the way the diffu...

The influence on calcium carbonate crystallization of a series of biopolymers that contain carboxylic acid or sulfate functional groups was studied using pH and turbidity measurements, optical microscopy, and scanning electron microscopy. Without biopolymer, single calcite (104) rhombohedra were formed. In the presence of nongelling biopolymers (xanthan and gellan) in the conditions used, (104) rhombohedra formed aggregates that were “stack-like”, but in the presence of gelling biopolymers (pectin, κ-carrageenan, and sodium alginate) the aggregates were “rosette-like”. The “rosettes” were proposed to form by the nucleation of calcite on a gelled microparticle template to form a hollow shell. Low methoxy (LM) pectin was particularly effective at directing the growth of calcite rosettes and led to aggregates of radially aligned crystals. The influence of biopolymer concentration on calcite crystallization was studied for LM pectin and κ-carrageenan. In the former case, an increasingly favorable influence of the pectin molecules on the surface energy of calcite nuclei was proposed to result in an enhanced propensity for nucleation, until the pectin concentration was so high that all of the calcium was sequestered. In the latter case, an increase in calcium binding with increasing κ-carrageenan concentration decreased the solution supersaturation and hence decreased the propensity for calcite formation.

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We study the spontaneous size selection in lyotropic cholesteric (W/O) and thermotropic nematic (O/W) liquid crystal emulsions. The droplet sizes have been characterized by dynamic light scattering, which indicates a narrow monomodal distribution of droplets achieved spontaneously even without emulsion filtration. Anchoring of the director, provided by the chosen surfactant on the interface, may generate a topological defect inside the droplet. Below the critical radius R = K/W, determined by the ratio of Frank elastic and the surface anchoring constants, the effective anchoring strength is weak and droplets are not topologically charged; this allows them to coalesce freely, depleting the size distribution in this range. Large droplets possess a topological charge of +1 and present a high elastic energy barrier for pair coalescence; the resulting size distribution is skewed, with R > R, and effectively frozen.