Adhesion of a free-standing newton black film onto a solid substrate.
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Biomedical subjects
Ophelia K C Tsui
Publications and source records attributed to Ophelia K C Tsui.
Unconventional spinodal surface fluctuations on polymer films.
We study the temporal growth pattern of surface fluctuations on a series of spinodally unstable polymer films where the instability can be adjusted with the film thickness, h0. For the most unstable film studied (whose /(h0 - h(sp))/h(sp)/ = 0.988; h(sp) is the thickness where the second derivative of the interfacial potential of the film equals zero), the growth rate function of the surface modes as a function of the wavevector fits well to the mean-field theory. When the film thickness is increased such that /(h0 - h(sp))/h(sp)/ < or = 0.977, the mean-field theory demonstrates marked disagreement with experiment, notwithstanding the provision of the known corrections from nonlinear effects and thermal noise. We show that the deviations arise from large-amplitude fluctuations induced by homogeneous nucleation, which are not considered in the conventional treatments.
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Liquid crystal pretilt control by inhomogeneous surfaces.
We consider the pretilt alignment of a nematic liquid crystal (LC) on inhomogeneous surface patterns comprising patches of homeotropic alignment domains in a matrix favoring homogeneous alignment, or vice versa. We found that the resultant LC pretilt generally increases continuously from the homogeneous limit to the homeotropic limit as the area fraction of the homeotropic region increases from 0 to 1. For any given homeotropic area fraction, the variations are qualitatively different depending on how the distance between adjacent patches compares to the extrapolation lengths of the anchoring domains. Our results agree with those previously found in stripe patterns. The present finding provides useful guidelines for designing inhomogeneous alignment surfaces for variable LC pretilt control.
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Dewetting induced by complete versus nonretarded van der Waals forces.
Spontaneous rupture of some polymer films upon heating is commonplace. The very criterion for this instability is the system free energy, G(L), possessing a negative curvature. In films that are apolar with h < or = 100 nm van der Waals (vdW) interactions usually constitute a major contribution to G(L) for which the approximate form G(L) = -A/12piL(2) (where A is the Hamaker constant), ignoring retardation, has been widely used. In this work, we investigate the limits to this approximation by calculating the complete vdW interactions for popular polymer film systems in dewetting experiments including air-polystyrene-SiO2-Si, air-polystyrene-poly(methyl methacrylate)-Si, and air-poly(methyl methacrylate)-polystyrene-Si based on the theory of Dzyaloshinskii, Lifshitz, and Pitaevskii (DLP). We found that retardation effects could produce significant modifications to G(L) even when the thickness of the polymer and/or the interlayer is only 1-2 nm, contrary to conventional presumption.
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First-order liquid crystal orientation transition on inhomogeneous substrates.
In a recent experiment, we uncovered an unconventional liquid crystal (LC) orientation transition on microtextured substrates consisting of alternating horizontal and vertical corrugations. When the period of alternation was decreased toward approximately 1 microm, the LC alignment underwent an abrupt transition from inhomogeneous planar to a more uniform configuration with a large pretilt angle ( approximately 40 degrees ). With the aid of a model based on the competition between the Frank-Oseen elastic energy and a phenomenological surface potential of the form W(theta,phi)=(1/2)W((2))(theta) sin(2) theta+(1/4)W((4))(theta) sin(4) theta+(1/2)W(phi) cos(2) theta sin(2) phi(x,y) (where theta and phi are, respectively, the pretilt and azimuthal angles of the LC director and W((2))(theta), W((4))(theta), and W(phi) are constants) that demonstrated good agreement with experiment, we investigated the microscopic origin of the observed transition. It was found that this transition comprises two steps. First, the LC director homogenizes toward the phi=45 degrees azimuthal direction in the plane to relax the elastic energy. The resulting rise in azimuthal anchoring energy subsequently drives the LC to adopt a finite pretilt. The values of the W's deduced from the model reveal that the polar anchoring energy is about approximately 1/10 of the typical values, with the sin(4) theta term dominating the sin(2) theta term. We present a possible explanation for this unexpected finding.
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Liquid crystal orientation transition on microtextured substrates.
A uniform alignment of liquid crystal (LC) with finite pretilt was observed on microtextured substrates that were lithographically fabricated with alternating horizontal and vertical corrugations. As the period of alternation was decreased toward 0.8 microm, the nematic LC alignment on these substrates changed from inhomogeneous in plane, copying the substrate corrugations, to a uniform configuration with a large pretilt of approximately 40 degrees. This transition is pertinent to a frustrated boundary wherein a lowering in the LC elastic energy due to spatial variation in the LC orientation compromises an increase in the surface anchoring energy. A model based on this idea demonstrates good agreement with the experiment. This result may open up a new arena for tailoring substrate characteristics for LC alignment.
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