Biomedical subjects

B M Law

Publications and source records attributed to B M Law.

10 recordsLinked to original sources

PubMed2006

Ellipsometric search for vapor layers at liquid-hydrophobic solid surfaces.

We use precision ellipsometry to evaluate the existence of nanometer thick vapor films at the surface between a liquid and a hydrophobic alkylsilane coated Si wafer. We find no evidence for such vapor films. All of our fluid-solid ellipsometry measurements can be explained using a double layer model consisting of an oxide plus silane layer between the fluid and bulk Si substrate. We have carefully checked our ellipsometer for residual phase shifts which might, under certain circumstances, cause a mis-interpretation of the experimental results. We find that the most reliable ellipsometric results for thin films (which are relatively immune to the presence of small residual phase shifts) are collected at the Brewster angle. The dielectric constant of the native oxide coating is also compared with similar measurements for two thick (approximately 100-300 nm) thermally grown oxide coatings on a Si wafer.

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PubMed2005

Adsorption at the liquid-vapor surface of a binary liquid mixture.

In a binary liquid mixture, the component possessing the lowest surface tension preferentially adsorbs at the liquid-vapor surface. In the past this adsorption behavior has been extensively investigated for critical binary liquid mixtures near the mixture's critical temperature T(c). In this fluctuation-dominated regime the adsorption is described by a universal function of the dimensionless depth zxi where xi is the bulk correlation length. Fewer studies have quantitatively examined adsorption for off-critical mixtures because, in this case, one must carefully account for both the bulk and surface crossover from the fluctuation-dominated regime (close to T(c)) to the mean-field dominated regime (far from T(c)). In this paper we compare extensive liquid-vapor ellipsometric adsorption measurements for the mixture aniline+cyclohexane at a variety of critical and noncritical compositions with the crossover theory of Kiselev and co-workers [J. Chem. Phys. 112, 3370 (2000)].

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PubMed2005

Critical adsorption at silicon surfaces in binary liquid mixtures.

In critical binary liquid mixtures the preferential adsorption that occurs at liquid-vapor or liquid-solid surfaces is expected to be described by a universal surface scaling function. In this paper, we show that aniline strongly adsorbs at an oxide-coated Si wafer surface from a critical mixture of aniline + cyclohexane where this solid-liquid adsorption can be described by the same universal function found at liquid-vapor surfaces. For a tetrabromoethane + n-dodecane critical mixture the n-dodecane adsorption on an alkylsilane coated Si wafer cannot be described by previously determined adsorption functions. We speculate that this discrepancy is caused by chemical heterogeneities at the alkylsilane surface due to differing surface distributions of -CH3 and -CH2- groups within the silane layer.

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PubMed2005

X-ray specular reflectivity study of a critical binary fluid mixture.

We have used direct inversion of x-ray reflectivity data to extract the liquid-vapor interface composition profile and the related critical scaling function of a binary mixture of dodecane and tetrabromoethane. The mixture was in the one-phase region above its critical point. The results indicate the formation of a monolayer of the lower surface tension component followed by an abrupt change to a mixed composition which gradually relaxes to the bulk composition deep within the fluid.

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PubMed2001

Casimir force in a critical film formed from an electrolytic solution.

We have studied the thickness of vapor adsorbed films of the critical binary liquid mixture acetic acid plus nonane adsorbed onto a silicon wafer substrate as a function of temperature near the critical temperature. This critical film possesses opposite boundary conditions (+-) at its two surfaces and, due to the dissociation of acetic acid, both the electrostatic force and the dispersion force affect the adsorbed film thickness. On approaching the critical temperature T(c), an increase in the film thickness L is observed, implying that the sign of the universal Casimir amplitude Delta(+-) is positive, consistent with theoretical predictions. However, we find quantitative discrepancies in the value of Delta(+-) and the form of the critical Casimir pressure scaling function vartheta(+-) compared with previous experimental results. We attribute these discrepancies to the complex nature of the critical system studied in this experiment.

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PubMed2001

Critical adsorption in the weak surface field limit.

We study critical adsorption in the small surface field (h(1)) limit using a homologous series of critical liquid mixtures. The experiment data, in the one-phase regime, is accurately described by a universal surface scaling function G+(z/xi(+),z/l(h)) at distance z from the interface with correlation length xi(+) and surface field length l(h) approximately absolute value of (h(1))(-nu/Delta(1)), where h(1) approximately Deltasigma, the surface energy difference between the two components.

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PubMed2001

Line tension approaching a first-order wetting transition: experimental results from contact angle measurements.

The line tension values of n-octane and 1-octene on a hexadecyltrichlorosilane coated silicon wafer, are determined by contact angle measurements at temperatures near a first-order wetting transition T(w). It is shown experimentally that the line tension changes sign as the temperature increases toward T(w) in agreement with a number of theoretical predictions. A simple phenomenological model possessing a repulsive barrier at l(0)=5.1+/-0.2 nm and a scale factor of B=78+/-6 provides a quantitative description of the experiments.

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PubMed2000

Dipole orientational order at the critical interface.

We present experimental evidence that dipoles exhibit orientational order at the critical interface of mixtures of polar plus nonpolar liquids using the technique of ellipsometry. In this technique the ellipticity rho; at the critical interface for all nonpolar or weakly polar fluids or fluid mixtures diverges as t(beta-nu) where t=(Tc-T)/Tc is the reduced temperature relative to the critical temperature Tc and beta=0.328, nu=0.632 are critical exponents. For polar fluids, however, the dipole-image dipole interaction at the interface can cause long-range orientational order resulting in deviations from this power-law divergence. Theoretical results predict that the surface orientational order parameter alpha2(z) approximately m(*4)[d2v(z)/dz2], where m(*) is the reduced dipole moment and v(z) is the local composition at position z within the interface. We find quantitative agreement with these predictions for two different critical binary liquid mixtures composed of a highly polar plus nonpolar component.

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PubMed1999

Universal surface scaling function for critical adsorption.

We present an accurate determination of the one- and two-phase universal surface scaling function which describes critical adsorption at noncritical interfaces of critical binary liquid mixtures in the strong surface field limit. The 95% confidence levels for this function are also determined. This function quantitatively describes ellipsometric critical adsorption data for three upper critical and one lower critical binary liquid mixtures.

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PubMed1999

Spreading dynamics of terraced droplets.

The liquid crystal (-)7S5 spreads as a two-terraced droplet on an oxide covered (100) Si wafer. The thickness of the upper and lower terraces are respectively approximately 200 and approximately 40 A. This is the simplest system for which the de Gennes and Cazabat (dGC) terraced spreading model [C. R. Acad. Sci. II 310, 1601 (1990)] is applicable. We find that soon after the upper terrace acquires a flat top a hole develops in the center of this terrace. The hole propagates down to the depth of the first terrace. In this contribution we demonstrate that the dGC model is unstable to the formation of a hole in the center of the upper terrace for a two-terraced droplet. Our extended dGC model, which includes a hole in the upper terrace, provides a reasonable description of the average spreading dynamics for this system. However, this model has difficulties quantitatively accounting for all of the features exhibited by the dynamics, perhaps because experimentally the inner and outer borders of the upper terrace become irregular with time. These irregularities in the borders have not been included within the model.

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