Bruce M. Law

Wetting, adsorption and surface critical phenomena

Abstract Our understanding of interfacial phenomena at the surfaces of critical systems, and in particular at the surfaces of critical binary liquid mixtures, has improved significantly in the past decade. There is now substantial agreement between theory and experiment. In this paper we review recent experimental progress, provide a conceptual framework within which the majority of these experiments can now be understood, and discuss critically any remaining unresolved discrepancies between experiments or with theory.

Fluctuations in fluids out of thermal equilibrium

After a brief review of dynamic correlations in equilibrium fluids, we consider the long-range correlations between the fluctuations in a fluid subjected to a large stationary temperature gradient. These long-range correlations enhance and modify the Rayleigh spectrum of the fluid. We elucidate that the modifications of the Rayleigh line are determined by the coupling of the entropy fluctuations to the transverse velocity fluctuations. Recent attempts to test the theoretical predictions with the aid of light-scattering experiments are discussed.

Model calculations of the ellipsometric properties of inhomogeneous dielectric surfaces

Model calculations of the reflectivity ratio rp/rs are presented for various inhomogeneous surface dielectric profiles representing (i) liquid-vapour surfaces, (ii) liquid on solid surfaces, (iii) leached glass surfaces, as a function of the angle of incidence and the thickness of the inhomogeneous layer. The sensitivity of ellipsometric measurements to profile shape is analysed.

Nanoparticle Adsorption at Liquid–Vapor Surfaces: Influence of Nanoparticle Thermodynamics, Wettability, and Line Tension

We developed a statistical mechanical theory that describes the adsorption of nanoparticles (NPs) at liquid-vapor surfaces. This theory accounts for the surface to bulk NP thermodynamic equilibrium, as well as the NP mechanical equilibrium, wettability, and line tension at liquid-vapor surfaces. The theory is tested by examining the adsorption of 5 nm diameter dodecanethiol-ligated gold NPs at the liquid-vapor surface of a homologous series of n-alkane solvents, from n-nonane to n-octadecane, where the NP wettability decreases with an increasing n-alkane chain length.

Wetting morphologies and their transitions in grooved substrates

When exposed to a partially wetting liquid, many natural and artificial surfaces equipped with complex topographies display a rich variety of liquid interfacial morphologies. In the present article, we focus on a few simple paradigmatic surface topographies and elaborate on the statics and dynamics of the resulting wetting morphologies. It is demonstrated that the spectrum of wetting morphologies increases with increasing complexity of the groove structure. On elastically deformable substrates, additional structures in the liquid morphologies can be observed, which are caused by deformations of the groove geometry in the presence of capillary forces. The emergence of certain liquid morphologies in grooves can be actively controlled by changes in wettability and geometry. For electrically conducting solid substrates, the apparent contact angle can be varied by electrowetting. This allows, depending on groove geometry, a reversible or irreversible transport of liquid along surface grooves. In the case of irreversible liquid transport in triangular grooves, the dynamics of the emerging instability is sensitive to the apparent hydrodynamic slip at the substrate. On elastic substrates, the geometry can be varied in a straightforward manner by stretching or relaxing the sample. The imbibition velocity in deformable grooves is significantly reduced compared to solid grooves, which is a result of the microscopic deformation of the elastic groove material close to the three phase contact line.

Strong critical adsorption at the liquid-vapor surface of a nonpolar mixture

For a critical binary liquid mixture where the surface tension difference between the two components is very large, the component with the lowest surface tension completely saturates the liquid-vapor surface. The variation in the local volume fraction v(z), with depth z into the liquid mixture, is described by a universal surface scaling function P±≡P±(z/ξ±), which takes differing forms in the one- (+) and two-phase (−) regions, where ξ represents the bulk correlation length. Carpenter et al. [Phys. Rev. E 59, 5655 (1999); 61, 532 (2000)] determined P± using the ellipsometric critical adsorption data of four different critical binary liquid mixtures. A deficiency of this prior study was that each of the liquid mixtures possessed at least one polar component, which could have generated distortions in the function P±(z/ξ±). In this publication, we demonstrate that P±, determined in the previous study, provides an excellent description of the nonpolar critical binary liquid mixture 1,1,2,2-tetrabromoethane+n...

2D imaging ellipsometric microscope

A two‐dimensional (2D) imaging ellipsometric microscope (IEM) has been constructed. It overcomes several problems inherent in existing 2D imaging ellipsometers. One can use IEM to measure and map the 2D film thickness profile with high spatial resolution and thickness sensitivity. The performance of the device is demonstrated through the study of the thin‐film profile of a spreading liquid drop on a molecularly smooth silicon wafer surface.

Improved in situ spring constant calibration for colloidal probe atomic force microscopy

In colloidal probe atomic force microscopy (AFM) surface forces cannot be measured without an accurate determination of the cantilever spring constant. The effective spring constant k depends upon the cantilever geometry and therefore should be measured in situ; additionally, k may be coupled to other measurement parameters. For example, colloidal probe AFM is frequently used to measure the slip length b at solid/liquid boundaries by comparing the measured hydrodynamic force with Vinogradova slip theory (V-theory). However, in this measurement k and b are coupled, hence, b cannot be accurately determined without knowing k to high precision. In this paper, a new in situ spring constant calibration method based upon the residuals, namely, the difference between experimental force-distance data and V-theory is presented and contrasted with two other popular spring constant determination methods. In this residuals calibration method, V-theory is fitted to the experimental force-distance data for a range of systematically varied spring constants where the only adjustable parameter in V-theory is the slip length b. The optimal spring constant k is that value where the residuals are symmetrically displaced about zero for all colloidal probe separations. This residual spring constant calibration method is demonstrated by studying three different liquids (n-decanol, n-hexadecane, and n-octane) and two different silane coated colloidal probe-silicon wafer systems (n-hexadecyltrichlorosilane and n-dodecyltrichlorosilane).

The scaling behavior of critical adsorption in critical polymer solutions

The critical adsorption ellipsometric measurements of five solutions of polystyrene in cyclohexane for different polystyrene molecular weights collapse to a single universal curve when scaled as a function of nBξ/λ, where nB is the polymer solution refractive index, ξ=ξ0Nnt−ν is the correlation length, and λ is the wavelength of incident light in vacuum. From this universal feature we deduce the value of the polymerization critical exponent n=0.258±0.017. We consider both the volume fraction order parameter (φ) and a symmetrized order parameter (ψs) together with both the renormalization group (RG) and Monte Carlo (MC) simulation forms for the surface scaling function P+(x). The symmetrized order parameter gives significantly better agreement with experiment than the volume fraction order parameter. The combination of RG and ψs provides better agreement with experiment than does the combination of MC and ψs.

Ellipsometric study of the liquid/pyrex surface: Evidence for orientational order and layering

Abstract In this paper we use a phase-modulated ellipsometer to study the liquid/Pyrex surface. The Pyrex surface is shown to be well characterized by a hydrated surface with a thickness of ∼ 10 A. The hydrated Pyrex model explains the ellipsometric data for air, carbon tetrachloride, cyclohexane, and methanol in contact with Pyrex and also gives good agreement with the background ellipticity for the two-phase region of the binary liquid mixture cyclohexane-methanol in the methanol-rich phase where no intruding wetting layer exists. At the aniline/Pyrex surface the ellipsometric results suggest that the aniline electron lone pair on the nitrogen atom and the π bonds on the aromatic carbon ring both form hydrogen bonds with the underlying water layer. This surface-induced orientational order decays to bulk liquid structure over ∼ 7 molecular diameters in qualitative agreement with previous results for layering of liquids against solid surfaces.