Stephen Garoff

The enhancement of Raman scattering, resonance Raman scattering, and fluorescence from molecules adsorbed on a rough silver surface

The enhancements of normal Raman scattering, resonance Raman scattering, and fluorescence from molecules adsorbed on identical, well‐characterized, silver‐island films are reported. The enhancement arises from the electromagnetic interaction between the molecules and the electronic plasma resonance of the silver islands. A hierarchy of enhancement ratios is found, with typical values of 105 for RS, 103 for RRS and 10−1 to 10 for fluorescence, depending on the quantum yield of the molecular fluorescence. A model, developed on heuristic grounds and substantiated using the density matrix formalism, describes the light scattering processes and the effects of the plasma resonance. This model presents a unified picture of the surface‐induced enhancement effects and is consistent with the experimental values. The comparison of all the forms of optical scattering leads to a complete determination of the role of the plasma resonances in the various portions of the scattering process. The excitation of the electron...

Physics of contact angle measurement

Abstract Contact angles can be of great value; however, making meaningful contact angle measurements and interpreting those measurements is complex. For years, researchers have addressed a wide variety of issues concerning contact angles. Some questions have been qualitatively answered; others remain open. In this paper, we focus on three issues which are particularly important for the measurement and use of contact angles: the appropriate definitions and use of macroscopic and microscopic contact angles, a brief survey of the length scales relevant to phenomena controlling contact angles, and the role of vibrations in determining contact angles. We emphasize contact angle issues relevant to heterogeneous surfaces, specifically, ambient surfaces prevalent in nature and industry.

On identifying the appropriate boundary conditions at a moving contact line: an experimental investigation

Over the past decade and a half, analyses of the dynamics of fluids containing moving contact lines have specified hydrodynamic models of the fluids in a rather small region surrounding the contact lines (referred to as the inner region) which necessarily differ from the usual model. If this were not done, a singularity would have arisen, making it impossible to satisfy the contact-angle boundary condition, a condition that can be important for determining the shape of the fluid interface of the entire body of fluid (the outer region). Unfortunately, the nature of the fluids within the inner region under dynamic conditions has not received appreciable experimental attention. Consequently, the validity of these novel models has yet to be tested. The objective of this experimental investigation is to determine the validity of the expression appearing in the literature for the slope of the fluid interface in the region of overlap between the inner and outer regions, for small capillary number. This in part involves the experimental determination of a constant traditionally evaluated by matching the solutions in the inner and outer regions. Establishing the correctness of this expression would justify its use as a boundary condition for the shape of the fluid interface in the outer region, thus eliminating the need to analyse the dynamics of the fluid in the inner region. Our experiments consisted of immersing a glass tube, tilted at an angle to the horizontal, at a constant speed, into a bath of silicone oil. The slope of the air–silicone oil interface was measured at distances from the contact line ranging between O.O13 a . and O.17 a , where a denotes the capillary length, the lengthscale of the outer region (1511 μm). Experiments were performed at speeds corresponding to capillary numbers ranging between 2.8 × 10 -4 and 8.3 × 10 -3 . Good agreement is achieved between theory and experiment, with a systematic deviation appearing only at the highest speed. The latter may be a consequence of the inadequacy of the theory at that value of the capillary number.

Fluorescent lifetimes of molecules on silver-island films

The fluorescence of molecules on silver-island films exhibits nonexponential decay and is several orders of magnitude more rapid than on a silica surface, while the total emission intensity is slightly increased. This behavior results from the electromagnetic coupling between the fluorescing molecules and the plasmon resonances of the silver islands. We obtain good agreement with a model that uses the same theory for the electromagnetic coupling as has been used to account for surface-enhanced Raman scattering.

Excitation spectra of surface-enhanced Raman scattering on silver-island films

Both the spectral dependence and the magnitude of the excitation spectra of surface-enhanced Raman scattering are found to be quantitatively related to the absorption spectra for silver-island films. This relationship is maintained even when the local electromagnetic enhancement at the islands is varied by coating the film with a layer of absorbing dye molecules. The results illustrate a simple experimental way to determine independently the role of the electronic plasma resonances in the enhancement process on these films and shed new insights into the details of the electromagnetic interactions responsible for surface-enhanced Raman scattering.

An investigation of microscopic aspects of contact angle hysteresis : pinning of the contact line on a single defect

We examine the microscopic pinning of a contact line on a single heterogeneity. We find that many features of this behavior are predicted by models based on the same energy functionals which describe macroscopic contact angle hysteresis on surfaces with many such imperfections. As the defect emerges from the undistorted contact line, the contact line pins, increasingly distorts, slides across the defect, and then jumps back to an undistorted position. The distortion of a stably pinned contact line is described by a logarithmic dependence. While the force pinning the contact line depends on the defect microstructure, the elastic restoring force follows Hookes law and is dependent on the properties of the fluid and contact angle on the underlying surface. The sliding motion and unstable jumps of the contact line occur at small capillary numbers.

Optical absorption resonances of dye-coated silver-island films

We have used Mie theory to account for the features of the transmission spectra of dye-coated silver-island films. The unusual behavior of the spectra arises from the effect of the dispersion and damping of the optical response of the absorbing coating on the structural resonances of the silver islands. Our modeling indicates that the power dissipation in the dye coating is enhanced compared with the dissipation of dye deposited on a nonresonant structure. These results help elucidate the excitation mechanisms of Raman and fluorescent emission of adsorbates at rough silver interfaces.

Contact angle hysteresis and the shape of the three-phase line

The effects on contact angle and capillary rise hysteresis of a specific pattern of mixed wettability are examined. The capillary rise of a liquid onto a vertical plate consisting of closely spaced alternating horizontal stripes characterized by two different contact angles is considered. For each value of the average rise height, the approximate shape of the liquid surface is determined by an energy minimization technique. An assumed straight horizontal contact line is shown to be unstable to small perturbations when the average rise height lies near those stripe boundaries where the material with smaller equilibrium contact angle lies above the other material. A second family of solutions, suggested by experiment, is shown to be less energetic in these regions. The calculated variation of total energy with mean rise height exhibits multiple energy minima; these metastable states can explain the hysteresis phenomenon. The energy barriers between adjacent minima are found to be significantly smaller than previous calculations suggest, the difference arising from the contact line deformation. This suggests a potential decrease in contact angle hysteresis. In addition, a virtual insensitivity of the energy barrier heights to the intensity of the wall wettability variation is revealed.

Surface interactions of adsorbed molecules as probed by their optical properties

Abstract Polarized absorption and fluorescence spectroscopies have been used to probe fundamental interactions at interfaces both between adsorbed dye molecules and between the adsorbate and the substrate. We have observed optical manifestations of long range dipole-dipole coupling between adsorbates as well as aggregation of dye molecules. A silica interface acts as a simple dielectric medium which confines the molecules to a two-dimensional arrangement; a rough silver interface participates intimately in the electromagnetic interactions between the adsorbate molecules.

The effects of substrate roughness on ultrathin water films

Real surfaces are rough and chemically heterogeneous on many length scales and wetting phenomena on these surfaces are affected by this inhomogeneous nature. We have used x‐ray reflectivity to examine the structure of static precursing films which precede the macroscopic meniscus of water on glass. We have examined the thickness of the film, the roughness of the interfaces bounding the film, and the conformality or correlations between the roughness features of these bounding interfaces. The wetting film consists of three basic components: the bulk meniscus, a thin (≤100 A) precursing wetting film which has a sudden termination, and beyond that, a much thinner film characteristic of the glass surface in contact with water vapor only. The roughness and chemical heterogeneity of the solid manifest themselves in the roughness of the water/vapor interface of the film, in the irregularity of the terminating boundary of the precursor wetting film, and in the hysteretic pinning of that boundary. The roughness of...