Interfacial roughness

The aspect of sample preparation and characterization is usually hidden in the smallprint of articles and many details are often not mentioned at all. It is, however, a very crucial point, especially with surface and interface investigations since there might be many unknown parameters with respect to surface contaminations, surface conformations, built-in stresses, lateral sample inhomogeneities, roughness, interfacial contact etc. This is in particular important when surfaces and interfaces are investigated on a molecular scale where those effects may be quite pronounced. Thus special care has to be taken to prepare well defined and artifact free specimens, which is of course not always simple to check. Many of these points are areas of... 

Typically, the analysis of samples involves inclusion of several components such as the outer roughness, interfacial Aims, and the bulk of the sample. Therefore, the overall optical response has to be deconvoluted into signal contributions from the respective parts of the sample. For a reasonable assessment in the calculation, additional knowledge is desirable. Roughness, for example, can be determined in AFM experiments and wiU be used in Section 2.4.1. 

It is helpful to consider qualitatively the numerical magnitude of the surface tensional stabilization of a particle at a liquid-liquid interface. For simplicity, we will assume 6 = 90°, or that 7sa = 7SB- Also, with respect to the interfacial areas, J sA = SB, since the particle will lie so as to be bisected by the plane of the liquid-liquid interface, and. AB = rcr - The free energy to displace the particle from its stable position will then be just trr 7AB- For a particle of l-mm radius, this would amount to about 1 erg, for Tab = 40 ergs/cm. This corresponds roughly to a restoring force of 10 dyn, since this work must be expended in moving the particle out of the interface, and this amounts to a displacement equal to the radius of the particle. 

Ellipsometry can be sensitive to layers of matter only one atom thick. For example, oxidation of freshly cleaved single-crystal graphite can be monitored from the first monolayer and up. The best thicknesses for the ellipsometric study of thin films are between about 1 nm and 1000 nm. Although the spectra become complicated, films thicker than even 1 pm can be studied. Flat planar materials are optimum, but surface and interfacial roughness can be quantitatively determined if the roughness scale is smaller than about 100 nm. Thus ellipsometry is ideal for the investigation of interfacial surfaces in optical coatings and semiconductor struc-... 

A typical structure capable of being analyzed is shown in Figure 3, consisting of a substrate, two films (thicknesses q and t ), two roughness regions (one is an interfacial region of thickness and the other is a surfiice region of thickness One of... 

If contact with a rough surface is poor, whether as a result of thermodynamic or kinetic factors, voids at the interface are likely to mean that practical adhesion is low. Voids can act as stress concentrators which, especially with a brittle adhesive, lead to low energy dissipation, i/f, and low fracture energy, F. However, it must be recognised that there are circumstances where the stress concentrations resulting from interfacial voids can lead to enhanced plastic deformation of a ductile adhesive and increase fracture energy by an increase in [44]. 

As the scale of roughness becomes finer, the effective increase in A can become enormous. Consequently Fg may be raised to very high value. Indeed, as many engineering surfaces are fractal in nature [36], we can only retain the concept of area at all, if we accept that it can be considered as indefinitely large. The practical adhesion does not become infinite, because the joint with a strong interfacial region will fail (cohesively) in some other region where Fg is smaller [89],... 

The above equations (68) and (69) do not guarantee wetting of a rough substrate and express only the fact that the interfacial free energy of a thick... 

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