Surface bulk phenomenon

A surface-bulk phenomenon of technical importance is the diffusion of an evaporated metal surface layer into the bulk of the backing metal. Fig.9 shows how such a process can be followed in an electron spectrum as a function of time. The time evolution of the Au4f lines is observed as the evaporated gold diffuses into the various substrates, Z.n,... 

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

The physical meaning of the parameter 2FNG/I is obvious It expresses the time required to form a monolayer of oxide ions on a surface with NG adsorption sites when the oxide ions are supplied at a rate I/2F. This proves that NEMCA is a surface phenomenon (not a bulk phenomenon and not a phenomenon at the tpb) taking place over the entire gas-exposed catalyst electrode surface. 

As noted previously in Chapter 3, the improved dimensional stability of wood as a result of anhydride modification has been found to be a function of WPG only, irrespective of the anhydride used for modification (Stamm and Tarkow, 1947 Hill and Jones, 1996b Li etal., 2000b). This shows that improved dimensional stability arises due to a bulking phenomenon, caused by the volume occupied by the bonded acyl adducts in the cell wall. The dimensional stabilization of wood modified with crotonic anhydride has also been reported ( etin and Ozmen, 2001). Reductions in the EMC have also been attributed to a bulking phenomenon (Papadopoulos and Hill, 2003), and in at least one example of decay protection (Papadopoulos and Hill, 2002). Further research is needed to determine if this is the case for resistance to other decay organisms. Only with the phenomenon of surface wettability does the relationship between a physical property of chemically modified wood and WPG seem to be broken (Hill and Jones, 1996c) (Table 4.1). 

Such a surface/bulk reactivity phenomenon may in part be responsible for the low (or zero) reactivity reported for BaP deposited on, or present in, a variety of substrates and exposed to ambient levels of 03 (e.g., 100 ppb) in air (see Grosjean et al., 1983 and Coutant et al., 1988). These observations can be rationalized by assuming that, while BaP in fact does react rapidly with 03 in ambient particles, not all of it is at (or close enough to) the surface to be available for reaction (Atkinson et al., 1988a Arey, 1998a). 

Despite limitations, the most common sorption medium is activated charcoal — a form of carbon treated in such a way as to open a large number of pores. The surface energy of the material and the pores combine to produce a material that can first attract and then trap small organic molecules. The attraction is via adsorption rather than absorption. Adsorption applies to attachment to the surface absorption is a bulk effect. Extraction is a bulk phenomenon. Simply put, adsorption is a function of surface area while absorption is a mass effect. 

The onset of photoconductivity is determined from the intersection point of the x-axis (dark current) and the extrapolated line from the linear part of the conductivity. All spectra show a pronounced onset of photoconductivity at 2.40 eV (Na S), 2.25 eV (K S), 2.15 eV (Na Se), and 2.10 eV (K Se), respectively, which is found to be in remarkable agreement with the optical gap. This proves that the photoconductivity is a bulk property instead of a surface conduction phenomenon. 

As described above, the initial cause of membrane fouling is concentration polarization, which results in deposition of a layer of material on the membrane surface. The phenomenon of concentration polarization is described in detail in Chapter 4. In ultrafiltration, solvent and macromolecular or colloidal solutes are carried towards the membrane surface by the solution permeating the membrane. Solvent molecules permeate the membrane, but the larger solutes accumulate at the membrane surface. Because of their size, the rate at which the rejected solute molecules can diffuse from the membrane surface back to the bulk solution is relatively low. Thus their concentration at the membrane surface is typically 20-50 times higher than the feed solution concentration. These solutes become so concentrated at the membrane surface that a gel layer is formed and becomes a secondary barrier to flow through the membrane. The formation of this gel layer on the membrane surface is illustrated in Figure 6.6. The gel layer model was developed at the Amicon Corporation in the 1960s [8],... 

Application of the Mossbauer effect, which is essentially a bulk phenomenon, to the study of surfaces has received significant attention in recent years. The usefulness of this technique lies in its ability to determine the electronic environment and symmetry of the surface nucleus, and it offers a method of investigation that is clearly complementary to other physical methods for the characterization of solid surfaces. Mossbauer spectroscopy has the attractive advantage that it may be used at a variety of pressures and can be applied to the study of heterogeneous catalysis and adsorption processes to probe the nature of the solid surface and its electronic modification when holding adsorbed species. 

Adsorption The process whereby the molecule in question only reacts with the surface of the compound which is doing the adsorption. In contrast to absorption, which is a bulk phenomenon. 

F is the bulk collision constant, A is a positive dimensionless factor, Vf is the Fermi velocity and R the particle radius. From a classical point of view, this modification is supported by the fact that, when the radius is smaller than the bulk mean free path of the electrons, there is an additional scattering factor at the particle surface. This phenomenon, known as the mean free path effect, is abundantly discussed in [19]. In a quantum approach, the boundary conditions imposed to the electron wave functions lead to the appearance of individual electron-hole excitations (Fandau damping) [21] resulting in the broadening of the SPR band proportional to the inverse of the particle radius as in Eq. (8) [22]. A chemical interface damping mechanism has also been considered, leading to the l/R dependence of F [23]. 

No assumptions have been invoked to obtain this result. As illustrated below, the mass flux term with respect to a stationary reference frame, V p, v, contains contributions from bulk fluid flow (i.e., convection) and molecular mass transfer via diffusion. In fact, whenever the divergence of a flux appears in a microscopic balance expression, its origin was a dot product of that flux with the outward-directed unit normal vector on the surface of the control volume, accounting for input and output due to flux across the surface that bounds V(t). The divergence of a flux actually represents a surface-related phenomenon that has been transformed into a volume integral via Gauss s law. 

Adsorption is a surface phenomenon. Absorption is a bulk phenomenon. They must not be confused. 

The consensus appears to favor the position that band formation is a bulk phenomenon and not due to instability or surface effects however wall effects may produce gap dependence. 

The electroosmotic mobility and bulk EOF velocity are sensitive to the physicochemical properties of the solution and channel walls. Because electroosmosis is a surface-driven phenomenon, EOF fluid transport is characterized by plug flow. Except in the thin EDL, there is no shear in the fluid flow and so the velocity profile is flat. At the same time, the individual species charged permanently or spontaneously in the bulk fluid also undergo electric forces. 

The adhesional friction, then, can be considered to be truly a surface as well as a bulk phenomenon. 

So far, we have discussed the properties of soft shape-memory materials as a function of their nanoarchitectonics. At the same time, material microarchitectonics, also known as material forms, should be tailored to specific applications as SME in material was generally treated with bulk phenomenon. As such, designing an SMP object at the submicron to microscale is challenging. In particular, the synthesis of well-regulated nanoscale structures built up as meso- or macroscopic materials remains challenging. Recent rapid evolution in SMPs has been developed in efforts to meet various requirements in diverse potential applications. As a result, not only traditional film, tube, and sponge (foam) forms, but also microparticles, surface as well as micro/nanofiber that exhibit SMEs at mesoscopic to microscopic scales, have attracted much attention (Fig. 5.2.4). 

There are two types of humidity sensors capacitive and resistive. The working principle of the capacitive sensors is based on a bulk phenomenon (Kulwicki, 1991) unlike the resistive sensors, which are more a surface phenomenon. Resistive sensors are also... 

Data reported on the low-temperature crystallization (at —90 C) of (PDMS), by. Y-ray and proton n.m.r., show that crystallization is a bulk phenomenon and not surface induced. Dielectric studies on amorphous and crystalline polymers have been reported. Dynamic Kerr-effects and dielectric relaxation studies of a poly(methylphenyl siloxane) have been reported in the region of its relaxation progress, and it is suggested that a substantial contribution to the process is due to motions about the silicon-aromatic ring bond. ... 

According to Alpert [8], the separation mechanism in HILIC is primarily based on partitioning. While retention in conventional NPLC is governed by surface adsorption phenomenon, Alpert suggested that the retention mechanism for HILIC is a partitioning between the bulk acetonitrile-rich eluent and a water-rich layer, partially immobilized onto the hydrophilic stationary-phase surface (see Figure 7.1) [6]. 

---