Particles liquid-like

A similar comparison of rate constants for 0.6 and 5% Pt/alumina and 1,8% Ag/alumina catalysts at 673 K in oxygen atmosphere reveals that sintering rates for the Ag catalyst are roughly 40-50 times higher than for either Pt catalyst. Thus, Pt is clearly much more thermally stable than Ag under oxidizing conditions. These results are consistent with those from a model catalyst study [44] of sintering of Pt and Ag on alumina in vacuum in which it was observed that Pt/alumina was thermally stable in vacuum to about 873 K, above which temperature liquid-like particle migration was observed, while Ag/alumina was stable to only 723 K, above which temperature evaporation of the metal was observed. This latter result is... 

Figure 12.7 shows values of xy calculated in two different ways as a function of temperature for panicles of dilTerent. size. One set of curves (solid lines) was calculated front the phenomenological relationships (12.5) and (12.6). For the solid particles (low temperature). MD simulations were used to obtain the diffusion coefficient and surface tension that appear in (12.6). For the liquid-like particles (high temperature), data for the viscosity and surface tension that appear in (12.5) were obtained from experimental results reported in the literature. The dashed lines in Fig. 12.7 were calculated directly from MD simulations of the decrease in the moment of inertia of two coalescing spheres. 

These are by far the most commonly used systems for the formulation of insoluble solids. The solid can be hydrophobic, such as most organic materials that are used in pharmaceuticals, agrochemicals, and paints the solid can also be hydrophilic, such as silica and clays. With some pigments and inks the particles need to be very small - that is, in the nanosize range - and these are referred to as nanosuspensions. Latexes may also be considered as suspensions, particularly if the particles are solid-like at ambient temperatures. With many of the latexes that are used in paints the particles are liquid-like at below and ambient temperature, but when applied to a surface these liquid-like particles coalesce to form a uniform film. The system may then be considered as an emulsion. 

Fig. 12. Snapshot of a cross-section of a critical nucleus of a hard-sphere crystal at a liquid volume fraction (j) = 0.5207. The figure shows a three-layer thick slice through the center of the crystallite. Solid-like particles are shown in yellow and liquid-like particles in blue. The layers shown in the figure are close-packed hexagonal crystal planes. The stacking shown in this figure happens to be fcc-like, i.e. ABC-stacking — however, analysis of many such snapshots showed that fee and hep stackings were equally likely...

Otlier possibilities for observing phase transitions are offered by suspensions of non-spherical particles. Such systems can display liquid crystalline phases, in addition to tire isotropic liquid and crystalline phases (see also section C2.2). First, we consider rod-like particles (see [114, 115], and references tlierein). As shown by Onsager [116, 117], sufficiently elongated particles will display a nematic phase, in which tire particles have a tendency to align parallel to... 

Flard spherocylinders (cylinders witli hemispherical end caps) were studied using computer simulations [118]. In addition to a nematic phase, such particles also display a smectic-A phase, in which tire particles are arranged in liquid-like layers. To observe tliis transition, ratlier monodisperse particles are needed. The smectic-A phase was indeed observed in suspensions of TMV particles [17]. 

The pores in question can represent only a small fraction of the pore system since the amount of enhanced adsorption is invariably small. Plausible models are solids composed of packed spheres, or of plate-like particles. In the former model, pendulate rings of liquid remain around points of contact of the spheres after evaporation of the majority of the condensate if the spheres are small enough this liquid will lie wholly within the range of the surface forces of the solid. In wedge-shaped pores, which are associated with plate-like particles, the residual liquid held in the apex of the wedge will also be under the influence of surface forces. 

Perlite and Solka-floc are finely divided powders manufactured from a volcanic mineral and from wood pulp respectively, which have filtration properties very similar to those of diatomite. Like diatomite, they are inert to a wide range of process liquids. Like diatomite, they are available in a range of particle-size distributions to give the desired clarity and flowrate in different applications. On a cost-of-use basis, they are as economical as, or more economical than, diatomite. 

FIG. 13 A colloidal liquid crystal. The rod-like particles point to a preferred diree-tion, called the nematic director. The solvent is disordered. 

Smectic A and C phases are characterized by a translational order in one dimension and a liquid-like positional order in two others. In the smectic A phase the molecules are oriented on average in the direction perpendicular to the layers, whereas in the smectic C phase the director is tilted with respect to the layer normal. A simple model of the smectic A phase has been proposed by McMillan [8] and Kobayashi [9] by extending the Maier-Saupe approach for the case of one-dimensional density modulation. The corresponding mean field, single particle potential can be expanded in a Fourier series retaining only the leading term ... 

In the sections above, only infinite planar interfaces between air and an aqueous phase or two immiscible liquids like water and DCE were considered. Reducing the question to this class of surfaces only would be a severe limitation in the scope of the review as more reports appear in the literature debating on the SH response from small centro-symmetrical particles [107-110]. It is the purpose of this section to discuss the SHG response from interfaces having a radius of curvature of the order of the wavelength of light. 

The final main category of non-Newtonian behaviour is viscoelasticity. As the name implies, viscoelastic fluids exhibit a combination of ordinary liquid-like (viscous) and solid-like (elastic) behaviour. The most important viscoelastic fluids are molten polymers but other materials containing macromolecules or long flexible particles, such as fibre suspensions, are viscoelastic. An everyday example of purely viscous and viscoelastic behaviour can be seen with different types of soup. When a thin , watery soup is stirred in a bowl and the stirring then stopped, the soup continues to flow round the bowl and gradually comes to rest. This is an example of purely viscous behaviour. In contrast, with certain thick soups, on cessation of stirring the soup rapidly slows down and then recoils slightly. 

When fluidized, the particles are suspended in the gas, and the fluidized mass (called a fluidized bed) has many properties of a liquid. Like a liquid, the fluidized particles seek their own level and assume the shape of the containing vessel. Large, heavy objects sink when added to the bed, and light particles float. 

To obtain more detailed information on the ultrastructure of lipid dispersions and the morphology of the particles, electron microscopy is usually performed on replicas of freeze fractured or on frozen hydrated samples. These techniques aim to preserve the liquid-like state of the sample and the organization of the dispersed structures during preparation. By using special devices, the sample is frozen so quickly that all liquid structures, including the dispersion medium, solidify in an amorphous state. 

The earliest work in this area assumed that particles in the atmosphere were solid and that the uptake of SOC involved adsorption to a solid or solid-like surface. It was subsequently recognized that many atmospheric particles are liquid or have liquid-like outer layers, and hence the uptake of gases could be treated as absorption into a liquid. These approaches are summarized in the following. It should be noted that these treat the equilibria between the gas- and condensed-phase species i.e., it is assumed that thermodynamics rather than kinetics controls the distribution between the phases. The implications of this assumption are discussed later. 

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