SURFACE EXPANSION

Patent Number US 5350544 A 19940927 METHOD OF PREPARING A CROSSLINKED, POLYETHYLENE FOAM PRODUCT BY SURFACE EXPANSION OF A FOAM... 

There is no simple, direct relationship between elasticity and emulsion or foam stability because additional factors, such as film thickness and adsorption behaviour, are also important [204]. Nevertheless, several researchers have found useful correlations between EM and emulsion or foam stability [131,201,203], The existence of surface elasticity explains why some substances that lower surface tension do not stabilize foams [25]. That is, they do not have the required rate of approach to equilibrium after a surface expansion or contraction as they do not have the necessary surface elasticity. Although greater surface elasticity tends to produce more stable bubbles, if the restoring force contributed by surface elasticity is not of sufficient magnitude, then persistent foams may not be formed due to the overwhelming effects of the gravitational and capillary forces. More stable foams may require additional stabilizing mechanisms. 

Unlike in three dimensions, where liquids are often considered incompressible, a surfactant monolayer can be expanded or compressed over a wide area range. Thus, the dynamic surface tension experienced during a rate-dependent surface expansion, is the result of the surface dilational viscosity, the surface shear viscosity, and elastic forces. Often, the contributions of shear and/or the dilational viscosities are neglected during stress measurements of surface expansions. Isolating interfacial viscosity effects is difficult because, since the interface is connected to the substrate on either side of it, the interfacial viscosity is coupled to the two bulk viscosities. 

Substrate Face Bulk Spacing(A) Surface Expansion(%) Spacing(A) Method... 

There appear then three primary mechanisms for stabilizing (or destabilizing) a three phase foam. The first derives from the micelle structuring in the film and depends directly upon surfactant concentration and electrolyte concentration. The second is a surface tension gradient (Marangoni) mechanism which relates to the short range intermolecular interactions and the rate of surface expansion. And the third is an oil droplet size effect which depends upon the magnitude of the dynamic interfacial tension. 

Figure 1.17. Successive stages of the breakaway of a liquid drop from the orifice of a stalagmometer. Details of the process depend on the dynamics of the surface expansion, mechanical vibrations and oscillations. The little additional drop in the last sketch is called a satellite. Sometimes there are more of these.

A different situation occurs for Pt(100) planes where hydrogen adsorption deconstructs the hex-reconstructed Pt(100) surface. This fact that above a critical temperature of 35 K, the surface is partially deconstructed by molecular hydrogen was first found by some authors [5,6] and later by Wandelt et al. [7], By the analysis of low-energy electron diffraction (LEED) data, a surface expansion of 3% can be obtained for the deconstruction of Pt(100) from the (1 x 1) to the hex structure. The initial heat of adsorption (at a negligible coverage) for the unreconstructed surface is —90 kJ mol-1, whereas it is —98 kJ mol-1 for the hex structure [8] the difference attributed to the presence of steps during reconstruction. 

FIGURE 11.5 Foaming properties of some proteins (solution of 0.25 mg per ml) in relation to the dynamic surface tension, y is surface tension d In A/dt is the surface expansion rate d is the approximate average bubble diameter. (After results by H. van Kalsbeek, A. Prins. In E. Dickinson, J. M. Rodriguez Patino, eds. Food Emulsions and Foams. Roy. Soc. Chem., Cambridge, 1999, pp. 91-103.)... 

Many surfactant solutions show dynamic surface tension behavior. That is, some time is required to establish the equilibrium surface tension. If the surface area of the solution is suddenly increased or decreased (locally), then the adsorbed surfactant layer at the interface would require some time to restore its equilibrium surface concentration by diffusion of surfactant from or to the bulk liquid. In the meantime, the original adsorbed surfactant layer is either expanded or contracted because surface tension gradients are now in effect, Gibbs—Marangoni forces arise and act in opposition to the initial disturbance. The dissipation of surface tension gradients to achieve equilibrium embodies the interface with a finite elasticity. This fact explains why some substances that lower surface tension do not stabilize foams (6) They do not have the required rate of approach to equilibrium after a surface expansion or contraction. In other words, they do not have the requisite surface elasticity. 

The rubber being isotropic, the local linear expansion resulting from the absorption of liquid is the same in all directions [15], Thus, the surface expansion A u,t) is as follows ... 

The surface expansion factor of 1.525 is not shown, but the surface of the sheet in contact with the liquid increases in this way. 

Fig. 8.8. The supermodified simplex, (a) The responses observed at W and R would indicate an expansion of the modified simplex, (b) With this response surface expansion would not be the best movement. A P value of about 1.3 would produce a larger response, (c) For the concave surface the R vertex is maintained.

Typical examples include studies of the underpotential deposition of various metals on metallic substrates. The structure of the upd-layer [33, 34], the position of adsorbed anions and water molecules on top of the upd-layer and the respective bond angles and lengths could be elucidated [35, 36]. Surface reconstruction caused by weakly adsorbed hydrogen [37], surface expansion effects of low-index platinum and gold surfaces correlated with adsorption/desorption of solution species [38] and... 

In this section we briefly discuss the surface expansion of the group IB metals, Cu, Ag and Au, focusing on the close-packed (111) surfaces as they have been studied in the most detail (in fact there have been no published SXS studies of the Cu(lOO) and Cu(llO) surfaces in the electrochemical environment). In terms of surface expansion effects, the IB metals are more difficult to study than Pt as no Hupd is formed, and so it is difficult to correlate stmctural changes with weU-defined adsorption processes. Furthermore, the Au(hkl) surfaces reconstruct at negative potential, which limits the potential window where the surfaces are in the unreconstructed state. Despite these difficulties, relaxation at the Au(lll) surface was recently studied by a combination of SXS and surface stress measurements. For potentials on the positive side of the potential of zero charge (pzc), where the surface is unreconstructed, increasing positive surface charge... 

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