Thin spot

For ordinary atmospheric exposure, it is usually possible to arrange that thin spots of the film, such as the contact points of the anodising jigs, are located in relatively unimportant positions on the article and are hidden from view. 

If a critical film thickness is not reached during film drainage, the drops separate from each other. Conversely, if the critical film thickness is reached, the film ruptures—as a result of van der Waals forces—and the drops coalesce. This generally occurs at thin spots, because van der Waals forces are inversely proportional to h (Verwey and Overbeek, 1948). The value of bent can be determined by setting the van der Waals forces equal to the driving force for film drainage, giving (Verwey and Overbeek, 1948)... 

Another possible approach to burn-out prediction is to study film breakdown due to hydrodynamic effects. Presumably, if thin spots occur in a film for any reason, the film becomes hotter, the surface tension is reduced, and increased vaporization tends to cause a break in the liquid layer. Although studies of surface tension and wetting angle in thin-film flow have been made, no successful correlation of burn-out in these terms has yet been offered. 

The surface elasticity of the solution is an especially important property because it is at this point that the air is entrapped. The phenomenon of surface transport, by which a spreading monolayer drags with it significant quantities of the underlying solution and rapidly repairs thinning spots in a lamella, was suggested to occur in quality foaming solutions. 

Alternatively, the surfactant concentration may be equalized by a second mechanism. Surfactant molecules may migrate from the bulk-liquid/gas interface. If this happens at a taster rate than surfactant initiation along the interface, then the thinned spot will not heal and the film will become weak and the foam unstable (Fig. 8d). 

A. i.e,. E — da/din A. If the film of liquid separating two neighboring bubbles in a foam develops a thin spot, the surface tension gradient in the vicinity of the thin spot will induce a Muraugoni flow of liquid toward Ihe direction of higher cr. This flow of liquid toward the thin spot helps heal the fluctuation and thus keeps the neighboring bubbles from coalescing. 

In another modem machine, the glass flows downward from an orifice in a continuous stream that passes between rollers that flatten it into a ribbon with alternate thick and thin spots. The ribbon is picked up by a horizontally moving support in which voids coincide with the thick portions... 

This is not surprising since the calculations are based on a perfectly uniform wall thickness of 1.2 pm for all spheres. In actual practice, as it was mentioned above, there are frequently thin spots in the glass shells, and some spheres have shells thinner than the mean value. Since the apparent modulus of the hollow spheres depends... 

When applying the glaze, coat the object evenly to avoid thick and thin spots in the glaze finish. 

Figure 2.3 Step coverage of deposited metal. Poor step covoage leads to thin spots in the metal, resulting in poor electromigration performance.

Bleeding oesophageal varices may be triggered by three pathogenetic mechanisms (7.) critical pressure increase in the varices, especially when it is sudden (= explosion hypothesis), (2.) occurrence of flow turbulences with bidirectional blood flow in the varices (= circulation hypothesis), and (5.) inflammatory lesion at the varix surface with the formation of thin spots (= erosion hypothesis). 

Thompson, G. Gibson, S. 1991. Subcontinental mantle plumes, hotspots and pre-existing thin-spots. Journal of the Geological Society, London, 148, 973-977. 

Qualitatively, in a number of cases, foam stability has been correlated with viscosity of the surface film, but the relation is not really clear. There are stable foams in which the viscosity of the surface film is not particularly high and viscous monolayers that do not produce particularly stable foams. However, it appears well accepted that if the viscosity of the surface film is either very low (a gaseous monomolecular film) or very high (a solid monomolecular film), the foam produced will be unstable. In both of these cases film elasticity is low. In addition, too high a surface viscosity can slow down self-healing of thinned spots in the film by the surface transport mechanism. 

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