Rupture, films

Film Rupture. Another general mechanism by which foams evolve is the coalescence of neighboring bubbles via film mpture. This occurs if the nature of the surface-active components is such that the repulsive interactions and Marangoni flows are not sufficient to keep neighboring bubbles apart. Bubble coalescence can become more frequent as the foam drains and there is less Hquid to separate neighbors. Long-Hved foams can be easHy... 

G. Reiter. Unstable thin polymer films rupture and dewetting process. Langmuir 9 1344-1351, 1993. 

Erratic behaviour caused by random film rupture. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

Fig. 8.2 Strain-generated active path mechanisms, (a) Often referred to as the film rupture model and (b) the slip step dissolution model. In both cases growth is by dissolution film rupture is the rate controlling step, not the mechanism of crack growth...

Lin, T. R., Analysis of Film Rupture and Reformation Boundaries in a Finite Journal Bearing with Micropolar Fluids, Wear, Vol. 161,1993,pp. 145-153. 

Reiter, G. (1993) Unstable thin polymer films Rupture and dewetting processes. Langmuir, 9, 1344-1351. 

For example, a synergistic defoaming occurs when hydrophobic solid particles are used in conjunction with a liquid that is insoluble in the foamy solution [652]. Mechanisms for film rupture by either the solid or the liquid alone have been elucidated, along with explanations for the poor effectiveness, which are observed with many foam systems for these single-component defoamers. 

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)... 

Hence, film rupture is much faster than either film transport or film drainage and filling. In this initial analysis we therefore neglect any time lag for the rupture event and assess breakage as... 

Film reactors, Chemithon, 23 544-547 Film reactor systems, 23 544 Film rupture, 12 14 Film sharpness, in color photography, 19 264-265... 

Craddng or checking Shrinkage of the coating resulting in film rupture... 

Alligatoring Film rupture resulting from the application of a brittle film over a more flexible coating... 

A.J. de Vries Foam Stabflity. IV. Kinetics and Activation Energy of Film Rupture. Rec. Trav. Chim. Pays-Bas Belgique 77, 383 (1958). 

The regime governed by coalescence was examined in more detail. The process of film rupture is initiated by the spontaneous formation of a small hole. The nucleation frequency. A, of a hole that reaches a critical size, above which it becomes unstable and grows, determines the lifetime of the films with respect to coalescence. A mean field description [19] predicts that A varies with temperature T according to an Arrhenius law ... 

In order to understand the basis for the prevention of bubble coalescence and hence the formation of foams, let us examine the mechanical process involved in the initial stage of bubble coalescence. The relatively low Laplace pressure inside bubbles of reasonable size, say over 1 mm for air bubbles in water, means that the force required to drain the water between the approaching bubbles is sufficient to deform the bubbles as illustrated in Figure 8.2. The process which now occurs in the thin draining film is interesting and has been carefully studied. In water, it appears that the film ruptures, joining the two bubbles, when the film is still relatively thick, at about lOOnm thickness. However, van der Waals forces, which are attractive in this system (i.e. of air/water/air), are effectively insignificant at these film thicknesses. 

Figure 8.3 Surface correlated wave model for water film rupture.

The polymer concentration sets the osmotic pressure of the system. This is increased until film rupture occurs, resulting in coalescence, which is easily seen under the microscope since the original emulsions are monodisperse. 

Since monodisperse creams of a range of droplet sizes can readily be prepared, it is possible to study the effect of droplet size on the critical osmotic pressure required for film rupture, n. This was found to increase with increasing droplet size. The critical osmotic pressure is, in effect, the disjoining pressure as smaller droplets have higher disjoining pressures (due to a smaller radius of curvature),... 

A stabilising effect in the presence of salt was also noted by Aronson and Petko [90]. Addition of various electrolytes was shown to lower the interfacial tension of the system. Thus, there was increased adsorption of emulsifier at oil/water interface and an increased resistance to coalescence. Salt addition also increased HIPE stability during freeze-thaw cycles. Film rupture, due to expansion of the water droplets on freezing, did not occur when aqueous solutions of various electrolytes were used. The salt reduced the rate of ice formation and caused a small amount of aqueous solution to remain unfrozen. The dispersed phase droplets could therefore deform gradually, allowing expansion of the oil films to avoid rupture [114]. 

II Figure 1). Adsorption continually occurs around the bubbles to replace protein in areas of the interface where coagulation or stretching of the film is occurring. The actual bubble size in the foam depends upon the rate of protein adsorption as well as upon the ease of film rupture. The protein films on adjacent bubbles come in contact and trap the liquid, preventing it from flowing freely. This restriction is governed by the viscosity of the colloidal solution. The polypeptides of denatured proteins situate to positions where their hydrophobic side chains are directed outward toward each other. Because liquid... 

Now we can understand what happens in the case of soap bubbles. As discussed in Chapter 7, the soap molecules spread on both air-water interfaces enclosing a film of water and provide the repulsion necessary to maintain a water film, similar to the case we discuss in Example 11.3. In the absence of soap molecules, the (attractive) air-air van der Waals forces through the water film rupture the film so that the bubbles are unstable. 

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