Film rupturing, definition

The n(fc) isotherms of different types of foam films are shown in Fig. 7.8. The surfactant (NaDoS) and electrolyte (NaCl) concentrations were the same as those used in the experiments with foams. The equilibrium thickness of thin films and CBF decreased with the increase in pa = II. Films ruptured in a definite range of capillary pressure (marked with arrows on curves 1 and 2). The thickness of NBF did not change and they ruptured at a definite capillary pressure (marked with an arrow on curve 3). 

FIGURE 4.86 Effect of duplex oil film on rupture of pseudoemulsion film by an axially symmetrical particle with many edges hinging in oil—water interface at rugosity jj = 1 (see Figure 4.84 for definition of particle geometry). Film rupture only requires that 0ow > 90°. 

The rest time is measurable evidence of the rate of thinning of the benzene film which separates the drop from the main pool of water. It has been postulated that when this film has been reduced to some definite critical film thickness, rupture will occur. But several authors have... 

In addition to electric charge between particles other factors are in some cases operative in preventing actual contact, thus the medium may be strongly adsorbed by the surface, and the thin film may not readily be displaced on collision of the two neutral particles, in other cases a tough elastic film may be formed, possessing definite mechanical strength and necessitating a violent impact to ensure rupture. 

The average lifetime r from the moment of formation of a film with a definite radius until the moment of its rupture serves usually as a measure of black film stability. Obviously, such a kinetic measure of stability can be applied to thermodynamically unstable films or to metastable films. 

Temperature dependence of the critical concentration Ce of a foam bilayer formation. The Cc concentration (see Eq. (3.129)) of formation of DMPC foam bilayer was determined on the basis of observations of the final state which the foam film reached during its drainage (see Section 3.2), i.e. either rupture at a definite critical thickness without formation of black spots occurs, or formation of foam bilayer via black spots is observed. Rupture at critical thickness occurred at lower DMPC concentrations in the solution (C < Cc) and black spots were formed at higher concentrations (C > Cc). These black spots encountered the film turning it into a foam bilayer of constant radius. At each temperature a series of observations were carried out at various DMPC concentrations for the determination of Cc (the minimum DMPC concentration at which a foam bilayer is formed). This concentration is... 

Equation 5.26 predicts that as the antifoam concentration of species i becomes very large, Vaf> d therefore F, both tend to zero because we must have by definition Qihigh concentrations, they eliminate any tendency for foam to form during aeration (see, e.g.. Figure 4.2). On the other hand, if c, —> 0, then F 1 even if 0. However, it should be noted that at extremely low concentrations of antifoam Af,(r,)(= A,W(r,)C ) will become <1. The latter implies that we can in principle have qt —> 0, and therefore F 0, if q Q and 0 < N r < 1 even though the latter means that some films will be without antifoam Clearly then the physical basis of supposing that the probability of a bubble surviving without rupture is f (and therefore of Equations 5.25 and 5.26) will be invalidated if A, (r,) < 1. 

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