Foam destruction avalanche-like

Systematic studies of the influence of border pressure on the kinetics of foam column destruction and foam lifetime have been performed in [18,24,41,64-71], Foams were produced from solution of various surfactants, including proteins, to which electrolytes were added (NaCI and KC1). The latter provide the formation of foams with different types of foam films (thin, common black and Newton black). The apparatus and measuring cells used are given in Fig. 1.4. The rates of foam column destruction as a function of pressure drop are plotted in Fig. 6.11 [68]. Increased pressure drop accelerates the rate of foam destruction and considerably shortens its lifetime. Furthermore, the increase in Ap boosts the tendency to avalanche-like destruction of the foam column as a whole and the process itself begins at higher values of foam dispersity. This means that at high pressure drops the foam lifetime is determined mainly by its induction period of existence, i.e. the time interval before the onset of its avalanche-like destruction. This time proves to be an appropriate and precise characteristic of foam column destruction. 

These pressures are clearly seen in Fig. 6.14, plotted for a foam from silicon-organic compounds. For a Triton-X-100 foam (Fig. 6.13) a weak decrease in the rp(Ap) dependence is also observed within the range of pressure drops higher than 10 kPa. The avalanche-like destruction occurs after a period of some minutes, i.e. after a certain change in the initial dispersity, and probably, after a bubble distribution by size that influences the process of structural rearrangement. 

Other experiments showed that the foam column destruction was initiated by the rupture of individual films within the range of maximum pressure (close to the porous plate). For a Triton-X-100 foam this critical pressure was 8-9 kPa. This means that under critical pressure drop corresponding to an avalanche-like foam column destruction, the equilibrium... 

All the results presented so far give reason to conclude that the avalanche-like destruction of a foam column at definite temperature, pressure drop and foam dispersity, depends mainly on the equilibrium pressure reached. However, in order to establish the mechanism of action of the critical pressure drop, further studies of single foam films and foams are required. They should be performed under conditions that reveal the role of all elements of the foam (films, borders and vertexes) in the process of foam destruction. 

For NaDoS foams at Ap0 < 20kPa an avalanche-like destruction is not realised. Critical pressure in the foam is determined by the type of foam films and the type of the surfactant solution used to obtain the foam. For example, for foams from aqueous protein solution Apcr is by two orders of magnitude less than Apcr of foams from the non-ionic surfactant NP20. 

Thus, on the one hand, the foam film type (and therefore the type of stabilisation due to long-range and short-range forces) is the determining factor for the course of tp(Ap0) dependences. On the other hand, in some systems an avalanche-like destruction occurs at A/ cr. A reasonable question arises as to why a foam built up of different types of foam films destructs before reaching Aplr. If the foam is considered as a system built up of equilibrium foam films, then they should be infinitely stable. However, the foam is a more complex system, built up of foam films and borders, subjected to the effect of several other factors (gas diffusion transfer, coalescence of bubbles and changes in the foam film size, external actions, local stretching, collective effects of destruction, etc.) which can lead to its destruction. 

It is necessary to mention that an avalanche-like destruction is also observed in a NaDoS foam but it occurs at significantly higher pressure drops with respect to the equilibrium pressure pa (see Fig. 6.12,a). That is why it is important to distinguish between the destruction at equilibrium critical Apcr,e and at non-equilibrium critical Apcrne pressures since probably the causes are different. Foam destruction at Apcre is perhaps due to foam film rupture while at Apcr ne the destruction results from other phenomena occurring in the disperse... 

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