Dynamic foam, coalescence

More conclusive data about the coalescence kinetics are obtained for dynamic foams where the surface coalescence appears to be the main process (see Chapter 7). 

Dynamically, foams may be subjected to any number of environmental stresses that will act to precipitate bubble coalescence and ultimate foam collapse. Regardless... 

Foams Two excellent reviews (Shedlovsky, op. cit. Lemlich, op. cit.) covering the literature pertinent to foams have been published. A foam is formed when bubbles rise to the surface of a liquid and persist for a while without coalescence with one another or without rupture into the vapor space. The formation of foam, then, consists simply of the formation, rise, and aggregation of bubbles in a hquid in which foam can exist. The hfe of foams varies over many magnitudes—from seconds to years—but in general is finite. Maintenance of a foam, therefore, is a dynamic phenomenon. 

In a third paper by the Bernard and Holm group, visual studies (in a sand-packed capillary tube, 0.25 mm in diameter) and gas tracer measurements were also used to elucidate flow mechanisms ( ). Bubbles were observed to break into smaller bubbles at the exits of constrictions between sand grains (see Capillary Snap-Off, below), and bubbles tended to coalesce in pore spaces as they entered constrictions (see Coalescence, below). It was concluded that liquid moved through the film network between bubbles, that gas moved by a dynamic process of the breakage and formation of films (lamellae) between bubbles, that there were no continuous gas path, and that flow rates were a function of the number and strength of the aqueous films between the bubbles. As in the previous studies (it is important to note), flow measurements were made at low pressures with a steady-state method. Thus, the dispersions studied were true foams (dispersions of a gaseous phase in a liquid phase), and the experimental technique avoided long-lived transient effects, which are produced by nonsteady-state flow and are extremely difficult to interpret. 

In the case of desorption the processes have the opposite direction.) Such interfacial expansions are typical for foam generation and emulsification. The rate of adsorption relaxation determines whether or not the formed bubbles/drops will coalesce upon collision and, in final reckoning how large the foam volume and the emulsion drop-size will be. - Below, we focus on the relaxation time of surface tension, X , which characterizes the interfacial dynamics. 

The problem of dynamic adsorption layers does not only arise in connection with adsorption-desorption processes, it can have a substantial effect on processes of interaction between bubbles or drops and thus on coagulation and coalescence processes in foams and emulsions (Chapter 12). 

Loss of Gas Blockage. In steady-state flow of foam, the major mechanism of foam decay is dynamic capillary suction coalescence (41, 51). This process may occur in earlier stages of an experiment with gasblocking foam but is expected to be less frequent later in the experiment. 

Wasan and his research group focused on the field of interfacial rheology during the past three decades [15]. They developed novel instruments, such as oscillatory deep-channel interfacial viscometer [20,21,28] and biconical bob oscillatory interfacial rheometer [29] for interfacial shear measurement and the maximum bubble-pressure method [15,29,30] and the controlled drop tensiometer [1,31] for interfacial dilatational measurement, to resolve complex interfacial flow behavior in dynamic stress conditions [1,15,27,32-35]. Their research has clearly demonstrated the importance of interfacial rheology in the coalescence process of emulsions and foams. In connection with the maximum bubble-pressure method, it has been used in the BLM system to access the properties of lipid bilayers formed from a variety of surfactants [17,28,36]. 

Destabilization of foams is a dynamic process that includes disproportionation, coalescence in addition to drainage of the thin film between bubbles. In addition to the bulk phase viscosity, all of these processes involve interfacial film properties [2, 4, 7, 10, 11]. The greater stabilizing effect may be attributed to a greater enhancement of the local viscosity in a foam lamellae which tends to inhibit film drainage, as well as to increased thickness of the mixed adsorbed layer which tends to enhance steric stabilization and inhibit bubble coalescence [3],... 

The understanding of foam persistence, or stability, and bubble coalescence is an analytic dilemma. Unquestionably, this is a dynamic... 

As liquid foams are dynamic, once generated, foams disappear gradually by drainage, coalescence, and disproportion. A certain volume of foam is prepared and the changes occurring in it are monitored as a function of time. To improve our understanding of foam stability, it should be characterized as a... 

As seen from the mechanism of foam formation illustrated in Fig. 2, the details of the foam stractures are critically influenced by the bubble size of the liberated hydrogen. In particular, the coalescence kinetics of hydrogen bubbles is of significant importance since the variation of bubble size during metal deposition process detemunes the pore size distribution inside the foam. The coalescence of bubble is known to be driven by the hydrophobic force of bubbles. When this force is sufficient to overcome the hydro-dynamic repulsive force needed to expel water molecules between two bubbles, bubbles start to approach each other and are eventually combined. Accordingly, controlling the hydrophobic force is a key to achieving the control of bubble coalescence and hence the pore size and distribution of the foam stractures. 

Researchers on the bubble dynamics have suggested that the inhibition of bubble coalescence in aqueous solutions can be realized by the addition of specific additives that affect the water structure and hence the hydrophobicity of bubbles. " Among the possible additives, acetic acid is known to be a strong bnbble-stabilizer. Further, it doesn t introduce any metal ions that could be co-deposited as impurity to the copper deposits. Figure 11 shows the effect of acetic acid on the pore size of the foam. The size of surface pores created in the solution containing 0.1 M acetic acid, as shown in Fig. 11(b), was about half of the size of the pores created in the same solution without acetic acid, as shown in Fig. 11(a). This indicates that coalescence of hydrogen bubbles are effectively suppressed by the addition of acetic acid. However, it is noted that the foam layer was about 20 % thinner with the addition of 0.1 M acetic acid, implying that the overall deposition rate may be somewhat suppressed as well. 

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