Coalescence, process film drainage

The flocculation and coalescence processes of a polydis-persed lamella or film can be divided into two processes film drainage and film rupture. To model the film-mpture process of polydispersed emulsions, film stress-relaxation experiments were carried out. In these experiments, the film was quickly expanded and then the relaxation of the film was measured. To characterize the film-drainage process, dynamic film-tension measurements were conducted in which the film was continuously and slowly expanded while the film tension was monitored. Single interfaces were also studied by forming a drop at the eapillary (7). 

It is argued that the kinetics of the limited coalescence process is determined by the uncovered surface fraction 1 - t and by the rate of thinning (drainage) of the films formed between the deformable droplets [46,47], The homogeneous and monodisperse growth generated by limited coalescence is intrinsically different from the polydisperse evolution observed for surfactant-stabilized emulsions. As noted by Whitesides and Ross [48], the mere fact that coalescence halts as a result of surface saturation does not provide an obvious explanation of the very... 

One of the most important factors regulating the rate of foam collapse (especially coalescence process) is the surfactant kind, along with the additives, both affecting the equilibrium film thickness, film stability, rate of film thinning and rate of drainage. Unfortunately, data about the kinetics of internal foam collapse for foams from various surfactants under comparable conditions are very poor. 

When two drops first come into contact in the process of coalescing, a film of continuous phase becomes trapped between them. The film is compressed at the point of encounter until it drains away and the two drops can merge. Decreasing the viscosity of the continuous phase, by heating or by addition of a low-viscosity diluent, may promote drop coalescence by increasing the rate of film drainage. Surface-active impurities or surfactants, when present, also can affect the coalescence rate, by accumulating at the surface of the drop. Surfactants tend to stabihze the film and reduce coalescence rates. Fine... 

Coalescence being the secondary process, the number of distinct droplets decreases leading to a stage of irreversibility and finally complete demulsification takes place. Coalescence rate very likely depends primarily on the film-film repulsion, film drainage and on the degree of kinetics of desorption. Kinetically, coalescence is a unimolecular process and the probability of merging of two droplets in an aggregate is assumed not to affect the stability at other point of contact (32). 

Hagesffither et al [27] derived a model for film drainage in turbulent flows and studied its predictive capabilities. It was concluded that the film drainage models are not sufficiently accurate, and that adequate data on bubbly flows are not available for model validation. For droplet flows it was found that the pure drainage process (without interfacial mass transfer fluxes) was predicted with fair accuracy, whereas no reliable coalescence criterion was found (similar conclusions were made by Klaseboer et al [41, 42]). Furthermore, it was concluded that a head on collisions are not representative for all possible impact parameters. Orme [86] and Havelka et al [32], among others, noticed that the impact parameter is of great importance for the droplet-droplet collision outcome in gas flows. However, no collision outcome maps have been published yet for bubble-bubble collisions. 

Since film drainage and rupture is a kinetic process, coalescence is also a kinetic process. If the number of particles n (flocculated or not) is measured at time t,... 

The presence of solids at the interface usually retards film drainage rates, thereby reducing the probability of coalescence. A few suspension-polymerization processes use solid particles as suspending agents. 

This is the one to be dealt with here and later to be considered as a reference based on which other cases could be discussed. This breaking process comprises several steps (a) long-distance approach between drops or between drop and flat interface, (b) interdrop film drainage and, finally, (c) coalescence (10-13). 

In addition to particle breakup, the coalescence process may be affected as well. It has been speculated that exfoliated clay platelets or well-dispersed nanoparticles may hinder particle coalescence by acting as physical barriers [19,22]. Furthermore, it has been suggested that an immobilized layer, consisting of the inorganic nanoparticles and bound polymer, forms around the droplets of the dispersed phase [50]. The reduced mobility of the confined polymer chains that are bound to the fillers likely causes a decrease in the drainage rate of the thin film separating two droplets [44]. If this is the case, this phenomenon should be dependent on filler concentration this is shown in Figure 2.8, which shows the effect of nanoclay fillers on the dispersed particle size of a 70/30 maleated EPR/PP blend [19]. 

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 second method used by Cockbain and McRoberts [36] involves plotting the results as a distribution curve. The number Nj of droplets that have not yet coalesced within a time t is plotted versus time. This distribution curve consists of two fairly well defined regions, one in which N, is nearly constant with t, followed by a region in which N( decreases with time in an exponential fashion. The first region corresponds to the process of drainage of the thin liquid film of the continuous phase from between the droplets and the planar interface, whereas the second region is that where rupture of the thin film and coalescence take place. Since... 

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