Films drainage kinetics

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). 

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,... 

It is remarkable that the 1993 and 1994 base wines compare so well within the experimental errors, at the local scale with same kinetics of film drainage and same aggregates, and at the global scale with same order of magnitude of the three foam parameters. It is amazing that the foam expansion E, the foam lifetimes Ln and L(2, and the foamability X all have smaller values with the 1990 wine than with the two others. Not less remarkable is the comparison between the 1990 and the dilute YGP model solution, the main difference between the wine and the model solution being the existence of residual foams with wines which do not exist in the case of the model solution. And finally the behaviours of the films and foams of the MBSA solutions are completely different from the wine and YGP solutions ones. 

Analysis of the films data helps to qualitatively understand which model is valid. Film thicknesses are plotted versus time in Figure 7. All films drain fast, but the ones formed from the 1990 wine do it much more rapidly than the others toward an equilibrium thickness hf s 16 nm. Films formed from the 1993 and 1994 wines have the same drainage rate towards a minimum thickness h 25 nm but it is slower than the 1990 wine film, and they thicken back. It is again remarkable that the drainage kinetics of the YGP solution is the same as the... 

Figure 7 Drainage kinetics of films formed from base wines and YGP model solutions 1990 wine one,...

Drainage kinetics laws are usually dependent upon the interfacial viscous stresses, which exist in the film and at the film interfaces,16 and also of the dynamic viscosity of the liquid. 

The increased temperature of the system may lower the interfacial shear viscosity, which may lead to an improved rate of film drainage between adjacent drops. Menon and Wasan (62) demonstrated the decrease in interfacial viscosity of shale oil/water at increased temperatures and showed analogous behavior at increased demulsifier concentration. Thompson et al. (133) reported that, at increased temperatures, some incompressible nomelaxing films tend to relax and the rate of buildup of the resistance to film compression increases. Demulsifiers then reduce the kinetic barrier to coalescence. 

The influence of a block copolymer on the droplet breakup and coalescence in model immiscible PEP/PPO polymer blends was investigated by Ramie et fd. [18], who found that the addition of 0.1 wt% or 1.0 wt% of PEO-b-PPO-b-PEO [poly (ethylene oxide)-poly(propylene oxide) copolymer] triblock copolymers facilitated breakup and inhibited coalescence. The steady-state droplet size resulting from breakup was reduced only slightly by the addition of 0.1 wt% copolymer, but more substantially by addition of lwt%. However, the kinetics of coalescence were suppressed effectively even when 0.1 wt% of copolymer was added. In these systems, the copolymer seems to reduce the efficiency of both droplet collision and film drainage and/or rupture. 

The film drainage step tends to be rate limiting. Most investigators have been concerned with describing the kinetics of... 

Equations (249) and (250), along with the balance of the stresses on the interface, Eq. (81), and the adsorption isotherm or the kinetic rate expression (see Secs. III.B and III.C), describe the influence of the surfactant on the film drainage. From Eq. (250), a generalization of the known formula of Charles and Mason [467] can be derived ... 

Cascao-Pereira, L. G., C. Johansson, C. J. Radke, and H. W. Blanch. 2003. Surface forces and drainage kinetics of protein-stabilized aqueous films. Langmuir 19 (18) 7503-7513. 

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... 

The kinetics of establishing equilibrium pressure provides information not only about foam drainage but also for other important parameters, such as the time for reaching equilibrium state in the borders and films and the radius of border curvature and border profile during liquid flow and drainage under pressure drop. 

Eq. (5.35) describes satisfactorily the kinetics of the change in capillary pressure of a foam at the initial and final drainage stages. In t vs. 1/r2 co-ordinates, Fig. 5.11 presents the experimental data 4-5 min after drainage initiation. They are well approximated by a linear dependence. Furthermore, the time for establishing an equal capillary pressure depends considerably on the surfactant kind and foam film type. Although the cylindrical model is. 

The comparison of border and film hydroconductivities [7,14] shows that the contribution of liquid flowing through films can be neglected. That is why more realistic proves to be the model of liquid flow through borders. On the basis of this model the following kinetic dependence with three constants a, b and k has been proposed in one of the first papers dedicated to foam drainage [66]... 

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. 

The contemporary level of knowledge in foam science enables the solution of many problems related to the kinetics of various processes in a foam (such as film thinning and rupture, foam drainage, diffusion, development of film deformation accounting for the Gibbs and dynamic elasticity, etc.) and to establish the equilibrium conditions of the individual foam elements (films and borders). Thus, it allows the qualitative, and sometime semi-quantitative, interpretation of foam stability. 

As already mentioned the real foam is not a simple sum of films. Its behaviour and response to different disturbances is much more complex than the behaviour of individual foam films. This is so because the films in the foam have different size and shape, and the kinetics of establishing film equilibrium is more complicated since the process of foam drainage exerts significant influence. 

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