Foam dispersity

Foaming (and emulsion). The low gas and liquid velocities in packing suppress foam formation. The large open area of the larger random packing promotes foam dispersal. Both attributes make... 

Subsequent chapters provide many examples of foams in industry and everyday life. Solid foams, dispersions of gas in a solid, are not, in general, covered in this book. 

Many of the negative properties of hydrocarbon oils can be overcome by the use of additives which inhibit oxidation, reduce foaming, disperse contamination,... 

Foam dispersity is characterised by the average bubble size, by bubble size distribution or by the specific foam surface e. There are three different specific foam surfaces... 

Pentagonal dodecahedron [76-80], compact tetradecahedron [73,80,82] and minimal tetrakaidecahedron [67,68] are most often used as models of foam cells in the calculation of foam electrical conductivity and hydroconductivity, foam dispersity and in the process of adsorption accumulation of foam. 

The average bubble radius (as well as the biggest and smallest size), the maximum distance between the opposite walls of the bubble (relative diameter) [8] and the specific liquid/air interface are involved in the estimation of dispersity. However, the distribution of bubbles by size, for example by radius of equivalent spheres, reveals completely the foam dispersity. Additional information about dispersity which takes into account the difference in polyhedral shapes is gathered from the number and shape of polyhedron faces (see Section 1.2). 

If foam dispersity is expressed in terms of a sphere radius with volume equal to the volume of the dodecahedral cell then the geometric coefficients will be respectively C d =... 

The capillary pressure and foam dispersity were determined simultaneously. After drying the reduced pressure under the porous plate sharply changed to lower values leading to an immediate liquid suck in from the porous plate into the foam. As a result the foam expansion ratio decreased to values at which the condition for polyhedricity was not fulfilled. The capillary pressure and the expansion ratio were measured again for the new state of the foam obtained. 

The average foam dispersity in the experiments performed varied within the limits of aVL = 6.10"2-3.5.10 1 mm the degree of polydispersity significantly increased in the process of foam coarsening. It can be seen that curves 1 and 2 fit well at expansion ratio n > 300. At low expansion ratio (20 < n < 40) the difference between rjr (n) and rjrn(n) grows to 15% but if the longitudinal curvature is accounted for then this difference is about 7%. This means that the difference in size of the individual bubbles in a polydisperse foam does not influence strongly the course of the ra lrn (n) dependence as compared to the monodisperse model system. 

The prototype of these techniques has been proposed by Derjaguin [43], It was designed to estimate the initial foam dispersity. The foam was placed in a container, then destroyed by applying strong compression and the excess pressure of the gas was measured [44]. 

An important condition which has to be fulfilled when using this method for foam dispersity determination is the absence of an excess hydrostatic pressure in the foam liquid phase. This pressure is equalized to a considerable extent when an equilibrium distribution the foam expansion ratio and the border pressure along the height of the foam column is established. This can be controlled by measuring the pressure in the Plateau borders at a certain level of the foam column by means of a micromanometer. However, if this condition is overlooked, the hydrostatic pressure can introduce a considerable error in the results of bubble size measurements, especially in low expansion ratio foams. Probably, it is the influence of the unrecorded hydrostatic pressure that can explain the lack of correspondence between the bubble size in the foam and the excess pressure in them, observed by Aleynikov[49]. The... 

A device for foam dispersity determination by measuring the local foam expansion ratio and the pressure in Plateau borders is illustrated in Fig. 4.4. It consists of a glass container equipped with platinum electrodes and a micromanometer. The container bottom is a porous plate (a sintered glass filter). The pressure Ap is measured with a capillary micromanometer and the expansion ratio is determined by the electrical conductivity of the foam. The manometer and the electrodes are positioned so that to ensure a distance of 1.0-1.5 cm between them and the porous plate. When the distance is small the liquid in the porous... 

Fig. 4.4. Schematic of the device for determination of foam dispersity by the local foam expansion ratio ...

Values of experimenlal atxp and calculated acai cell sizes of a foam from NP-20. Foam dispersity is determined by the method for measuring local expansion ratio and capillary pressure. 

Another variant of this counting method for determination of foam dispersity has been proposed by Onken and Brentrup [53]. In their device the gas bubbles are moved through a vertical tube. A lamp and a photoelement are set at a specially chosen place. A recording device is registering the time for which each bubble passes through. A relevant graduation allows the determination of the bubble size. 

There have been several attempts [60,61] to determine foam dispersity by the extinction of a luminous flux passing through the foam. Clark and Blackman [62] have proposed the following link between the specific surface area of a foam and the extinction of a luminous flux... 

The rate of foam drainage is determined not only by the hydrodynamic characteristics of the foam (border shape and size, liquid phase viscosity, pressure gradient, mobility of the Iiquid/air interface, etc.) but also by the rate of internal foam (foam films and borders) collapse and the breakdown of the foam column. The decrease in the average foam dispersity (respectively the volume) leads the liberation of excess liquid which delays the establishment of hydrostatic equilibrium. However, liquid drainage causes an increase in the capillary and disjoining pressure, both of which accelerate further bubble coalescence and foam column breakdown. 

The liquid outflow from the foam represents the last stage of a process which includes film thinning and rupture and liquid outflow through borders and films. That is why the term syneresis is often used in a wider sense involving the behaviour of foam dispersion medium (achievement of equilibrium liquid distribution, liquid sucking into the foam, etc.). In order to... 

Each term in Eq. (5.48) can be determined if the border profile in the flowing direction and its change with time as well as the change in foam dispersity are known. 

The qualitative correspondence between the experimentally obtained foam dispersity vs. foam drainage rate and that of Eq. (5.60) can be seen in Fig. 5.19 [21]. The dispersity change was achieved by blowing air through sintered glass filters of definite pore radii. 

The decrease in foam dispersity results from both bubble coalescence and diffusion bubble expansion. So, depending on the surfactant kind and the time elapsed after foam formation, one of these processes can have a prevailing effect on the rate of foam collapse. 

There is a qualitative agreement between the results for the rate of decrease in the specific foam surface area estimated by the increase in surfactant concentration during gravitational foam breaking and those from the kinetic studies of foam dispersity changes. 

Pertsov et al. [19] and Kann [20] have proposed a logarithmic time dependence of the foam expansion ratio in order to determine the rate of bubble expansion. This approach is reasoned by the fact that at hydrostatic equilibrium further increase in foam expansion ratio occurs only when excess liquid is released with decreasing foam dispersity. It was experimentally established that at the final stage of internal foam collapse this increase in foam expansion ratio can be expressed by an exponential function [19,21]... 

The experiments performed in [49] reveal that the foam lifetime depends strongly on the humidity of the blowing air. This is illustrated in Fig. 6.8. However, a quantitative verification of Eq. (6.31) is not possible for the lack of data about film thickness, foam dispersity and rate of evaporation. 

Fig. 6.9. Kinetic curve of foam dispersity change at different pressure drops in the foam liquid phase (a)...

The most important factor regulating the rate of foam column destruction are the surfactant kind, electrolyte concentration and additives that determined the structural characteristics of the foam (dispersity, film type and thickness, etc.) and foam film stability. 

The total degradation time is often employed as a characteristic of the kinetics of foam column decay. Sometimes the time needed for breaking a certain part of the foam column (for example, 1/2 or 1/5 of its height) is also used. Obviously, either of these characteristics depend strongly on foam column height and foam dispersity. 

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. 

The lifetime of a foam being subjected to a pressure drop is affected by the mode of foam formation, the foam column height and foam dispersity. The foam column height... 

A more detailed study on foam behaviour and the features of foam column destruction has been performed in [69-71]. Various kinds of surfactants, different foam column heights, foam dispersity and temperatures, were investigated at Ap pgH, including the range of critical pressure drops pcr. The kinetics of establishing a capillary pressure was also accounted for. Used were ionic (NaDoS) and nonionic (Triton-X-100) surfactants as well as some silicon-organic compounds which differed by the number of siloxane, dimethylsiloxane, oxyethylene and oxypropylene groups (KS-1, BS-3 and KEP-2). 

Fig. 6.13 depicts the xp(Ap) dependence of Triton-X-100 (5 1 O 4 mol dm 3 and 0.4 mol dm"3 NaCl) foams studied at (a) different foam column heights but equal dispersity and (b) different foam dispersity but equal foam column height (1 cm). Fig. 6.14 presents the same tp(Ap) dependence for foams from silicon-organic compounds. 

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. 

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