Foam dispersity determination

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... 

The density of a syntactic foam is determined by the relative proportions (volume fractions) of microspheres, resin matrix, and dispersed air. The theoretical lower limit of the density in a two-phase system (without dispersed air) is determined by the close packing rules for spheres. Various packing possibilities in systems containing uniform-size spheres are summarized below 8> 8S) ... 

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. 

A second group of methods for dispersity determination are based on the dependence between the specific foam surface area and the average excess capillary pressure in foam bubbles. 

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. 

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. 

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 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. 

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. 

A more important fact is the change in the mechanism of foam column destruction with the increase in the applied pressure drop. For example, at small pressure drops a slow diffusion bubble expansion along with the corresponding slow rate of structural rearrangement (either zero or very slow rates of coalescence) occurs in a NaDoS foam with CBF or NBF. This is expressed in the layer-by-layer reduction of foam column height ending with the disappearance of the last bubble layer. In such a foam the critical pressure of the foam column destruction is not reached at any dispersity, and only the foam column height and the rate of internal foam collapse determine the foam lifetime. 

The results from the additional experiments with pressure dump [72] have shown that the main reason for change in the slope angles in the dependence considered is the inaccuracy of foam expansion ratio (dispersity) determination. 

Eqs. (10.11) and (10.12) indicate that when T = const and n = const, the accumulation ratio is determined by foam dispersity, while for r = const and a - const, it is determined by the foam expansion ratio and the capillary pressure. 

Other factors (pH, temperature, foam dispersity, foam column height, rate of gas and liquid feed, etc.) also affect the accumulation effectiveness (parameter //). Some of these, such as pH, temperature, type and concentration of the collector, are changing the adsorption, others, like dispersity and foam column height, are changing the drainage rate that determines foam stability and expansion ratio. The book of Rusanov et al. [23] summarises the results on the effect of these factors on foam accumulation of surfactants. 

In investigations of the structure and properties of disperse materials, particularly plastic foams, it is necessary to find out the distribution pattern of one phase in the material. It is simpler to consider the distribution of the gas phase in a solid body, i.e. the gas-filled cell distribution within a foam bulk] In their turn, the cells, as shown above, may be characterized by several parameters (size, shape, volume and surface area). The cell size distribution pattern is the most comprehensive characteristic of the dispersity of the gas structural elements of plastics. Furthermore, the cell size can, be determined by one of the methods which will be discussed below, and the foam dispersity is expressed in terms of the nominal cell diameter distribution function. 

The compressibility of a foam is determined by (i) the ability of the gas to compress and (ii) its wetting power, which is determined by the properties of the foaming solution [4]. As with any disperse system, a foam may acquire the properties of a solid body - that is, it can maintain its shape and it possesses a shear modulus (see below). 

The stability of a foam is determined through the interplay of a number of factors that involve bulk solution, interfacial properties, and also external forces. We have summarized some of the effects on foam stability of gravity drainage, capillary suction, surface elasticity, viscosity, electric doublelayer repulsion, dispersion force attraction, and steric repulsion. Foams are such complex systems that Lucassen (56) has stated that any attempt to understand their properties in terms of a simple all-embracing theory is doomed to failure. Nevertheless, we have attempted to provide an introduction to the occurrence, properties, and importance of foams as they relate to the petroleum industry. More detailed aspects are taken up in the subsequent chapters of this book. 

Mechanisms of Single-Foam Film Stability. Soap bubbles and soap films have been the focus of scientific interest since the days of Hooke and Newton (2—9). The stability and structure of foams are determined primarily by the relative rate of coalescence of the dispersed gas bubbles (10). The process of coalescence in foams is controlled by the thinning and rupture of the foam films separating the air bubbles. Experimental observations suggest that the lifetime (stability) of foam films is determined primarily by the thinning time rather than by the rupture time. Hence, if the approaching bubbles have equal size, the process of coalescence can be split into three stages ... 

In practical application, it was reported that the platinum particles dispersed in highly porous carbonized polyacrylonitrile (PAN) microcellular foam used as fuel-cell electrocatalyst160 have the partially active property. The fractal dimension of the platinum particles was determined to be smaller than 2.0 by using the potentiostatic current transient technique in oxygen-saturated solutions, and it was considered to be a reaction dimension, indicating that not all of the platinum particle surface sites are accessible to the incoming oxygen molecules. 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

Several features of this experiment (5 — 7) as a radiotracer method are as follows (1) since the relative adsorbability has been determined from the ratio of radiocounts of dried samples, it is not affected by errors in the absolute values for radioactivity and (2) specific activity, which has to be large usually for the study of surface phenomena, may be very small and yet the radioactive measurements yield accurate results. The concentrations of solutions examined were all close to but less than the c.m.c. values of respective surfactant solutions. The foams were stable enough to carry the experiment in this concentration range and the mole ratio of counterions in the bulk was equal to the stoichiometric mole ratio —i.e., there was no shift in mole ratio of dispersed counterions owing to the micelle formation at the present study. 

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