Foams drainage

Following foam formation, the liquid begins to drain out of the foam, with the excess liquid draining first into the plateau borders and then flowing down from the upper to the lower foam layers (under gravity) until the gradient of the capillary pressure equalizes the gravity force, 

Simultaneously with drainage from films into borders, the liquid begins to flow out from the foam when the pressure in the lower foam films outweighs the external pressure. This process is similar to gel syneresis, and is sometimes referred to as foam syneresis or foam drainage.  

The foregoing discussion leads to the question of whether actual foams do, in fact, satisfy the conditions of zero resultant force on each side, border, and comer without developing local variations in pressure in the liquid interiors of the laminas. Such pressure variations would affect the nature of foam drainage (see below) and might also have the consequence that films within a foam structure would, on draining, more quickly reach a point of instability than do isolated plane films. 

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Foam Drainage and Overflow The rate of foam overflow on a gas-free basis (i.e., the total volumetric foamate rate Q) can be estimated from a detailed theoiy for foam drainage [Leonard and Lemlich, Am. Jn.st. Chem. Eng. J., 11, 18 (1965)]. From the resulting relationship for overflow [Fanlo and Lemlich, Am. Jn.st. Chem. Eng. Symp. Ser, 9, 75, 35 (1965)], Eq. (22-55) can be employed as a convenient approximation to the theory so as to avoid tri and error over the usual range of interest for foam of low hquid content ascending in plug flow ... 

Figure 2. Foam drainage and water loss from the bubble wall concentrate is diluted into a premix, and the foam is generated from the premix. When foam is generated, hydrophobic R groups become part of the bubble wall (X = width of bubble wall). X decreases as water drains downward under the influence of gravity. The polymer strands prevent narrowing of bubble wall, decreasing drainage time and stabilizing the foam.

The foam drainage, surface viscosity, and bubble size distributions have been reported for different systems consisting of detergents and proteins. Foam drainage was investigated by using an incident light interference microscope technique. 

The foam drainage profiles for untreated stabilized and pipeline Nmian crude oils showed that there was a marked difference in stability between the two samples. Furthermore, conventional silicone oils (e.g., 60.000-cSt (60 000 mm /s) oil) did nut cause any significant collapse of the foam formed by depressurizing pipeline oil. Closer examination of the foam formed by the pipeline crude... 

The advantage of this method is that samples can be studied without dilution, and it is quicker than electron microscopy methods. It is believed that the method will provide valuable information on foams in the future. Using this method also makes it possible to measure the rate of foam drainage and collapse, as well as the gas fraction in the foam. 

In fact, extremum tendencies expressing the dominant mechanisms in systems like turbulent pipe flow (Li et al, 1999), gas-liquid-solid flow (Liu et al, 2001), granular flow, emulsions, foam drainages, and multiphase micro-/nanoflows also follow similar scenarios of compromising as in gas-solid and gas-liquid systems (Ge et al., 2007), and therefore, stability conditions established on this basis also lead to reasonable descriptions of the meso-scale structures in these systems. We believe that such an EMMS-based methodology accords with the structure of the problems being solved, and hence realize the similarity of the structures between the physical model and the problems. That is the fundamental reason why the EMMS-based multi-scale CFD improves the... 

One of the simplest representations for foam drainage, in terms of the drained volume of liquid, is given by ... 

Recently a new technique has been introduced for the study of foam drainage under pressure drop. The especially constructed apparatus allows automated calculation of foam expansion ratio at any instant of time (see Section 5.3.4). 

The pressure difference Ap causes an acceleration of foam drainage which runs until it is equalised by the capillary pressure. In order to reach capillary pressures higher than atmospheric pressure fine porous filter should be used and the pressure difference should be created by increasing pressure in the space above the foam. Under these conditions when hydrostatic equilibrium is reached the pressure of the liquid in Plateau borders and the capillary pressure will be, respectively, equal to... 

Approximate calculations for a gravitational foam drainage have shown [5] that at a 0.1 mm, a =30 mN/m and film thickness h = 0.1-0.2 pm (this state is usually reached 5-7 min after foam formation), the amount of liquid in Plateau borders exceeds more than 10 times that in films. This applies for a foam layer situated 4-5 cm above the porous plate. For low dispersity foams (a = 1 mm or more) the condition h r a is fulfilled at this height even at h 1 pm, i.e. practically at the moment of foam formation by the barbotage method. 

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 techniques employed in foam drainage investigations involve the determination of liquid quantity that drains from the foam per unit time. Different foam numbers were introduced as indirect drainage characteristics of low expansion ratio foams produced by the... 

A widely used characteristic of foam drainage is the time during which half of the initial foam liquid volume flows out [14,17,18]. 

Upon creating a pressure drop Ap in the foam liquid phase (with a porous plate, see Section 1.4) the liquid begins to flow out from the foam. During the process of foam drainage the radius of border curvature rb decreases while the capillary pressure increases, thus leading to reduction of the pressure drop Apr... 

In order to describe quantitatively the hydrodynamic phenomena occurring during foam drainage at high pressure drop as well as during surfactant solution flow through a high... 

In the calculation of foam drainage, apart from the geometrical border length L = aH, the full border length which includes the affluents is to be considered. It is proportional to the total foam liquid volume and to the total border numbers N. The total border length expressed in terms of Eq. (5.3) is... 

Trapeznikov [46] indicate that the transition temperature of a 0.1% NaDoS solution is raised from 31°C to 38°C when the dodecanol concentration is increased from 0.001 to 0.1%. It is suggested that the sharp reduction of surface viscosity is the reason for increase in rate of foam drainage. 

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