Foams bubble coalescence

Film Rupture. Another general mechanism by which foams evolve is the coalescence of neighboring bubbles via film mpture. This occurs if the nature of the surface-active components is such that the repulsive interactions and Marangoni flows are not sufficient to keep neighboring bubbles apart. Bubble coalescence can become more frequent as the foam drains and there is less Hquid to separate neighbors. Long-Hved foams can be easHy... 

Foam Coalescence Coalescence is of two types. The first is the growth of the larger foam bubbles at the expense of the smaller bubbles due to interbubble gas diffusion, which results from the smaller bubbles having somewhat higher internal pressures (Adamson, The Physical Chemlstiy of Suifaces, 4th ed., Wiley, New York, 1982). Small bubbles can even disappear entirely. In principle, the rate at which this type of coalescence proceeds can be estimated [Ranadive and Lemhch,y. Colloid Inteiface Sci., 70, 392 (1979)]. 

In a gas and liquid system, when gas is introduced into a culture medium, bubbles are formed. The bubbles rise rapidly through the medium and dispersion of the bubbles occurs at surface, forming froth. The froth collapses by coalescence, but in most cases the fermentation broth is viscous so this coalescence may be reduced to form stable froth. Any compounds in the broth, such as proteins, that reduce the surface tension may influence foam formation. The stability of preventing bubbles coalescing depends on the film elasticity, which is increased by the presence of peptides, proteins and soaps. On the other hand, the presence of alcohols and fatty acids will make the foam unstable. 

The primary factor controlling how much gas is in the form of discontinuous bubbles is the lamellae stability. As lamellae rupture, the bubble size or texture increases. Indeed, if bubble coalescence is very rapid, then most all of the gas phase will be continuous and the effectiveness of foam as a mobility-control fluid will be lost. This paper addresses the fundamental mechanisms underlying foam stability in oil-free porous media. 

Bubble Coalescence, Foams and Thin Surfactant Films... 

BUBBLE COALESCENCE, FOAMS AND THIN SURFACTANT FILMS... 

In order to understand the basis for the prevention of bubble coalescence and hence the formation of foams, let us examine the mechanical process involved in the initial stage of bubble coalescence. The relatively low Laplace pressure inside bubbles of reasonable size, say over 1 mm for air bubbles in water, means that the force required to drain the water between the approaching bubbles is sufficient to deform the bubbles as illustrated in Figure 8.2. The process which now occurs in the thin draining film is interesting and has been carefully studied. In water, it appears that the film ruptures, joining the two bubbles, when the film is still relatively thick, at about lOOnm thickness. However, van der Waals forces, which are attractive in this system (i.e. of air/water/air), are effectively insignificant at these film thicknesses. 

Aeration. This interphasic property of protein products, also called foaming or whipping, depends on the ability of the protein to form protective films around gas bubbles. Coalescence, and subsequent break-up of the membrane-like system around the bubbles by the proteins is thereby prevented (55-60). 

If a gas and a liquid are mixed together in a container and then shaken, a foam will be formed. A foam structure can always be formed in a liquid if bubbles of gas are injected faster than the liquid between bubbles can drain away. Even though the bubbles coalesce as soon as the liquid between them has drained away, a temporary dispersion is formed. An example would be the foam formed when bubbles are vigorously blown into a viscous oil1. Such a foam, comprising spherical, well separated... 

Overall there will be a concentration of the hydrophobic particles in the upper regions of the froth layer and a concentration of the hydrophilic particles in the lower regions. Figure 10.7 provides an illustration of how froth structure changes from the bottom to the top of the froth layer. At the very top of the froth bubble, coalescence and rupture will be occurring as well. The amount of ore that can be separated for any given amount of liquid is proportional to the surface area of the foam. It has been estimated that a foam with a specific surface area of 0.2 m2g-1 can separate a thousand times more ore, by mass, than the mass of its own liquid [629]. 

The objective is to reduce volatiles to below 50-100-ppm levels. In most devolatilization equipment, the solution is exposed to a vacuum, the level of which sets the thermodynamic upper limit of separation. The vacuum is generally high enough to superheat the solution and foam it. Foaming is essentially a boiling mechanism. In this case, the mechanism involves a series of steps creation of a vapor phase by nucleation, bubble growth, bubble coalescence and breakup, and bubble rupture. At a very low concentration of volatiles, foaming may not take place, and removal of volatiles would proceed via a diffusion-controlled mechanism to a liquid-vapor macroscopic interface enhanced by laminar flow-induced repeated surface renewals, which can also cause entrapment of vapor bubbles. 

Dispersity of gas emulsions and polyhedral foams is a very important parameter which determines many of their properties and processes occurring in them (diffusion transfer, drainage, etc.) and, therefore, their technological characteristics and areas of application. The kinetics of changes in dispersity indicates the rate of foam inner destruction resulting from coalescence and diffusion transfer. In real foams bubble size varies in a wide range (from micrometers to centimetres). Only by means of special methods it is possible to obtain foam in which bubble size varies in a narrow interval, i.e. foam that can be regarded as monodisperse. 

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 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. 

In wet steady-state foams the bubble coalescence in the foam volume can be neglected. Then, it can be assumed that film rupture only occurs at the upper foam layers. In the other foam layers the average number of bubbles remains constant, the volume of the liquid outflow is equal to the liquid volume entering with the bubbles. 

In addition to these bubble generation events, bubble-bubble coalescence events can increase the average bubble size. As a result, unless injected as a fine foam with bubble sizes below those of the pore channels, or at rates above those possible in actual reservoirs, foam is expected to advance through the pore channels as single layer bubble trains with the bubbles separated... 

Eor the cells of simple mode (Eig. 6A), feed is introduced into the liquid pool under the following three assumptions there is no appreciable bubble coalescence in the rising foam with the column foam leaving the liquid surface is in equilibrium with the completely mixed bulk solution and the bulk solute concentration is equal to the drain solute concentration. For a sufficiently long column, the separation ratio and the stripping ratio LJL. are given by Eqs. (15) and (16) ... 

Coalescence. This is caused by rupture of the film between two emulsion drops or two foam bubbles. The driving force is the decrease in free energy resulting when the total surface area is decreased, as occurs after film rupture. The Laplace equation (Section 10.5.1) plays a key role. 

Most foams are far less stable to coalescence than most emulsions. This is because foam bubbles are large. This means that (a) the number of bubbles is small, implying that relatively few coalescence events are needed to produce a very coarse foam and (b) the films are large and flat and thereby relatively unstable to rupture. However, in most foods it takes a while for films to drain to a thickness allowing rupture. Fairly thick films can also rupture, but that is due to the presence of (extraneous) small hydrophobic particles some mechanisms have been proposed to explain the film rupture. 

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