Foam syneresis

Simultaneously with drainage from films into borders the liquid begins to flow out from the foam, when the pressure in the lower foam layers overweighs the external pressure. By analogy with gel syneresis Arbuzov and Grebenshchikov [ 1 ] have called the liquid outflow from the foam foam syneresis . Today both terms foam syneresis and foam drainage are used in literature. 

Kann, K. B., Some laws of foam syneresis. Outflow, Colloid Journal, Vol. 40, No. 5, 1978. 

Other useful parameters are whipping time and estimation of syneresis (serum separation from the foam). 

The spontaneous shrinking of a colloidal dispersion due to the release and exudation of some liquid frequently occurs in gels and foams but also occurs in flocculated suspensions. Mechanical syneresis refers to enhancing syneresis by the application of mechanical forces. Micro-syneresis is a special case of syneresis in which the polymer molecules cluster together while retaining some of the original bulk gel structure. This process creates regions of free liquid within the gel network. 

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... 

Food products As thickeners, to provide specific rheology, to form gels, to inhibit syneresis, to stabilize emulsions, to stabilize suspensions, to emulsify, to bind/hold water, to form films and coatings, to inhibit crystallization, to improve texture/mouthfeel, as processing aids, as whipping agents, to stabilize foams, as extrusion aids, as binders, to stabilize proteins, to encapsulate, to flocculate... 

The continuous liquid phase in a foam layer is a surfactant solution whose volume and interface vary in the course of evolution. The system evolves, its volume varies because of syneresis, and the interface area varies because of the diffusion gas exchange between the system and the ambient medium. For a system with variable volume V(t) and surface area S(t), we can write out the balance equation for the surfactant mass in the system in the form... 

Syneresis. The most actual problem in the study of foam systems is the spatial redistribution of liquid phase under the action of external fields and internal inhomogeneities. 

The outflow of a liquid from foam under the action of gravity field was considered in [15] and termed syneresis. Later on, syneresis was attributed to capillary effects, primarily due to the gradients of capillary rarefaction. The importance of these effects was already mentioned in [266], but the gradient of capillary rarefaction was rightly set to be zero, since only steady-state flows of liquids through foam layer were considered there. The fundamental role of the gradient of capillary rarefaction in the process of evolution of a foam layer in syneresis was also pointed out in [215, 335]. 

Hydroconductivity. A phenomenological theory of syneresis was proposed in [239, 243-247]. There, in accordance with the linear theory of syneresis, for the local density q of flux of the extensive variable V, which is the volume fraction of the liquid phase (the reciprocal of the foam multiplicity K), the following expression was proposed [245] ... 

Equation of syneresis. Syneresis coefficient. Together with the liquid flux (7.4.1) relative to the skeleton phase, there also exists translational transfer VU determined by the local velocity field U of the entire foam (or of its skeleton phase). In this case, the law of conservation of liquid mass has the form... 

In [246] the hydroconductivity and syneresis coefficients were calculated for the channel version of hydroconductivity of polyhedral foam ... 

Gravitational syneresis. Distribution of the foam multiplicity. In [247], Eq. (7.4.4) was solved for a vertical steady-state (dV/dt = 0) stagnant (U = 0) nonirrigated (q = 0) foam column under the condition that the foam multiplicity attains its minimum (Kmm) at the interface between the foam and the liquid (in the cross-section Z = 0). The following quadratic dependence of the foam multiplicity on the height was obtained ... 

Transient problems of syneresis are of great interest. For example, the transient syneresis in a stagnant foam layer (U = 0) under the action of constant mass forces is governed by a complex nonlinear parabolic equation. Some self-similar solutions and traveling wave type solutions were found in [152] for some special forms of this equation. For one-dimensional barosyneresis (g = 0), Eq. (7.4.4) has the form... 

In [242], an additional boundary condition at the upper boundary of the foam column is also discussed for the case of nonstationary syneresis. 

Krotov, V. V., The theory ofsyneresis of foams and concentrated emulsions. 4. Some analytical solutions of the one-dimensional syneresis equation, Colloid Journal, Vol. 43, No. 2,1981. 

Chapter 7 deals with the basic concepts and properties of very specific technological media, namely, foam systems. Important processes such as surfactant interface accumulation, syneresis, and foam rupture are considered. 

Using the theory of hydrostatic foam stability [21, 26], the behaviour of foams in contact with fabrics was studied [181]. Under these conditions, the drainage of liquid from the foam occurs under the influence of the following processes 1) suction of liquid due to capillarity of high-dispersion foams 2) imbibition of foam by the capillary structure of fabrics. The production of foams with a controlled structure allows to achieve a more uniform application of a dye to a fabric. Foamer formulations were developed containing surfactants and a water-soluble polymer, which allow to achieve a minimum initial syneresis rate up to 10 m/s [182]. 

The minimum initial syneresis rate Wo was calculated by the fomiula rr = —=---------------, where H is the foam... 

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