Foam-dilatational viscosity

The rheology of wet foams was analysed by Wasan et al. (1992) and a relation was derived (Edwards et al. 1991) connecting the foam dilational viscosity Kp to the surface dilational elasticity E ... 

Equation 6 clearly shows that the foam-dilatational viscosity is directly proportional to surface viscosity and inversely to foam film thickness. The terms in the parenthesis (in eq 6) represent the disjoining pressure contribution and the Plateau border curvature radius. 

The foam-dilatational viscosity, K, arises because of two primary mechanisms (37) (1) viscous flow within the thin films, and (2) interfacial tension gradients acting along the foam bubble surfaces. The effect of interfacial tension gradients is to increase the foam viscosity as they impede flow near the surfaces of the thin foam films by contributing to a larger film stress. As in the wet foam (eq 6), the foam dilatational viscosity for a dry foam, K, is inversely proportional to film thickness as well (eq 9). 

Figure 11 displays the dependence of the foam dilatational viscosity upon the rate of foam expansion for Na = 1 (21). The foam viscosity, K, increases for foam expansion, as the foam films thin and velocity gradients occurring over the thickness of the film increase in magnitude, whereas, for opposite reasons, the foam viscosity, K, decreases as the foam is compressed. 

Figure 11. Dependence of the foam dilatational viscosity upon rate of dilatation for = 1. (Reproduced from reference 22 Copyright 1986 American Chemical Society.)...

It has been shown (16) that a stable foam possesses both a high surface dilatational viscosity and elasticity. In principle, defoamers should reduce these properties. Ideally a spread duplex film, one thick enough to have two definite surfaces enclosing a bulk phase, should eliminate dilatational effects because the surface tension of an iasoluble, one-component layer does not depend on its thickness. This effect has been verified (17). SiUcone antifoams reduce both the surface dilatational elasticity and viscosity of cmde oils as iUustrated ia Table 2 (17). The PDMS materials are Dow Coming Ltd. polydimethylsiloxane fluids, SK 3556 is a Th. Goldschmidt Ltd. siUcone oil, and FC 740 is a 3M Co. Ltd. fluorocarbon profoaming surfactant. 

There is good correlation between the concentration giving the maximum surface dilatational viscosity and that giving the best foam performance. The nonylphenol 10 EO is a low-foaming nonionic surfactant with a maximum foam height of 150 ml in this test, whereas AOS produced 670 ml of foam. Figure 12 clearly shows that there is an optimum surfactant concentration for a dynamic process such as foam generation. 

Ed is the dilatational elasticity, and rid is the dilatational viscosity. It is characteristic for a stable foam to exhibit a high surface dilatational elasticity and a high dilatational viscosity. Therefore effective defoamers should reduce these properties of the foam. 

The theoretical analysis indicated that asymmetric drainage was caused by the hydrodynamic instability being a result of surface tension driven flow. A criterion giving the conditions of the onset of instability that causes asymmetric drainage in foam films was proposed. This analysis showed as well that surface-tension-driven flow was stabilised by surface dilational viscosity, surface diffusivity and especially surface shear viscosity. 

Unlike the wet foam calculation where primary viscous stresses are localized within Plateau border regions and derived from interfacial viscous properties, the total viscous stress for a dry foam (i.e., dispersed-phase volume fraction approaching 1) is distributed throughout the thin liquid films. The leading-order dilatational viscosity of a dry foam composed of a spatially periodic array of tetrakaidecahedron (39) bubbles was given by Edwards and Wasan (40) as follows ... 

The viscosity emphasized in this chapter, called elongational or extensional viscosity, was originally designated (34) tensile viscosity. When this bulk-phase parameter is near the interface, its two-dimensional equivalent is called the surface dilational viscosity. The importance of this parameter in the foaming of coatings, which arises from differences in surfactant structures, has been discussed (35). In cosmetic applications, foam and gel structures are important and probably reflect the reason the hydrophobically modified acrylic acid polymers were emphasized in the last section of Ghapter 7. 

Ida et. al. (17) have postulated that the effect of surface-dilational viscosity (18,19) is not negligible in the motion of a small bubble when anti-foaming additive is added to base oil, and this effect is more dominant when initial bubble radius is small. This effect restrains the natural vibration of bubbles, and delays the transient response under sudden decompression. 

Clearly, when qj q the droplets behave as rigid spheres and [qj approaches the Einstein limit of 2.5. In contrast, if qj q, (as is the case for foams), [qj = 1. In the presence of viscous interfacial layers, equation (5.8) is modified to take into account the surface shear viscosity qg and surface dilational viscosity m... 

In summary, it can be said that the various aspects of foam formation and persistence are related to the actions of surfactant molecules and additives at the various interfaces in the system, coupled with the rheological characteristics of the system, including the dilational viscosity of the interfacial layers and the bulk rheological properties of the system. Depending on whether foam is wanted, the choice of surfactants and additives for a formulation must address all of those factors in the context of the system being prepared and its end use. 

This has been verified for polydimethylsiloxanes added to crude oils. The effect of the dilatational elasticities and viscosities on crude oil by the addition of polydimethylsiloxanes is shown in Table 21-1. Under nonequilibrium conditions, both a high bulk viscosity and a surface viscosity can delay the film thinning and the stretching deformation, which precedes the destruction of a foam. There is another issue that concerns the formation of ordered structures. The development of ordered structures in the surface film may also stabilize the foams. Liquid crystalline phases in surfaces enhance the stability of the foam. 

The viscoelastic properties of the surface layer are important parameters. The most useful technique for studying the viscoelastic properties of surfactant monolayers is surface scattering. When transversal ripples occur, a periodic dilation and compression of the monolayer occurs, and this can be accurately measured, enabling the viscoelastic behaviour of monolayers under equilibrium and nonequilibrium conditions, to be obtained, without disturbing the original sate of the adsorbed layer. Some correlations have been found between surface viscosity and elasticity and foam stability an example of this is the addition of lauryl alcohol to sodium lauryl sulphate, which tends to increase the surface viscosity and elasticity [10]. 

Because protein-ba sed foams depend upon the intrinsic molecular properties (extent and nature of protein-protein interactions) of the protein, foaming properties (formation and stabilization) can vary immensely between different proteins. The intrinsic properties of the protein together with extrinsic factors (temperature, pH, salts, and viscosity of the continuous phase) determine the physical stability of the film. Films with enhanced mechanical strength (greater protein-protein interactions), and better rheological and viscoelastic properties (flexible residual tertiary structure) are more stable (12,15), and this is reflected in more stable foams/emulsions (14,33). Such films have better viscoelastic properties (dilatational modulus) ( ) and can adapt to physical perturbations without rupture. This is illustrated by -lactoglobulin which forms strong viscous films while casein films show limited viscosity due to diminished protein-protein (electrostatic) interactions and lack of bulky structure (steric effects) which apparently improves interactions at the interface (7,13 19). 

With respect to the rheological parameters fliey come to the conclusion that surface elasticity effects are superior to surface viscosity effects. This, however, apphes to pure surfactant layers and may be different for pure protein or mixed surfactant/protein adsorption layers. It has been stressed also by Langevin (26), in her review on foams and emulsions, fliat studies on the dynamics of adsorption and dilational rheology studies for mixed systems, in particular surfactant-polymer systems, are desirable in order to understand these most common stabilizing systems. 

It was discussed quite extensively, that interfacial dynamics and rheology are key properties of liquid disperse systems, such as foams and emulsions. The stability of such systems depends for example on the dilational elasticity and viscosity, however, surely not on the elasticity modulus (Borwankar et al. 1992). Here, the interfacial rheology with its frequency dependence comes into play, and data at respective frequencies will possibly correlate with the stability behaviour. 

Callaghan and Neustadter [31] have made a study of the foam stabilities of air-crude oil and natural gas-crude oil systems using a variety of light crude oils of viscosities 14 mPa s. This study, at ambient temperature using a sparging method, concerned so-called dead oils from which natural gas had been separated. It also involved a comparison of the foam behavior with critical film rupture thicknesses, bulk phase, and surface shear viscosities together with dilatational surface properties. 

As already mentioned, the surfactants are used to stabilize the liquid films in foams, in emulsions, on solid surfaces, and so forth. We will first consider the equilibrium and kinetic properties of surfactant adsorption monolayers. Various two-dimensional equations of state are discussed. The kinetics of surfactant adsorption is described in the cases of dijfusion and barrier control. Special attention is paid to the process of adsorption from ionic surfactant solutions. Theoretical models of the adsorption from micellar surfactant solutions are also presented. The rheological properties of the surfactant adsorption mono-layers, such as dilatational and shear surface viscosity and suiface elasticity, are introduced. The specificity of the proteins as high-molecular-weight surfactants is also discussed. 

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