Surface elasticity and viscosity

The mechanical characteristics of thin films on liquids are described in a similar way to the three-dimensional case. The surface elasticity [618] is defined as 

A is the total surface area. 1 /Ea is also called compressibility. It represents a measure for the resistance of the film against compression. When measuring the surface elasticity we have to keep in mind that there may be different phases in the film which may have different mechanical properties [619], 

The surface viscosity if is defined as follows. Two parallel line elements are moved parallel to each other. Between them (if we consider Newtonian behaviour) a constant speed gradient dv/dy will exist. The force required to maintain the movement is 

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Surface rheology Viscoelasticity of the monolayer differentiation between fluid and solid phases. Surface elasticity and viscosity in the transversal and longitudinal mode wave damping characteristics. Relaxation processes in monolayers. Mechanical stability of the monolayer. Interpretation often complicated because several molecular processes may be involved and because viscous and elastic components may both contribute. 

E.H. Lucassen-Reynders, Surface Elasticity and Viscosity in Compression/ Dilation, in Anionic Surfactants Physical Chemistry and Surfactant Action E.H. Lucassen-Reynders, Ed., Marcel Dekker (1981). (Review of dllatlonal rheology mode, emphasis on Gibbs monolayers includes discussion on 2D equations of state.)... 

Another demonstration of a critical phenomenon, the rate of coalescence of emulsions in dependence of surface elasticity and viscosity, based on the work of Boyd et al. (1972), can also be found in the review of Malhotra Wasan (1988) shown in Fig. 3.20. 

Lucassen-Reynders, E.H., "Surface Elasticity and Viscosity in Compression/Dilatation", in Surfactant Science Series, Vol. 11, (1981)173 Lucassen-Reynders, E.H., Marcel Dekker, Surface Science Series, 11(1986)1 Lucassen-Reynders, E.H., J. Colloid Interface Sci., 117(1987)589 Lucassen-Reynders, E.H., Colloids Surfaces, 25(1987)231 Ludviksson, E.N. and Lightfoot, J., AlChE J., 14(1968)674 Lunkenheimer, K. and Kretzschmar, G., Z. Phys. Chem. (Leipzig), 256(1975)593 MacLeod, C.A. and Radke, C.J., J. Colloid Interface Sci., (1993)... 

The formation of pimples was discovered by Yanitsios and Davis (139) in computer calculations for emulsion drops from pure liquids, without any surfactant. Next, by means of numerical calculations, Cristini et al. (140) established the formation of a pimple for emulsion drops covered with insoluble surfactant in the case of negligible surface diffusion their computations showed that rapid coalescence took place for h < h . A complete treatment of the problem for the formation of pimples was given in Ref 138, where the effects of surface and bulk diffusion of surfactant, as well as the surface elasticity and viscosity, were taken into account, and analytical expressions were derived. 

Lucassen-Reynders, E.H., Surface elasticity and viscosity in compression/dilation, in Anionic Surfactants Physical Chemistry of Surfactant Action, E.H. Lucassen-Reynders (ed.), Marcel Dekker, New York, 1981, p. 173. 

Three techniques may be applied for measurement of the dilational surface elasticity and viscosity (25). The first method applies surface waves to the interface (with frequency co). The dilational elasticity, e, is given by the expression... 

The method demonstrates a high sensitivity of the measured signal to change of the surface elasticity and viscosity, which depends on the presence of surfactants in the solution and on the mechanisms of surface relaxation. The resonance frequency increases with the surface elasticity and the maximum of the amplitude decreases with the surface viscosity. The phase angle changes significantly with both, surface elasticity and viscosity. For example, a strong increase... 

A combination of surfactants gives slower drainage and improved foam stability. For example, mixtures of anionic and nonionic surfactants or anionic surfactant and long-chain alcohol produce much more stable films than the single components. This could be attributed to several factors. For example, the addition of a nonionic to an anionic surfactant reduces the c.m.c. of the anionic surfactant. The mixture can also produce a lower surface tension than the individual components. The combined surfactant system also has a high surface elasticity and viscosity when compared with the single components. 

Gibbs/Maragoni effect, surface elasticity and viscosity... 

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. 

The ratio (p/G) has the units of time and is known as the elastic time constant, te, of the material. Little information exists in the published literature on the rheomechanical parameters, p, and G for biomaterials. An exception is red blood cells for which the shear modulus of elasticity and viscosity have been measured by using micro-pipette techniques 166,68,70,72]. The shear modulus of elasticity data is usually given in units of N m and is sometimes compared with the interfacial tension of liquids. However, these properties are not the same. Interfacial tension originates from an imbalance of surface forces whereas the shear modulus of elasticity is an interaction force closely related to the slope of the force-distance plot (Fig. 3). Typical reported values of the shear modulus of elasticity and viscosity of red blood cells are 6 x 10 N m and 10 Pa s respectively 1701. Red blood cells typically have a mean length scale of the order of 7 pm, thus G is of the order of 10 N m and the elastic time constant (p/G) is of the order of 10 s. 

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 stability of foams in constraining media, such as porous media, is much more complicated. Some combination of surface elasticity, surface viscosity and disjoining pressure is still needed, but the specific requirements for an effective foam in porous media remain elusive, partly because little relevant information is available and partly because what information there is appears to be somewhat conflicting. For example, both direct [304] and inverse [305] correlations have been found between surface elasticity and foam stability and performance in porous media. Overall, it is generally found that the effectiveness of foams in porous media is not reliably predicted based on bulk physical properties or on bulk foam measurements. Instead, it tends to be more useful to study the foaming properties in porous media at various laboratory scales micro-, meso-, and macro-scale. 

We have also measured the surface compression elasticity and viscosity of DTAB-PAMPS mixed surface layers. These two coefficients describe the resistance of the layer to a uniaxial compression in the surface plane. They were measured with a device in which surface waves are excited at a frequency of a few hundred hertz. It was found that as expected, the layers start to exhibit a measurable elasticity at surfactant concentrations much less than with pure surfactant solutions (Figure 5). The elasticity (both real and imaginary parts, r and j respectively) exhibits a maximum around CAC and decreases to zero around CMC. 

All the phenomena depend on the elasticity and viscosity of the surface, and high elasticities and viscosities are thus expected to stabilize the foam. However, since the foam is formed rapidly, the polymer has probably no time to reach its equilibrium adsorption in the CAC region and the foam behaviour should be governed by the surfactant adsorption as observed. 

First of all, surface rheology is completely described by four rheological parameters elasticity and viscosity of compression/dilatation and of shear. In every case surface flow is coupled with the hydrodynamics of the adherent liquid bulk phase. From interfacial thermodynamics we know that the integration over the deviation of the tangential stress tensor from the bulk pressure represents the interfacial tension y (after Bakker 1928). 

Using the classical equation of Boussinesq (1913) for the surface dilational viscosity, a relationship between surface tension change, surface elasticity and surface dilatation viscosity is obtained. 

Rapid spreading of a drop of oil that has a low surface tension over the lamella can cause rupture by providing a weak spot (46). The spreading oil lowers the surface tension, increases the radius of curvature of the bubbles, alters the original surface elasticity, and also changes the surface viscosity. Thus the interfacial film loses its foam-stabilizing capability. If S is negative, then the oil should not spread at the interface. 

During the 1870 s, Carlo Marangoni, who was apparently aware of Carra-dori s work but not of Thompson s, formulated a rather complete theory of surface tension driven flow (M2, M3). He noted that flow could result from surface tension variations as they are caused by differences in temperature and superficial concentration, and that, conversely, variations in temperature and concentration could be induced by an imposed surface flow. Marangoni ascribed several new rheological properties to the surface (notably surface viscosity, surface elasticity, and even surface plasticity), while remarking that perhaps some of these properties could be associated only with surface contamination. Most present-day authors ascribe the first explanation of surface tension driven flow to Marangoni, and term such flow a Maragoni effect. ... 

The viscoelasticity is a complex number determined by the dilatational elasticity and viscosity [19, 94, 95]. The viscoelasticity modulus (or surface dilatational modulus) incorporates a real and imaginary constituent, elasticity and viscosity, respectively. 

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