Marangoni surface elasticity

Although many factors, such as film thickness and adsorption behaviour, have to be taken into account, the ability of a surfactant to reduce surface tension and contribute to surface elasticity are among the most important features of foam stabilization (see Section 5.4.2). The relation between Marangoni surface elasticity and foam stability [201,204,305,443] partially explains why some surfactants will act to promote foaming while others reduce foam stability (foam breakers or defoamers), and still others prevent foam formation in the first place (foam preventatives, foam inhibitors). Continued research into the dynamic physical properties of thin-liquid films and bubble surfaces is necessary to more fully understand foaming behaviour. Schramm et al. [306] discuss some of the factors that must be considered in the selection of practical foam-forming surfactants for industrial processes. 

The elasticity that is determined from nonequilibrium dynamic measurements depends upon the stresses applied to a particular system, is generally larger in magnitude, and is termed the Marangoni surface elasticity, jEm (6, 22) (equation 12). The time-dependent Marangoni elasticity is il-... 

Marangoni Surface Elasticity See Film Elasticity, Marangoni Effect. 

Many surfactant solutions show dynamic surface tension behavior. That is, some time is required to establish the equilibrium surface tension. If the surface area of the solution is suddenly increased or decreased (locally), then the adsorbed surfactant layer at the interface would require some time to restore its equilibrium surface concentration by diffusion of surfactant from or to the bulk liquid. In the meantime, the original adsorbed surfactant layer is either expanded or contracted because surface tension gradients are now in effect, Gibbs—Marangoni forces arise and act in opposition to the initial disturbance. The dissipation of surface tension gradients to achieve equilibrium embodies the interface with a finite elasticity. This fact explains why some substances that lower surface tension do not stabilize foams (6) They do not have the required rate of approach to equilibrium after a surface expansion or contraction. In other words, they do not have the requisite surface elasticity. 

The Marangoni elasticity can be determined experimentally from dynamic surface tension measurements that involve known surface area changes. One such technique is the maximum bubble-pressure method (MBPM), which has been used to determine elasticities in this manner (24, 26). In the MBPM, the rates of bubble formation at submerged capillaries are varied. This amounts to changing A/A because approximately equal bubble areas are produced at the maximum bubble pressure condition at all rates. Although such measurements include some contribution from surface dilational viscosity (23, 27), the result will be referred to simply as surface elasticity in this work. 

Capillary Ripples Surface or interfacial waves caused by perturbations of an interface. When the perturbations are caused by mechanical means (e.g., barrier motion), the transverse waves are known as capillary ripples or Laplace waves, and the longitudinal waves are known as Marangoni waves. The characteristics of these waves depend on the surface tension and the surface elasticity. This property forms the basis for the capillary wave method of determining surface or interfacial tension. 

Film Elasticity The differential change in surface tension of a surface film with relative change in area. Also termed surface elasticity, dilata-tional elasticity, areal elasticity, compressional modulus, surface dilata-tional modulus, or modulus of surface elasticity. For fluid films, the surface tension of one surface is used. The Gibbs film (surface) elasticity is the equilibrium value. If the surface tension is dynamic (time-dependent) in character, then for nonequilibrium values, the term Marangoni film... 

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 first involves a study by Malysa et al. [5] of the surface elasticity and dynamic stability of wet foams for a homologous series n-alcohols (C4-Cxo). On ascending the series, the authors found that foam stability passed through a maximum with the Cg-Cg alcohols. This effect was found to be unrelated to the Marangoni dilational modulus. However, the authors also determined a parameter they called the effective elasticity which depended on the kinetics of adsorption. They foimd that at short time scales (0.05-0.10 s), the effective elasticity correlated well with foam stability, reaching a maximum value with the C7 alcohol. 

Surface elasticity, sometimes referred to as the self-heahng effect, is caused by surfactants. The mechanism behind this phenomenon is called the Gibbs-Marangoni effect and is illustrated in Figure 13.14. 

The drop rim is described as a toroidal tube of oil, that is located at the drop radius position R(t), having a (time-dependent) mass m. A time-dependent spring parameter k(t), that takes all the springlike forces acting on the rim into account, is used. These forces are surface tensions, Marangoni stress, as well as the drop surface elasticity due to the surfactant monolayer. The spring parameter can be seen as the opposite of an effective spreading coefficient (Seff(t) = —k t)/2n). 

We will see in the following that the ability of the surface layer to oppose the resistance to smface motion is also a very important factor. Surface flow induces surface concentration gradients leading to surface tension gradients responsible for very large surface forces (Marangoni effect). This can be quantified by the surface elasticity sq defined as... 

A is the area of the surface. In a foam, where the surfaces are interconnected, the time-dependent Marangoni effect is important. A restoring force corresponding to the Gibbs elasticity will appear, because only a finite rate of absorption of the surface-active agent, which decreases the surface tension, can take place on the expansion and contraction of a foam. Thus the Marangoni effect is a kinetic effect. 

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