Tension-film

Mechanism of Action A bulk-producing laxative that decreases surface film tension by mixing liquid and bowel contents. Therapeutic Effect Increases infiltration of liquid to form a softer stool. 

SURFACE TENSION. Fluid surfaces exhibit certain features resembhng the properties of a stretched elastic membrane hence the term surface tension. Thus, one may lay a needle or a safety-razor blade upon the surface of water, and it will lie at rest in a shallow depression caused by its weight, much as if it were on a rubber air-cushion. A soap bubble, likewise, tends to contract, and actually creates a pressure inside, somewhat after the manner of a rubber balloon. The analogy is imperfect, however, since the tension in the rubber increases with the radius of the balloon, and the pressure inside, which would otherwise decrease, remains approximately constant while the liquid film tension remains constant and the pressure in the bubble falls off as the bubble is blown. 

Figure 5 shows dynamic film tension of soybean oil films stabilized by 0.5 wt % SPAN 80 emulsifier between aqueous phases under expansion by various flow rates. The increase in film tension from equilibrium is higher at higher rates of interface expansion because the flux of surfactant that can adsorb during expansion is lower at higher rates. 

Figure 5. Dynamic film tension of soybean oil film containing 0.5 wt% SPAN 80 between water phases, expanding with various flow rates as a function of the relative film area. Rc=0.32 mm, h=0.03 mm, pH=7, at 25 °C.

The lamellae in a foam contain two gas/liquid interfaces separated by a layer of fluid, thin film, each interface having a surface tension. For this reason the term film tension is sometimes used, the film tension being equal to twice the surface tension of the surfaces. It should be noted that, while the film tension is twice the surface tension of the surfaces, this is not necessarily the same as twice the surface tension of the bulk solution. In fact, the surface tension of a fluid film surface is similar to that of the bulk solution when the fluid film is thick, but departs from the bulk solution value as the fluid film thins. The situation is similar for the thin films between droplets in a concentrated emulsion. 

From Eqs. (1.12) and (1.13) it follows that the film separating two bubbles is flat only when the bubbles are of equal size. When the difference in size is large (7 i R2), then R] Rj. Expressions (1.11) - (1.13) are valid when the separating film is very thick and the film tension does not differ from the surface tensions of both bulk phases. 

In the calculation of the curvature of thin films the change in film tension should be taken into account. Thus, for a bubble with thin films Eq. (1.12) transforms [55] in... 

One of the most important thermodynamic characteristics of foam films is the contact angle appearing at the contact of the film with the bulk phase (solution) from which it is formed. The measurement of the contact angle and the film tension related to it is an... 

When a vertical foam film is illuminated, part of the light is scattered by thermal fluctuational microwaves in the film surface. Measuring the intensity of the scattered light makes possible the calculation of the film tension and the energy of molecular interactions in the film [89,90]. 

The elasticity modulus is evaluated from the measured film tension and local film thickness as follows... 

The same principle of formation of a spherical foam film is used in the device (Fig. 2.23) for direct measurement of the film tension y in static [126] and dynamic [127] conditions. The lower part of the capillary is placed in a closed space filled up with a gas, whose pressure can be measured precisely and by means of a special pump can be varied at different rates. The manometer registers the difference with the atmospheric pressure Ap, which is equal to the capillary pressure pa of the spherical foam film... 

The radius of the curvature of the film Rf is determined from the photographs taken with a horizontal microscope. The equilibrium values of /are depicted at a fixed pressure difference Ap = pa [126], while the dynamic film tension yis evaluated by photographing the film and the simultaneous registration of Ap at a given moment when it is varied at a certain rate [127]. 

With geometrical parameters R and Al precisely measured from photographs, Eq. (2.16) allows to determine the film tension y. Furthermore, if the film deformation by the electric field is measured under various conditions (concentration and temperature), this equation could be used in the study of CBF/NBF transition. 

From here a Gibbs-Duhem type of equation for the film tension follows [7]... 

Fig. 3.64 plots the difference between film tension and surface tension of the bulk phase, 2o(l-cos0o), as a function of the electrolyte concentration Cei in the initial solution [251], It is obvious that the course of the dependence changes sharply at the CBF/NBF transition. Cationic surfactant also follow a similar trend of behaviour [324],... 

Fig. 3.64. Dependence of the difference between film tension of a black films from NaDoS solution and...

The relation between film stability, spreading coefficient on a substrate and surface pressure can be found using the method proposed by Frumkin [20] and Derjaguin [538]. A diagram is drawn of the dependence film tension versus area of a mole of the substance in the film (Ao = I/O, T being the number of moles of the substance in a unit area. At small substance concentrations (pressures) in the gas phase, its adsorption is close to Gibbs adsorption. 

This new DI technique therefore presents a method for determining and relating the pseudoemulsion film tension to spreading... 

The interactions between an oil phase and foam lamellae are extremely complex. Foam destabilization in the presence of oil may not be a simple matter of oil droplets spreading upon foam film surfaces but may often involve the migration of emulsified oil droplets from the foam film lamellae into the Plateau borders where critical factors, such as the magnitude of the Marangoni effect in the pseudoemulsion film, the pseudoemulsion film tension, the droplet size and number of droplets may all contribute to destabilizing or stabilizing the three phase foam structure. 

Equation [5.2.5a] agrees with intuition. The spreading coefficient can only be positive if the work of adhesion exceeds that of cohesion. When the SG interface carries an adsorbate, these equations remain valid on the understanding that ai Gi jg lower by an amount of n. When this surface carries a liquid film, 12 1 gi has to be replaced by the corresponding film tension, say. For thick films... 

Equation 5.136 predicts a rather weak dependence of the film tension y on the disjoining pressure, n, for equilibrium thin films (small h). By means of Equations 5.128 and 5.129, Equation 5.135 can be transformed to read ... 

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