Capillary pressure definition

The correlation between the stability of single O/W emulsion films, single drops under oil/water interfaces and real emulsions found in [514,516] also deserves attention. As revealed in the beginning of this Section the correlation between emulsions and emulsion films was studied in various aspects and always provide information about stability of such systems. Model studies of emulsion systems are worth further development especially if the correlation films/real emulsion is done at definite conditions which are as close as possible in both cases, for example, at equal capillary pressure, film size, emulsion dispersity, etc., as it is done in the correlation foam films/foam (see Chapter 7). 

The n(fc) isotherms of different types of foam films are shown in Fig. 7.8. The surfactant (NaDoS) and electrolyte (NaCl) concentrations were the same as those used in the experiments with foams. The equilibrium thickness of thin films and CBF decreased with the increase in pa = II. Films ruptured in a definite range of capillary pressure (marked with arrows on curves 1 and 2). The thickness of NBF did not change and they ruptured at a definite capillary pressure (marked with an arrow on curve 3). 

A schematic presentation of one of the most convenient modifications of these cells is given in Fig. 8.2. It represents a glass cuvette 1 (internal dimensions 5x3 cm and 7 x 1.7 cm), made of transparent optical glass, sintered glass filters 2 for creating a definite capillary pressure and electrodes 3 for measuring foam electrical conductivity. 

Figure 5 Example of capillary pressure and mercury volume versus time (A). Definition of different parameters of complexity. Figure (B) shows detail of figure (A) and defines different orders of penetration thresholds taking into account the fluctuations of capillary pressure. 

In the original definition of disjoining pressure by Deryaguin [11, 12], only the first two terms on the right-hand side of Equation (16.6) were considered. At low electrolyte concentrations, double layer repulsion predominates and can compensate the capillary pressure that is = P. This results in the formation of an equilibrium-free film which is usually referred to as the thick common film CF ( 50nm thickness). This equihbrium-metastable film persists until thermal or mechanical fluctuations cause mpture. The stability of the CF can be described in... 

The concept of the safety factor has caused some difficulty. In its simplest terms it may be thought of as the amount by which the capillary pressure has to be raised before oedema, i.e. swelling of the tissue spaces, occurs. This is a carefully constructed definition note it says nothing about the flow of fluid across the capillary wall - this may be substantially increased but as long as no swelling, i.e. no accumulation of water, in tissue spaces occurs, then there is no oedema. 

If we can assume that always both fluids are present, or, that by regularization, we replace domains with vanishing phases by low saturation domains, we can stay with the pressure pressure formulation which does not degenerate in this case. This approach has the advantage that boimdary conditions can be set for the phase pressures and phase fluxes. Moreover, the definition of interface conditions between porous layers of different characteristics is much simpler It is sufficient to assume continuity of phase pressures and phase fluxes, whereas in the saturation-based cases, the continuity of the capillary pressure has to be bought by a jump in the basic variable, which complicates the numerical schemes. 

In porous media, by definition, the capillary pressure is positive when water is the more wetting phase and, accordingly, the capillary pressure is negative when oil is the more wetting phase. Capillary pressure becomes zero at the oil-water interface, also called the free fluid level that would exist outside of the porous medium. 

By definition, the cake saturation (5) at any point on a capillary pressure curve is given by equation (4.30)... 

In particular, the definitions of wetting and nonwetting phases used to determine capillary pressure and surface tension angle are taken as a statistical average over the entire medium, obscuring local effects which differ from the whole. Because the dropletA)ubble... 

Let be the radius of the equilibrium droplet. According to the definition of the capillary pressure. 

Let p be the radius of the curvature of a meniscus in a flat capillary (a meniscus between two parallel plates). According to the definition of the capillary pressure,... 

In diying solids it is important to distinguish between hygroscopic and nonhygroscopic materials. If a hygroscopic material is maintained in contact with air at constant temperature and humidity until equilibrium is reached, the material will attain a definite moisture content. This moisture is termed the equilibrium moisture content for the specified conditions. Equilibrium moisture may be adsorbed as a surface film or condensed in the fine capillaries of the solid at reduced pressure, and its concentration will vaiy with the temperature and humidity of the surrounding air. However, at low temperatures, e.g., 15 to 50°C, a plot of equilibrium moisture content versus percent relative humidity is essentially independent of temperature. At zero humidity the equilibrium moisture content of all materials is zero. 

In the capillary method, the time required for a liquid to flow through a capillary tube is determined. The melt under investigation flows with a constant rate through a tube with a small, definite cross-sectional area, such as a cylindrical capillary. The viscosity can be measured in an absolute way from the pressure drop. This method can yield the most reliable absolute data, the viscosity being given by a modified Hagen-Poiseuille equation ... 

The spherical foam films can be obtained by blowing a bubble from a vertical capillary tube. The principle of formation of such a bubble is illustrated in Fig. 2.22. A vertical capillary tube is placed in a vessel with the surfactant solution so that its upper orifice is close to the solution surface. When a gas (air) with a definite pressure is introduced into the tube over the solution surface a foam film is formed acquiring the shape of a hemisphere. 

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