Conjoining pressure

The total interaction free enthalpy can also be expressed as an excess pressure compared to the bulk. When the interaction free enthalpy is repulsive this pressure is called the disjoining pressure, or conjoining pressure when the interaction free energy is attractive (see Ref. [23]). 

A film can only break up into droplets after a disturbance the film locally thins to less than t)q)ically 1000 nm (see Fig. 6.40). In this region the interaction force (van der Waals, electrical double layer, for example) between the liquid-solid and liquid-air surface of the film becomes important. Attraction forces can rupture the thin film and a dry patch is nucleated. Such a film is called a non-wetting film. When the interaction between the two film interfaces is repulsive the so-called disjoining pressure (see also p. 162) of the film, i.e. the pressure difference between the film and bulk liquid, is negative. In the other case of negative disjoining pressures, it may also be called conjoining pressure. 

Instabilities in UTR polymer films are manifested in two main ways, namely, (i) defects resulting from the coating process, substrate nonuniformities, and conjoining pressure, and (ii) discontinuihes in the thermophysical properties of the hlms due to interfacial effects and polymer cooperative and surface dynamics. 

Disjoining or conjoining pressure, important for films less than 1 xm thick. 

Comparison of the proposed dynamic stability theory for the critical capillary pressure shows acceptable agreement to experimental data on 100-/im permeability sandpacks at reservoir rates and with a commercial a-olefin sulfonate surfactant. The importance of the conjoining/disjoining pressure isotherm and its implications on surfactant formulation (i.e., chemical structure, concentration, and physical properties) is discussed in terms of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of classic colloid science. 

Figure 4. A conjoining/disjoining pressure isotherm for the constant- potential and weak overlap electrostatic model.

We can conclude that the stability of static foam in porous media depends on the medium permeability and wetting-phase saturation (i.e., through the capillary pressure) in addition to the surfactant formulation. More importantly, these effects can be quantified once the conjoining/disjoining pressure isotherm is known either experimentally (8) or theoretically (9). Our focus... 

Conversely, whenever the stretched lamella of local thickness h exerts a conjoining/disjoining pressure 11(h) which lies above the value of P, then fluid transports out of the... 

Figure 6. Evolution of the lamella thickness as it transports down the periodic pore for the conjoining/disjoining pressure isotherm of Figure 4. Three capillary pressures are considered in curves 1 through 3. These capillary-pressure values are also labelled in Figure 4. Curve 2 defines the critical or marginally...

ENORDET AOS 1618) in an 81- fjtm permeability sandpack. Using the parameters listed and the constant- charge electrostatic model for the conjoining/disjoining pressure isotherm, the data are rescaled A ... 

II - dimensionless conjoining/disjoining pressure, defined under Equations 1 and 2... 

Pressure in thin liquid films (thickness usually less than 10 m) is different from the pressure in the bulk liquid. This difference is caused by the action of additional forces, which are referred to as surface or colloidal forces. This force over the area of the film is the disjoining pressure. The additional pressure can be either disjoining or conjoining. However, the term disjoining is used in literature because of historical reasons. 

If h is bigger than 0.1 pm (Fig. 2), then the interaction force is equal to zero. However, if h < 0.1 pm, then an interaction force appears. This force can depend on the thickness h in a particular way. The interaction forces divided by the surface area of the plate have a dimension of pressure and are referred to as the disjoining pressure [1]. Note that the term disjoining pressure is misleading, because the force can be either disjoining (repulsion between surfaces) or conjoining (attraction between surfaces). However, disjoining is used because of historical reasons. 

---