Conjoining forces

Fig. 6.40. A flat coated film of non-wetting liquid (a) usually greater than 1 pm thick can dewet if a disturbance thins the film (b) to the extent (ordinarily less than 1 pm) that the effect of conjoining force is to dewet (c) the solid substrate. (After Kheshgi and Scriven [73].)...

Fig. 6.43. Surface tension gradient-driven flow draws liquid from regions of locally low tension (a,b). Film thinning might continue until conjoining forces become appreciable (c) and rupture the film (d). (After Kheshgi and Scriven [73].)...

For spin-coated ultrathin films, nucleation occurs by a spinodal decomposition phenomena or by airborne particles falling on the surface of the film. Spinodal decomposition proceeds by amplification of surface disturbances on the free surface of the film due to thermal fluctuations or mechanical vibrations. Conjoining forces overwhelm the tendency for surface tension to level the film, thus driving the growth of the surface modulations until they reach the substrate to nucleate a hole. 

Unite the one with the other, so that the two waters may be one This [conjoined] stream will possess the forces of each of them, mixed together, just as the fountain of Jupiter Hammon is hot and cold at the same time. 

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. 

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. 

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