Some Consequences of Capillary Pressure

As a consequence of the pressure difference across a curved interface, such an interface will resist deformation by exerting an external force. The larger this pressure difference is, the larger the resistance is. Therefore, smaller drops or bubbles are less easily deformable than larger ones. This phenomenon is relevant for the preparation and stability of emulsions and foams (see Chapter 8). 

FIGURE 6.6 Droplet with varying capillary pressure along its irregularly shaped perimeter. 

FIGURE 6.7 Cross section through contacting foam lamellae. The pressure in the curved Plateau border is reduced. 

FIGURE 6.8 Capillary instability induced by a sinusoid-like perturbation. Due to different capillary pressures at A and B, the cylindrical jet of liquid may break up into droplets. 

If Pb Pa, liquid in the jet flows from B to A and deformation of the jet will be restored. This occurs when 2%R. i liiR, Pa Pb and the ensuing flow of liquid from A to B will cause rupture of the jet. Applications of the phenomenon of capillary instability are found, for example, in sprayers that produce aerosols, in emulsifying devices, and in spray drying. 

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