Shape Regimes for Fluid Particles

Bubbles and drops tend to deform when subject to external fluid fields until normal and shear stresses balance at the fluid-fluid interface. When compared with the infinite number of shapes possible for solid particles, fluid particles at steady state are severely limited in the number of possibilities since such features as sharp corners or protuberances are precluded by the interfacial force balance. 

Bubbles or drops which are prevented from moving under the influence of gravity by a flat plate are termed sessile (see Fig. 2.3a and 2.3b). When bubbles or drops remain attached to a surface with gravity acting to pull them away, they are called pendant (see Fig. 2.3c and 2.3d). Floating bubbles or drops, shown in Fig. 2.3e, are those which sit at the interface between two fluids. 

The profiles of pendant and sessile bubbles and drops are commonly used in determinations of surface and interfacial tensions and of contact angles. Such methods are possible because the interfaces of static fluid particles must be at equilibrium with respect to hydrostatic pressure gradients and increments in normal stress due to surface tension at a curved interface (see Chapter 1). It is simple to show that at any point on the surface 

Bubbles and drops in free rise or fall in infinite media under the influence of gravity are generally grouped under the following three categories  

Moreover, ellipsoidal bubbles and drops commonly undergo periodic dilations or random wobbling motions which make characterization of shape particularly difficult. Chapter 7 is devoted to this regime. 

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