Formation of liquid drops

The formation of liquid drops when flow occurs through thin tubes is a common daily phenomenon. In some cases, such as eyedrops, the size of the drop plays a significant role in the application and dosage of the medicine. The drop formed when liquid flows through a circular tube is shown in Figure 2.10. 

The surface tension for a liquid takes on an important part in the formation of liquid drop, bubble, or gas bubble in a liquid. Compared with the solid skin premelting, a liquid surface tends to solidify at temperatures below the bulk melting point. Normally, the temperature dependence of the surface tension of a molten substance follows a linear relation to the temperature of testing [49-54] ... 

If a drop is formed in an immiscible liquid, show that the average surface available during formation of the drop is l2nr2/5, where r is the final radius of the drop, and that the average time of exposure of the interface is 3///5, where tf is the time of formation of the drop. 

Another notable example of the dripping mechanism is the slow formation of a drop by breakaway from a liquid film on the bottom surface of a flat, horizontal plate under the action of gravity. The size of the drop formed can be estimated using the following expression derived on the basis of a force balance between gravity and surface tension)1 ... 

When formation of condensation nuclei just starts, we may assume p to be only a little greater than px. If the radius of the nucleus is known, the 7 can be computed from Eq. (63). Usually, however, the 7 in this equation is supposed to be equal to the surface tension of the liquid in bulk also for the smallest drops. This supposition was rejected by several scientists see, e.g., Ref.94, p. 63. Thus, application of W. Thomson s equation even to the formation of liquid droplets is beset with difficulties. 

In many processes (such as oil recovery, blood flow, underground water), one encounters liquid flow through thin (micrometer diameter), noncircular-shaped tubes, or pores. In the literature, one finds studies that address these latter systems. In another context of liquid drop formation, for example, in an inkjet nozzle, this technique falls under a class of scientifically challenging technology. The inkjet printer demands such quality that this branch of drop-on-demand technology is much in the realm of industrial research. All combustion engines are controlled by oil drop formation and evaporation characteristics. The important role of capillary forces is obvious in such systems. 

Some field measurements of HN03 suggest that the formation of liquid or solid Type I PSCs depends on the initial background sulfate aerosols on which the PSCs form. If they are liquid, then liquid ternary solution PSCs tend to form first as the temperature drops below 192 K, whereas if the sulfate particles are initially solids, solid Type lc PSCs may be generated (Santee et al., 1998). 

Johnstone, Feild, and Tassler (7D) have developed a Venturi atomizer to produce dense fogs of liquid drops. This equipment is similar to that used by Comings, Adams, and Shippee (IB). Durbin (6D) has studied the formation of condensation particles in supercooled hypersonic air flow. 

In view of the importance of macroscopic structure, further studies of liquid crystal formation seem desirable. Certainly, the rates of liquid crystal nucleation and growth are of interest in some applications—in emulsions and foams, for example, where formation of liquid crystal by nonequilibrium processes is an important stabilizing factor—and in detergency, where liquid crystal formation is one means of dirt removal. As noted previously and as indicated by the work of Tiddy and Wheeler (45), for example, rates of formation and dissolution of liquid crystals can be very slow, with weeks or months required to achieve equilibrium. Work which would clarify when and why phase transformation is fast or slow would be of value. Another topic of possible interest is whether the presence of an interface which orients amphiphilic molecules can affect the rate of liquid crystal formation at, for example, the surfaces of drops in an emulsion. 

Stupin and Kister (loc. cit.) relate the flattening of the curve in Fig. 14-76 at low liquid loads to the formation of more, smaller, easier-to-entrain liquid drops when the liquid load is lowered beyond the limiting liquidload. It follows that devices that can restrict the formation of smaller drops may be able to approach the system limit capacity predicted by Stupin s original equation [Eq. (14-167)] even at low liquid loads. 

Drop spreading The PolyJet process is based on the formation of liquid films, rather than single drops, during the brief time interval between the drop s contact with a surface and the moment of UV exposure. Therefore, the surface tension (and the viscosity) is of major importance. The surface tension affects the magnitude of... 

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