Interfaces liquid droplet example

In the example given by Figure 2.4, there is no heat transfer across the gas-liquid interface (i.e., q = 0), and we can assume that air does not diffuse into the liquid droplet (i.e., Ng= 0). 

The process of wetting involves the creation of new types of interfaces at the expense of others. For example, when a liquid droplet spreads over a solid, more of the SL and LG interface is created, whereas (psud of the) SG interface has vanished. All these changes are accompanied by chemges in thermodynaunlc characteristics, such as the Gibbs or Helmholtz energies of the system. Here, thermodynamics is useful to... 

Emulsion stability is determined by the strength of the interfacial film and the way the adsorbed molecules in it are packed. If the adsorbed molecules in the film are closely packed, and it has some strength and viscoelasticity, it is difficult for the emulsified liquid droplets to break the film. In other words, coalescence is difficult. The emulsion is therefore stable. The molecular structure and the properties of the emulsifiers in the film affect the film s properties. The molecules in the film are more closely packed if the emulsifier has straight chains rather than branched chains. The film strength is increased if mixed emulsifiers are used rather than a single one. The reasons are that (1) the molecules in the film are closely packed, (2) mixed liquid crystals are formed between droplets, and (3) molecular complexes are formed in the interface by emnlsifier compositions. For example, an oil-soluble surfactant mixed with a water-solnble snrfactant works very well to stabilize emulsions (Kang, 2001). 

Not all such flows, however, are linear—as, for example, in the case of non-Newtonian creeping flows around spherical particles (B4a, B4b, Cl, D3, F9, FIO, G5, L8, LIO, Rl, S2, SIO, T4, T7, W2, W3, W3a, W3b, W4, W5, W6, Zl). Similarly, owing to the unknown shape of the interface at the outset, free-boundary problems involving liquid droplets in nonuniform flows (Section II, C, 2, b) are intrinsically nonlinear despite the possible linearity of the equations of motion (and boundary conditions) inside and outside of the droplet. 

In these chapters, most emerging tunable and non-tunable microlenses based on various mechanisms have their optical axes perpendicular to their substrates, thus requiring optical alignments of different layers. This means that complicated structures are required for applications such as labs on chips. Chapter 7, Horizontal Microlenses Integrated in Microfluidics, presents examples of microlenses whose optical axes are parallel to substrates of microfluidic networks. These horizontal microlenses include two-dimensional polymer lenses tunable and movable liquid droplets as lenses hydrodynamically tuned cylindrical lenses liquid core and liquid cladding lenses air-liquid interface lenses and tunable liquid gradient refractive index lenses. 

Another theory of liquid-liquid explosion comes from Board et al. (1975). They noticed that when an initial disturbance, for example, at the vapor-liquid interface, causes a shock wave, some of the liquid is atomized, thus enhancing rapid heat transfer to the droplets. This action produces further expansion and atomization. When the droplets are heated to a temperature equal to the superheat temperature limit, rapid evaporation (flashing liquid) may cause an explosion. In fact, this theory resembles the theory of Reid (1979), except that only droplets, and not bulk liquid, have to be at the superheat temperature limit of atmospheric pressure (McDevitt et al. 1987). 

Water-cooling in towers operates on the evaporative principles, which are a combination of several heat/mass transfer processes. The most important of these is the transfer of liquid into a vapor/air mixture, as, for example, the surface area of a droplet of water. Convective transfer occurs as a result of the difference in temperature between the water and the surrounding air. Both these processes take place at the interface of the water surface and the air. Thus it is considered to behave as a film of saturated air at the same temperature as the bulk of the water droplet. 

It is cavitation in a heterogeneous medium which is the most studied by sonoche-mists. When produced next to a phase interface, cavitation bubbles are strongly deformed. A liquid jet propagates across the bubble towards the interface at a velocity estimated to hundreds of metres per second. At a liquid-liquid interface, the intense movement produces a mutual injection of droplets of one liquid into the other one, i. e. an emulsion (Fig. 3.3). Such emulsions, generated through sonication, are smaller in size and more stable than those obtained conventionally and often require little or no surfactant to maintain stability. It can be anticipated therefore that Phase Transfer Catalysed (PTC) reactions will be improved by sonication. Examples are provided later in this chapter. 

Surface tension and contact angle phenomena play a major role in many practical things in life. Whether a liquid will spread on a surface or will break up into small droplets depends on the above properties of interfaces and determines well-known operations such as detergency and coating processes and others that are, perhaps, not so well known, for example, preparation of thin films for resist lithography in microelectronic applications. The challenge for the colloid scientist is to relate the macroscopic effects to the interfacial properties of the materials involved and to learn how to manipulate the latter to achieve the desired effects. Vignette VI provides an example. 

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