Multicomponent droplet

A microemulsion droplet is a multicomponent system containing oil, surfactant, cosurfactant, and probably water therefore there may be considerable variation in size and shape depending upon the overall composition. The packing constraints which dictate size and shape of normal micelles (Section 1) should be relaxed in microemulsions because of the presence of cosurfactant and oil. However, it is possible to draw analogies between the behavior of micelles and microemulsion droplets, at least in the more aqueous media. 

The most widely studied deformable systems are emulsions. These can come in many forms, with oil in water (O/W) and water in oil (W/O) the most commonly encountered. However, there are multiple emulsions where oil or water droplets become trapped inside another drop such that they are W/O/W or O/W/O. Silicone oils can become incompatible at certain molecular weights and with different chemical substitutions and this can lead to oil in oil emulsions O/O. At high concentrations, typical of some pharmaceutical creams, cosmetics and foodstuffs the droplets are in contact and deform. Volume fractions in excess of 0.90 can be achieved. The drops are separated by thin surfactant films. Selfbodied systems are multicomponent systems in which the dispersion is a mixture of droplets and precipitated organic species such as a long chain alcohol. The solids can form part of the stabilising layer - these are called Pickering emulsions. 

It is known that, in a water phase, immiscible liquids such as gasoline or other petroleum products may form multicomponent droplets of various forms and sizes, under dispersive conditions. These droplets are transported by convection and diffusion, which contributes to the contamination of fresh water systems. However, during droplet transport, more volatile substances partition to the gas phase at the droplet surface, leaving less volatile material that volatilizes more slowly. More volatile material still exists in the droplet interiors, and it tends to diffuse toward the surface because of concentration gradients created by prior volatilization. Different components in a droplet have different volatilization rates, which may vary significantly during droplet transport, and as a result, the contamination of fresh water is affected accordingly. 

To represent the above phenomena, the present simulations consider the fuel droplets to be multicomponent, consisting of a solid high-energy fuel core surrounded by a liquid carrier. For example, cubane has been used as the core material embedded in n-heptane. n-Heptane was chosen because of the availability of experimental data, but in principle any other carrier liquid could be used in the model. An infinite conductivity model is used to account for droplet... 

Most food products and food preparations are colloids. They are typically multicomponent and multiphase systems consisting of colloidal species of different kinds, shapes, and sizes and different phases. Ice cream, for example, is a combination of emulsions, foams, particles, and gels since it consists of a frozen aqueous phase containing fat droplets, ice crystals, and very small air pockets (microvoids). Salad dressing, special sauce, and the like are complicated emulsions and may contain small surfactant clusters known as micelles (Chapter 8). The dimensions of the particles in these entities usually cover a rather broad spectrum, ranging from nanometers (typical micellar units) to micrometers (emulsion droplets) or millimeters (foams). Food products may also contain macromolecules (such as proteins) and gels formed from other food particles aggregated by adsorbed protein molecules. The texture (how a food feels to touch or in the mouth) depends on the structure of the food. 

A multicomponent gas flow contains a uniform distribution of small droplets of an organic solvent called A. The droplets have a diameter d and a number density Q [m-3]. The solvent evaporation rate m"k (kg/s-m2) depends on the gas-phase concentration of A. It may be assumed that the volume occupied by the droplets is negligible. 

Let us consider a solution composed of hard-sphere droplets of a single size g present in a multicomponent solvent. The expression for the osmotic pressure due to the hard spheres allows us to calculate the chemical potentials of the components in the mixed solvent. Subsequently, the Gibbs—Duhem equation is used to calculate the chemical potentials of the hard-sphere droplets.26 The mole fraction X, of the components and their volume fraction d> are related via the expressions... 

Atmospheric aerosols are multicomponent particles ranging from 0.001 to 10 pm in diameter. Particles are introduced into the atmosphere by combustion processes and a variety of other anthropogenic and natural sources. They evolve by gas-to-particle conversion and coagulation, are augmented due to the formation of fresh particles by nucleation, and are removed by deposition at the earth s surface and scavenging by airborne droplets. Atmospheric aerosols are the main cause of the visibility degradation accom-... 

This chapter complements Refs. 21 and 22 in reviewing the progresses made on the transient, convective, multicomponent droplet vaporization, with particular emphasis on the internal transport processes and their influences on the bulk vaporization characteristics. The interest and importance in stressing these particular features of droplet vaporization arise from the fact that most of the practical fuels used are blends of many chemical compounds with widely different chemical and physical properties. The approximation of such a complex mixture by a single compound, as is frequently assumed, not only may result in grossly inaccurate estimates of the quantitative vaporization characteristics but also may not account for such potentially important phenomena as soot formation when the droplet becomes more concentrated with high-boiling point compounds towards the end of its lifetime. Furthermore, multi-... 

In the next section some of the important time scales are identified and transient droplet heating effects during the spherically symmetric, single-component droplet vaporization are reviewed. Spherically symmetric, multicomponent droplet vaporization and droplet vaporization with nonradial convection are discussed in later sections. 

General Discussion. It was shown in the previous section that the bulk vaporization characteristics of a single-component droplet do not depend too sensitively on the detailed description of the internal heat transfer mechanisms. However, for multicomponent droplet vaporization qualitatively different behavior is expected for different internal transport mechanisms. This is because the vaporization characteristics (for example, the vaporization rate, the flame temperature and location, and the... 

The General Discussion of the previous section is equally applicable here, except now proper multicomponent descriptions of the gas-phase transport and the interfacial phase change should be used (50,51, 52). By assuming the gas-phase reactions are again confined to a flame-sheet where the reactants are consumed in a species-weighted stoichiometric proportion, explicit expressions can be derived (50) for y, Tf, H, and the fractional mass evaporation rate of the i species, as functions of the temperature and vapor concentration at the droplet surface. 

Experimental Observations. Most of the experimental observations on multicomponent droplet vaporization use two-component droplets 49, 62, 63,-64), The observations on the temporal behavior of the droplet temperature (49,64), size (62,63), and composition (64) all indicate that the vaporization processes are controlled by the volatihty differentials rather than by hquid-phase mass diffusion. Since the fuels used in these experiments are quite nonviscous, the above results then indicate that internal circulation of suflBcient strength has been generated by the prevailing forced and/or natural convection. 

General Discussion. We have shown that for the vaporization of practical, multicomponent droplets, qualitatively different vaporization behavior results when extreme internal transport rates are assumed. Since diffusive transport is always present during the transient, it is the... 

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