Droplet characteristics

Many of the most important physicochemical and functional properties of emulsions are governed by the presence of the droplets that they contain [3]. Knowledge of the characteristics of emulsion droplets is therefore important for understanding and controlling emulsion properties. 

The droplet concentration is normally expressed in terms of the disperse phase volume fraction which is equal to the volume of all the emulsion droplets (V ) divided by the total 

The droplet concentration can be measured using traditional proximate analysis techniques (e.g. drying, solvent extraction, and density measurements) or by using more sophisticated modern analytical techniques (e.g. light scattering, electrical pulse counting, and ultrasonic spectroscopy). 

The size of the droplets in an emulsion is largely determined by the emulsifier type and concentration, the physicochemical properties of the component phases, and the homogenization conditions used to prepare it [3]. A manufacturer normally specifies a pre-established desirable droplet size distribution for a particular product. If the product does not meet this specification it typically must be reprocessed or even discarded. 

CkiUoidal interactions govern whether emulsion droplets aggregate or remain as separate entities thereby impacting the characteristics of any aggregates formed (e.g. their size, shape, porosity, and deformability) [3]. The bulk physicochemical properties and stability of many emulsions depend on the extent of droplet aggregation and the characteristics of any aggregates formed. The interactions between two emulsion droplets can be described 

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In the second method, i.e., th particle method 546H5471 a spray is discretized into computational particles that follow droplet characteristic paths. Each particle represents a number of droplets of identical size, velocity, and temperature. Trajectories of individual droplets are calculated assuming that the droplets have no influence on surrounding gas. A later method, 5481 that is restricted to steady-state sprays, includes complete coupling between droplets and gas. This method also discretizes the assumed droplet probability distribution function at the upstream boundary, which is determined by the atomization process, by subdividing the domain of coordinates into computational cells. Then, one parcel is injected for each cell. 

In the case of spray towers it has been shown by Thornton 10 that ur is well represented by Hod — j) where u0 is the velocity of a single droplet relative to the continuous phase, and is termed the droplet characteristic velocity. The term (1 — j) is a correction to m0 which takes into account the way in which the characteristic velocity is modified when there is a finite population of droplets present, as opposed to a single droplet. It must be seen therefore that for very dilute dispersions, that is as j -> 0, w0(l — j) o- On the other hand, as the fractional hold-up increases, the relative velocity of the dispersed phase decreases due to interactions between the droplets. Substituting for ur, equation 13.32 may be written as ... 

Flooding-point data may be correlated by equations 13.34 and 13.35 using the droplet characteristic velocity concept as discussed by Thornton and Pratt(34), since coalescence is absent. 

Achieve reproducible (from run-to-run) spray droplet characteristics as they arrive at the tablet surface (see later discussion)... 

Information on droplet characteristics in sprays and spray flames is presented. The distribution of OH and CH species in spray and gas-fueled flames is presented in order to decouple the effect of droplet vaporization. As the droplet size in the spray flame becomes smaller, its signature should begin to resemble that of the gas-fueled flame, due to the extremely short evaporation time of the droplets. 

As discussed previously, atomizing and pattern air should ideally be set to provide suitable droplet size and spray pattern with minimal bed disturbance. Atomization involves an appropriate balance of air velocity and volume at the required spray rate. As spray is increased in a scale-up, an appropriate increase in atomizing air velocity and volume would be needed to maintain similar droplet characteristics from the same nozzle, which may disturb the bed. Potential nozzle configuration changes required for scale-up include use of larger nozzle tip and/or cap orifices. These will... 

Table 2.25 Cloud droplet characteristics including standard deviation low-level stratiform...

Secondary droplet characteristics can also be predicted from semi-empirical models. This subject is not addressed here but the reader is referred to the various reviews (e.g., [24]) which analyze the models proposed for impacts onto wetted and cold surfaces (e.g., [16, 17, 19]). 

Samenfink W, ElsSPer A, Dullenkopf K, Wittig S (1999) Droplet inteiactimi with shear-driven liquid films analysis of deposition and secondary droplet characteristics. International Journal of Heat and Fluid Flow 20 462-469. 

Much research has, in fact, been carried out to achieve this imderstandmg [37-39]. The spray structure has been investigated using visualization techniques such as laser sheet photography [40] and laser-induced fluorescence (LIF) [36], but detailed information of droplet characteristics cannot be obtained by planar measurements. Point measurements such as phase Doppler anemometer (PDA) can provide very high temporal resolution of droplet diameter and velocity, but lacks the ability to provide information about the spatial structure of the spray. Due to incomplete information about the spray, it is still difficult to optimize the pressure swirl injector. Computational studies of spray structure have revealed details of the... 

Nolan et al. (1990), Moodie and Ewan (1990), and lohnson (1991) have studied two-phase source terms on the laboratory scale. Johnson (1991) has presented results on the deposition of contaminant liquid versus liquid superheat for six substances. Nolan et al. (1990) and Moodie and Ewan (1990) have also presented measurements on droplet characteristics, including size and number concentration. 

Polymer-dispersed liquid crystals (PDLCs) is a relatively new class of promising material for many applications such as, switchable windows, display devices, infrared shutters, angular-discriminating filters, thermoelectrooptic switches, memories, gas-flow sensors, optical sensors, and optical gratings etc. These materials are examples of combined application of polymers and liquid crystals and command the attention of the display industry as well as the researchers. These consist of LC droplets which are dispersed in a polymer matrix. These tiny droplet characteristics are responsible... 

Lastly, droplet distribution curves and droplet size characteristic measurements were made for each of the sprays. Figure 15.13 shows the droplet size characteristics for the pressure swirl injector and Fig. 15.14 shows the droplet size characteristics for the liquid jet injector. The droplet size distribution is shown in both a cumulative and normalized sense. The cumulative volume fraction is used to determine DvO.l, DvO.5, and DvO.9 measurements. The calculated values for DIO, D31, and SMD are also shown on each plot. In general, the pressure swirl injector analyzed had a tighter distribution of injected droplets. The tighter distribution means that there are fewer larger droplets measured, which create smaller values for all of the key droplet characteristics, measured (D31, DIO, DvO.l, Dv05, DvO.9, and D32). 

A key parameter for lipase activity is the composition of the siufactant system. Because the lipase works at the water-oil interface (see Sec. Ill A), its activity depends on the oil droplet characteristics such as charge and size, which depend on the type of surfactant. However, surfactants may also have more specific effects, which result in boosting or impairing the enzymatic activity, according to their structure and the type of lipase. The lipase may be truly activated by surfactants which create a favorable enviromnent at the oil-water interface. The surfactant-lipase complex is then probably more surface active than the lipase alone, making the interaction with the oil easier [27]. At high concentrations however, surfactants may induce the enzyme denaturation and/or the blockage of the active site. 

Fluorescent nanoparticles, such as CdSe QDs, have been used to probe fluid-fluid interfadal nanopartide segregation, as well as the relationship between nanopartide size and various droplet characteristics, including nanopartide transport across the interface, in-plane interfadal mobility, and phase separation of different-sized nanoparticles. Fluorescence confocal microscopy is particularly useful for observing the stmrture and behavior of QD-stabilized... 

Figure 9.16. Multiple emulsion classification based on droplet characteristics in the primary emulsion.

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