Droplet velocity distribution emulsion

Figure Bl.14.13. Derivation of the droplet size distribution in a cream layer of a decane/water emulsion from PGSE data. The inset shows the signal attenuation as a fiinction of the gradient strength for diflfiision weighting recorded at each position (top trace = bottom of cream). A Stokes-based velocity model (solid lines) was fitted to the experimental data (solid circles). The curious horizontal trace in the centre of the plot is due to partial volume filling at the water/cream interface. The droplet size distribution of the emulsion was calculated as a fiinction of height from these NMR data. The most intense narrowest distribution occurs at the base of the cream and the curves proceed logically up tlirough the cream in steps of 0.041 cm. It is concluded from these data that the biggest droplets are found at the top and the smallest at the bottom of tlie cream.

Although NMRI is a very well-suited experimental technique for quantifying emulsion properties such as velocity profiles, droplet concentration distributions and microstructural information, several alternative techniques can provide similar or complementary information to that obtained by NMRI. Two such techniques, ultrasonic spectroscopy and diffusing wave spectroscopy, can be employed in the characterization of concentrated emulsions in situ and without dilution [45],... 

Spatially-resolved measurement of the droplet size distribution can be accomplished by the implementation of velocity compensated pulse sequences, such as the double PGSTE [81] in a spatially resolved imaging sequence. Accurate measurements of spatially resolved droplet size distributions during flow and mixing of emulsions would provide truly unique information regarding flow effects on the spatial distribution of droplets. 

Determining the droplet size distribution of an emulsion by ultrasonic spectrometry involves two steps. First, the ultrasonic velocity and (or) attenuation coefficient of the emulsion is measured as a function of the frequency — preferably over as wide a range as possible. Second, the experimental measurements are compared with theoretical predictions of the ultrasonic properties of the emulsion, and the droplet size distribution providing the best fit between theory and experiment is determined. 

The size of the droplets in an emulsion has a strong influence on many of its physicochemical and sensory properties, e.g., shelf life, appearance, texture, and flavor (1,2, 4). For example, the stability of an emulsion to gravitational separation or droplet aggregation can be greatly improved by decreasing the droplet size. This is because the velocity of sedimentation is proportional to the square of the droplet size. 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 (4). A food manufacturer normally specifies a preestablished desirable droplet size distribution for a particular product. If the product does not meet this specification, it typically must be reprocessed or even discarded. 

Droplet size depends on a number of factors such as the type of oil, brine composition, interfacial properties of the oil-water system, surface-active agents present (added or naturally occurring), flow velocity, and nature of porous material. For the study of OAV emulsions, McAuliffe (9) varied emulsion droplet sizes and size distributions by increasing the sodium hydroxide concentration in the aqueous phase, as shown in Figure 10. Higher NaOH concentration neutralizes more of the surface-active acids in the crude oil and produces an emulsion that has droplets of smaller diameters and is also more stable. Emulsion droplet size distribution can also be varied by varying the concentration of a surfactant added to the crude oil, as shown in Figure 11. 

The influence of wall shear stress on the droplet size distribution for a 4.8-pjn SPG membrane is shown in Figure 16.17 and Figure 16.18. With increasing the wall shear stress from 1.3 to 30 Pa, the droplet size distribution curve shifts to smaller droplet diameters and becomes narrower and narrower. For the given pore size and experimental conditions, an emulsion with a narrow droplet size distribution (span = 0.43) was even produced at a- = 0.37 Pa, corresponding to Vt = 0.3 m/sec and laminar flow inside the membrane tube (Figure 16.13). Williams et al. [49] obtained a span value of 0.82 at the mean tube velocity of Vt = 0.6 m/sec in a semicontinuous... 

Figure 4.8 Creaming of emulsion without added polymer, (a) Volume fraction profiles measured using the ultrasonic creaming monitor showing the progression from uniform (horizontal line) to fully creamed state over 37 days. (Inset) Droplet size distribution inferred from the creaming data, (b) Height of eight ordinates y]-y8 with time. The gradient of each ordinate is related to the Stokes velocity of the size fraction represented by each volume fraction...

By microchanneling (Figure 20.8, left), monodisperse emulsions may be produced in the absence of shear forces [63]. A strongly non-cylindrical geometry at the microchannel exit followed by a terrace is responsible for this effect [64, 65]. The droplet formation mechanism is limited to a critical velocity of the dispersed phase, above which the droplet sizes are widely distributed [51]. 

Three microscopic pictures of the DE emulsion structure treated with the EXMIX nozzle are inserted in Fig. 23.13 (bottom). To keep the microstructure of such DE unchanged, an INMIX nozzle is obviously the better choice for a GLR up to ca. 0.5. On the other hand, the GLR also determines the spray drop/spray particle size distributions of the end-use prilled or spray-dried powders, which is an additional criterion for the choice of the spray nozzle. The comparison of, r5o,3 Nozzle and. 90,3 Nozzle with 50,3 Initial and 3 903 Initial for the same emulsion at GLR = 0 (Fig. 23.12 (top) and Fig. 23.13) indicates the dispersing capacity of the velocity field (and related to locally acting shear and elongation stresses) of the pure liquid phase flow in the nozzle which was explored further in detail (section Impact of Pure Liquid-Cap Nozzle Flow on Secondary Droplet Size of SE, DE ). 

The speed of each fraction of oil (contour) can thus be used to infer the effective hydrodynamic size distribution of the droplets. The inset to Figure 4.8(a) shows the droplet diameters obtained from the creaming contours. The size distribution is consistent with that measured using a laser diffraction particle sizer on the initial emulsion, showing that the assumption of individual droplet movement according to Stokes velocity is justified. 

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