Mean droplet size variation

The variations of the mean droplet size and the droplet size distribution with axial distance in a spray generated by pressure swirl atomizers have been shown to be a function of ambient air pressure and velocity, liquid injection pressure, and initial mean droplet size and distribution 460]... 

Thus, both the mean droplet size and the size distribution may be predicted using these correlations [Eqs. (26), (27), (28), or (29) and Eqs. (30), (31)] for given process parameters and material properties. For a given atomizer design, the standard deviation of droplet size distribution has been found to increase with the melt flow rate, but appears to be less sensitive to the gas flow rated5 Moreover, the variation of the standard deviation is very atomizer- and melt-specific. An empirical correlation which fits with a wide range of atomization data has the following form ... 

Generally, the wheel atomizer produces a spray of high homogeneity within a wide range of mean droplet size. The size distribution of droplets can be controlled by changing the wheel speed. Feed rate variation produces much less effect. Wheel atomizers are very flexible and can handle a wide assortment of liquids with different physical properties. Factors influencing the wheel atomizer performance are specified in Ref [25] for instance. 

FIGURE 16.23 The variation of the mean droplet size and the span of distribution with time during stationary storage of 0/W emulsions prepared with different SPG membranes. (From Vladisavljevid, G.T. and Scbubert, H., J. Membr. Set, 225 (1-2), 15-23, 2003. With permission.)... 

The nebulization concept has been known for many years and is commonly used in hair and paint spays and similar devices. Greater control is needed to introduce a sample to an ICP instrument. For example, if the highest sensitivities of detection are to be maintained, most of the sample solution should enter the flame and not be lost beforehand. The range of droplet sizes should be as small as possible, preferably on the order of a few micrometers in diameter. Large droplets contain a lot of solvent that, if evaporated inside the plasma itself, leads to instability in the flame, with concomitant variations in instrument sensitivity. Sometimes the flame can even be snuffed out by the amount of solvent present because of interference with the basic mechanism of flame propagation. For these reasons, nebulizers for use in ICP mass spectrometry usually combine a means of desolvating the initial spray of droplets so that they shrink to a smaller, more uniform size or sometimes even into small particles of solid matter (particulates). 

To test the reliability of the previous method, the authors compared it to an independent measurement of oj. They thus propose an extended version of the previous mean-fleld model, valid at any stage of the coalescence regime, even in presence of broad droplet size distributions. It is obtained by considering that the variation of the total number of coalescence events is proportional to the total surface area per unit volume developed by the droplets of different sizes. The total number of drops and total surface are replaced by summations over all the granulometric size intervals ... 

The absorption of ozone from the gas occurred simultaneously with the reaction of the PAH inside the oil droplets. In order to prove that the mass transfer rates of ozone were not limiting in this case, the mass transfer gas/water was optimized and the influence of the mass transfer water/oil was studied by ozonating various oil/water-emulsions with defined oil droplet size distributions. No influence of the mean droplet diameter (1.2 15 pm) on the reaction rate of PAH was observed, consequently the chemical reaction was not controlled by mass transfer at the water/oil interface or diffusion inside the oil droplets. Therefore, a microkinetic description was possible by a first order reaction with regard to the PAH concentration (Kornmuller et al., 1997 a). The effects of pH variation and addition of scavengers indicated a selective direct reaction mechanism of PAH inside the oil droplets... 

We have compared these theoretical predictions of the low-frequency modulus to experimental measurements on compressed emulsions and concentrated dispersions of microgels [121]. The emulsions were dispersions of silicone oil (viscosity 0.5 Pas) in water stabilized by the nonionic surfactant Triton X-100 [102, 121]. The excess surfactant was carefully eliminated by successive washing operations to avoid attractive depletion interactions. The size distribution of the droplets was moderately polydisperse with a mean droplet diameter of 2pin. The interfacial energy F between oil and water was 4mJ/m. The contact modulus for these emulsions was thus F 35 kPa. The volume fraction of the dispersed phase was easily obtained from weight measurements before and after water evaporation. Concentrated emulsions have a plateau modulus that extends to the lowest accessible frequencies, from which the low-frequency modulus Gq was obtained. Figure 11 shows the variations of Gq/E"" with 0 measured for the emulsions against the values calculated in the... 

To make the significance of the NMR technique as an experimental tool in surfactant science more apparent, it is important to compare the strengths and the weaknesses of the NMR relaxation technique in relation to other experimental techniques. In comparison with other experimental techniques to study, for example, microemulsion droplet size, the NMR relaxation technique has two major advantages, both of which are associated with the fact that it is reorientational motions that are measured. One is that the relaxation rate, i.e., R2, is sensitive to small variations in micellar size. For example, in the case of a sphere, the rotational correlation time is proportional to the cube of the radius. This can be compared with the translational self-diffusion coefficient, which varies linearly with the radius. The second, and perhaps the most important, advantage is the fact that the rotational diffusion of particles in solution is essentially independent of interparticle interactions (electrostatic and hydrodynamic). This is in contrast to most other techniques available to study surfactant systems or colloidal systems in general, such as viscosity, collective and self-diffusion, and scattered light intensity. A weakness of the NMR relaxation approach to aggregate size determinations, compared with form factor determinations, would be the difficulties in absolute calibration, since the transformation from information on dynamics to information on structure must be performed by means of a motional model. 

FIGURE 16.19 The variation of mean droplet to mean pore size ratio with the wall shear stress at the disperse phase content of 9 1 vol% (emulsion formulation as in Figure 16.15). (From Vladisavljevic, G.T.,... 

In most cases d 2 (the volume/surface average or Sauter mean) is used, while the width of the size distribution can be given as the variation coefficient. The latter is the standard deviation of the distribution weighted with d divided by the corresponding average d. Generally C2 will be used which corresponds to d 2 An alternative way to describe the emulsion quahty is to use the specific surface area A (the surface area of all emulsion droplets per unit volume of emulsion). 

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