Mean droplet size, measurement from

Fig. 9 Comparison of mean droplet sizes resulting from HPH or ultrasonication. The dispersed phase viscosity was adjusted by dissolving different amounts of PMMA in the monomer. The droplet size distribution was measured with dynamic light scattering (DLS) the surfactant concentration was 10 mmol SDS with respect to the continuous phase...

To characterize a droplet size distribution, at least two parameters are typically necessary, i.e., a representative droplet diameter, (for example, mean droplet size) and a measure of droplet size range (for example, standard deviation or q). Many representative droplet diameters have been used in specifying distribution functions. The definitions of these diameters and the relevant relationships are summarized in Table 4.2. These relationships are derived on the basis of the Rosin-Rammler distribution function (Eq. 14), and the diameters are uniquely related to each other via the distribution parameter q in the Rosin-Rammler distribution function. Lefebvre 1 calculated the values of these diameters for q ranging from 1.2 to 4.0. The calculated results showed that Dpeak is always larger than SMD, and SMD is between 80% and 84% of Dpeak for many droplet generation processes for which 2left-hand side of Dpeak. The ratio MMD/SMD is... 

The studies on the performance of effervescent atomizer have been very limited as compared to those described above. However, the results of droplet size measurements made by Lefebvre et al.t87] for the effervescent atomizer provided insightful information about the effects of process parameters on droplet size. Their analysis of the experimental data suggested that the atomization quality by the effervescent atomizer is generally quite high. Better atomization may be achieved by generating small bubbles. Droplet size distribution may follow the Rosin-Rammler distribution pattern with the parameter q ranging from 1 to 2 for a gas to liquid ratio up to 0.2, and a liquid injection pressure from 34.5 to 345 kPa. The mean droplet size decreases with an increase in the gas to liquid ratio and/or liquid injection pressure. Any factor that tends to impair atomization quality, and increase the mean droplet size (for example, decreasing gas to liquid ratio and/or injection pressure) also leads to a more mono-disperse spray. 

An acoustical particle counter for counting and sizing fog droplets has been evaluated by Singh and Reist.161 Fog droplets, mostly in the size range of 5-30 pm, were measured by the acoustical particle counter as well as an optical and an electron microscope for comparison. The mean droplet diameters estimated from the acoustical particle counter were in agreement with the microscope values. A Rich 100 condensation nuclei monitor was also operated simultaneously during the fog droplet counting to monitor condensation nuclei counts. 

A two-component phase Doppler interferometer (PDI) was used to determine droplet size, velocity, and number density in spray flames. The data rates were determined according to the procedure discussed in [5]. Statistical properties of the spray at every measurement point were determined from 10,000 validated samples. In regions of the spray where the droplet number density was too small, a sampling time of several minutes was used to determine the spray statistical characteristics. Results were repeatable to within a 5% margin for mean droplet size and velocity. Measurements were carried out with the PDI from the spray centerline to the edge of the spray, in increments of 1.27 mm at an axial position (z) of 10 mm downstream from the nozzle, and increments of 2.54 mm at z = 15 mm, 20, 25, 30, 35, 40, 50, and 60 mm using steam, normal-temperature air, and preheated air as the atomization gas. 

Mean droplet size as measured from the holograms and photographs was consistent with the visual observations of spray formation and spray quality. 

This procedure has been used to determine droplet size in sprays. Oseillations in the curve relating x and D can be smoothed out by the use of an incident laser beam having a broad speetral bandwidth [83]. An accumulation of independent scattering intensities from multiple scatterers ean be used to measure the mean droplet size of a group [84]. This procedure has been applied to water sprays and the experimental data confirmed by phase Doppler anemometry [85]. The applicability of the polarization ratio technique is strongly influenced by the complex refractive index of the dispersed media and is limited to media having a relative refractive index below about 1.44 [86]. 

Depending on the operating conditions, mean droplet sizes of 0.8-2.5 pm with standard deviations ranging from 0.4 to 1.3 pm were measured. As expected, the mean droplet size decreased with increase in total flow rate. For instance, on increasing the flow rate from 0.3 to 0.9 Lh, the droplet diameter decreased by a factor of almost two. 

Figure 13.16 gives the voltage signal obtained from the pulse laser photometer against the Sauter diameter measured by the Coulter LS 230. The peaks between the different pressures are due to air bubbles and for this reason they are neglected. It depicts the good correlation between mean droplet size and optical density of an emulsion. 

Recently, Razumovskid441 studied the shape of drops, and satellite droplets formed by forced capillary breakup of a liquid jet. On the basis of an instability analysis, Teng et al.[442] derived a simple equation for the prediction of droplet size from the breakup of cylindrical liquid jets at low-velocities. The equation correlates droplet size to a modified Ohnesorge number, and is applicable to both liquid-in-liquid, and liquid-in-gas jets of Newtonian or non-Newtonian fluids. Yamane et al.[439] measured Sauter mean diameter, and air-entrainment characteristics of non-evaporating unsteady dense sprays by means of an image analysis technique which uses an instantaneous shadow picture of the spray and amount of injected fuel. Influences of injection pressure and ambient gas density on the Sauter mean diameter and air entrainment were investigated parametrically. An empirical equation for the Sauter mean diameter was proposed based on a dimensionless analysis of the experimental results. It was indicated that the Sauter mean diameter decreases with an increase in injection pressure and a decrease in ambient gas density. It was also shown that the air-entrainment characteristics can be predicted from the quasi-steady jet theory. 

It is probable that numerous interfacial parameters are involved (surface tension, spontaneous curvature, Gibbs elasticity, surface forces) and differ from one system to the other, according the nature of the surfactants and of the dispersed phase. Only systematic measurements of > will allow going beyond empirics. Besides the numerous fundamental questions, it is also necessary to measure practical reason, which is predicting the emulsion lifetime. This remains a serious challenge for anyone working in the field of emulsions because of the polydisperse and complex evolution of the droplet size distribution. Finally, it is clear that the mean-field approaches adopted to measure > are acceptable as long as the droplet polydispersity remains quite low (P < 50%) and that more elaborate models are required for very polydisperse systems to account for the spatial fiuctuations in the droplet distribution. 

An increase in droplet size with axial position is observed for all three gases. However, the relative trend of smallest droplet mean size with steam and largest with normal (unheated) air remains unchanged. As an example, at 50 mm downstream from the nozzle exit at r = 0, droplet mean size for steam, preheated air, and normal air were found to be 69, 86, and 107 pm, respectively see Fig 16.3. The droplet size with steam is also significantly smaller than air at all radial positions see Fig. 16.3. The droplet size with preheated air is somewhat smaller than normal air due to the decreased effect of preheated air at this location and increased effect of combustion. Early ignition of the mixture with preheated air (see Fig. 16.1) must provide a longer droplet residence time which results in a smaller droplet size. In addition, the increased flame radiation with preheated air increased droplet vaporization at greater distances downstream from the nozzle exit. Indeed, the results indicate that the measured droplet sizes with preheated atomization air are smaller than normal air in the center... 

Platnick and Twomey (1994) have applied Eq. (KK) to marine clouds off the coast of California and southern Africa, to fogs in central California, and to ship tracks. Figure 14.42 shows a typical range of susceptibilities as a function of cloud droplet size. The measured susceptibilities in these studies covered three orders of magnitude, from 5 X 10-5 cm3 for fogs to 0.8 X 10-3 cm3 for marine clouds off south Africa and 2 X 10 2 cm3 for thin stratus clouds off the California coast. Similarly, Taylor and McHaffie (1994) report cloud susceptibilities in the range from 10-4 to >8 X 10-3 at various locations around the world. The highest susceptibilities were those with the smallest aerosol particle concentrations below the cloud base. As the particle concentration increased beyond 500 cm3, the susceptibility was relatively constant at 5 X 10"4 cm3. This means that the addition of new particles to a relatively clean air mass is far more effective than for a polluted one in terms of the effect on clouds. In short,... 

Eq. 1 showed that in the case of unrestricted diffusion the echo attenuation value R depends upon the durations 8 and A. This is also true in the case of restricted diffusion, although in a different manner. The dependence of the R-value upon these two parameters is shown in Fig. 5. This figure clearly shows that the echo attenuation factor R steadily decreases with increasing A in the case of unrestricted diffusion, but becomes independent of this parameter in the case of restricted diffusion. It may be deduced from this figure that it is necessary to determine the parameters of the log-normal droplet size distribution R as a function of A or by measuring R as a function of 8 for a fixed large value of A. Measurement of only one R-value, at a chosen 8 or A, is not sufficient for a careful determination of the droplet size distribution in Fig. 5 a given In R-value can be found on more than one In R versus A-5/3 curve. This means that the In R-values have to be determined for different values of A and/or 8. 

Two experiments were conducted using No. 2 diesel fuel atomized at different air/fuel ratios to show that changes in droplet size caused by atomization can be measured. During the first of these experiments the air/fuel ratio was set at 1/1 while the second was conducted with an air/fuel ratio of 2/3. All other conditions were identical. The measured mean droplet diameters 40 in. downstream of the nozzle were 76.5 and 84.8 /un, respectively, thus indicating better atomization dining the first experiment resulting from the higher air/fuel ratio. 

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... 

The predictions presented above agree with measured concentration/size dependencies measured in clouds that are not heavily influenced by anthropogenic sources. Noone et al. (1988) sampled droplets from a marine stratus cloud and calculated that the volumetric mean solute concentration of the 9-18-pm droplets was a factor of 2.7 smaller than in the 18-23-pm droplets. Ogren et al. (1989) reported similar results for a cloud in Sweden. On the other hand, similar measurements for cloud and fog droplets in heavily polluted environments suggest that solute concentrations decrease with increasing droplet size (Munger et al. 1989 Ogren et al. 1992). No satisfactory explanation exists for such behavior. 

The reservoir incorporated within the Circulaire nebulizer system may help increase the inhaled proportion of aerosol from a given fill volume compared to the same driving nebulizer without incorporation of the valved reservoir bag system. Like the Halolite, this implies reconsideration of prescribed doses and volume fills of drug solution to avoid overdosing. The extent to which the droplet size from conventional nebulizers is affected by adaptations in Halolite and Circulaire nebulizer system is not clear. Meaning in vitro measurements (e.g., European standard) characterizing droplet size from either system is not available. 

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. 

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