Collision between droplets

In the impinging streams of gas-liquid systems, high relative velocity between phases and collision between droplets favor surface renewing of droplets, resulting in reduced liquid film resistance and thus increased overall mass transfer coefficient. 

Because of the collisions between droplets and the shearing effect of the gas flow carrying the droplets, the original droplets may be re-atomized, yielding an increased interface area. 

The results on the influence of the impinging distance are shown in Fig. 7.17 as a plot of 77s versus the dimensionless impinging distance, S/d(). In the range tested the sulfur-removal efficiency decreases continuously as S/d() reduces. The most likely reason is that at smaller impinging distance the concentration of droplets in the impingement zone increases, giving enhanced collision between droplets and an increased tendency of the droplets to coalescence, thus reducing the interface area. On... 

Miniemulsion copolymerization of a 50 50 styrene/methyl methacrylate monomer mixture, using hexadecane as hydrophobe, was carried out by Rodriguez et al. [78]. The mechanism of mass transfer between miniemulsion droplets and polymer particles in the miniemulsion copolymerization of styrene-methyl methacrylate (AIBN as initiator, hexadecane as hydrophobe) was studied, analyzing the mass transfer of highly water-insoluble compounds from miniemulsion droplets to polymer particles by both molecular diffusion and collisions between droplets and particles [79,80]. 

Candau and co-workers were the first to address the issue of particle nu-cleation for the polymerization of AM [13, 14] in an inverse microemulsion stabilized by AOT. They found that the particle size of the final microlatex (d 20-40 nm) was much larger than that of the initial monomer-swollen droplets (d 5-10 nm). Moreover, each latex particle formed contained only one polymer chain on average. It is believed that nucleation of the polymer particle occurs for only a small fraction of the final nucleated droplets. The non-nucleated droplets also serve as monomer for the growing particles either by diffusion through the continuous phase and/or by collisions between droplets. But the enormous number of non-nucleated droplets means that some of the primary free radicals continuously generated in the system will still be captured by non-nucleated droplets. This means that polymer particle nucleation is a continuous process [ 14]. Consequently, each latex particle receives only one free radical, resulting in the formation of only one polymer chain. This is in contrast to the large number of polymer chains formed in each latex particle in conventional emulsion polymerization, which needs a much smaller amount of surfactant compared to microemulsion polymerization. 

We shall then account for the observed phenomena by the orientation of transient aggregates (postulated few years ago (24), recently observed in attractive systems by neutron scattering O) ) formed during sticky collisions between droplets. Such an explanation agrees with the first experimental fact (non linear behavior of B versus ) and with the measured characteristic times at low volume fraction (1 to 10 ps). The characteristic rotation time for a single droplet with R=50 A would be 70ns which is much shorter than the measured times. 

The mechanism of disrqrpearance of miniemulsion droplets other tiian by becoming polymer particles is still not resolved. Although initially it was expect that all droplets could be successhilly entered by a free radical thereto becoming polymer particles, only a relatively small fraction 20%) succeed in this. Collision between droplets and existing particles, and diffusion are the two medianisms often cited as possibilities to explain this disappearance, although the former explanation seems favoured. This will be seen later in tiie S/MMA copolymerization studies (Section 20.3.2.2(b)) [7]. 

Saffinan and Turner (43) considered collisions between droplets due to turbulence in rain clouds. Under turbulent conditions, droplet collision is governed by two different mechanisms isotropic turbulent shear and turbulent inertia. The choice of regime applicable to a droplet is determined by its size in relation to the Kolmogorov microscale denned earlier. Droplets of diameter d > t are subjected to die former of these processes (small-scale motion). Spatial variations in the flow give neighboring droplets different velocities and fliis result in collisions. Droplets of diameter d > T] are subjected to turbulent inertia. In this case, collisions result from the relative movement of droplets in the surrounding fluid. Droplets of different diameter will have different inertias and this results in collisions. Droplets of equal diameter, however, will not collide under this mechanism as fliey have the same inertia. 

M. Rieber, A. Frohn Three-dimensional Navier-Stokes simulation of binary collisions between droplets of equal size, J. Aerosol Sci. 26(Suppl. 1), S929-S930 (1995). 

A spectrum of droplet sizes is formed at each position in a cloud for reasons discussed previously, among others. Collisions between droplets, and between drops and droplets, can occur because droplets and drops of different sizes have differing fall velocities large droplets or drops fall faster than smaller ones, overtaking and collecting some fraction of those lying in their paths. Electrical fields may promote additional collections. 

Various types of spontaneous processes take place in microemulsions. The surfactant and cosurfactant exchange between the interfacial film separating water and oil domains and the bulk phases. Also collisions between droplets with temporary merging of the collided droplets ( sticky collisions) have been evidenced. The kinetics of these processes (and of other ones) is reviewed in Chapter 5, Sections VI.F and VIII, and Chapter 10. 

Critical behavior of, and electrical conductivity percolation in, microemulsions is much dependent on the dynamics of stick/ collisions between droplets of oil (water) present in oil-in-water (water-in-oil) microemulsions (see Chapter 5). Sticky collisions refer to collisions that bring assembly cores into contact. During the time of contact matter may be transferred from one assembly to another. Such collisions occur in systems with strong attractive interactions between assemblies. 

Table 5.2 Examples of Values of Rate Constants Associated with the Exchange of Material upon Collisions Between Droplets and Influence of Important Parameters...

To account for the experimental finding, as noted above, it was proposed that only a small Iraction of monomer-swollen droplets initially present in w/o microemulsion are nucleated. The nucleated particles grow as monomer is supplied from the non-nucleated monomer droplets either by diffusion through the continuous phase or by sticky collisions between droplets, as shown in Fig. 3.12. Because of the growth process and the presence of large amount of surfactant in the initial... 

In Fig. 7, a typical time series of simulation snapshots are used. The background coloring shows the gas hold up. The particles are color coded to indicate their size. Due to collisions between droplets and solid particles, the particles grow in size. Due to the presence of a wet layer, the collisional properties of the particles change. The collisions become much more dissipative. The formation of (semi) permanent Hquid bridges between particles has not yet been accounted for in this simulation. 

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