Droplet rupture

When sheared at 50 s-1, the crude premixed emulsion remains unchanged (Fig. 8a,b). The applied stress is insufficient to cause droplet rupture. At 1000 s-1, one can deduce from the image 8c, that fragmentation has taken place since the resulting droplets are significantly smaller than in the mother emulsion. How-... 

Since a droplet ruptures at values of the Weber number which exceed the critical value, Eq. (12-27) may be solved for the maximum value of the droplet diameter to give... 

A neat direct method for studying droplet rupture and coalescence, allowing one actual visual observation of individual droplets, was developed by Parfenova et al. [38,39]. A very small droplet (about 0.3 mm in diameter) is immersed into another liquid phase and stretched under controlled conditions (see Figure 4.14). The interfacial tension and rupture force are measured at the point when the droplet assumes cylindrical shape due to deformation. This corresponds to uniaxial tension resulting from asymmetric uniaxial stretching of the film (membrane) at the interface between the polar and nonpolar phases (which can represent both the dispersed phase as well as dispersion medium). In subsequent compression of two half droplets, one measures the critical force causing coalescence,/.oai, that is, the rupture of a double-sided emulsion film. These results correspond to a symmetrical double-sided axisymmetric stretch of the membrane. 

The study of droplet rupture and coalescence by direct visual observation has been utilized in numerous essential studies [39-43]. Of principal importance are the experimental studies by Amelina et al. on the analysis of colloid stability in artificial blood substitutes [40-43]. These studies involved the use of various nonpolar phases, including perfluo-rinated systems, such as perfluorodecalin (PFD), perfluorotributylamine (PFTBA), per-fluoromethylcyclohexylpiperidine (PFMCHP), and conventional hydrocarbons, such as heptane. Stabilizing agents included Pluronic surfactants (ethylene oxide (EO)/propylene oxide (PO) block copolymers), as well-fluorinated surfactants, such as perfluorodiisononyl-ene with 20 mol of EO (( )-PEG). Tables 4.1 and 4.2 show some very characteristic results. 

There have been some studies of the equilibrium shape of two droplets pressed against each other (see Ref. 59) and of the rate of film Winning [60, 61], but these are based on hydrodynamic equations and do not take into account film-film barriers to final rupture. It is at this point, surely, that the chemistry of emulsion stabilization plays an important role. 

After vessel rupture, the superheated liquid vaporized in a white cloud consisting of vapor and fine droplets. After ignition, the flame propagated through the cloud, forming a fireball. Fireball size increased as combustion proceeded, and the fireball was lifted by gravitational buoyancy forces. 

Experiments by Schmidli et al. (1990) were focused on the distribution of mass on rupture of a vessel containing a superheated liquid below its superheat-temperature limit. Flasks (50-ml and 100-mI capacity) were partially filled with butane or propane. Typically, when predetermined conditions were reached, the flask was broken with a hammer. Expansion of the unignited cloud was measured by introduction of a smoke curtain and use of a high speed video camera. Large droplets were visible, but a portion of the fuel formed a liquid pool beneath the flask. Figure 6.5 shows that, as superheat was increased, the portion of fuel that... 

The defoamer is dispersed in fine droplets in the liquid. From the droplets, the molecules may enter the surface of the foam. The tensions created by this spreading result in the eventual rupture of the film. 

As dispersion proceeds drops come into close contact with each other and may coalesce. Coalescence is commonly divided into three sequential steps (Chesters, 1991) collision or close approach of two droplets, drainage of the liquid between the two drops, and rupture of the film (see Fig. 26). 

Subjected to steady acceleration, a droplet is flattened gradually. When a critical relative velocity is reached, the flattened droplet is blown out into a hollow bag anchored to a nearly circular rim which contains at least 70% of the mass of the original droplet. Surface tension force is sufficient to allow the bag shape to develop. The bag, with a concave surface to the gas flow, is stretched and swept off in the downstream direction. The rupture of the bag produces a cloud of very fine droplets presumably via a perforation mode, and the rim breaks up into relatively larger droplets, although all droplets are at least an order of magnitude smaller than the initial droplet size. This is referred to as bag breakup (Fig. 3.10)T2861... 

Figure 1.13. Mechanism of rupturing. Under shear, drops elongate into long cylinders (a) that undergo a Rayleigh instability leading to identical aligned droplets (b). (Adapted from [149].)...

T.G. Mason and J. Bibette Shear Rupturing of Droplets in Complex Fluids. Langmuir 13, 4600 (1997). 

The first mechanism proposes that metal volatilisation causes rupture of molten droplets (as with magnesium), whereas the second considers the production of a volatile oxide such as CO inside materials such as steels that contain an excess of 0.1% carbon. The third mechanism involves the formation of oxy-nitride compounds which decompose at high temperatures, liberating nitrogen (as with titanium). 

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