Submicron droplets

Figure 4.1.2 is a photograph of a coimterflow burner assembly. The experimental particle paths in this cold, nonreacting, counterflow stagnation flow can be visualized by the illumination of a laser sheet. The flow is seeded by submicron droplets of a silicone fluid (poly-dimethylsiloxane) with a viscosity of 50 centistokes and density of 970 kg/m, produced by a nebulizer. The well-defined stagnation-point flow is quite evident. A direct photograph of the coimterflow, premixed, twin flames established in this burner system is shown in Figure 4.1.3. It can be observed that despite the edge effects.

Since they act as surfactants, copolymers are added in only small amounts, typically from a thousandth parts to a few hundredth parts. Theoretically, Leibler [30] showed that only 2% of a diblock copolymer may thermodynamically stabilize an 80%/20% incompatible blend with an optimum morphology (submicronic droplets). However, in practice kinetic control and micelle formation interfere in this best-case scenario. To a some extent, compatibilization increases with copolymer concentration [8,31,32], Beyond a critical concentration (critical micellar concentration cmc) little or no improvement is observed (moreover, for high amounts, the copolymer can act as a plasticizer). Copolymer molecular weight influence is similar to that of the concentration effect. For example, in a PS/PDMS system [8,31,32], when the copolymer molecular weight increases, domain size decreases to a certain extent. Hu et al. [31] correlated their experimental results with theoretical prediction of the Leibler s brush theory [30]. Leibler distinguishes two regimes to characterize the behaviour of the copolymer at the interface... 

The word homogenization is somewhat inconclusive and is typically defined as used in context. Two processes are considered here the first is a fine clearance valve homogenizer, and the second is a rotor-stator-type mechanical homo-genizer. Homogenization is similar to sonication and produces submicron droplets by a combination of mechanical shearing and cavitation. 

The rotor-stator-type mechanical homogenizer generates submicron droplets by forcing the emulsion through small openings in the stationary... 

Davis, E.J., Ray, A.K. (1977) Determination of diffusion coefficients by submicron droplet evaporation. J. Chem. Phys. 67, 414-419. 

P. Contal, J. Simao, D. Thomas, T. Frising, S. Call, J.C. Appert-Collin, D. Berner, Clogging of fibre filters by submicron droplets. Phenomena and influence of operating conditions, Aerosol Science, vol. 35, 263-278, 2004. 

There is a lack of understanding of the mechanism of production of submicron droplets and the roles of surfactants and cosurfactants. 

The production of small (submicron) droplets requires the application of large amounts of energy, as the process of emulsification is generally very inefficient (as illustrated below). 

Attempts to fabricate nanoparticles directly from emulsions were unsuccessful, even when submicron emulsion droplet size has been used. The spontaneous aggregation of primary particles and droplets leads to the final particle size of 0.1 mm. To avoid aggregation, the new method of synthesis in emulsions has been suggested for the preparation of oxide particles of 5 10 nm size [186]. Thermally stable water-oil emulsions prepared with the addition of surfactant to obtain a narrow distribution of submicron droplets of uniform size and volume are shown in Fig. 5.8. 

Besides numerous applications in surface nanopatterning, the tools presented in this paper can also be used for more fundamental studies of wetting processes at submicron scale. To illustrate this point, the evaporation of droplets produced by NADIS with volumes smaller than the femtoliter was performed. When manipulating liquids at submicron scale, evaporation becomes an important issue. It is therefore essential to study experimentally the evaporation of submicron droplets since most of the previous comprehensive studies of evaporation have been limited to microliter droplets (i.e., diameters in the millimeter range). ... 

Steric stabilization is believed to be the primary mechanism by which coalescence is reduced due to the presence of a copolymer at the interface [42, 43]. Simdararaj and Macosko [33] have provided a schematic illustration of steric supression of coalescence due to the presence of a copolymer at the interface, Fig. 5.12. The surface coverage of copolymer required to inhibit coalescence of submicron droplets has been estimated [1, 42]. Most commercial reactively compatibilized blends have quite sufficient chemical... 

Grossiord et al. discuss similar method based on emulsified microemul-sions. The idea is to disperse an oil phase within water by surfactant and to form L2 phase (water-in-oil microemulsion) (Grossiord et al., 1998). This phase is further emulsified with water to form multiple emulsion (Figure 7.3). The problem is that there is no evidence of the formation of multiple emulsions and that the internal phase remains, after the second emulsification process, a L2 phase of a submicronal droplets in size with intrinsic thermodynamic stability. 

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