Drop stabilizer

Based on a lot of experimental observations, criteria for the drop stability can be defined as below the U curve, namely We < We.cn, the interfacial stress can equilibrate the shear stress, and the drop will only deform into a stable prolate ellipsoid. Above this curve, the viscous shear stress becomes larger than the interfacial stress. The drop is at first extended and finally breaks up into smaller droplets. 

Drop stabilization methods rely on the immediate stabilization of drops by encapsulation with thin polymer films or surfactants [219-221] a photomicrographic method has been employed usually after encapsulation of drops. However this method cannot always be used due to incompatibility of the encapsulating materials with some systems. The method also has the disadvantage of the influence of the chemical treatment on drop size. A special sampling apparatus has been developed to withdraw a sample of dispersed phase from the mixing vessel to stabilize drops with a surfactant and to force the dispersed sample through a capillary with a photometer assembly to measure both droplet size and concentration [222]. 

Coalescence is also controlled by the condition of drop surfaces. Surfactants reduce the interfacial tension and help preserve drop stability, therefore affecting drop sizes. Surface-active materials are important in suspension/emulsion polymerization processes. 

A simple test to characterize relative drop stability is to measure the time it takes to separate a giveu dispersion into two distinct clear layers. Table 9.13 gives rough guidelines for such a characterization. 

In later stages, the precipitation of a salt film is a secondary stabilizing factor. This salt layer may serve as a reservoir for locally high halide concentrations and a barrier to take over a large portion of the potential drop stabilizing pitting at lower levels of the local current density. This precipitation is ruled by the product of Sand s equation (Eqs. 4 and 6). For i t < i /r, open pits are stabilized by accumulated halides, whereas for i > i Jx, a salt layer stabiKzes additionaUy. 

Why don t oil drops stabilized by sodium stearate coagulate to form larger oil drops ... 

Effect of Surfactants on Drop Stability and Thin Film Drainage... 

In the paper Effect of Surfactants on Drop Stability and Thin Film Drainage presented by Professor Krassimir Danov (Sofia University, Bulgaria) the stability of suspensions/emulsions is under consideration. Traditional consideration of colloidal systems is based on inclusion only Van-der-Waals (or dispersion) and electrostatic components, which is refereed to as DLVO (Derjaguin- Landau-Verwey-Overbeek) theory. Professor Danov s contribution shows that not only DLVO components but also other types of the inter-particle forces may play an important role in the stability and colloidal systems. Those contributions are due to hydrodynamic interactions, hydration and hydrophobic forces, steric and depletion forced, oscillatory structural forces. The hydrodynamic and colloidal interactions between drops and bubbles emulsions and foams are even more complex (as compared to that of suspensions of solid particles) due to the fluidity and deformability of those colloidal objects. The latter two features and thin film formation between the colliding particles have a great impact on the hydrodynamic interactions, the magnitude of the disjoining pressure and on the dynamic and thermodynamic stability of such colloidal systems. 

Drop stability It is well known that the phase separation of the emulsions is controlled by the factors which influence the coalescence of drops. Therefore, we will now attempt to apply the developed Theory of Coalescence to the present situation. 

In Figure 5 are given relations of water drop stability at the flat interface (Curve I), adsorption of SFA aluminum soap on emulsion drops (Curve II) and the critical shearing strength of the interfacial layer (Curve III) depending on the equilibrium soap concentration in solution. Greater equilibrium soap concentrations in solutions lead to higher stabilities due to adsorption and formation of a stable structure at the interface. 

Abstract We explored the structural effects induced by the addition of salt on lipid-based liquid crystalline drops stabilized in aqueous media by charged sphere-like colloids. This allows us to distinguish two different stabilization regimes. In one case, the internal liquid crystalline phase has the ability to reorganize upon the coalescence of the drops and in the other not. This in turn depends mainly on the contact angle and the internal phase viscosity. 

Fig. 1 X -ray scattering curves for PT TC 50 50 drops stabilized by 0.5 wr% TM with 0 mM, 1 mM, and 10 mM added NaQ (up panel) 2 wr TM with 0 mM and 10 mM added NaCl (bothnn panel). The symbols are displayed in both panels...

FIGURE 4.56 Ratio of the coalescence rates of emulsion 1 and emulsion 2 vs. the dimensionless distance, hlR, for water drops stabilized by sodium alkyl sulfates (C S04Na) in hexadecane at concentrations close to the CMC. 

Danov, K.D., Effect of surfactants on drop stability and thin film drainage, in Fluid Mechanics of Surfactant and Polymer Solutions, Starov, V. and Ivanov, LB. (Eds.), Springer, New York, 2004, p. 1. 

Suspension stability is maintained by good agitation and by the use of drop stabilizers. Removal of the stabilizing agents, after polymerization, may not be complete and some contamination of the final product is sometimes inevitable. Suspension polymerization usually requires larger reactor volumes than bulk processes because the vessels often contain about 50% of the continuous phase. Suspension polymerization has been reviewed previously by Munzer and Trommsdorff [1], Bieringer et al. [2], Warson [3], Brooks [4], Yuan et al. [5], Vivaldo-Lima et al. [6], and Arshady [7]. 

The chemical events that occur inside drops of the dispersed phase are similar to those found in bulk polymerization. The drops contain monomer (or monomers), radical generators (often called initiators), and polymer. Sometimes chain-transfer agents are added also. The continuous phase is often regarded as chemically inert, but drop stabilizers are usually present in it and, in some cases, those stabilizers participate in the polymerization process. For a discussion of stabilizer types, see Section 5.2.1. 

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