Homogenization droplet coalescence

The coalescence-redispersion (CRD) model was originally proposed by Curl (1963). It is based on imagining a chemical reactor as a number population of droplets that behave as individual batch reactors. These droplets coalesce (mix) in pairs at random, homogenize their concentration and redisperse. The mixing parameter in this model is the average number of collisions that a droplet undergoes. 

Emulsifier Type and Concentration For a fixed concentration of oil, water, and emulsifier, there is a maximum interfacial area that can be completely covered by an emulsifier. As homogenization proceeds, the size of the droplets decreases and the interfacial area increases. Once the emulsion interfacial area increases above a certain level, there may be insufficient emulsifier present to completely cover the surface of any newly formed droplets. This will not only increase the energy required for subsequent droplet dismption but also increase the probability for droplet coalescence. The minimum size of stable droplets that can be produced during homogenization is governed by the type and concentration of emulsifier present ... 

Experiments have shown that the smallest droplet size that can be achieved using a high-pressure valve homogenizer increases as the disperse phase volume fraction increases (52). There are a number of possible reasons for this, (1) increasing the viscosity of an emulsion may suppress the formation of eddies responsible for breaking up droplets, (2) if the emulsifier concentration is kept constant, there may be insufficient emulsifier molecules present to completely cover the droplets, and (3) the rate of droplet coalescence is increased. 

Figure 14.6 Lignosulfonates stabilize v etshie oil emulsions, here demonstrated with soya bean oil and sunflower oil. No viscosity modifier was added so a creaming layer formed upon storage. Before measurement of the droplet size the emulsions were gently stirred for homogenization. No coalescence or flocculation was observed and the droplet size remained constant.

Various novel applications in biotechnology, biomedical engineering, information industry, and microelectronics involve the use of polymeric microspheres with controlled size and surface properties [1-31. Traditionally, the polymer microspheres larger than 100 /urn with a certain size distribution have been produced by the suspension polymerization process, where the monomer droplets are broken into micron-size in the existence of a stabilizer and are subsequently polymerized within a continuous medium by using an oil-soluble initiator. Suspension polymerization is usually preferred for the production of polymeric particles in the size range of 50-1000 /Ltm. But, there is a wide size distribution in the product due to the inherent size distribution of the mechanical homogenization and due to the coalescence problem. The size distribution is measured with the standard deviation or the coefficient of variation (CV) and the suspension polymerization provides polymeric microspheres with CVs varying from 15-30%. 

It is argued that the kinetics of the limited coalescence process is determined by the uncovered surface fraction 1 - t and by the rate of thinning (drainage) of the films formed between the deformable droplets [46,47], The homogeneous and monodisperse growth generated by limited coalescence is intrinsically different from the polydisperse evolution observed for surfactant-stabilized emulsions. As noted by Whitesides and Ross [48], the mere fact that coalescence halts as a result of surface saturation does not provide an obvious explanation of the very... 

M 14] [P I3]The fluorescent droplet is moved towards the non-fluorescent droplet and the coalesced droplet is held in place [97] (see also an initial experiment in [98]). By this action, the fluorescent droplet moves underneath the non-fluorescent one. From a top view, a homogeneous texture is yielded however, a vertically segregated fluidic system exists (see Figure 1.37). Mixing takes then place by diffusion and needs about 90 s to be completed. 

In this process, one starts with the phase that should become the dispersed phase. We call this phase A. One then slowly adds the other phase (B) to phase A while the system is agitated (or rapidly flowing, or homogenized in a suitable machine). Initially, droplets of phase B are formed, which are broken up into small droplets by the agitation. In time, more and more of these droplets are formed However, one ultimately wants to have an emulsion of in 5 therefore, the surfactant system is dissolved in phase B and not in phase A. Thus, at a certain time, the emulsion becomes so concentrated and the viscosity becomes so high, that the droplets of B are sheared apart droplets of B then start to coalesce. As soon as this coalescence sets in, all droplets start to coalesce, as a snowball effect. So, suddenly the droplets of B combine and start to form a continuous phase, taking up droplets of phase A, which starts to be the dispersed phase. 

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