Droplet formation membrane emulsification

Both direct and premix emulsification can be obtained with a continuous phase flowing along the membrane surface (i.e., crossflow, stirring) (Figure 21.2(b)). However, it is important to distinguish between the droplet-formation mechanism and the macroscopic operation procedure. In other terms, often, in the literature, the... 

Emulsification devices where the membrane is immersed in a stirred vessel containing the continuous phase, so as to obtain a batch emulsification device operating in deadend emulsification mode, have also been developed (Figure 21.13). Both flat-sheet and tubular membranes are used. In this membrane emulsification device, the continuous phase kept in motion creates the shear stress at the membrane surface that detaches the forming droplets. In a different operation mode, that is, when the continuous phase is not stirred, droplet formation in quiescent conditions is obtained. 

From the theoretical point of view the key problem of the membrane emulsification is to explain and predict the dependence of the mean droplet diameter, Dmembrane emulsification parameters. Important quantities such as droplet-formation time can thus be successively predicted by the mean droplet diameter and disperse-phase flux. 

Droplet formation during direct membrane emulsification and in particular in crossflow emulsification has been described using models different in the scale and in the considered mathematical and physical phenomena, such as ... 

The global balance models are less accurate than the other methods, however, they are easier to handle and more instructive. The latter feature is crucial to acquire the necessary understanding of the physical causes at the basis of the droplet formation and detachment. The balance methods are versatile and permit analysis of the influence of many membrane emulsification parameters with limited computational time, useful in process optimizations. Starting from these considerations, in this section more attention will be paid to the proposed torque and force balances. 

In the microfluid dynamics approaches the continuity and Navier-Stokes equation coupled with methodologies for tracking the disperse/continuous interface are used to describe the droplet formation in quiescent and crossflow continuous conditions. Ohta et al. [54] used a computational fluid dynamics (CFD) approach to analyze the single-droplet-formation process at an orifice under pressure pulse conditions (pulsed sieve-plate column). Abrahamse et al. [55] simulated the process of the droplet break-up in crossflow membrane emulsification using an equal computational fluid dynamics procedure. They calculated the minimum distance between two membrane pores as a function of crossflow velocity and pore size. This minimum distance is important to optimize the space between two pores on the membrane... 

Vladisavljevic, G.T., Shimizu, M., and Nakashima, T., Direct Observation of Droplet Formation in Membrane Emulsification, Proceedings of the DDS Seminar on Development of Lipid Microcarriers and Their Appheation to Drug Dehvery Systems, Sadowara, Japan, August, 2003. 

A.f. Abrahamse, R. van Lierop, R.G.M. van der Sman, A. van der Padt, R.M. Boom, Analysis of droplet formation and interactions during cross-flow membrane emulsification, /. Membr. Sci., 2002, 204, 125-137. 

Figure 20.8 Conventional and innovative membrane emulsification principles spontaneous droplet formation at microchannels (left), droplet formation and detachment at conventional membranes (middle) and jet formation at microengineered microsieves (right) as a function of dispersed phase pore velocity...

By direct membrane emulsification (conventional membrane emulsification), the phase to be dispersed has to be pressed through a microporous membrane. Small droplets are formed and detached from the membrane by a flow of the continuous phase (Figure 13.5). For an appropriate droplet formation, the surface of the membrane has to be wetted by the continuous phase, for example a hydrophilic membrane has to be used to produce an o/w emulsion. 

Figure 13.5 Droplet formation by direct membrane emulsification process.

Membrane and microporous emulsification processes are associated with specific advantages and limitations that can be traced back to the nature of droplet formation taking place in the process. Since single droplets are formed at the exit of a... 

Van der Graaf S, Schroen CGPH, Van Der Sman RGM, Boom RM. 2004. Influence of dynamic interfacial tension on droplet formation during membrane emulsification. / Colloid Interface Sci 711 456-463. 

Van der Graaf S, Steegmans MU, Van Der Sman RGM, Schroen CGPH, Boom RM. 2005b. Droplet formation in a T-shaped microchannel junction A model system for membrane emulsification. Colloids Surf A 266 106-116. 

Ultrasound-assisted emulsification in aqueous samples is the basis for the so-called liquid membrane process (LMP). This has been used mostly for the concentration and separation of metallic elements or other species such as weak acids and bases, hydrocarbons, gas mixtures and biologically important compounds such as amino acids [61-64]. LMP has aroused much interest as an alternative to conventional LLE. An LMP involves the previous preparation of the emulsion and its addition to the aqueous liquid sample. In this way, the continuous phase acts as a membrane between both the aqueous phases viz. those constituting the droplets and the sample). The separation principle is the diffusion of the target analytes from the sample to the droplets of the dispersed phase through the continuous phase. In comparison to conventional LLE, the emulsion-based method always affords easier, faster extraction and separation of the extract — which is sometimes mandatory in order to remove interferences from the organic solvents prior to detection. The formation and destruction of o/w or w/o emulsions by sonication have proved an effective method for extracting target species. 

Some preparation methods specific to the formation of nanoparticle suspensions are provided in References [20,62,63]. Many such methods are simply conventional colloidal suspension preparation methods that have been extended to produce smaller particle sizes, but others involve novel approaches. Some ofthese involve making nanoemulsions as a first step. For example, membrane, microfluidic and nanofluidic devices have been used to make nanoscale emulsions of all kinds, as already noted earlier, and the emulsion droplets so generated can be used in turn to make sohd microparticles and nanoparticles. If the nanoparticles are intended to encapsulate other materials, then a double emulsification technique can be used, at elevated temperature, to prepare a multiple emulsion (i.e. 

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