Microfluidic emulsification technique

It has been stated that the development of emulsification methods for production of mono-sized droplets must be rooted in one of two possible manufacturing approaches (Williams et al., 2001a) (1) reduction of process length scales of the turbulent perturbations and enhancement of their uniformity in the mixing processes that rupture the liquids, and (2) the creation of droplets individually (drop by drop). The production of emulsions using membrane and microfluidic devices represents a typical example of the second approach. This chapter aims to introduce the latest development on the utilization of the membrane and microfluidic emulsification techniques for the preparation of double emulsions, as well as micro- and nanoparticles from double-emulsion precursors. 

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. 

Microfluidic techniques have been recently used for the synthesis of microgel particles with dimensions of 1-30 pm. In these methods, microfluidic devices are used that provide emulsification of polymer solutions followed by physical [27, 28] or chemical [29] crosslinking. 

Importantly, in practically all common techniques of formation of drops and bubbles, the liquids are deformed geometrically with the use of a force of choice, and then they spontaneously break into smaller bits by the action of the interfacial tension. As we will discuss it below, emulsification in microfluidic devices constitutes a very different route to emulsification. 

The effects of flow rate(s), monodispersity, energy input, and ease of parallelization are evaluated to indicate the potential of microfluidic techniques for large-scale application. Cross-flow membrane emulsification will be used as benchmark technology as it is already available and relatively easy to scale up by using several membranes in series or in parallel. Table 1 shows a summary of various effects of which more detailed information is given in the remainder of this section. 

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