Droplet fusion

Percolation in microemulsions and concomitant microstructural changes are the focal points of this review. A complete understanding of percolation phenomena in reverse microemulsions will require an understanding of droplet interactions and the associated thermodynamics of droplet fusion, fission, aggregation to form clusters of varying fractal... 

In terms of measuring emulsion microstructure, ultrasonics is complementary to NMRI in that it is sensitive to droplet flocculation [54], which is the aggregation of droplets into clusters, or floes, without the occurrence of droplet fusion, or coalescence, as described earlier. Flocculation is an emulsion destabilization mechanism because it disrupts the uniform dispersion of discrete droplets. Furthermore, flocculation promotes creaming in the emulsion, as large clusters of droplets separate rapidly from the continuous phase, and also promotes coalescence, because droplets inside the clusters are in close contact for long periods of time. Ideally, a full characterization of an emulsion would include NMRI measurements of droplet size distributions, which only depend on the interior dimensions of the droplets and therefore are independent of flocculation, and also ultrasonic spectroscopy, which can characterize flocculation properties. 

The droplet size and size distribution seems to be controlled by a Fokker-Planck type dynamic rate equilibrium of droplet fusion and fission processes, i.e., the primary droplets are much smaller directly after sonication, but colloidally unstable, whereas larger droplets are broken up with higher probability. This also means that miniemulsions reach the minimal droplet sizes under the applied conditions (surfactant load, volume fraction, temperature, salinity, etc.), and therefore the resulting nanodroplets are at the critical borderline between stability and instability. This is why miniemulsions directly after homogenization are called critically stabilized [19,20]. Practically speaking, miniemulsions potentially make use of the surfactant in the most efficient way possible. 

In both the procedures, by varying to, the dimensions of the synthesised particles can be altered. The pictorial representations of the above protocols are illustrated in Fig. 6.2. As can be seen, the internal phenomenon of droplet fusion followed by fission takes place. The materials formed during fusion by reaction get distributed among the droplets upon fission. By probability, some droplets may remain empty which is more in dilute solution of the reactants. The occurrence of the process of fusion and fission has been established by the TRFQ (time-resolved fluorescence quenching method [ 18-20]). The internal dynamics of the disperse particles essentially guide the formation characteristics of nanoparticles. 

Fig. 2 Droplet fusion devices, (a) Fusion based on electrocoalescence. Reproduced with permission from [40]. (b) Droplet fusion based on surface energy patterning. Reproduced with permission from [41]. (c) Fusion based on changing the concentration of surfactant in continuous phase. Reproduced with permission from [42]. (d) Pillars assisted droplet fusion. Reproduced with permission from [43]...

Hung, L.-H., et al.. Alternating droplet generation and controlled dynamic droplet fusion in microfluidic device for CdS nanoparticle synthesis. Lab on a Chip, 2006, 6 174-178. 

A second factor that differs between oil-in-water and water-in-oil systems is the role of fat crystals. In oil-inwater systems, such crystals induce coalescence. However, the finalization of the droplet fusion is also slowed... 

Droplet Fusion and Droplet Loading, Fig. 1 Pictographs for microfluidic structures for droplet loading (left), droplet merging (right), and related pre- and postprocessing steps including distance control (top). 

Droplet Fusion and Droplet Loading, Fig. 3 Microstructures and microfluidic sysirans for droplet-based microfluidics... 

Enzyme Assay in Microfluidics, Fig. 3 Schematic of operations inside the microdroplet platform, (a) The mixing of two reagents (amino acid mix and nucleic acid mix) while the droplets are generated, (b) Droplet fusion between OpdA-containing droplets and coiunaphos... 

Figure 4.5 Schematic (not to scale) of experimental setup for studying reaction kinetics in fused droplets. (Inset) The droplet fusion center is the intersection of the two droplet streams. Most fusion events take place in a circle (dotted) of about 500 fim surrounding the droplet fusion center [101]. Reproduced from Lee, j.K., Kim, S., Nam, H.C., Zare, R.N. (2015) Microdroplet Fusion Mass Spectrometry for Fast Reaction Kinetics. Proc. Natl. Acad. Sci. USA 112 3898-3903 with permission from PNAS...

Fidalgo LM, AbeU C, Huck W Siarface-induced droplet fusion in microfluidic devices. Lab Chip 7 984-986, 2007. 

A) Static percolation SCHEME 3.3 Dynamics of droplet fusion. 

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