Droplet Micromixer

Z.B. Stone and H.A. Stone, Imaging and Quantifying Mixing in a Model Droplet Micromixer, Phys. Fluids, 17, 063103 (2005). 

Capillary reactor Droplet micromixer Droplet-based microreactor... 

Z. B. Stone, H. A. Stone, Imaging and quantifying mixing in a model droplet micromixer. Phys. Fluids, 2005, 17,... 

A micromixer in which the fluid can be stirred by periodically pumping through the side channels is shown in Figure 3.45 [481]. The periodic perturbation applied via the side channel allows liquids A and B to be mixed. Other mixers based on oscillating pressure-pumped flow have also been reported [482,483]. Two droplets (600 pL) were merged and mixed by a push-pull (shuttling) method in a PDMS device consisting of a hydrophobic microcapillary vent (HMCV) [364]. 

Figure 11. Equivalence between the droplet diffusion model (81) and the IEM model for a zero-order reaction and a second-order reaction in a CSTR. The Damkohler numbers are such that f = 0.5 for perfect micromixing. The agreement is excellent for the second-order reaction, more approximate for the zero-order one.

The effect of coalescence and break-up of droplets on the yield of chemical reactions was studied by Villermaux (33). Micromixing effects may occur even in batch reactors if there is a drop size distribution and mass-transfer control. Although practical rules for the design and scale-up of liquid-liquid reactors are available as Oldshue showed in the case of alkylation (152), many problems remain unsolved (.5) mass transfer effects, high hold-up fractions (> 20 %), large density differences, high viscosities, influence of surfactants. 

Figure 4.19 Schematic illustration of high-throughput screening used for the isomerization of allylic alcohols to ketones with the illustration of the micromixer used to generate droplet flow (courtesy of Wiley-VCH) [211],...

Harada and co-workers (HI) developed two coalescence-redispersion models to describe micromixing in a continuous-flow reactor. In the first model, the incoming dispersed-phase fluid is assumed to consist of uniformly sized droplets. These droplets undergo 0 to n coalescences and redispersions with surrounding droplets of a constant average concentra-... 

In the chemical industry (on the mega- as well as the micro-scale) fine emulsions have many useful applications in, e.g., extraction processes or phase transfer catalysis. Additionally, they are of interest for the pharmaceutical and cosmetic industry for the preparation of creams and ointments. Micromixers based on the principle of multilamination have been found to be particularly suitable for the generation of emulsions with narrow size distributions [33]. Haverkamp et al. showed the use of micromixers for the production of fine emulsions with well-defined droplet diameters for dermal applications [38]. Bayer et al. [39] reported on a study of silicon oil and water emulsion in micromixers and compared the results with those obtained in a stirred tank. They found similar droplet size distributions for both systems. However, the specific energy required to achieve a certain Sauter mean diameter was 3-1 Ox larger for the macrotool at diameters exceeding 100 pm. In addition, the micromixer was able to produce distributions with a mean as low as 3 pm, whereas the turbine stirrer ended up with around 30 pm. Based on energy considerations, the intensification factor for the microstirrer appears to be 3-10. 

Tung K, Li C, Yang J (2009) Mixing and hydrodynamic analysis of a droplet in a planar serpentine micromixer. Microfluid Nanofluid 7(4) 545-557... 

Another example of microextraction more related to organic chemistry was the transfer of acetone in hexane/water mixtures [32]. In this study, mixing by a T-sha-ped glass micromixer was first used to create small droplets (about 20 pm) to increase the exchange surface area. Additionally, a settler was used to allow phase separation. The chosen example resulted in a drop-free organic phase, with only some drops found in the water phase. Coalescence of the phases readily occurred during the settling period. The authors mention that this spontaneous droplet combination would not occur in many systems of industrial interest. 

The problem of spontaneous separation of micromixer-produced droplets was investigated using dodecane and water as the model system [33]. To enhance spontaneous separation the emulsion from the micromixer was fed into a rectangular channel fabricated from aluminum foil as a spacer between two flat plates of glass and/or PTFE. In the case of PTFE coalescence occurred readily whereas glass did not promote the separation. This study demonstrated that the wall material and its surface properties (wettability) can have a big influence on coalescence behavior. 

The ability of micromixers to create small droplets to increase the exchange surface is well established, while the field of coalescence in microdevices needs much more research. Micromixing will make mass transfer very rapid, but after this stage the coalescence of the droplets has to occur. The small droplets produced by micromixers often give rise to stable emulsions but, for an effective extraction, sepa-... 

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