Microfluidic droplet reactors

Microfluidic systems also provide the possibility to spatially and temporally monitor and control reactions by adding reagents at precise time intervals during the reaction progress. This was demonstrated by Shestopalov et al. [191] by carrying out a multistep synthesis of quantum dots using the microfluidic droplet reactor. 

In droplet-based microfluidics, these reaction vessels are formed by droplets of a dispersed phase, which are embedded into a continuous phase. Both liquid phases are immiscible. A huge amount of such droplet reactors can be generated, transported, controlled, and processed in parallel in a droplet-based lab-on-a-chip device. These devices can be characterized as application specific microfiuidic networks that implement and automate a conventional laboratory workflow in a microfluidic chip device or system. They are built up by appropriately intercoimecting microfluidic operation units, which provide the required laboratory operations at the microscale. Consequently, for each conventional laboratory operation, its microscale counterpart is required. 

Droplet reactors are basic components of digital microfluidics. There are still a number of opportunities in droplet reactor research. The future directions can be categorized into fundamentals and applications. Fundamental research could result in other platforms for droplet reactors. While electrowetting has been widely reported in the past, microfluidic platforms based on thermocapillary and other forces are still underrepresented. More research on the systematic design of droplet-based reactors is needed to secure industrial adaptation and commercial... 

In this chapter, we present an overview of the transport of fluids in microchannels with a focus on the formation and manipulation of emulsion droplets. The next section deals with defining terminology and describing the physics of fluid flow in small channels. We briefly summarize approaches that are used to drive the fluid flow in the microchannels and dispersion of immiscible phases (oil and water) to create well-defined droplets. In the last section, we focus on the applications of droplet-based microfluidics. In particular, we review microparticle formation and biochemical reactions in small droplet reactors. 

Yamada, M., Sugiyama, N., Seki, M., Microfluidic reactor array for multistep droplet reactions, Micro Total Analysis Systems 2000, Proceedings pTAS 2002 symposium, 6th, Nara, Japan, Nov. 3-7, 2002, 2000, 43M4. 

Chang et al. [5] utilized microtubes to generate micro-segmented flow. Upon surface modification, the prepared nanoparticles were mixed with a monomer and emulsified into uniform droplets in a capillary-based microfluidic device. The microchannel-based reactor offered reliable control over the nanocomposite products by precisely adjusting the interfacial tension. 

In a continuous flow system, reactions are performed at steady state, which makes it possible to achieve better control and reproducibility. Furthermore, the ability to manipulate reactant concentrations in both space and time also provides a high level of reaction control than that of bulk stirred reactors. The spatial and temporal controls of chemical reactions in microfluidic devices are useful to control and alter chemical reactivity according to the prefiminary design. And usually multistep synthesis can produce particles with fairly complex shapes and functionalities. However, the coalescence between droplets, the stability of flows after several times of mixing, and the controllability of the fluid by multistep stiU remain to be improved. 

An industrial batch reactor has neither an inflow nor an outflow of reactants or products while the reaction is being carried out. Batch reactions can be carried out in droplet microreactors, where nanoliters of fluid are individually manipulated using techniques such as electrowetting on dielectric (EWOD) and surface tension control. Semibatch reactors are used in cases where a by-product needs to be removed continuously and to cany out exothermic batch reactions where a reactant has to be added slowly. Microfluidics allows precise control of concentration and temperature, which allows batch and semibatch reactions to be carried out in a continuous manner. Figure 1 shows the general components of a simple industrial-reactor semp, compared with a laboratory-scale setup to carry out a reaction with microfluidic chips. 

Polymer Synthesis Within Microfluidic Reactor, Fig. 1 (a-e) Scanning electron microscopy images of polymer microbeads obtained by polymerizing TPGDA in droplets obtained in regimes A, B, C, and D, respectively, after removing a SO core. Inset cross section of the... 

In microreactor applications, the segmented flow pattern (Figure 1.3) is most common. Here, discrete droplets behave as separate reactor vessels that are con-vected along in the microfluidic network by the continuous carrier liquid. Similarly, the longitudinal dispersion of the continuous phase can be suppressed by the use of discrete droplets or bubbles of the segmenting phase. 

The topic of handling liquids of different viscosities for dispersion processes in microfluidic networks with two microreactors ( droplet emitters ) is addressed by Unilever R D (Vlaardingen, The Netherlands) in a patent [14,15]. The patent deals with the flow distribution in a microfluidic network on the background of dispersion processes with two supply sources for the two phases, which supply via upstream channel systems two microfluidic reactors called drop emitters (Figure 22.6). The dispersion leaves the microreactors via downstream channels. The major point of the patent is that the flow distribution is controlled by a design which creates a high pressure drop in the upstream channels. The pressure drop in the downstream... 

EGDA is a cross-linker that has been employed repeatedly in iCVD polymerization to enhance the mechanical properties of the film [67,68]. p(PFDA-co-EGDA) was deposited in channels of poly(dimethylsiloxane) (PDMS) microfluidic devices as a barrier coating to prevent diffusion and swelling [50] or to provide an adequate interface for the synthesis of nanoparticles in droplet-flow microfluidic reactors [52]. p(PFDA-co-EGDA) was used to robustly encapsulate IL marbles [51]. IL marbles were fabricated rolling off a droplet of IL in a Petri dish with FI FE microparticles. Next, the Petri dish was placed inside the vacuum chamber and the marbles were conformally coated over the entire surface. 

Chapter 19 Droplet-Based Microfluidics Picoliter-Sized Reactors for... 

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