Droplet segmented flow

Layers of calcium nitrate (40 mmol T1 in water CaN03 4 H20), potassium carbonate (40 mmol 1 1 in water) and a separation layer of water are fitted into each other by means of the concentric separation mixer [53], The reaction temperature is 22 °C. The reaction solution forms droplets in a dodecane reservoir and inserted as such a segmented flow in the tubular reactor [137, 138],... 

Figure 4.14 (a) Segmented flow of oil and water in a channel with a medium hydrophobic properties - the interface boundary and the channel surface are in contact (b) droplet flow of oil and water in a channel with hydrophilic properties - the interface boundary and the channel surface are not in contact. 

Segmented flow microfluidics describes the principle of performing an assay within small liquid droplets immersed in a second immiscible continuous phase (gas or liquid). For process automation, the droplets are handled within microchannels, where they form alternating segments of droplets and the ambient continuous phase. 

In principal, droplets of a dispersed liquid phase are immersed in a second continuous gas (two-phase gas-liquid) or liquid (two-phase liquid-liquid) phase within a microchannel. Thereby, the inner liquid droplets are separated by the continuous carrier liquid along the channel. If the size of the iimer phase exceeds the cross sectional dimensions of the channel, the droplets are squeezed to form non-spherical segments, also called plugs . Following this flow scheme, the platform is called segmented flow microfluidics. 

Table 3 gives an overview of the microfluidic unit operations and applications that have been already implemented on the segmented flow platform. They all take advantage of the enclosed reaction confinement within the droplets, either for analytical applications (cell analysis, single organism analysis, DNA assays, drug screening, protein crystallization) or chemical synthesis. 

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. 

Segmented (Droplet-Based) Flow Self-Assembly Fabrication... 

Segmented flow (or droplet-based flow) means a multiphase flow where the individual phases are divided in consecutive segments. 

Segmented flow. When the ratio of flow rates for the wetting to the non-wetting phases is close to unity, the dispersed phase forms droplets or bubbles that span most of the cross-section of the charmel. Two consecutive droplets confine the continuous liquid phase between them. 

Droplets and bubbles in microchannels are very stable, in particular when surfactants are added. As a result, one should focus on the conditions at the inlet of the channel. Once formed, droplets and bubbles of a given size often remain intact when carefully expanded within a microchannel network. As is shown in Figure 1.5, a two-phase stream that was generated as a segmented flow may well become a bubbly or foam-like flow. 

In many applications, a capillary instability is expected to produce liquid-liquid [62, 63, 66] or gas-liquid [67, 68] segmented flows with uniform droplet or bubble sizes. Several attempts have focused on characterizing the rich dynamic behavior of segmented microflows that also includes very irregular flow behavior. 

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. 

At the small capillary numbers associated by most microfluidic applications, the pressure drop over a bubble is significant with respect to the viscous losses in the continuous liquid. For single-phase laminar flow, velocity and pressure drop are linearly related and the pressure drop is proportional to the length Az of the channel. For segmented flow, the pressure drop also contains a term that depends on n, the number of segments (i.e. bubbles or droplets) in that channel. For round channels, the two terms are... 

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