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Segmented Flow: The Droplet as a Reactor

Micro-structured reactors can be used to process multi-phase fluid systems (e. g. liquid/liquid or gas/liquid) with high precision in the form of segmented flows which opens up new opportunities for interesting applications. In specially developed microfluidic structures, we segment fluids continuously into a second liquid phase by shearing them off or constricting them in the form of droplets or bubbles. The size of the droplets or bubbles formed and the frequency of the segmentation can be controlled very precisely through the selected flow conditions, reactor geometries and other process parameters. The droplets or fluid segments function as closed reaction vessels having no chemical interaction with the transport phase. Within the fluid segments each with a volume of just a few nanolitres syntheses of high value products can be performed by suppressing cross-contamination, dilution and dispersion effects caused by convection and diffusion. Moreover, mixing of the reagents in the nanolitre segment is strongly intensified by advection.

On the other hand, segmented flow processing can also be used to deliberately intensify interaction between two-phase systems. By providing large interfacial areas, we can significantly accelerate the mass transport over the phase boundary layers compared to macroscopic processes. Typical applications are phase transfer catalysis and other two-phase organic syntheses.

Segmented flow processes in microreactors are also used at the Fraunhofer ICT for manufacturing microscale particulate products. For example, the precise control of the droplet size is used to synthesise monomodal spherical polymer particles in the microreactor or to manufacture polymer microcapsules, which can be filled with a wide range of active ingredients during the process. The size of the polymer particles is infinitely adjustable over a broad range, e.g. between 5 m and 300 m; typical wall thicknesses of polymer microcapsules lie in the range
of approx. 100 nm.

Microreactor for segmented flow processes.


Generation of segmented flow in a microchannel (high-speed video).

Dosing reagents into droplets within a microchannel (high-speed video).


Droplet fusion in a microchannel (high-speed video)


Production of spherical polymer particles.


Chemical reaction by merging droplets (high-speed Video).

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Continuous synthesis of filled microcapsules: Monitoring via high-speed microscopy.

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Synthesis of droplet-in-droplet composites in microreactors (high-speed videos)


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