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Generate droplets in a microfluidic capillary

Introduction to droplets generation in a microfluidic capillary

microfluidic millifluidic t and cross junction droplets generation in microfluidic capillary 300x141 1
Fig.1: Photograph of chromatography tools: (a): a T-junction – (b): a Cross-junction

We describe in details what is digital microfluidics in the review about microfluidic droplets, and how to achieve it with a pressure controller in this Elveflow® application note (Digital microfluidics using pressure driven flow).. It is possible to make droplets in a microfluidic capillary with commercial tools, especially chromatography tools. Here we will focus on how to do it. One can easily make droplets using a cross or a T junction (Fig.1).

These tools are respectively the macroscopic equivalents of flow focusing & cross flowing microfluidic methods.

The main difference is in the required setup time. The main drawback is the manufacturing dependence. It means that you cannot choose precisely your channel dimensions, you have to choose between the proposed dimensions (from 100 µm to 1 mm). There is a main setup protocol.

This protocol describes how to make fluid-fluid dispersion with:
A/ Flow focusing method
B/ Cross flowing method

The general protocol in a microfluidic capillary is the same; you bring two phases in a junction. One phase will be the continuous phase and the other one the dispersed phase (droplets) (Fig.2).

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    Droplet generation in a microfluidic capillary

    Setup schematic

    T junction droplet generation inside a capillary2

    Microfluidic droplets generation at a T junction

    Microfluidic droplets moving into a capillary

     
    To discover more tips and tricks about droplet-based microfluidics, please check our new droplet userguide!

    There are some details however, which differentiate the two methods.

    A- Flow focusing with a cross junction

    1. The main channel is the channel where the droplets will flow

    2. It is very important to connect the continuous phase perpendicularly to the main channel

    3. The dispersed phase has to be connected to the channel in the continuity of the main channel

    4. Control droplets sizes with input pressure driven flow

    microfluidic flow focusing cross junction
    Fig.3: Scheme of droplets formation at a cross junction (flow focusing)

    B- Cross flowing method

    1. The main channel is the channel where the droplets will flow
    2. It is very important to connect the dispersed phase perpendicularly to the main channel
    3. The continuous phase has to be connected to the channel in the continuity of the main channel
    4. Control droplets size with input pressure driven flow
    microfluidic flow focusing cross junction b
    Fig.4: Schema of droplets’ formation at a T junction (cross flowing)

    In conclusion, by connecting submillimeter tubes to submillimeter T and cross junctions, it is possible to generate droplets in a microfluidic capillary like in microfluidic devices. It is an easy method to manage the production of droplets. There is no simple alternative to the co-flowing method. But the two main methods used in microfluidics are easily done with chromatographic tools. As described in another Elveflow® application note (Digital microfluidics using pressure driven flow), droplets size are pre-determined by the characteristic dimensions of the tubes and junctions. There are more flexibilities on droplets size with flow focusing method (cross junction). Refer to [1,2] where the control of sizes is well described.

    For more reviews about microfluidics, please have a look at: «Microfluidics reviews».  The photos in this article come from the Elveflow® data bank, Wikipedia or elsewhere if precised. Article written by F. Bertholle, G. Velvé Casquillas, A. Hassan-Zahraee and T. Houssin.

    1. G. F. Christopher and S. L. Anna. Microfluidic methods for generating continuous droplet streams. Journal of Physics D: Applied Physics, 40(19): R319, (2007).
    2. A. R. Abate, A. Poitzsch, Y. Hwang, J. Lee, J. Czerwinska, and D. A. Weitz. Impact of inlet channel geometry on microfluidic drop formation. Phys. Rev. E, 80(2), (2009).

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