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The mechanism of microdroplet coalescence is a fundamental issue for droplet-based microfluidics. We developed an asymmetric expansion (a rectangular groove) along one side of a microchannel to achieve multiple-microdroplet trapping, collision, and coalescence. Compared with reported symmetric expansions, this asymmetric groove could easily trap microdroplets and control two or three microdroplet coalescences precisely without a requirement for temporal and spatial synchronization. To reveal the mechanisms of multiple-droplet coalescences in a groove, we observed five different coalescence patterns under different flow conditions. Moreover, we characterized the flow behavior quantitatively by simulating the velocity vector fields in both the microdroplets and continuous phase, finding good agreement with experiments. Finally, a map of coalescence forms with different capillary numbers ( $$0.001< {Ca}

Recently, with the development of lab-on-a-chip researches, the use of microfluidics to precisely control the droplet behaviors in microchannel has received more and more research attention. This article firstly introduces the basic theory and mechanism of the droplets coalescence. Then, it carefully reviews the passive methods of droplets coalescence used in current researches by changing microchannel geometry and adding surfactants. The active methods to trigger droplet fusion such as applied electric field, magnetic fields, temperature gradients, surface acoustic wave and focused lasers are also introduced briefly. We also introduce the fluid dynamics of the droplets coalescence and forecast its progress at last to provide useful guidance for microfluidic devices design and wide use of the drop-based microfluidics.