Coalescences of microdroplets at a cross-shaped microchannel junction without strictly synchronism control

In addition to the generation of micrometer scaled droplets, microchannel device can also serve as unique platform to control microdroplet coalescence. In this study, a cross-shaped microchannel junction is designed to realize head-on microdroplet coalescence without strict control of droplet synchronism. The flow characters of contacting microdroplets in confined microchannels are carefully examined and a critical capillary number (Ca∗ ≈ 0.005) for the occurrence of coalescence is identified for different working systems. Experimental analysis on the film drainage time and the droplet contact time are carefully discussed and a comparison between microdroplet coalescences in confined and free flowing spaces are taken for the in-depth understanding of microdroplet coalescence process.

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► Head-on microdroplet coalescence is obtained without synchronism control.
► The critical Ca for microdroplet coalescence is identified as 0.005.
► The critical Ca is larger in confined microchannel than in free flowing space.