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Thecoalescence between liquid flow and droplets is usually encounteredin liquid filling or some new microdevices; a deep understanding ofits underlying mechanism is required. In this work, therefore, thedynamic behaviors of this atypical phenomenon in microchannels werevisually investigated with droplets generated by photothermally inducedevaporation and condensation. The effects of droplet quantity andposition, inlet pressure, and microchannel size were also explored.Experimental results showed that the coalescence accelerated the liquidmovement as a result of lowered pressure at the interface. Parametricstudies indicated that large droplet quantity and small distance betweenthe inlet and droplets yielded a large velocity increment ratio asa consequence of lowered liquid pressure at the interface and increasedpressure gradient, respectively. A high peak velocity increment butwith low velocity increment ratio was obtained at high inlet pressurebecause of high liquid flow velocity. Moreover, the microchannel witha small size yielded a strong acceleration rate of the liquid movementbecause of more obvious capillary action in the small channel. Resultsobtained will be helpful for the design and operation of those kindsof microdevices that may face this atypical phenomenon. [ABSTRACT FROM AUTHOR]