Effects of two-phase periodic velocity on droplet coalescence inside microchannels

A novel method based on periodic change of two-phase velocity for the droplet coalescence in microchannels is proposed. The feasibility of the method is justified by investigating the droplet coalescence in several combinations of the velocity pairs. Once the droplet pairs have been generated, the frequency of the droplet coalescence can be divided into a liquid slug of continuous phase dominated region and a real velocity ratio dominated region. Since the liquid slug between the droplet pairs and the real velocity ratio of the droplet pairs are determined by the flow rate ratios, the critical value of the flow rate ratio is determined to distinguish whether the frequency of the droplet coalescence is liquid slug dominated or real velocity ratio dominated. Compared with the traditional passive method, the droplet pairs are alternatively generated by the periodic change of the two-phase velocity. The velocity gradient of the droplet pairs can be generated spontaneously which avoids the introduction of an expansion structure. The mixing performance inside the coalesced droplet is improved due to the reorganized inner circulation when the droplets is coalesced. The quantitative mixing of the reagents can be achieved to satisfy special application needs by a precise control of the volume of the droplet pairs.