Study of drop coalescence and mixing in microchannel using Ghost Particle Velocimetry

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The coalescence of drops formed in a flow focusing microfluidic device at Reynolds number 0.1<Re<1 was studied experimentally using high speed video-recording and Ghost Particle Velocimetry. It was shown that in the confined microfluidic geometry the presence of both ionic and non-ionic surfactants can facilitate drop coalescence for surfactants dissolved in either the dispersed or the continuous phase. Drop merging was accompanied by strong convection inside the drops with maximum velocity exceeding the superficial liquid velocity by one order of magnitude. Intensity of convection increased with a decrease of drop size and decreased with a decrease of interfacial tension between continuous and dispersed phase. Effect of drop size was particularly strong when the drop size exceeded 80% of the channel width due to the considerably thinner film of continuous phase separating dispersed phase from the channel wall, slower expelling of continuous phase surrounding growing neck between merging drops and therefore slower neck thickening. When merging drops of different sizes was considered, the convection was much stronger in the small drop and movement of the contents of the smaller drop towards the larger drop was observed.