Surface-tension-driven droplet coalescence has received significant attention due to its significant role in microfluidics, coalescence-induced droplet jumping, fluid mixing, and microscale heat and mass transfer. However, the study of internal flow characteristics of merging droplets remains a challenge due to visualization difficulty, limited droplet size control, poor droplet manipulation, and insufficient droplet front tracking. Here, in order to study droplet coalescence dynamics, a droplet dispensing and visualization system was developed. To control the size of droplets, monodispersed droplets with diameters of ≈ 40 μm were dispensed onto a superhydrophobic surface, enabling the target droplets to accumulate in volume and grow in radii. To track the internal flow front, an ethanol (20 wt %)-water mixture was used as the working fluid. Due to the unequal evaporation rate of water and ethanol, a density gradient was introduced at the liquid-gas interface of the droplets, resulting in a lower refractive index compared to that in the bulk liquid. The coalescence process of droplets with diameters of ≈ 232 μm and ≈ 1400 μm was imaged. We observed jet-like internal flow and vortex rings (“swimming jellyfish”) inside the merging droplet. It was shown that the jet-like flow underwent a significant deceleration while the vortex-ring experienced slow radial expansion. Our work not only presents a powerful platform capable of visualizing droplet coalescence hydrodynamics, but also provides insights into the internal flow dynamics during droplet coalescence.

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