The coalescence dynamics of an ethanol droplet freely falling on a sessile ethanol droplet is investigated experimentally using a high-speed imaging system. The regime maps showing the partial coalescence and spreading behaviors in the plane of the Weber number (We) and the volume of the sessile droplet (V-p) normalized with the volume of the impacting droplet (V-i) have been presented. The partial coalescence phenomenon is observed when the ratio of the volume of the sessile droplet to that of the impacting droplet (V-p/V-i) is greater than two. For Vp/Vi = 2, the size of the daughter droplet is found to be about 0.1 times as that of the impacting droplet, which increases with the increase in the We and normalized volume of the sessile droplet. In the present study, the negative curvature of the droplet coupled with the presence of the substrate leads to a different coalescence dynamics. Published under license by AIP Publishing.
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