Crown behavior during a concentric collision of a falling droplet onto a sessile droplet

This paper reported experimental observations of the splashing morphology and the crown behavior of concentric impact between a falling droplet and a sessile droplet on a glass surface. To control the initial shape of the sessile droplet, we used two syringe pumps, to generate the sessile droplet by pumping the liquid through a holed glass substrate, and to produce the falling droplet, respectively. Both of the two droplets had the same liquid of ethylene glycol. Our experimental cases covered a wide range of the impact Weber number (i.e., We = 270–1500) and four different volume ratios of the sessile to the falling droplets (i.e., V_s/V_f = 1.5, 2.7, 4.3, and 5.7). It is found that the crown had a bowl shape, which it differs from the crown shape of the impact onto a uniform liquid film. We investigated the crown wall break-up phenomena, which occurs when the Weber number is higher than 964. Both of the upper diameter and the height of the crown were quantitatively analyzed, and fitted correlations were derived successfully for both of them.