Observation of a mixed regime for an impinging droplet on a sessile droplet

This article reports experimental results of the presence of a mixed regime where rebound or coalescence occurs when a liquid droplet with low impact velocity impinges on a static sessile droplet, deposited on a flat aluminum surface. In particular, the present study suggests possible reasons why this mixed occurs with nearly the same probabilities with the use of the sequential images captured by two high-speed cameras from the 767 experimental cases. It is also demonstrated that the mixed regime is substantially affected by static droplet volumes, Weber numbers, and offset distances between impinging and static droplets. In the mixed regime, rebound may occur because a relatively large air-film maintains to prevent film drainage for coalescence. The lubrication theory is utilized to estimate film thickness between two droplets and is compared to the critical film thickness. The upper limit of the Weber numbers in the mixed regime decreases with an increase in the offset distance, owing to a rotational droplet motion causing air-film elongation. The contact time during rebound slightly increases with the static sessile droplet volume. It is observed that the measured contact times increases with a decrease in impact velocity, showing cushioning effect by air-film. On the other hand, a coalescence is observed to intermittently in the mixed regime because the air-film layer existing at the interface has an irregular shape by oscillation of an impinging droplet.