Coalescence may not occur immediately when droplets impact a liquid film.
Despite the prevalence of the high-temperature condition during the impact
process in many applications, the effect of droplet temperature on droplet
coalescence is rarely considered. In this study, we experimentally investigate
the droplet coalescence during the impact of hot droplets on a liquid film by
using color interferometry, high-speed imaging, and infrared imaging. We find
that the coalescence of the hot droplet with the liquid film can be delayed
which is mediated by the intervening gas layer between the droplet and the
film. Compared with droplets at room temperature, the residence time of hot
droplets can increase by more than two orders of magnitude. We find that the
thickness of the gas layer increases with the droplet temperature, explaining
that the thermal delay of coalescence is due to the thicker gas layer. During
the hot droplet impact, the temperature gradient at the bottom of the droplet
induces Maranogni flow, which can delay the drainage of the intervening gas
layer. The results also show that as the Weber number increases, the residence
time of the droplet decreases because of the thinner thickness of the gas
layer.

Cet article explore les excursions dans le temps et leurs implications.

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