Article ID: 25-00227
The dynamic contact behavior of a minute liquid droplet upon collision with a high-temperature solid is investigated using total reflection imaging. An inkjet water droplet collides with a high temperature surface of a sapphire prism (and a quartz glass prism) and then splashes away. The contact behavior captured from the back side using a nanosecond lighting stroboscope varies significantly with the contact temperature Tc, as determined based on heat-conduction theory, rather than the temperature Ts of the bulk solid. The contact behavior can be classified into four regions: (I) film evaporation, (II) nucleate boiling, (III) spontaneous nucleation and (IV) supercritical state. The contact area decreases significantly in region II and exhibits a minimum at a temperature close to the superheat limit for the liquid. It then increases in region III to reach a maximum at a temperature close to the critical temperature before it decreases at higher temperatures. Even at a contact temperature so high as to exceed the critical temperature, the liquid still contacts the solid surface over a significant area for several microseconds before the surface dries out. The fine bubbles generated due to spontaneous nucleation hinder contact due to the formation of a local dried area as the contact temperature approaches the superheat limit, whereas contact is enhanced at higher temperatures due to the dynamic action of spontaneous nucleation. Similar behavior is observed for the quartz glass prism in the same range of contact temperature.
