Liquid-solid contact behavior of a minute liquid droplet impinging on a hot solid surface

The dynamic contact behavior of a minute liquid droplet upon the collision with a high temperature solid is investigated using total internal reflection imaging. An inkjet water droplet collides with a high temperature surface of sapphire and quartz glass prisms then splashes away. Contact behaviors captured from back using a nanosecond lighting stroboscope vary dramatically with the contact temperature Tc based on the heat conduction theory rather than the solid temperature Ts and are classified into four regions, (I) film evaporation, (II) nucleate boiling, (III) spontaneous nucleation and (IV) supercritical state regions. Contact area decreases significantly in the region (II) to show a minimum at a temperature close to the limit of liquid superheat, then increases in the region (III) to reach a maximum at a temperature close to the critical temperature before decreasing at higher temperatures. Even at a contact temperature so high as to exceed the critical temperature, liquid still contacts the solid surface over a significant area for several microseconds before drying up of the surface. The fine bubbles generated due to spontaneous nucleation hinders the contact due to the formation of the local dried area as the contact temperature approaches the superheat limit, whereas the contact is rather enhanced at further higher temperatures due to the dynamic action of spontaneous nucleation. Similar behaviors are observed for the quartz glass prism in the same range of the contact temperature.