Ionic liquid drop impact onto heated surfaces

Ionic liquids (ILs), molten salts at low temperatures, are nonionizing, thermally stable, and with low vapor pressure, thereby offering promising applications for lubrication, cooling, and combustion, where drop impact on a heated surface plays a vital role. Drop impact on heated surfaces has been extensively investigated with molecular liquid drops, such as water and ethanol, but rarely explored with ILs. We experimentally investigate the impact dynamics of three types of IL drops onto a heated flat surface under broad ranges of impact velocity (0.18≤U≤4.22 m/s) and surface temperature (18≤Ts≤455°C). The impact events observed with the ILs include spreading, spreading with bubbling, and splashing with bubbling. However, the dynamic Leidenfrost effect with an insulating vapor film causing droplet rebound, typically recorded for molecular liquid drops under an initial impact velocity, is not observed for the IL liquids under similar Weber number (We≈2) and surface temperature (at Ts=350°C). This suppression is attributed to the low gas pressure underneath the IL droplet, induced by evaporation and thermal decomposition of ILs, and can significantly benefit various thermal applications such as cooling and coating. Finally, the maximum spreading factor of the IL drops is modeled using an energy conservation concept and is consistent with experimental results.