Aqueous droplets in a Leidenfrost state on near room temperature sulphuric acid

Droplets that hover above a condensed phase of matter, on a cushion of gas, are in the Leidenfrost state [1]. A typical example of this is a water droplet levitated by its own vapor above a heated pan [2,~3]. In the last few decades more exotic systems such as oil droplets hovering above warm oil [4,~5] or self-propelled acetone droplets hovering above warm water have attracted attention [6]. Acknowledging the dangers of adding water to an acid, we demonstrate that aqueous droplets at room temperature can be prepared in a Leidenfrost state above sulphuric acid at slightly higher temperatures. Guided and supported by experiments, possible mechanisms underlying non-coalescence of the droplets with the acid are discussed.\\ \noindent [1] D.\ Qu\'er\'e, Annu.\ Rev.\ Fluid Mech.\ \textbf{45}, 197--215 (2013).\\ \noindent [2] J.\ G.\ Leidenfrost, Ovenius (1756).\\ \noindent [3] A.-L.\ Biance \textit{et al.}, Phys.\ Fluids \textbf{15}, 1632--1637 (2003).\\ \noindent [4] G.\ P.\ Neitzel and P. Dell'Aversana, Annu.\ Rev.\ Fluid Mech.\ \textbf{34}, 267--289 (2002).\\ \noindent [5] M.\ Geri \textit{et al.}, J.\ Fluid Mech.\ \textbf{833}, R3, (2017).\\ \noindent [6] S.\ D.\ Janssens \textit{et al.}, Phys.\ Fluids \textbf{29}, 032103 (2017).