Universality of oscillating boiling in Leidenfrost transition

The Leidenfrost transition leads a boiling system to the boiling crisis, a state in which the liquid loses contact with the heated surface due to excessive vapor generation. Here, using experiments of liquid droplets boiling on a heated surface, we report a phenomenon, termed oscillating boiling, at the Leidenfrost transition. We show that oscillating boiling results from the competition between two effects: separation of liquid from the heated surface due to localized boiling and rewetting. We argue theoretically that the Leidenfrost transition can be predicted based on its link with the oscillating boiling phenomenon and verify the prediction experimentally for various liquids.

Figure 1

Snapshots showing the wetted area, measured by the total internal reflection (TIR) technique [11, 12, 13, 14], of (a) the boiling process and (b) the rewetting process for acetone droplets with velocity $V_0=2.7\mathrm{m}{\mathrm{s}}^{-1}$ and surface temperature $T=150{}^{\circ }\mathrm{C}$. (c) Wetted area $A_w$ vs $t$ in the early stage of a droplet impacting on the top surface of a heated sapphire prism. (d) Wetted area $A_w$ vs $t$ during the entire impact process; the shaded area corresponds to the wetted area shown in (c). The inset: snapshots show the side view (upper panel) and the bottom view (lower panel) of the impact at different times. (e) Frequency $f$ of oscillation vs $V_0$ for different values of $T$ showing a weak dependence of $f$ on $V_0$. (f) Frequency $\overline{f}$ averaged across $V_0$ vs $T$. The solid line is the frequency $V_{\mathrm{re}}/R_d$, where $V_{\mathrm{re}}$ is the rewetting velocity [Eq. (1)] and $R_d\approx 1\mathrm{mm}$ is the droplet radius. All scale bars indicate a length scale of 1 mm.

Figure 2

(a) Critical size $2r_c$ of a vapor embryo (solid line) and evaporation time ${\tau }_h$ (dashed line) vs surface temperature $T$. Upper panel: schematic showing vapor embryos before and after merging to create a vapor layer. (b) Snapshots showing the rewetting process for acetone droplets impacting with velocity $V_0=2.3\mathrm{m}{\mathrm{s}}^{-1}$ at low temperatures (left panel) and high temperatures (right panel); arrows indicate the motion of the three-phase contact line. Scale bars represent the length scale of 0.3 mm. The time stamps are measured from the first contact of impact. (c) Rewetting velocity $V_{\mathrm{re}}$ vs surface temperature $T$ for acetone droplets for different impact velocities. (d) $V_{\mathrm{re}}$ vs $T$ for different impact velocities for ethanol, isopropyl alcohol (IPA), and water. The solid lines show the prediction according to Eq. (1).

Figure 3

(a) Phase diagram of characteristic boiling regimes of acetone. The solid line represents the theoretical prediction of the Leidenfrost transition [Eq. (2)]; the dashed line and the dashed-dotted line, respectively, mark the Leidenfrost transition measured by TIR and side-view recordings. (b) Comparison between Leidenfrost temperatures obtained from theory ($T_L^P$) and those from experiments ($T_L^M$). The open markers on the diagonal line indicate $T_L^P$. Upward solid triangles indicate $T_L^M$ obtained from TIR; downward solid triangles indicate $T_L^M$ from side-view recordings. All other solid markers are $T_L^M$ from previous studies [10, 12, 13, 17, 18, 19]. The shaded area indicates $\pm 15\%$ deviation from the theoretical values.