Triple Leidenfrost Effect: Preventing Coalescence of Drops on a Hot Plate

We report on the collision-coalescence dynamics of drops in Leidenfrost state using liquids with different physicochemical properties. Drops of the same liquid deposited on a hot concave surface coalesce practically at contact, but when drops of different liquids collide, they can bounce several times before finally coalescing when the one that evaporates faster reaches a size similar to its capillary length. The bouncing dynamics is produced because the drops are not only in Leidenfrost state with the substrate, they also experience Leidenfrost effect between them at the moment of collision. This happens due to their different boiling temperatures, and therefore, the hotter drop works as a hot surface for the drop with lower boiling point, producing three contact zones of Leidenfrost state simultaneously. We called this scenario the triple Leidenfrost effect.

Figure 1

(a) Experimental setup. (b) Direct coalescence of two water drops in Leidenfrost state. (c) Consecutive bouncing of an ethanol droplet (transparent one) against a water droplet (tinted with methylene blue) during several seconds before coalescing. (d) Path of an acetonitrile droplet (tinted in blue) bouncing several times against a water drop. The snapshots show that the water droplet, initially transparent, turns bluish suddenly when the droplets coalesce. In (b)–(d), the elapsed time is indicated in seconds in each snapshot, with $t=0\mathrm{s}$ corresponding to the moment of coalescence [scale bars: (b) and (c) 10 mm, (d) 5 mm].

Figure 2

(a) Evaporation time $\tau$ vs plate temperature $T_p$ (red squares) and $\tau (T_L)$ (blue dots), for 0.5 ml drops of different liquids. (b) $T_L$ vs $T_B$ and (c) ${\mathrm{\Theta }}_L$ vs ${\mathrm{\Theta }}_B$, for the liquids in (a), with the best linear fit: ${\mathrm{\Theta }}_L=1.17{\mathrm{\Theta }}_B+0.05$ (see text).

Figure 3

Coalescence time $t_c$ for a $250\mu \mathrm{l}$ Drop 2 of the indicated liquids with a 1 ml Drop 1 of water as a function of (a) $\mathrm{\Delta }\sigma$ and (b) $\mathrm{\Delta }T$. Blue numbers denote the ethanol concentration (percent) in Drop 2 composed of water-ethanol mixtures.

Figure 4

(a) Radial bouncing distance $r_b$ and (b) diameter $D_2$ of Drop 2 as a function of time $t$, for drops of different liquids of initial volume $V_0=250\mu \mathrm{l}$ before coalescing with water. For clarity, only the last 20 s of bouncing are shown (coalescence occurs at $t=0\mathrm{s}$). A complete evolution of $D_2(t)$ can be found in Supplemental Material [36]. In (b), gray lines exhibit the linear decrease of $D_2$ vs $t$. (c) Coalescence size $D_c$ of Drop 2 as a function of $V_0$ for different liquids. For reference, we show $D_c$ for the water Drop 1 (open circles) coalescing with the Drop 2 of ethanol, and the initial diameter $D_0$ of the latter considering a pancakelike shape (dashed line). (d) Log-log plot of $t_c$ vs $V_0$ of a Drop 2 of methanol with a water Drop 1 of three different initial volumes $V_0^{*}$. The green line highlights a power-law behavior of the form $t_c=a{V}_0^n$. In (a)–(d), the experiments were performed at $T_p=250°\mathrm{C}$. (e) $D_c$ for methanol and water drops when they coalesce, as a function of $T_p$ (their respective initial volumes $V_0$ and $V_0^{*}$ are indicated).

Figure 5

(a) Sketch of the triple Leidenfrost effect (see text). (b) Snapshots taken at 500 fps showing the vapor layer generated when droplets of ethylene glycol (transparent) and chloroform (blue) collide ($T_p=350°\mathrm{C}$). (c) Explosive coalescence of chloroform (blue) with ethylene glycol. (d) Together but not scrambled: water (blue) and toluene join but remain unmixed because these liquids are immiscible (scale $\mathrm{bars}=5\mathrm{mm}$).