Minimum Leidenfrost Temperature on Smooth Surfaces

During the Leidenfrost effect, a thin insulating vapor layer separates an evaporating liquid from a hot solid. Here we demonstrate that Leidenfrost vapor layers can be sustained at much lower temperatures than those required for formation. Using a high-speed electrical technique to measure the thickness of water vapor layers over smooth, metallic surfaces, we find that the explosive failure point is nearly independent of material and fluid properties, suggesting a purely hydrodynamic mechanism determines this threshold. For water vapor layers of several millimeters in size, the minimum temperature for stability is $\approx 140°\mathrm{C}$, corresponding to an average vapor layer thickness of $10–20\mu \mathrm{m}$.

Figure 1

(a) Sequence of images showing a single pure water Leidenfrost drop that remains levitated over a heated aluminum surface below the boiling point (Supplemental Material [45], Video S1). (b) Failure temperature $T_{-}$ as a function of drop radius. Each data point represents a different water drop. The upper Leidenfrost temperature, $T_{+}=190°\mathrm{C}\pm 20°\mathrm{C}$, is indicated by the dashed line and shaded region.

Figure 2

(a) Experimental setup for high-speed measurements of the vapor layer dynamics. The dashed box indicates the complex impedance, which varies in time. (b) Equivalent circuit for the complex impedance of the vapor layer, as described in the main text. Both $C_v$ and $C_{\lambda }$ are time dependent. (c) Simplified geometry for modeling the capacitance of the vapor layer. (d) Capacitance data from the COMSOL model (Supplemental Material [45]) and [48] using a range of values for $H$ and $d$. Symbols correspond to different values of $d$. The dashed line is Eq. (4), with $c=0.58$ and $C_0=1.85\mathrm{pF}$. (e) $d$ vs $T_s$ for a nickel-coated copper electrode in pure water. The electrode was first heated (red triangles) and then subsequently cooled (blue circles). $T_{+}=240°\mathrm{C}\pm 30°\mathrm{C}$ (red dashed line) was the average temperature when the vapor layer formed, and $T_{-}=140°\mathrm{C}\pm 10°\mathrm{C}$ (blue dashed line) was the average temperature when the vapor layer collapsed. The shaded regions show the standard deviation from multiple experiments.

Figure 3

Resistance (a) and capacitive reactance (b) of the Leidenfrost cell during collapse with 0.02 M NaCl. The insets show 2 ms of the data right before and after collapse. At 30 s after collapse, the system is quiescent. Capillary waves are visible as oscillations in the reactance prior to collapse. (c) Total capacitance of the liquid-vapor-solid interface just before collapse. The enormous increase is due to the formation of an ionic double layer at the liquid-solid contact. The images show a time sequence of the initial collapse point, as indicated by the arrow, where bubbles are generated as the wetting front spreads rapidly. The blue points in the data correspond to the indicated images (Supplemental Material [45], Video S4).

Figure 4

(a) Spatially averaged vapor layer thickness $d$ as a function of substrate temperature $T_s$ during the cooling of titanium, brass, and copper electrodes. The visible discontinuities in the data, indicated by the dashed lines, correspond to vapor layer failure at temperature $T_{-}$ and thickness $d_c$ (b) Thickness at failure $d_c$ as a function of the temperature at failure $T_{-}$. Each data point represents varying aqueous NaCl concentrations and liquid level $H$ for each metal. The dashed lines show the mean value of $d_c$ for each metal, while the average of $T_{-}$ was $140°\mathrm{C}\pm 10°\mathrm{C}$ for all experiments.