Dynamic Leidenfrost Effect: Relevant Time and Length Scales

When a liquid droplet impacts a hot solid surface, enough vapor may be generated under it to prevent its contact with the solid. The minimum solid temperature for this so-called Leidenfrost effect to occur is termed the Leidenfrost temperature, or the dynamic Leidenfrost temperature when the droplet velocity is non-negligible. We observe the wetting or drying and the levitation dynamics of the droplet impacting on an (isothermal) smooth sapphire surface using high-speed total internal reflection imaging, which enables us to observe the droplet base up to about 100 nm above the substrate surface. By this method we are able to reveal the processes responsible for the transitional regime between the fully wetting and the fully levitated droplet as the solid temperature increases, thus shedding light on the characteristic time and length scales setting the dynamic Leidenfrost temperature for droplet impact on an isothermal substrate.

Figure 1

Schematic of the experimental setup with synchronized side-view and TIR imaging.

Figure 2

Sequence of ethanol droplets impacting on a sapphire substrate at $T_s=$ (a) 150, (b) 170, (c) 180, and (d) $220°\mathrm{C}$ ($U=1.3\mathrm{m}/\mathrm{s}$ for all cases). The columns show images at different elapsed times after the impact; $t=0$, 0.5, 1.0, 1.5, and 6.5 ms from left to right. The images in the upper row in each pair show the side view, while the lower one consists of TIR images in original gray scale (the upper part) and calculated color height scale (the lower part; the image is the same as the upper one but is horizontally flipped). The colored TIR images show the distance from the substrate surface according to the height map shown in the right bottom. The cutoff height is 91 nm, and any distance more than this is shown in blue corresponding to the largest thickness. The wet area measured from the TIR images of $T_s=150°\mathrm{C}$ are drawn by red half circles as a measure of the diameter of the spreading front. The corresponding movies are available in the Supplemental Material [28].

Figure 3

(a) Schematic diagrams of the three different boiling regimes identified in the present study, (i) contact, (ii) transition, and (iii) Leidenfrost, showing the differences between the radius of the spreading front $R_s$ and the one of the (partially) wetting region $R_w$. Phase diagram of the boiling regimes for (b) ethanol and (c) FC-84, with its more than ten times lower latent heat (as compared to ethanol). The plots indicate the three different boiling behaviors: the contact boiling regime (the solid circle), the transition regime (the open circle), and the Leidenfrost regime (the square). The dashed lines between different regimes are drawn as a guide for the eye. For comparison, static Leidenfrost temperatures are 160 and $125°\mathrm{C}$ for ethanol and FC-84, respectively.

Figure 4

Spreading of a circular liquid ring within the evanescent length scale at the beginning of the impact. (a) Color height images obtained from TIR images of the droplet base, for FC-84, $T_s=160°\mathrm{C}$ and $U=1.4\mathrm{m}/\mathrm{s}$. (b) Radius of the circular ring, $R_n$, corresponding to the closest area of the droplet to the substrate. The error bars represent the uncertainty due to the frame interval in the imaging (horizontal) and the pixel resolution of the image (vertical). The curves show the position of the maximum pressure area, estimated by a modified Wagner’s theory: $\sqrt{3R_{0}Ut}$. The inset shows the same data but in dimensionless form. The data collapse reveals the universality.