Drop Impact on Superheated Surfaces

At the impact of a liquid droplet on a smooth surface heated above the liquid’s boiling point, the droplet either immediately boils when it contacts the surface (“contact boiling”), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back (“gentle film boiling”), or both forms the Leidenfrost layer and ejects tiny droplets upward (“spraying film boiling”). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading $\gamma$ of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number We ($\gamma \sim {\mathrm{We}}^{2/5}$), which is much steeper than for the impact on nonheated (hydrophilic or hydrophobic) surfaces ($\gamma \sim {\mathrm{We}}^{1/4}$). We also interferometrically measure the vapor thickness under the droplet.

Figure 1

Schematic of the experimental setup (not drawn to scale) used to study droplet impact on heated surfaces. A liquid droplet of initial diameter $D_0\approx 2\mathrm{mm}$ falls on a heated plate P and spreads to its maximum diameter $D_m$. The plate P is a polished silicon plate (average roughness $\approx 5\mathrm{nm}$) in most experiments. In the case that a bottom view is needed, a polished sapphire plate is used instead of the silicon one. The plate is placed on a holder H which can be heated by six cartridge heaters embedded symmetrically inside. The heaters are monitored by a controller via a solid-state switch (Omega, Inc.). The maximum temperature that can be obtained with this system is $700°\mathrm{C}$, with accuracy within 1 K. The holder has a 2 cm-diameter hole in the center allowing bottom-view observation. The side view of the impact is recorded by camera S (Photron SA1), and the bottom view is recorded by camera B (Photron SA2) connected to a long working distance microscope C via a mirror M.

Figure 2

Series of images of representative water droplet impacts in three regimes. The Weber number in all three experiments is 32. Each image has both bottom view and side view of the impact. Images in the same column are taken at the same time after impact. (a) The surface temperature $T=380°\mathrm{C}$; contact boiling. The first sign of droplet ejection is seen at 0.6 ms after impact. (b) $T=500°\mathrm{C}$; gentle film boiling. (c) $T=580°\mathrm{C}$; spraying film boiling. The contrast of the side-view images was enhanced to show tiny droplets ejected upward at 1.9 ms. The inset bar (shown in upper left image) indicates a length scale of 2.5 mm.

Figure 3

Phase diagram for water droplet impact on a heated surface showing three separate regions: contact boiling regime (red solid diamonds), gentle film boiling regime (blue solid circles), spraying film boiling regime (green solid squares). Each region has an inset illustrating the typical droplet impact behavior in that regime. The three large symbols represent the experiments shown in Fig. 2. The dashed lines between different regimes are drawn to guide the eye.

Figure 4

Log-log plot of the maximum spreading diameter normalized by the drop’s diameter ($D_m/D_0$) versus the Weber number (We) for impact in both gentle and spraying film boiling regimes. Experimental data for water drops spreading on superhydrophobic surfaces at room temperature by Tsai et al. [15] (open downward triangles), ethanol in the gentle film boiling regime by Chaves et al. [16] (solid right triangles), FC-72 in the gentle film boiling regime (solid left triangles), water in the gentle film boiling regime (solid circles), water in the spraying boiling regime (solid squares). The Weber number $0.5\le \mathrm{We}\le 600$. The surface temperature $250°\mathrm{C}\le T\le 560°\mathrm{C}$. The solid line represents the best fit for the experimental data for $\mathrm{We}>10$ in the present study with the slope 0.39. The dashed line represents the scaling $D_m/D_0\sim {\mathrm{We}}^{1/2}$ resulting from the balance between the drop’s initial kinetic energy and its surface energy at maximum deformation. The dash-dotted line represents the scaling $D_m/D_0\sim {\mathrm{We}}^{1/4}$ resulting from a momentum argument [17].

Figure 5

(a) Interference pattern showing thickness variation of the vapor layer during impact of a droplet (the image was taken 0.3 ms after the camera detects the droplet). The image was taken from the bottom view of droplet impact using a color high-speed camera connected to a long working distance microscope at 10 000 frames per second. The Weber number is 3.5 and the surface temperature is $350°\mathrm{C}$. The inset bar indicates 0.2 mm. (b) Profile of the vapor thickness extracted from the color image.