Inverse-Leidenfrost phenomenon on nanofiber mats on hot surfaces

The Leidenfrost effect is a technically and industrially important phenomenon that severely restricts heat removal from high-heat-flux surfaces. A simple remedy to the Leidenfrost effect is provided by polymer nanofiber mats created and deposited by electrospinning on stainless steel surfaces. The influence of nanofiber mats on hydrodynamics and cooling efficiency of single drop impact onto hot surfaces has been investigated experimentally. The evolution of the drops has been recorded by a high-speed complimentary metal-oxide semiconductor camera, whereas the cooling temperature was measured by a thermocouple. A remarkable phenomenon was discovered: a mat of polymer nanofibers electrospun onto a heater surface can completely suppress the Leidenfrost effect, thereby increasing the rate of heat removal from the surface to the liquid drops significantly. The “inverse-Leidenfrost” effect is described qualitatively and quantitatively, providing clear physical reasons for the observed behavior.

Figure 1

Leidenfrost effect: water droplet levitating on a vapor pillow above a very hot surface.

Figure 2

Experimental setup for drop impact onto nanofiber mats.

Figure 3

Water drop impact on a bare steel foil at different initial foil temperatures. The initial foil temperature $T_{\mathrm{foil},\mathrm{init}}$ is equal to (a) 60°C, (b) 220°C, and (c) 300°C.

Figure 4

Ethanol drop impact on a bare steel foil at different initial temperatures. The initial foil temperature $T_{\mathrm{foil},\mathrm{init}}$ is equal to (a) 60°C, (b) 180°C, and (c) 300°C.

Figure 5

Water drop impact onto PAN+CB nanofiber mat with thickness $h$ $=$ 0.5 mm at different initial temperatures of the underlying stainless steel foil. The initial foil temperature $T_{\mathrm{foil},\mathrm{init}}$ is equal to (a) 60°C, (b) 220°C, and (c) 300°C.

Figure 6

Ethanol drop impact onto PAN+CB nanofiber mat with thickness $h$ $=$ 0.5 mm at different initial temperatures of the underlying foil. The initial foil temperature $T_{\mathrm{foil},\mathrm{init}}$ is equal to (a) 60°C, (b) 180°C, and (c) 300°C.

Figure 7

Evolution of foil temperature under the drop impact point, $T_{\mathrm{A}}$, for water drop impact. The initial foil temperature $T_{\mathrm{foil},\mathrm{init}}$ is equal to (a) 220°C and (b) 300°C.

Figure 8

Evolution of foil temperature under the drop impact point, $T_{\mathrm{A}}$, for the ethanol drop impact. The initial foil temperature $T_{\mathrm{foil},\mathrm{init}}$ is equal to (a) 140°C, (b) 180°C, (c) 220°C, and (d) 300°C.

Figure 9

Minimum foil temperature after drop impact as a function of the initial foil temperature for (a) water and (b) ethanol.

Figure 10

Drop evaporation time on nanofiber-coated foils compared to the one on bare foils for (a) water and (b) ethanol.

Figure 11

Water drop impact on a PAN+CB nanofiber mat with thickness $h$ $=$ 0.15 mm. The initial foil temperature is equal to (a) 80°C and (b) 180°C.

Figure 12

Water drop impact onto a damaged PAN nanofiber mat with thickness $h$ $=$ 1.5 mm. The initial foil temperature is equal to 220°.

Figure 13

Water drop impact onto a damaged PAN nanofiber mat with a thickness $h$ $=$ 0.25 mm. The initial foil temperature is equal to 220°C. This sequence of images illustrates the formation of a geyser.