Boiling regimes of impacting drops on a heated substrate under reduced pressure

We experimentally investigate the boiling behavior of impacting ethanol drops on a heated smooth sapphire substrate at pressures ranging from $P=0.13$ bar to atmospheric pressure. We employ frustrated total internal reflection imaging to study the wetting dynamics of the contact between the drop and the substrate. The spreading drop can be in full contact (contact boiling), it can partially touch (transition boiling), or the drop can be fully levitated (Leidenfrost boiling). We show that the temperature of the boundary between contact and transition boiling shows at most a weak dependence on the impact velocity, but a significant decrease with decreasing ambient gas pressure. A striking correspondence is found between the temperature of this boundary and the static Leidenfrost temperature for all pressures. We therefore conclude that both phenomena share the same mechanism and are dominated by the dynamics taking place at the contact line. On the other hand, the boundary between transition boiling and Leidenfrost boiling, i.e., the dynamic Leidenfrost temperature, increases for increasing impact velocity for all ambient gas pressures. Moreover, the dynamic Leidenfrost temperature coincides for pressures between $P=0.13$ and $0.54$ bar, whereas for atmospheric pressure the dynamic Leidenfrost temperature is slightly elevated. This indicates that the dynamic Leidenfrost temperature is at most weakly dependent on the enhanced evaporation by the lower saturation temperature of the liquid.

Figure 1

The FTIR images of ethanol drops at $t=0.21\mathrm{ms}$ after impacting a sapphire prism with impact velocity $U=1.0\mathrm{m}/\text{s}$ at atmospheric pressure. (a) For a substrate temperature of $T_{\mathrm{sur}}=138{}^{\circ }\mathrm{C}$ the spreading drop fully wets the surface and is in contact boiling. (b) At $181{}^{\circ }\mathrm{C}$ the drop partially wets the surface and is identified as transition boiling. (c) At $199{}^{\circ }\mathrm{C}$ the drop is only visible in gray values: It is fully levitated and hence in the Leidenfrost boiling state. The blue circles correspond to the radius of the wetted area of a drop in the contact boiling state.

Figure 2

Phase diagram for ethanol drops impacting on sapphire substrate at atmospheric gas pressure. The blue diamonds correspond to contact boiling, yellow triangles to transition boiling, and red squares to the Leidenfrost state. The gray crosses indicate measurements for which the boiling state is hard to identify. The red dotted line is a guide to the eye for the TB-LF boundary, the dynamic Leidenfrost temperature. The horizontal black line indicates the static Leidenfrost temperature on a silicon substrate for comparison.

Figure 3

Schematic representation of the setup used in our experiments. In addition to the side-view camera, we study the wetting of the drop by a bottom-view camera, using frustrated total internal reflection imaging. The sapphire prism was heated to set the desired surface temperature $T_{\mathrm{sur}}$.

Figure 4

Static Leidenfrost temperature $T_{\mathrm{L}}$ of ethanol drops on silicon changes with ambient pressure, as shown by red circles. Following Orejon et al. [8], a logarithmic function (2) is fitted through these points, with $A=0.0069\left(5\right)$ and $B=0.004\left(1\right)$. The dependence of the saturation temperature on the pressure of ethanol is shown as the blue solid line.

Figure 5

Comparison of ethanol drops impacting a heated sapphire prism under an ambient pressure of 0.29 and $1\mathrm{bar}$ (in the left and the right half of every panel, respectively). In addition to the side-view observations, the FTIR views are displayed, revealing a clear distinction between wet areas (black) and air-vapor regions (white). The spreading radius as found by the side-view observations is included in red as a reference, revealing the changes in boiling state with increasing temperature and reduction in pressure. The dashed line for the $T_{\mathrm{sur}}=159{}^{\circ }\mathrm{C}$ case indicates the field of view for this particular measurement.

Figure 6

The CB-TB boundaries for different pressures and impact velocities are shown in yellow and the dynamic Leidenfrost temperatures are shown in red. Both plots show the same data, but they are represented in a different way to show different effects more clearly. The boundaries are plotted in the top panel as a function of impact velocity, while in bottom panel the boundaries are plotted as a function of ambient pressure. It can be seen that the CB-TB boundary is almost independent of impact velocity and is in close agreement with $T_{\mathrm{L},\mathrm{static}}\left(P\right)$ indicated with the black dotted line in the bottom panel. The TB-LF boundary increases for most measurements with impact velocity and changes slightly with pressure.