Nonisothermal drop impact and evaporation on polymer nanofiber mats

The work describes the experimental and theoretical investigation of water drop impact onto electrospun polymer nanofiber mats deposited on heated stainless-steel foils. The measurements encompass water spreading over and inside the mat, as well as the corresponding thermal field. The results show that the presence of polymer nanofiber mats prevents receding motion of drops after their complete spreading and promotes the moisture spreading inside the mat over a large area of the heater, which facilitates a tenfold enhancement of heat removal as the latent heat of drop evaporation.

Figure 1

Scanning electron microscope images of PAN nanofibers at various magnifications. Scale bars are 500 μm, 20 μm and 2 μm, respectively from left to right.

Figure 2

Scanning electron microscope image of a PAN fiber containing carbon black nanoparticles. Scale bar is 1 μm.

Figure 3

Experimental setup for drop impact onto nanofiber mats.

Figure 4

Drop impact on a bare steel foil and on a nanofiber mat at a foil temperature of 60 °C. Top: bare steel; bottom: steel foil covered with a PAN nanofiber mat (mat thickness $h$ $=$ 1.05 mm).

Figure 5

Drop evaporation on a steel foil and on a nanofiber mat: (a.1)–(a.5) bare steel; (b.1)–(b.5) steel covered with a PAN nanofiber mat (mat thickness $h$ $=$ 1.05 mm). Scale bar is 10 mm.

Figure 6

Comparison of diameters of the wetted area evaluated using the high-speed (HS) and infrared (IR) images. The diameters $d_r$, $d_{\mathrm{out}}$, and $d_i$ are defined in Fig. 5(b.3).

Figure 7

Evolution of the cooled area (divided by π/4) in time at an initial foil temperature of 60 °C. (a) $d^2$ vs time. (b) The derivative $d$($d^2$)/$\mathit{dt}$ vs time. (c) The initial interval of $d$($d^2$)/$\mathit{dt}$ zoomed in.

Figure 8

Spreading diameter vs time for different initial foil temperatures. (a) PAN nanofiber mat with a thickness of $h$ $=$ 0.25 mm; (b) PAN with $h$ $=$ 1.05 mm.

Figure 9

Reduction of the relative maximum spreading diameter with increasing initial foil temperature.

Figure 10

(a) Maximum spreading diameter and (b) the corresponding evaporation time vs nanofiber mat thickness.

Figure 11

Spreading diameter vs time for different impact heights. (a) PAN $+$ CB nanofiber mat with thickness $h$ $=$ 0.15 mm; (b) PAN with $h$ $=$ 1.00 mm.

Figure 12

Foil temperature under the drop impact point, $T_A$, for different nanofiber mats and different initial temperatures. (a) The initial foil temperature $T_{\mathrm{foil},\mathrm{init}}=60\hspace{0.5em}{}^{°}\mathrm{C}$; (b) $T_{\mathrm{foil},\mathrm{init}}=100\hspace{0.5em}{}^{°}\mathrm{C}$; (c) $T_{\mathrm{foil},\mathrm{init}}=220\hspace{0.5em}{}^{°}\mathrm{C}$; (d) $T_{\mathrm{foil},\mathrm{init}}=260\hspace{0.5em}{}^{°}\mathrm{C}$.

Figure 13

Cooling efficiency of nanofiber mats as compared to the uncoated steel foil. (a) The minimum temperature and (b) the evaporation time vs the initial foil temperature.

Figure 14

Schematic of a liquid drop impact onto a single pore in the axisymmetric case.

Figure 15

Initial stage of cooling of a foil coated by the PAN $+$ CB nanofiber mat.

Figure 16

Measured scaled rate of temperature evolution of the foil (the heat flux at the foil surface) in the range $t_s<t<t_{\mathrm{drop},\mathrm{init}}$ in comparison with the theoretical predictions (16). (a) Different nanofiber mats at the initial temperature 60 °C. (b) PAN nanofiber mat of $h=0.25$ mm at different initial temperatures.