## 3 Results and Discussion
### Selective Wetting Property  
Water and oil contact angles on the fabricated PU foil were characterized to examine its hydrophobic and oleophilic properties. Considering that micropillars might fracture or even totally detach from the foil surface (although in a small area) in upscaled applications, a superhydrophobic and superoleophilic foil cross-section is also important for oil/water separation. Thus, the results of the micropillar surface and microporous cross-section are summarized and compared to a pristine reference (i.e., flat bulk sample). Figure 4a,b show the water and oil droplets on the flat PU surface. Corresponding results of the micropillar surface and microporous cross-section of a structured foil are revealed in Figure 4c-f, respectively. Apparently, all of the samples showed a selective wetting property, i.e., a comparably large water contact angle while a relatively small oil contact angle. Nevertheless, it is interesting to note that for micropillar and microporous structures, the disparity between the water and oil contact angles further widened if compared to the flat foil. More specifically, the water contact angle significantly increased from 110\({}^{\circ}\) to around 150\({}^{\circ}\), whereas the oil contact angle evidently declined from 52\({}^{\circ}\) to about 20\({}^{\circ}\) after embedding microstructures. The surface roughness of flat foil might also cause potential variations in contact angle, which was observed to be very small in our experiments (please see the error bars in Figure 5). However, this effect did not influence the overall trend of our results, i.e., microstructures triggered selective wetting. Thus, we conclude that topographical micropillar arrays and cross-sectional micropores could immensely enhance the hydrophobicity and oleophilicity (selective wetting property) of PU foil. The corresponding contact angle values also indicated the fascinating superhydrophobicity and superoleophilicity of our developed PU foil, which implied superior oil/water separating performance. Figure 3: Pore size distribution at the cross-section for three different PU foils. A similar Gaussian distribution pattern was observed for all samples, whereas the average pore size differed. The average value \(\mu\) and its corresponding standard deviation \(\sigma\) of each dataset are shown in the legend. The dashed curve is the corresponding fitting trend to each dataset. The foaming pressures for samples showing pore sizes from small to large were 20 MPa, 18 MPa, and 16 MPa, respectively. The foaming temperature was 60 \({}^{\circ}\)C.  
The dimensions of microstructures might affect the superhydrophobicity and superoleophilicity of the developed foil. Hence, the water and oil contact angles were further analyzed on samples with different pillar spacings, height/diameter ratios, or pore sizes. First, foils with pillar spacings of 5 \(\upmu\)m, 10 \(\upmu\)m, and 15 \(\upmu\)m were tested and the results were compared to a flat surface (see Figure 5a). Consistent with Figure 4, all of the micropillar surfaces displayed higher water contact angles but lower oil contact angles than the flat reference. Among them, the sample with 5 \(\upmu\)m pillar spacing demonstrated the most excellent superhydrophobicity and superoleophilicity. In terms of samples with 10 \(\upmu\)m and 15 \(\upmu\)m spacing, the selective wetting effect was slightly weakened compared to the foil with 5 \(\upmu\)m spacing, i.e., the difference between water and oil contact angles was reduced. Moreover, the results for these two samples (10 \(\upmu\)m and 15 \(\upmu\)m spacing) were similar. Pillar spacing smaller than 5 \(\upmu\)m might have led to a more apparent selective wetting effect. Nonetheless, such a dimension usually requires high-precision imprinting mold and also has the risk of pillar fracture or buckling during demolding. Thus, to achieve a robust selective wetting ability for oil/water separation, the subsequent PU foils were all fabricated with micropillars with 5 \(\upmu\)m spacing.