## 3 Results and Discussion
### Structural Properties  
SEM micrographs of the pillar topography and the porous cross-section of the fabricated PU samples are shown in Figure 2, taking the example of three different pillar spacings (s = 5 \(\upmu\)m (a, compacted), 10 \(\upmu\)m (b), and 15 \(\upmu\)m (c, spacious)). The pillar diameter was kept constant as 5 \(\upmu\)m. The height of the pillars obtained was approximately 12.5 \(\upmu\)m, 3.75 \(\upmu\)m, and 5 \(\upmu\)m, thus the respective aspect ratios shown here amounted to (a) 2.5, (b) 0.75, and (c) 1, respectively. Although the pillar height was sometimes slightly larger than the hole depth on the silicon template due to the tension forces during demolding, in most cases, the pillar height approximated the hole depth. Samples with dedicated spacing and  
Figure 1: Schematic procedure of the developed approach for fabricating micropillar and microporous structures on PU foil. (**a**) Flat PU chip between two silicon templates. (**b**) The whole setup was put into a supercritical CO2 environment for saturation. (**c**) Pressure quenching and depressurization caused CO2 to nucleate into numerous tiny pores within the polymer. (**d**) Pore size grew (foaming) with CO2 diffusion. (**e**) The resulting PU foil showed a micropillar array on the surface and micropores at the cross-section. (**f**) SEM image indicating the free layer (about 25 \(\upmu\)m thickness) without pores below the pillars. (**g**) Post hole array in hexagonal lattice on the topography of the silicon template. (**h**) Specifications of micropillar array. (**i**) A photo of the fabricated PU foil. For clarity, the schematic foil in (**a–e**) is not to scale and the thickness decrease after nanoimprinting as well as the volume increase due to foaming is not shown. aspect ratio were fabricated and characterized before the oil/water separation experiments. Nonetheless, it is also worth mentioning that the pillars with too high aspect ratio would bend or buckle during demolding (e.g., Figure 2a), potentially leading to worse selective wetting performance. Figure 2d-f show the pore structure of the PU sample prepared under different foaming pressures of 20 MPa, 18 MPa, and 16 MPa, resulting in average pore sizes of about 50 \(\upmu\)m, 75 \(\upmu\)m, and 125 \(\upmu\)m, respectively. The SEM micrographs were taken in the central region of the cross-section. Reduced foaming pressure generally yielded increasing pore size within a reasonable pressure range, while the pore size could not be unlimitedly increased. This trend followed the homogeneous nucleation theory in [37], which indicated reduced pore size but higher pore density with increased foaming pressure. In our study, the obtained pore size was mainly in the range of 20 \(\upmu\)m to 150 \(\upmu\)m.  
Figure 3 summarizes the pore size distribution at the cross-section of three different foils (corresponding to Figure 2d-f, respectively), which were fabricated under stepwise reduced foaming pressure. One can observe that the pore size on each foil manifested a similar Gaussian distribution pattern. It seems that the CO2 foaming process generated regularly distributed micropores independent of the foaming conditions. However, the Gaussian peak for each sample was located at a distinct position. Interestingly, the width of the Gaussian distribution also varied in terms of individual foil. The average pore size and its corresponding deviation were calculated and are shown in the figure legend. The sample with a larger pore size generally showed a wider distributing pattern. As a result, the calculated average pore size for each sample differed. Such a difference was triggered by the different foaming pressure of each sample, i.e., high pressure reduced the pore size and improved their uniformity. Figure 2: SEM images of the topographical micropillar arrays as well as the cross-sectional microporous structures on the PU foil. Pillar spacings of **(a)** 5 \(\upmu\)m, **(b)** 10 \(\upmu\)m, and **(c)** 15 \(\upmu\)m were fabricated.