## 3 Results and Discussion
### Oil/water Separation
As displayed in Figure 9a, both foils reached their full absorption within some seconds, while the absorbed maximum oil volume significantly differed. Although taking a longer absorption time, our structured foil demonstrated a dramatically higher absorption capacity than the unstructured foil of the same material. To carry out a quantitative comparison by a single parameter, the absorption efficiency  
Figure 8: Oil absorption capacity plotted as a function of (**a**) pore size and (**b**) porosity at the cross-section of the structured PU foil. On all examined samples, a micropillar array of \(s=5\)\(\upmu\)m and \(AR=1\) was fabricated on the surface. Each data point in the figure represents the measured absorption capacity of a newly fabricated, single-use foil. The foaming pressure for these foils ranged from 16 MPa to 20 MPa. of both foils was calculated. The results revealed 15.7% per second for the structured foil but only 4.4% per second for the unstructured sample. This indicated a nearly 4 times improvement of oil absorption efficiency owing to the introduction of micropillar and microporous structures to the PU foil. Moreover, the oil retention time within both foils was also evaluated and compared (Figure 9b). As both foils reached maximum absorption within 30 s (see Figure 9a), the starting time for this long-term retention evaluation was set to 30 s. After that, the absorbed oil was constantly retained within the structured PU foil in the subsequent 7 days, demonstrating a stabilized absorption capacity of above 400%. However, the oil absorption capacity of the unstructured foil gradually reduced from 42% to 12% over the same time period. This was probably due to oil loss from the surface of the flat foil, where oil residuals were temporarily adhered but not completely absorbed in the inner foil during absorption. Oil detached from the foil due to gravity over time, leading to lower oil retention by the foil. Consequently, it can be concluded that our developed micropillar and microporous PU foil showed superior oil absorption efficiency and oil retention ability if compared to the unstructured sample, making it an ideal element for oil/water separation applications. Moreover, our method was facile and environmentally friendly, showing great compatibility for other thermoplastic polymers. Nonetheless, dedicated experimental conditions (e.g., temperature or pressure) should be further studied. In terms of practical applications for industrial use, foil upscaling might be achieved by a roll-to-roll fabricating process [27], which allows the fabrication of large-sized polymeric foils with microstructures on the surface. Figure 9: Comparison of oil absorbing performance between the structured and unstructured PU foils. (**a**) Absorption capacity and time. (**b**) Absorption capacity as a function of oil retention time within both foils. The time axis is plotted as a logarithmic scale in (**b**) for easy illustration. The structured foil possessed a micropillar array of \(s=5\)\(\upmu\)m and \(AR=1\) on the surface. The mean pore size at the cross-section was characterized as around 130 \(\upmu\)m. Both foils were stored under ambient conditions (25 \({}^{\circ}\)C, in air) during the long-term retention experiment. Each experiment was conducted with a new, single-use foil.