## 3 Results and Discussion
### Oil/water Separation  
The fabricated structured PU foil was applied to remove the oil spill from water. Figure 7 shows the time-lapse of oil absorption by the foil. Once the foil contacted the oil, the oil spill (dyed in red) was quickly and completely absorbed within 10 s from water (dyed in blue), which was thereafter dabbed away from the water surface. This timescale was comparable to the results in references [11; 24]. Although the experimental conditions (e.g., oil type or amount) employed by us and in the references were not exactly the same, this outcome proved the excellent oil/water separation ability of our developed PU foil. To further investigate its maximum absorption capacity, the foil was subsequently immersed into a large amount of oil spill to achieve full absorption. After nearly 20 s, a maximum absorption volume was reached (nearly 400% weight of the original foil), indicating a high absorption efficiency. More specifically, the absorption capacity of the structured PU foil was quantitatively evaluated and plotted as a function of the average pore size at the cross-section (Figure 8a). The corresponding result of a flat, bulk foil is compared. With no pores at the cross-section, the oil absorption capacity of pristine PU foil was only less than 20%. Upon the introduction of porous microstructures, however, the oil absorption capacity significantly rose to several hundred percent, depending on the pore size. Larger pores usually allow higher oil absorption capacity. This outcome confirmed that cross-sectional porous microstructures  
Figure 6: Water contact angle on the porous cross-section of PU foil with different pore sizes. The result of a bulk sample was plotted as a comparison. Each bar in the figure represents an average value from three arbitrary measurements. The error bar is the corresponding statistical error. Figure 7: Time-lapse of the developed PU foil absorbing oil (in red) in water (in blue). The oil spill was completely removed within 10 s. A micropillar array of \(s\) = 5 \(\upmu\)m and \(AR\) = 1 was fabricated on the foil surface. The average pore size at the foil cross-section of the specific sample used in this experiment was around 124.6 \(\upmu\)m.  
mainly functioned as numerous tiny oil containers to enhance the oil absorption capacity. According to nucleation theory proposed by [37], lower foaming pressures allow increasing pore size but reduce the pore density, which also affects oil absorption capacity. Thus, porosity was calculated as the ratio of total pore volume to the foamed PU volume, which was dominated by both pore size and pore density. The impact of porosity on the oil absorption ability was analyzed (see Figure 8b). A similar increasing trend with pore size was observed. This was reasonable because high porosity is supposed to offer more containers for oil storage, which will lead to more oil absorption volume. In terms of our fabrication method, the pore size and porosity could be tailored by adjusting the saturation pressure or temperature during the supercritical CO2 foaming process. Nevertheless, both parameters could not be unlimitedly increased due to the actual implemental restraints of experimental conditions and the thickness of untreated foil. The largest absorption capacity of around 500% was accomplished by a structured foil with an average pore size of about 130 \(\upmu\)m and porosity of nearly 90%. Such performance was superior to the results of typical mesh sorbents [22; 23], but not as profitable as polymeric sponges [7; 18]. It is worth mentioning that such a comparison is extremely rough because the absorption capacity calculated from the mass was significantly influenced by the original weight of the sorbent (e.g., lighter sorbents are more likely to obtain higher absorption capacity). Nonetheless, our results demonstrate the excellent oil-absorbing capability of the developed PU foil. Finally, the oil absorbing performance (i.e., rate and capacity) of the structured PU foil was compared to a flat, bulk reference.