## 1 Introduction  
Accidental oil spillage has become a serious public problem as it has a severe negatively impact on aquatic safety and human health [1, 2]. Conventional techniques for oil spill cleanup mainly include in situ burning, chemical degradation [3, 4] and membrane filtration [5, 6]. However, burning or degradation usually causes potentially harmful residues, whereas the filtration method requires the collection of oil/water mixtures into the filtration setup by extra facilities such as pumps and containers. These methods are usually inconvenient for engineering applications. Thus, it is of great significance to develop a facile and sustainable approach for oil/water separation to tackle the oil pollution challenge. Cleaning oil by absorption [7, 8] is a promising method without secondary pollution or the need for extra facilities, which is favorable for practical applications. The used solvents are generally required to repel water but absorb oil (i.e., superhydrophobic/superoleophilic) [9] to accomplish efficient oil/water separation. Besides this, they should also possess characteristics of high absorption capacity and upscalable fabrication [10]. Researchers first investigated such materials from nature due to their potential benefits of low cost and large amount. Oil-absorbing materials including straw soot [11], carbon oxide [12], fly ash [13], and zeolites [14] have been reported. Nonetheless, shortcomings of low absorption capability and environmental incompatibility of these materials impede their upscaled industrial use [13]. Currently, superhydrophobic or superoleophilicsorbents are commonly artificially synthesized by modifying the chemical composition of materials with low surface energy or constructing a rough topography. According to the type of substrate, they are classified into fiber materials [15], aerogel materials [16], resin materials [17], etc. In most cases, these technical materials are fabricated as polymeric sponges [18; 19; 20], specially coated metal meshes [21; 22; 23], or metal-coated fabrics [5] by various chemical methods. The chemical substances used in their production [18] and disposal [21], however, might also cause toxic pollution. Alternatively, topographical structures, due to their easy fabrication and being environmental benign, producing no chemical residues, have gained more and more attention for oil/water separation applications. The most well-known example of a topographically structured oil/water separator from nature is the water-repellent leaves of _Salvinia molesta_[24; 25], which are covered by numerous trichome-like microstructures. Inspired by plants or animals from nature, many artificial structures have been invented and embedded in technical surfaces [8; 9] to achieve oil/water separation. Previous publications have already reported outstanding oil/water separation performance on artificial surfaces with microstructures, including nanofur [26; 27], nanofibers [28; 29], or microparticles/powders [30; 31]. However, the influence of structural dimensions such as size, spacing, or height/diameter ratio on selective wetting properties was rarely studied. In addition, most research focused on improving surface superhydrophobicity by topographical structures but ignored the function of cross-sectional architectures for enhancing oil absorption. Porous microstructures similar to sponges at the cross-section will very likely benefit oil absorbing volume and oil retention ability. Combining both surface structures and cross-sectional architecture might offer a new solution for highly efficient oil/water separation. Also, the dimension of these microstructures needs to be optimized to improve the oil/water separation ability. In this paper, we developed a novel polyurethane (PU) foil by imprinting micropillar arrays on the surface and fabricating microporous structures at the cross-section for efficient oil/water separation.