## 2 Materials and Methods  
PLA 6052D (\(\rho\) = 1.24 g/cm\({}^{3}\), the glass transition temperature is at about 58 \({}^{\circ}\)C, NatureWorks, Nebraska, USA), dichloromethane (DCM), and dimethyl formamide (DMF) (Tianjin Da Mao Chemical Reagent, Tianjin, China) were used as the fiber material and two solvents, respectively. All reagents are utilized without further purification. A plastic syringe (Foshan Lepton Precision Measurement & Control Technology, Foshan, Guangdong, China) with a stainless steel needle of different inner diameters (0.50, 0.67, 0.86, 1.26, and 1.60 mm respectively) was employed as the electrospinninging equipment. The samples were named F1, F2, F3, F4, and F5, respectively.  
An amount of PLA (10 wt.%) was dissolved in a mixture of DCM and DMF (9:1 weight ratio) at room temperature with stirring for 2 h to form the precursor solution. The solution was then put into a 20 mL plastic syringe. In order to form the PLA fibers with different diameters, metallic needles with inner diameters of 0.50, 0.67, 0.86, 1.26, and 1.60 mm were used. The high voltage applied between the needle tip and the collector was set to 18 kV, and the distance between them was 15 cm. The flow rate was fixed at 0.5 mL/h. A temperature of 25 \(\pm\) 2 \({}^{\circ}\)C and Relative humidity of 65 \(\pm\) 5% were used during the electrospinninging process. Afterwards, the porous fibers were annealed at 70 \({}^{\circ}\)C for over 12 h.  
To analyze the maximum oil absorption capacity using the procured fiber membranes, 10 g vacuum pump oil (dyed a green color) was poured into a beaker which contained 80 mL of water. A quantity of 0.5 g of the fiber was placed on the surface of the oil which was on the top of water. After 10 min, the wet absorbent was drained for 2 min until no residual oil droplets were left on the surface. Oil absorption capacities of all absorbents were determined by the following equation:  
\[\delta=\frac{\left[m_{f}-\left(m_{0}+m_{w}\right)\right]}{m_{0}} \tag{1}\]  
where \(\delta\) is the absorption capacity (g/g), \(m_{f}\) is the weight of the wet absorbent after 10 min of immersion, \(m_{0}\) is the initial weight of the absorbent, and \(m_{w}\) is the weight of adsorbed water (although the absorbent is placed in the up oil layer, practically, the water is usually absorbed at the same time. To exactly measure the absorbed oil and water weight, the porous fiber was imposed in pure water and pure oil respectively, and the absorbed weights were recorded). In pure oil medium without any water, \(m_{w}\) is equal to zero. To explore the selectivity of different oils, the same fibrous membranes were put into different types of oil (vacuum pump oil, peanut oil, and diesel oil) for absorption and to test their individual absorption capacity.  
The SEM images were taken using a scanning electron microscope (SEM, SU8220, Hitachi, Tokyo, Japan) and the diameters of fibers were measured using Image J software. Functional groups of the fibers were measured in absorbance mode by an Attenuated Total Reflection Flourier Transformed Infrared Spectrometer (ATR-FTIR, Nicolet IS50, Thermofisher, Massachusetts, USA). The wavenumber ranged from 400 to 4000 cm\({}^{-1}\). The contact angle was measured using a contact-angle system (JC2000D2, Shanghai Zhongchen Digital Technology Co. Ltd. Shanghai, China) at 25 \({}^{\circ}\)C, and the specific surface areas of five samples were measured using a Surface Area and Porosimetry Analyzer (BET, V-Sorb 2008B, Gold APP Instrument Corporation, Beijing, China).