An Electrospun Polystyrene Fibrous Membrane with a Mechanically Reduced Pore Size Exhibits Efficient Gravity‐Driven Separation of a Water‐in‐Oil Emulsion

Abstract Electrospun fibrous membranes are widely used for water‐oil separation. However, achieving effective separation often requires post‐processing to enhance membrane wettability or modify pore size, which can reduce productivity. In this study, a simple method is proposed for pore size control via mechanical compression of electrospun membranes. Polystyrene (PS) fibrous membranes (PFMs) are fabricated through electrospinning and subsequently compressed using a hand tool, reducing the membrane thickness by ≈87%. The proportion of pores smaller than 10 µm in diameter increase from 4.8% before compression to a maximum of 45.6% after compression. While the uncompressed membrane allowes sub‐10‐µm water droplets to pass through, the compressed membrane effectively blocked them. Due to the oleophilic nature of PS, oil permeation occurres rapidly via capillary action, while water droplets are retained within the membrane's internal pores, facilitating continuous demulsification. By combining electrospinning with mechanical compression, the optimized compressed PS fibrous membrane (CPFM) successfully separated water‐in‐oil emulsions under gravity, achieving a separation flux of 606 L m −2 h −1 with an oil purity of over 99.85%. This approach provides a simple, cost‐effective, and highly efficient method for water‐in‐oil emulsion separation.