Direct Electrospinning of Ultrafine Fibers with Interconnected Macropores Enabled by in Situ Mixing Microfluidics

Porous ultrafine fibers are of great importance to various applications. Herein, we report a method to directly fabricate macro-porous ultrafine fibers by an in situ mixing microfluidics which allows for the simultaneous electrospinning of solution immediately after mixing. The formation mechanism of macro-pores should be attributed to the incomplete mixing coupled with nonsolvent-induced phase separation, which was elucidated by systematical investigation of various solvent systems and mixing solvents. The diameter of the macro-porous fibers can be tuned from 1.80 ± 0.40 to 6.75 ± 0.48 μm by adjusting the solution concentration and the feeding rate of mixing solvent. The results indicated that macro-porous fibers exhibited higher specific surface area (48.66 ± 8.30 m² g^-1), larger pore size (116.73 nm) and pore volume (0.169 ± 0.007 cm³ g^-1) than conventional electrospun porous fibers, enabling the high oil absorption capacities of 95.68, 57.98, and 34.82 g g^-1 for silicon oil, motor oil, and peanut oil, respectively. Our method has greatly expanded the solution scope for electrospinning from stable solution systems to unstable or substable solution systems, thus providing intriguing opportunities for the investigation and fabrication of heterogeneous fibers by in situ mixing of various immiscible solvents/solutions. Our findings can serve as guidelines for the electrospinning of ultrafine fibers with interconnected macro-pores (>50 nm).