Abstract
Fabrication of air filtration materials (AFM) that allow air to easily flow through while retaining particles is a significant and urgent need due to the harmful airborne particulate matter pollution; however, this is still a challenging research area. Herein, we report novel slip-effect functional nanofibrous membranes with decreased air resistance (reduction rate of 40%) due to the slip flow of air molecules on the periphery of nanofibers. This was achieved through careful control over the diameters of electrospun polyacrylonitrile fibers and aperture size of fiber assembly. Fiber assembly with 86% of fiber diameters between 60–100 nm was found to be most effective for slip flow, as these diameters are close to the mean free path of air molecules (65.3 nm). Significantly, an equilibrium factor τ = df/d2 has been introduced to elucidate the effect of distance of adjacent fibers on the drag force of airflow. Furthermore, the most effective aperture size (>3.5 μm) for slip-effect has been determined. Ultimately, the new material displayed low air resistance of 29.5 Pa, high purification efficiency of 99.09%, good transmittance of 77%, and long service life. The successful fabrication of such materials can facilitate the development of high-performance AFMs for various applications.
Highlights
Fabrication of air filtration materials (AFM) that allow air to flow through while retaining particles is a significant and urgent need due to the harmful airborne particulate matter pollution; this is still a challenging research area
We designed the fibrous AFMs based on the following three criteria: (1) the nanofiber-based AFMs must consist of uniformly distributed and bead-free nanofibers; (2) the diameter of nanofibers should be similar to the mean free path length of air molecules, which would help air molecules to bypass the nanofibers with maximum probability; (3) the nanofiber-based AFMs should have optimal aperture size to avoid weakening or elimination of the slip-effect induced by the interaction of airflow stream around the periphery of adjacent fibers
The pristine PAN fibers displayed bead-on-string binary structures, comprised of slender fibers with diameters between 70–230 nm and microsized spindle shaped beads with an average size of 1.71 μm along the fiber length direction (Supplementary Fig. S1). The formation of this unique structure could be due to the use of a low viscosity polymer solution (Supplementary Fig. S2), which could disturb the balance between viscoelastic force, surface tension and electrostatic repulsion, inducing instability of the Taylor cone during electrospinning[32]
Summary
Fabrication of air filtration materials (AFM) that allow air to flow through while retaining particles is a significant and urgent need due to the harmful airborne particulate matter pollution; this is still a challenging research area. Fiber-based air filters have become a very feasible, efficient and promising technology to combat air pollution due to their reticular support structure and tortuous pore channels, which allow the effective passage of air molecules, while trapping the particulate matter[12,13] For this reason, various fiber-fabrication technologies have been developed to prepare high-performance fibrous materials for air filtration, such as melt-blown[14,15], needle-punched[16,17], and papermaking methods[18,19]. Conventional fibrous materials prepared by the above mentioned methods suffer from high air resistance caused by the direct impact of air molecules with the fibers, which results in the transition of airflow state due to the large diameter of fibers and dense structure of fiber assembly[20,21] These fibrous materials face the problem of low filtration efficiency due to large pore size[22]. When the fiber diameter is close to 65.3 nm, the velocity of airflow is non-zero on the surface of single fiber due to the slip flow phenomenon, which will result in significant reduction of drag force of the airstream[29]
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