Recently a new technique has been developed in which free-standing nanomats are formed by gas-phase neutralization of charged nanofibers with a cloud of charged small counter-ions. Here we report unusual optical and filtering properties of nonwoven free standing nanomats (FSN) manufactured by this technique from nylon-4,6. Almost invisible (>95% light transmission), extremely lightweight (10-20 mg/m2) FSN comprising of nanofibers with a diameter (3-15 nm) well below the mean free path of air molecules make the most efficient filtering material among the filtering fabrics reported thus far. They combine low resistance to airflow with low penetration of sub-micron aerosol particles. Furthermore, FSN are capable of selecting particles by size because of nearly calibrated pores, cutting off penetration of airborne and water-suspended particles when their size exceeds a threshold level. Using self-developed software, we studied statistics of pores distribution and showed that the fraction of large holes in FSN is greatly reduced as compared to that in purely random simulated nanomats and in real nanomats formed by deposition of electrospun nanofibers onto a solid substrate. Such unusual distribution of holes is explained by a preferential landing of charged nanofibers onto the largest holes because electrical charges on the free standing nanomat are not shielded by the mirror charges on electrode used in the conventional electrospinning technique. Particulate matter and microbial cells collected from air and from water may be directly visualized on the filters by electron and optical microscopy in the bright-field and fluorescent modes.
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