In this work filtration efficiency of commercially available filter media with fiber/pore sizes of the scale of 10 pm is dramatically increased by not only adding electrospun nanofibers, as is usually done, but also a layer of ultrafine supersonically blown 20-50 nm nanofibers. Three different commercially available base filters were modified with (i) electrospun nanofibers alone, (ii) solution-blown 20-50 nm nanofibers alone, and (iii) the dual coating with electrospun nanofibers deposited first and the solution-blown 20-50 nm nanolibers deposited on top of them Detailed observations of nanoparticle removal by these base and the above-mentioned modified filters revealed that the filters with dual electrospun nanolibers (deposited first) and the solution-blown 20-50 nm nanofibers deposited on top of them are the most effective in removing the below-200 nm Cu nanoparticles/clusters from aqueous suspensions. Experiments were conducted in two different time ranges: (a) for 8-15 s, and (b) for 8 min. It was found that the efficiency of the dual coated filters containing 20-50 nm fibers was significantly higher than those of the others at the lowest nanoparticle concentrations of 02-0.5 ppm in suspension. The experiments conducted for longer time revealed that the smallest nanofibers were as efficient in particle retention as in the shorter time experiments, and there was no visible breakage pattern of these nanofibers. The theory developed in the present work explains and describes how the smallest solution blown nanofibers introduce a novel physical mechanism of nanoparticle interception (the attractive van der Wools forces) and become significantly more efficient collectors compared to the larger electrospun nanofibers. The theory predicts the domain of nanoparticle collection due to the van der Waals forces. The theory also elucidates the morphology of the nanoparticle clusters being accumulated at the smallest nanofiber surfaces, including the clusters growing at the windward side, or in some cases also on the leeward side of a nanofiber. (C) 2015 Elsevier B.V. All rights reserved.