## 2 Materials and Methods
### Experimental Stand  
The experiments were carried out in a laboratory setup comprised of a small-scale vertical rectangular channel with a nanofibrous filter inside, seed particle generator, differential pressure gauge, gas flow meter, outlet suction fan, and PIV measuring system. Figure 4: SEM micrographs of metal mesh scaffold with PVDF nanofibrous filter for two magnifications. Electrospinning time of 10 min: magnification (**a**) 100\(\times\), (**b**) 10,000\(\times\). Fiber diameter distribution (**c**); b/w conversion of SEM image of the nanofibrous filter (**d**). A schematic of this setup is shown in Figure 5. A photograph of the experimental stand with a PIV system and CCD camera, and a nanofibrous filter channel mounted on a tripod is shown in Figure 6. The system was designed to study the distribution of the flow field velocity downstream of a nonwoven nanofibrous filter by the PIV method. The metal mesh with the deposited nanofibrous filter on its upstream side was fixed in the middle of the channel length, and the aerosol flowed from the bottom of the channel upwards. Figure 5: Schematic of the laboratory setup used to study the distribution of flow velocity field through nanofibrous filter by the PIV method. Figure 6: A photograph of PIV experimental stand for flow field measurement: (**a**) the laser, CCD camera, and nanofibrous filter channel mounted in the frame; (**b**) the photograph taken during the PIV measurements. The channel was made of an acrylic plate with 10 mm thickness. An optical glass window of 0.4 mm thickness was mounted in one of the side walls of the channel in order to record the seed particles' motion with the PIV camera. A Nikon FlowSense M2 CCD camera equipped with a macro-photography lens was used for the recording of the seed particle trajectories. The channel was of an inner square cross section of 15 \(\times\) 15 mm. A scaffold with a nanofibrous filter was placed in the middle of the channel length. The seed particles were injected at the inlet of the channel and drawn upwards through the channel by an outlet suction fan. The seed particles were produced by spraying Diethylhexyl-sebacate (DEHS) using an atomizer aerosol generator ATM 226 (TOPAS GmbH, Dresden, Germany). This aerosol generator complies with the VDI 3491 Standards (VDI Germany 2016) [92]. The size of the particles was measured by a laser aerosol particle size spectrometer LAP 322 (TOPAS GmbH, Dresden, Germany). Figure 7 shows a size distribution of DEHS seed particles used for PIV measurements. The mean size of these particles was about 1 \(\upmu\)m. The air flow rate was measured using flowmeter RTV-10-300 (ROTAMETR, Gliwice, Poland). The pressure drop through the nanofibrous filter was measured by the pressure gauge TESTO 512 (TESTO, Titisee-Neustadt, Germany). The measurements were carried out at ambient temperature and humidity. The Reynolds number for a DEHS particle in a flow is as follows:  
\[Re_{p}=\rho u_{0}d_{p}/\eta \tag{2}\]  
in which \(\rho\) is the air density, \(u_{0}\) is the average air velocity in the channel, \(d_{p}\) is the diameter of a particle, and \(\eta\) is the air dynamic viscosity. In normal conditions, for a particle with a diameter \(d_{p}\) = 1 \(\upmu\)m and air velocity \(u_{0}\) = 1 m/s, the Reynolds number for the particle is \(Re_{p}\) = 0.071. The Stokes number for a particle approaching a single fiber is  
\[St=\tau u_{0}/d_{f} \tag{3}\]  
Figure 7: Size distribution of seed particles produced by ATM 226 atomizer aerosol generator (TOPAS) from DEHS. where \(\tau\) is the characteristic relaxation time of the motion of a particle, \(u_{0}\) is the average air velocity in the channel, and \(d_{f}\) is the characteristic dimension of the obstacle (diameter of a fiber). The characteristic time for the particle can be determined from the following relation:  
\[\tau={\rho_{p}{d_{p}}^{2}}/{18\eta} \tag{4}\]  
where \(\rho_{p}\) is the particle density, \(d_{p}\) is the diameter of a particle, and \(\eta\) is the air dynamic viscosity.