## 3 Results and Discussion  
The prepared Kapton film with thru-holes was tested by measuring their single-pass efficiency for filtration of dust-particle sizes ranging from 0.3 \(\upmu\)m to 10 \(\upmu\)m. After this, in order to investigate the performance of the prepared filters under realistic working conditions, a closed-chamber air cleaning system was used, which is shown in Figure 1g. The structure of the air cleaning system is simple but includes the necessary components (a filter and fan) to simulate commercial air purifiers (Figure 1h). A pyrethrum mosquito coil was burned in the chamber for 1 min to produce dust particles and the air condition was measured without the air cleaning system operating. After cleaning the chamber, another pyrethrum mosquito coil was burned in the chamber, and the PM density of the air with the air cleaning system was measured in order to allow for comparison of the two situations. A commercial PM sensor (Air Quality Monitor JSM-136, Shenzhen Aocome Tech. Co., Guangdong, China) was used to measure the PM2.5 density of the air, which is shown in Figure 1g. Figure 1i,j show the optical microscopy (OM) and scanning electron microscopy (SEM) images of the as-prepared Kapton air filter, respectively. The SEM (Quanta 250 FEG, ThermoFisher Scientific Co., Waltham, MA, U.S.A.) and energy-dispersive X-ray spectroscopy (EDS; Apollo X, AMETEK Materials Analysis Division Co., Mahwah, NJ, U.S.A.) measurements were conducted using an acceleration voltage of 20 kV and a working distance of 10 mm. Uniformly spaced pores with diameters of 15 \(\upmu\)m are seen in the Kapton film. There are two notable observations. First, the polymer material was finely etched and micro-sized pores were bored without any burning. Second, the pores were not just formed on the surface but penetrated the film with an excellent aspect ratio. PMs with various sizes were generated and passed through the filter once. The single-pass filtering efficiencies of the filter for different PM sizes (0.3-10.0 \(\upmu\)m) were measured and listed in Table 1.  
Although the diameter of the thru-holes is 15 \(\upmu\)m, the Kapton air filter captures PMs with a diameter that is smaller than 10 \(\upmu\)m while 0.3-0.5 \(\upmu\)m sized PMs were also captured with an initial filtering efficiency of 27.9%. This single-pass efficiency is high enough to indicate that the repeated circulation of air through the filter will be able to effectively remove such particles. One of the mechanisms of capturing PMs was electrostatic force. Furthermore, the results imply that a strong electrostatic force is present at the surface of the Kapton film, which attracts PMs that are smaller than the thru-holes due to the electrostatic force created by friction from air flow. The high electrostatic force results from the large inherent work function of Kapton of 4.36 eV [24] as the large difference between the work functions of two materials produces a high electrostatic field at their interface. The electrostatic field is inversely proportional to the distance between the materials and the micro-sized thru-holes enhance the field effectively. A SEM image of the Kapton filter after capturing the dust particles is shown in Figure 2a-d. In order to test the performance of the filter in a realistic environment, a pyrethrum mosquito coil was burned for 1 min in a closed chamber. After burning and removing the coil, the measured PM density with and without the air cleaning system (the Kapton filter and fan; Figure 1h) was measured. \begin{table}
\begin{tabular}{c c} \hline \hline
**Particle Size (\(\upmu\)m)** & **Initial Fractional Efficiency (\%)** \\ \hline
0.3–0.5 & 27.9 \\
0.5–1.0 & 45.2 \\
1.0–2.0 & 55.3 \\
2.0–5.0 & 64.6 \\
5.0–10.0 & 78.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The single pass efficiency of the Kapton filter according to size of dust particulates. Figure 3a shows the evolution of the PM2.5 densities as a function of time, demonstrating the superior performance of the Kapton air filter.