## 4 Results and Discussion
In this experiment, the filtration system operated continuously, i.e., no interruptions of the aerosol flow were applied. In the first test, the filter cartridge became saturated with the oil (reached a steady-state dP value), then the filtration system was put on hold for approximately 1 day, and after this time the aerosol flow was resumed. For the element saturated with the oil, the pressure drop stabilized very quickly, thus the system operation time was set to 3 h. This procedure, i.e., 3 h of system operation and approximately 21 h of downtime, was applied for the next 4 days. The results are presented in Figure 4:8 \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & **Day 1** & **Day 2** & **Day 3** & **Day 4** & **Day 5** \\ \hline \multirow{2}{*}{dP} & initial & 714 & 6615 & 6690 & 6685 & 6725 \\ \cline{2-6}  & final & 6550 & 6720 & 6745 & 6790 & 6825 \\ \hline \multirow{2}{*}{efficiency} & initial & 99.93\% & 99.84\% & 99.85\% & 99.83\% & 99.80\% \\ \cline{2-6}  & final & 99.91\% & 99.79\% & 99.89\% & 99.72\% & 99.75\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Performance of the PDMS 0.2% filter within 5 consecutive days of testing. Figure 12: Experimental results for the cartridge elements. A relatively long-time operation using the same element multiple times confirmed that the performance parameters of the coalescing filter were maintained. These results confirm the stability of the applied coating in contact with the test oil and at the process conditions used in the tests. The presence of PDMS after the process was also confirmed using the SEM and FTIR of the material used for 5 days, which was washed with the IPA before analysis. However, these techniques provided qualitative information only--the PDMS was still present on the fibers. An interpretation of observed phenomena on the performance of coalescing filters with modified surface properties can be determined based on the principles of the film-channel model. This model constitutes a reliable theoretical approach, which allows for understanding the principles of the process and clarifies different saturation profiles across the media made of material, which differ in the wettability. The model assumptions were confirmed in numerous research studies; therefore, it accounts for well-established knowledge regarding the operation of coalescence filters. It can also be useful to explain observed phenomena in this research. The unmodified material used in this work is strongly oleophilic, so according to the model assumptions regarding the oil distribution at steady-state operation, the oil film is created on the downstream side of the filter (Figure 14). When the flow is interrupted and the drag force acting upon the liquid disappears, the oil collected on the outlet side of the filter can flow backward into the porous material driven by capillary forces. As a result, the filter is loaded with an extra amount of oil, which causes the increase in pressure drop when the flow is resumed. Although it seems that the process should be reversible and the steady-state dP value (neglecting the dP creep) could be regained, it was not observed in the studied system (i.e., material and oil properties, and the flow rate applied). This was probably due to a very strong adhesion of the oil to the fibers, which counteracted the dislocation of oil inside the fibrous material. A new configuration of oil inside the porous material was probably created, conducive to blocking the material (e.g., filling the pores or creating the oil \"sails\" between the fibers), which translated into a higher pressure drop after the restart. Figure 13: Fractional efficiencies for unmodified and PDMS 0.2% cartridge elements.