## 2 Problem Statement  
Most of the presented studies on gas-liquid coalescing filters concern the assessment of their efficiency in the initial state or, more importantly, from the practical point of view, after reaching saturation and stabilizing the pressure drop value (which is achieved for an ideal system without the content of solid particles). In their work, [PERSON] and [PERSON] attempted to assess the change in the efficiency of aerosol droplet separation caused by interruptions in operation and draining the oil from elements during downtime [20]. They presented the results in subsequent on-off cycles for different pauses of the flow, different modes of saturation of elements with the oil, and cleaning using the reversed flow. For selected structures, they reached the final steady-state values at a similar level, regardless of the system operation mode (saturation procedure and length of the break without aerosol flow). [PERSON] and [PERSON] discussed the \"dP creep\" and the liquid rearrangement inside the filter media for different flow velocities, loading rates, and saturation scenarios (i.e., starting from dry/clean filter or presaturated) [21]. They concluded that reaching steady-state saturation can correspond to different dP values depending on the experimental conditions and properties of filter media. Moreover, they advised that the saturation reaches a stable endpoint sooner than the pressure drop, and the creeping of the dP value may continue for the filter lifetime.  
The presented research focuses on the elimination of step changes in pressure drop and the accompanying decrease of the droplet separation efficiency due to the on-off operation. These flow interruptions were forced already in the initial stage during the saturation of the filters. The stepwise changes of the pressure drop were observed, which, as it turned out during the tests, were irreversible--i.e., the terminal values of flow resistance (pressure drop) for elements without modification of filters and continuous operation of the system were much lower than when airflow was interrupted and restarted multiple times. In extreme cases, this led to exceeding the range of the dP sensor before reaching a full saturation of the filter with the oil accumulated inside. For some applications, this may be the reason for exceeding the permissible parameters and the need to replace the element with a \"clean\" one, which could turn out to be a temporary solution anyway. It was concluded that the cause for this behavior is the transport of oil inside the porous structure due to capillary interactions. Therefore, research was focused on the affinity change of the liquid (oil) to the material surface by the application of chemical coatings to reduce or eliminate the transport of oil in the structure when the filtration system is down. In a typical design, the coalescing filters consist of the coalescence media and a drain layer. A major part of the pressure drop is related to the coalescence media, where the capillarity plays a much bigger role. Hence, in this work, the additional layer of media facilitating the drainage was not included in the filters used in the experimental program for flat panels to isolate the effects of the coalescing structure.