Nonwoven filters are widely used today to reduce emissions - for example, to improve indoor air quality or to protect technical equipment from dust exposure. Electret filters make it possible to increase the separation efficiency of a nonwoven filter without increasing the pressure drop. This allows the same cleaning performance to be achieved with lower energy consumption. In electret filters, the fibers are electrically charged so that electrostatic forces augment the purely mechanical separation mechanisms.

However, it is well known that the effect of this electrostatic charge diminishes with increasing operating time. The investigations presented here focus on liquid aerosol particles (droplets), since ageing caused by solid particles has previously been attributed mainly to charge neutralization [1]. In contrast, the influence of water droplets has already been examined experimentally and numerically [2,3]. If droplets remain within the filter medium, continued loading is expected to lead to coalescence, thereby altering both the electric field and the flow field within the filter structure. The interaction between droplets and the filter surface was investigated for various liquids using a contact angle measuring device (model OCA 15 EC, DataPhysics Instruments GmbH).

The liquids considered were water, ethanol, isopropanol, glycerin, DEHS, and paraffin oil. To additionally validate the results at the material level, measurements were performed on plastic plates made from five different raw materials that represent the common base materials of electret filters...