Electrical charges in fibrous filter media can lead to non-monotonic penetration curves because the dominant effect of filtration changes over time. The two orange curves in Figure 1 show how the penetration is low at the beginning of the experiment, then increases and finally decreases again.  At first, the charges are so high that most particles get trapped. Over time, as more mass deposits, the charges weaken, and fewer particles are trapped. Later still, as enough mass was deposited, captured particles contribute increasingly to the capture efficiency and more particles are trapped again.

Our earlier attempts to simulate this behavior could not reproduce this behavior, as illustrated by the simulation results on the left of Fig. 1. They were not taking decreasing charges into account. By making the electrical charges depend on the deposited dust, the digital filtration experiments reproduce the general behavior of the physical experiments as illustrated on the right of Fig. 1.

As there are many unknowns regarding how exactly the charge reduction depends on the deposited mass, we developed and present a general modeling framework that allows the detailed modelling of electrical charges, both depending on the deposited mass but also on properties of individual fibers, to improve the agreement of digital filter experiments with physical filter experiments.