The effect of an AC electric field on a colloidal particle near a permeable surface

Fouling of membranes during separation remains an operational drawback and, as such, has driven much research effort aimed at its mitigation. Some of the more recent approaches for fouling suppression include passive means, such as various surface modifications, as well as active means such as applying electric fields. However, use of an AC electric field has not received much attention, despite great potential shown in early studies. Notably, an AC electric field gives rise to a long-range force when applied to an electrolyte solution, due to differences in mobility and/or charge between the cation and the anion. In the present study, this mechanism is studied by solving the Poisson-Nernst-Planck equations numerically to obtain the time-averaged electric field and the resultant force acting on a charged foulant particle advected towards the membrane by permeation flow. The interplay between two oppositely acting forces (electric force and the permeation drag) gives rise to possible equilibrium positions of the particles, at a finite distance from the membrane, vs. cases where they are deposited. Mapping these equilibrium positions vs. the permeation velocity enables identification of a ‘critical’ flux, above which deposition occurs, and its dependence on system parameters. Notably, it is shown that there exists a ‘resonant’ frequency of the applied AC field, for which the critical flux is maximized with respect to the power input. The present study can provide potentially valuable guidelines for finetuning operational parameters for AC-based fouling mitigation in membrane filtration processes.