Fouling of ultrathin nanoporous membrane significantly inhibits their use in purification, separation and miniaturized therapeutic devices. Dielectrophoresis (DEP) is an electrokinetic force that induces migration of a polarizable particle in an inhomogeneous electric field [1]. Application of DEP as an anti-fouling mechanism will enhance the membrane performance and service life without introducing harmful chemicals or stopping the filtration process [2]. However, the DEP force (F_DEP) on nanoparticles (NPs) is significantly reduced due to their small particle radius, as indicated in the equation below [1]:

where r is the particle radius, ω is the angular frequency, E is the root mean square electric field and Re[K(ω)] is the real part of the Clausius-Mossotti factor. Though applying larger voltage to increase 𝛻|𝐸|² can increase F_DEP, this leads to undesirable consequences such as rapid joule heating and low energy efficiency.

We hypothesized that by using ultrathin membranes, significant 𝛻|𝐸|² can be achieved without the need for extreme voltages. Our work aims to prove this concept by combining molecularly-thin nanoporous silicon-nitride (NPN) membrane filters with DEP and demonstrate successful repelling of nanoparticles from membrane surface (i.e. antifouling).

This presentation will focus on testing our hypothesis by studying the effect of NPN geometry on F_DEP. Numerical analyses using COMSOL Multiphysics^® indicated that the thickness of NPN (~50 nm) can reduce the required voltage by 3-4 orders of magnitude compared to previous reports for similar insulator-based DEP-based NP manipulation [3,4,5]. We will also discuss our numerical analyses on the critical effect of pore density on NP rejection around the pore orifice. The presentation will also describe the microfluidic device developed for the experimental verification of the DEP-based NP rejection predicted by our theoretical studies. Using positively charged, fluorescein-labeled polystyrene NPs as a model foulant, we found that successful manipulation of NP trajectory above the NPN membrane can be achieved under a voltage bias of only 1V (compared to conventional voltage bias of ~1000V [1,4]). This simple, low-cost strategy offers a novel opportunity that combines DEP with ultrathin nanoporous membranes, enabling a potentially ground-breaking antifouling mechanism for ultrafiltration applications.

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