On the dielectrophoretic particle retention in porous media

Particle separation from liquids and especially multidimensional separation of particles smaller than 1μm is a challenging and unresolved task with applications in the chemical, pharmaceutical, and biomedical industry. Besides a variety of existing technologies with their respective drawbacks is dielectrophoresis (DEP), a very suitable particle manipulation technique that is able to solve a range of separation problems. DEP is based on the interaction of an inhomogeneous electric field with an induced dipole. It is label-free, very sensitive and, if applied correctly, highly selective towards a specific target particle. Apart from a few reports, DEP has mostly been researched and applied in analytical or bioanalytical chemistry to solve separation problems on lab-on-a-chip devices at μL min−1 throughputs. The DEP force depends on the target particle’s volume and on the gradient of the square of the electric field, ∇|E |2. A scale-up towards preparative or industrial-scale throughputs is difficult since ∇|E |2 and thus the trapping force rapidly drops with increasing distance from the electrode array. Trapping 100 nm particles already requires vast maximum values of∇|E |2 of approximately 1× 1017 V2 m−3. A promising approach to generate values of ∇|E |2 high enough to trap particles from a substantial volume flow is the distortion of an originally homogeneous field at the solid-liquid interface of a highly porous separation matrix, a process termed dielectrophoretic filtration. Proof-of-principle results presented in this thesis demonstrate the possibility to separate layer-by-layer-assembled PAH particles (340 nm) at a flow rate of 60 mL h−1 by applying 200 VRMS over a 2 mm-thick polyethylene filter with pores in the range of 20–60 μm at an efficiency of almost 65 %. This was achieved without knowledge of the field distortion mechanism at the solid-liquid interface and detailed dynamics of the dielectrophoretic particle trapping in the porous medium. Such detailed knowledge, however, is important in order to make an informed decision on the employed parameters and filters for a highly efficient high-throughput dielectrophoretic filtration process. This work scrutinizes the dielectrophoretic particle retention in a quasi two-dimensional array of posts as a model porous medium. This allows for easier description of the originally random complex medium without becoming too detached from the original problem. To gain deeper understanding of the field distortion mechanisms the polarization field of a single post is analyzed by investigating the multipolar moments of the post’s induced charge distribution a function of the post’s cross-sectional geometry and material. To understand the trapping mechanism the resulting polarization potentials were used to derive trajectories of particles subjected to DEP and fluid drag in the vicinity of the post. The trapping potential of single post was quantified by the critical distance a particle can initially have from it to just get trapped. The critical distance normalized by the post’s size can be understood as the trapping efficiency and, to quantify the parameter dependencies, it was analyzed with respect to operational and