Trapping of micrometre and sub-micrometre particles by high-frequency electric fields and hydrodynamic forces

We demonstrate that micrometre and sub-micrometre particles can be trapped, aggregated and concentrated in planar quadrupole electrode configurations by positive and negative dielectrophoresis. For particles less than 1 mu m in diameter, concentration is driven by thermal gradients, hydrodynamic effects and sedimentation forces. Liquid streaming is induced by the AC field itself via local heating and results, under special conditions, in vortices which improve the trapping efficiency. Microstructures were fabricated by electron-beam lithography and modified by UV laser ablation. They had typical gap dimensions between 500 nm and several micrometres. The theoretical and experimental results illustrate the basic principles of particle behaviour in ultra-miniaturized field traps filled with aqueous solutions. The smallest single particle that we could stably trap was a Latex bead of 650 nm. The smallest particles which were concentrated in the central part of the field trap were 14 nm in diameter. At high frequencies (in the megahertz range), field strengths up to 56 MV m(-1) can be applied in the narrow gaps of 500 nm. Further perspectives for microparticle and macromolecular trapping are discussed.