The influence of higher moments on particle behaviour in dielectrophoretic field cages

Three-dimensional multi-electrode systems with typical dimensions between 1 and 100 mu m can be fabricated by semiconductor technology on glass or silicon substrates and used for stable trapping and/or manipulation of individual cells or micro-particles. Electric field frequencies in the MHz-range are used to achieve negative dielectrophoresis (repelling forces) in aqueous solution. This allows the creation of closed, dielectric field cages that trap cells or particles singly or as aggregates in free solution. To date, dielectric field cages have been analysed by dipole approximation. We show that this approach gives a good estimate for particles smaller than about a quarter of the electrode spacing in rotating electric field cages. For larger particles, quadrupole forces are significant. This is of great importance for, e.g., the force calibration of laser tweezers in dielectric cages. A knowledge of higher moments is necessary for understanding particle behaviour in non-rotating octode field cages. We evaluate the trapping efficiency of cages for Brownian particles, both analytically and by numerical integration of Langevin's equation. Single particles with effective diameters down to 35 nm should reliably trapped in micron field cages. Advantages and possible applications of field cages in biology, physics and chemistry are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.