Separation of submicrometre particles using a combination of dielectrophoretic and electrohydrodynamic forces

The controlled spatial separation of submicrometre particles by a combination of dielectrophoretic and electrohydrodynamic forces is demonstrated for the first time. Using planar microelectrode arrays, a mixture of two differently sized particles (93 nm and 216 nm diameter latex spheres) has been separated into constituent components. The particles separate into distinct bands, spatially separated from each other. The separation pattern is reproduced across an entire electrode array, indicating that the method could be used as a means of rapid nanoparticle separation, positioning and processing. In spatially non-uniform ac electric fields, dielectric particles move as a consequence of the interaction of the dipole induced in the particle and the applied field gradient. This movement was termed dielectrophoresis (DEP) by Pohl [1]. In the years subsequent to Pohl’s early pioneering work, this developed into a successful technology for the manipulation, characterization and separation of cells and micro-organisms [2, 3]. More recently, through the use of advanced microelectrode fabrication techniques, the technology has moved into the submicrometre world so that particles such as latex spheres, macromolecules, DNA and viruses can now be characterized and separated [4–10]. The DEP force depends (amongst other things) on the particle volume, therefore high electric field gradients are required to move submicrometre particles. As a consequence of electrohydrodynamic (EHD) effects, the high fields gives rise to fluid movement and this fluid movement can cause particle movement through the viscous drag force. The total force on a polarizable particle in a nonuniform a.c. field can be written as the sum of a number of independently acting forces: F̄Total = F̄DEP + F̄Viscous + F̄Other. (1) Apart from the DEP force and the viscous force the other forces in this equation include sedimentation and thermal randomizing forces (which drive diffusion when it occurs). For particles of diameter less than 1 μm, thermal effects can dominate, but as has been demonstrated previously [4–10] the DEP force can be sufficient to produce deterministic particle movement. The dielectrophoretic force can be written as F̄DEP = Re{(m̄(ω) · ∇)Ē} (2) where Ē is the electric field, m̄(ω) is the induced dipole moment of the particle and Re{ } indicates the real part of. For a homogeneous dielectric sphere, the induced dipole moment is given by m̄(ω) = 4πεmaK(ω)Ē (3) where ω is the angular field frequency, a the particle radius and K(ω) the Clausius–Mossotti factor given by