Filtration at the microfluidic level: enrichment of nanoparticles by tunable filters.

We present an electrohydrodynamic device for filtration of nanometre-sized particles from suspensions. A high-frequency electric field is locally generated through the action of mutually parallel microelectrodes integrated into a microfluidic channel. Due to the mechanism of ohmic heating, a thermal gradient arises above these electrodes. In conjunction with temperature-sensitive properties of the fluid, an eddy flow behaviour emerges in the laminar environment. This acts as an adjustable filter. For quantification of the filtration efficiency, we tested a wide range of particle concentrations at different electric field strengths and overall external flow velocities. Particles with a diameter of 200 nm were retained in this manner at rates of up to 100%. Numerical simulations of a model taking into account the hydrodynamic as well as electric conditions, but no interactions between the point-shaped particles, yield results that are similar to the experiment in both the flow trajectories and the particle accumulation. Our easy technique could become a valuable tool that complements conventional filtration methods for handling nanometre-scaled particles in medicine and biotechnology, e.g. bacteria and viruses.