Multi-target pathogen detection using heterogeneous medical samples require continuous filtering, sorting, and trapping of debris, bioparticles, and immunocolloids within a diagnostic chip. We present an integrated AC dielectrophoretic ͑DEP͒ microfluidic platform based on planar electrodes that form three-dimensional ͑3D͒ DEP gates. This platform can continuously perform these tasks with a throughput of 3 L / min. Mixtures of latex particles, Escherichia coli Nissle, Lactobacillus, and Candida albicans are sorted and concentrated by these 3D DEP gates. Surface enhanced Raman scattering is used as an on-chip detection method on the concentrated bacteria. A processing rate of 500 bacteria was estimated when 100 l of a heterogeneous colony of 10 7 colony forming units /ml was processed in a single pass within 30 min.

Uniform and nonuniform ac electric fields sustained by internal microfabricated electrodes are shown to sensitively affect protein crystallization by reducing the number of nucleation sites and enhancing the quality of the crystals formed. Under conditions when massive spontaneous nucleation occurs, a properly tuned ac field can desolvate the protein molecules to form a gel matrix with only a few nucleation sites. A dielectrophoretic force generated by the nonuniform ac field further consolidates these crystallites. Upon field removal, the gel transforms into crystals around these nucleation sites to form high-quality large crystals.

We report a free-surface electrohydrodynamic flow phenomenon driven by an ionic wind mechanism induced by a high frequency gas-phase ac field ͑Ͼ10 kHz͒. Intense vortices Ͼ1 cm/s are generated above a critical voltage, beyond which the vortices break down to spawn off new vortex pairs leading to a cascade of vortices over a continuum of length scales; the mixing efficiency approaches a turbulent-like state. Colloidal particles are attracted and aggregated into planar crystal structures within the vortices by a combination of dielectrophoresis and shear-induced diffusion.

Rapid concentration and detection of bacteria in integrated chips and microfluidic devices is needed for the advancement of lab-on-a-chip devices because current detection methods require high concentrations of bacteria which render them impractical. We present a new chip-scale rapid bacteria concentration technique combined with surface-enhanced Raman scattering ͑SERS͒ to enhance the detection of low bacteria count samples. This concentration technique relies on convection by a long-range converging vortex to concentrate the bacteria into a packed mound of 200 m in diameter within 15 min. Concentration of bioparticle samples as low as 10 4 colony forming units ͑CFU͒/ml are presented using batch volumes as large as 150 l. Mixtures of silver nanoparticles with Saccharomyces cerevisiae, Escherichia coli F-amp, and Bacillus subtilis produce distinct and noticeably different Raman spectra, illustrating that this technique can be used as a detection and identification tool.

A complex and dynamic particle banding phenomenon in an electric field is reported. A single cylindrical vortex flow is generated by a dc-biased ac electro-osmotic flow on parallel electrodes. Charged particles are attracted to the vortex by positive dielectrophoresis ͑DEP͒ to form rotating cylindrical structures. As the particle concentration increases, the cylinder undergoes longitudinal symmetry breaking, producing concentrated rotating bands caused by field screening effects. The focusing of the particles into bands is shown to obey negative diffusion dynamics of a long-wave instability. Funnels and butterfly-like patterns also form because of secondary longitudinal DEP forces from nonuniform screening effects.