Micro-total-analytical systems (μTASs) for analyzing chemical/biological substances are now used across a wide variety of applications ranging from biological warfare agent detection to the healthcare industry. The first step in the operation of a μTAS consists of concentrating and separating the analytes of interest from the background matrix and positioning them into selected locations for subsequent analysis. The use of ac electric fields was demonstrated to have promising potential for a μTAS because the application of an ac field suppresses undesirable electrolytic effects in the liquid. The main purpose of this work is to study micro-scale phenomena in a flowing suspension subject to shear and high-gradient strong ac electric field.

A microfluidic device equipped with dielectrophoretic gates arranged perpendicular to the flow was designed and fabricated at Sandia National Laboratories. Experiments were conducted on flowing suspensions over a broad range of flow and electric field parameters to investigate how these characteristics affect the concentration and separation of particles. It was found that dipolar interactions between suspended particles subject to a high-gradient ac field and shear lead to a new many-body phenomenon of dielectrophoresis accompanied by the field-induced phase separation in a flowing suspension. As a result, shear and electric stresses strongly compress a layer enriched with particles. The predictions of the proposed electro-hydrodynamic model for the coupled shear, dielectrophoresis, and phase separation in a flowing suspension are shown to be consistent with experimental data even though the model contains no fitting parameters. Both the model and the experiments showed that the concentration volume could be increased up to about 50%. It was demonstrated that the field-induced dielectrophoresis accompanied by the phase separation provides a new method for concentrating particles in focused regions and for separating biological and non-biological materials, a critical step in the development of miniaturizing biological assays. Specifically, experiments were performed using I um diameter polystyrene latex beads and heat-killed (Staphylococcus aureus; Molecular probes) dispersed in deionized water.