We report observations of a new electric field-and shear-induced many-body phenomenon in the behavior of suspensions. Its origin is dielectrophoresis accompanied by the field-induced phase separation. As a result, a suspension undergoes a field-driven phase separation leading to the formation of a distinct boundary between regions enriched with and depleted of particles. The theoretical predictions are consistent with experimental data even though the model contains no fitting parameters. It is 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.
A thin flow-focusing microfluidic channel is evaluated for generating monodisperse liquid droplets. The microfluidic device is used in its native state, which is hydrophilic, or treated with OTS to make it hydrophobic. Having both hydrophilic and hydrophobic surfaces allows for creation of both oil-in-water and water-in-oil emulsions, facilitating a large parameter study of viscosity ratios (droplet fluid/continuous fluid) ranging from 0.05 to 96 and flow rate ratios (droplet fluid/continuous fluid) ranging from 0.01 to 2 in one geometry. The hydrophilic chip provides a partially-wetting surface (contact angle less than 90°) for the inner fluid. This surface, combined with the unusually thin channel height, promotes a flow regime where the inner fluid wets the top and bottom of the channel in the orifice and a stable jet is formed. Through confocal microscopy, this fluid stabilization is shown to be highly influenced by the contact angle of the liquids in the channel. Non-wetting jets undergo breakup and produce drops when the jet is comparable to or smaller than the channel thickness. In contrast, partially-wetting jets undergo breakup only when they are much smaller than the channel thickness. Drop sizes are found to scale with a modified capillary number based on the total flow rate regardless of wetting behavior.
A new method for diffusion coefficient measiirement, applicable to micro-fluidics is presented. The method utilizes an analytical model describing laminar dispersion in rectangiilar micro-channels. The inet'hod was verified through measurement of fluorescein diffusivit,y in water and aqueous polymer solutions of differing concentration. The diffiisivity of fluorescein was measured as 0.64 x 10-9m2/s in wat,er, 0.49 x 10-'m2/s in the 4 gm/dl dextran solution and 0.38 x 10-91~12/~ in the 8 gni/dl dextran solution.
We present a novel separation device for the front-end of a biodetection system to discriminate between biological and non-biological analytes captured in air samples. By combining AC dielectrophoresis along the flow streamlines and a field-induced phase-separation, the device utilizes “dielectrophoretic gating”to separate analytes suspended in a flowing fluid based on their intrinsic polarizability properties. The gates are integrated into batch fabricated self-sealed surface-micromachined fluid channels. We demonstrate that setting the gate to a moderate voltage in the radio frequency range removed bacteria cells from a mixture containing non-biological particles without the need for fluorescent labeling or antibody-antigen hybridization, and also validate experimentally basic relations for estimating the gate performance.
We describe a novel technique that utilizes simultaneous implementation of dielectrophoresis (DEP) and magnetophoresis (MAP) to focus magnetic particles into streams for optical analysis of biological samples. This technique does not require sheath flow and utilizes a novel interdigitated electrode array chip that yields multiple streams of flowing magnetic particles in single-file columns. The MAP force placed particles in close proximity to the microelectrodes where they were subjected to a strong DEP force that generated the particle focusing effect. Particle focusing efficiency was improved using this combination DEP-MAP technique compared to DEP alone: particle stream widths were reduced ∼47% and stream width variability was reduced 80% for focused streams of 8.5 μm diameter magnetic particles. 3 μm diameter magnetic particles were strongly focused with DEP-MAP (∼4 μm wide streams with sub-μm variability in stream width) while DEP alone provided minimal focusing. Additional components of a prototype detection system were also demonstrated including an integrated magnetic pelleting component, a hand-held MHz frequency signal generator and a bench-top near-confocal microscope for optical analysis of flowing particles. Preliminary testing of a sandwich assay performed on the surface of magnetic particles showed 50 ppb detection levels of a surrogate biotoxin (ovalbumin) in a raw milk sample.
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