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A key need for dynamic single-cell measurements is the ability to gently position cells for repeated measurements without perturbing their behavior. We describe a new method that uses a gentle secondary flow to trap and suspend single cells, including motile cells, at predictable locations in 3-D. Trapped cells can be more dense or less dense than the surrounding medium. The cells are suspended without surface contact in one of four steady streaming eddies created by audible-frequency fluid oscillation ([less than or equal to] 1000 Hz) in a microchannel containing a single fixed cylinder (radius = 125 [micro]m). Comparison of measured trap locations to computations of the eddy flow show that each trap is located near the eddy center, and the location is controlled via the oscillation frequency. We use the motile phytoplankton cell (Prorocentrum micans) to experimentally measure the trapping force, which is controlled via the oscillation amplitude. Trapping forces up to 30 pN are generated while exerting moderate shear stresses (shear stresses [less than or equal to] 1.5 N/[m.sup.2]) on the trapped cell. The magnitude of this trapping force is comparable to that of optical tweezers or dielectrophoretic traps, without requiring an external field outside the physiological range for cells (the shear stresses are comparable to those found in arterial blood flow). The unique combination of predictable 3-D positioning, insensitivity to cell and medium properties, strong adjustable trapping forces, and a gentle fluid environment makes hydrodynamic tweezers a promising new option for noncontact trapping of single cells in suspension.

Well-mixed reaction volumes are often sought in engineered microchemical devices and can be an important feature of naturally occurring physicochemical processes such as pitting corrosion. Steady streaming eddies can serve as well-mixed, easily controlled microliter chemical reactors for characterizing homogeneous chemical reactions. Here, steady streaming eddies are produced by oscillating a liquid-filled cuvette around a stationary cylindrical electrode (radius 406 [micro]m, length 1.6 cm) at audible frequencies (75 Hz). Oxidant (ferricyanide) electrochemically dosed at small rates ([less than or equal to] 30 nmol/s) from the cylindrical electrode accumulates to millimolar concentrations within the closed streamlines of each eddy, where it mixes and reacts with an antioxidant (vitamin C) present in the bulk solution. The composition in the eddy is controlled by varying the oxidant dosing rate and the bulk antioxidant concentration ([less than or equal to] 10 mM), as well as the cuvette oscillation amplitude. A simple algebraic mole balance is combined with Raman spectroscopy measurements of oxidant concentration in the eddy and bulk to determine the reaction rate law and homogeneous rate constant (45 [+ or -] 9 [M.sup.-1] [s.sup.-1]) for the antioxidant properties of vitamin C against ferricyanide. Numerical solutions to the full Navier--Stokes equations and species continuity equations illustrate the distribution of species during the reaction and general limitations to the assumption of a well-mixed eddy.

A technique is described for assembly of multiple batches of micro components onto a single substrate. The substrate is prepared with hydrophobic alkanethiol-coated gold binding sites. To perform assembly, a hydrocarbon oil, which is applied to the substrate, wets exclusively the hydrophobic binding sites in water. Micro components are then added to the water, and assembled on the oil-wetted binding sites. Moreover, assembly can be controlled to take place on desired binding sites by using an electrochemical method to deactivate specific substrate binding sites. By repeatedly applying this technique, different batches of micro components can be sequentially assembled to a single substrate. As a post assembly procedure, electroplating is incorporated into the technique to establish electrical connections for assembled components. Important issues presented are: substrate fabrication techniques, electrochemical modulation by using a suitable alkanethiol (dodecanethiol), electroplating of tin and lead alloy and binding site design simulations. Finally, we demonstrate a two-batch assembly of silicon square parts, and establishing electrical connectivity for assembled surface-mount light emitting diodes (LEDs) by electroplating. [839] Index Terms--Capillary force, electrochemical modulation, electroplating, hydrophobicity, interfacial energy, micro assembly, reductive desorption, self-assembled monolayer (SAM), self-assembly, surface tension.

During fabrication, local variations in convective-diffusive mass transfer and current distribution can give rise to undesirable variations in material composition for electrodeposited microelectromechanical systems (MEMS). Local transport characteristics are normally difficult to analyze in complex microstructures, especially the convective-diffusive conditions present during device fabrication. To tackle this problem, we have developed an experimental method called 'Process Archaeology.' The general approach involves the electrodeposition of NiFe alloys from an electrolyte formulated to give a local composition that is very sensitive to the local mass transfer rate. Spatially-resolved measurements of the alloy composition and growth rate are accomplished by combining scanning electron microscopy and energy dispersive x-ray spectroscopy. Using a simple inverse problem formulation, these experimentally measured quantities are related to the local convective-diffusive mass transfer coefficient (k) and the local Sherwood number present during fabrication. Here the Sherwood number is defined as Sh = kH/D, where H is the thickness of the lithographic mask and D is the diffusivity of a key transport-controlled species. Electrodeposited microgears are used as a test bed for demonstrating 'Process Archaeology.' Sherwood number images show the wide ranges of mass transfer rates and the complex mixing features present during microgear fabrication, especially in the region downstream of the microgear's central hub. [669] Index Terms--Electroplating, current distribution, LIGA, microfabrication, microfluidics, NiFe alloy.

The design and characterization of a dye-sensitized p-n heterojunction (DSH) consisting of a planar interface between n-TiO2 and p-CuSCN single layers are discussed. The wide-band-gap semiconductors, which contain a monolayer of sulforhodamine B dye, initiate electron transfer to the n-TiO2 and holes to the p-CuSCN upon photoexcitation by visible light with an absorbed-photon-to-current efficiency of more than 70%.