We have developed an electrokinetic process to rapidly stir micro- and nanoliter volume solutions for microfluidic bioanalytical applications. We rapidly stir microflow streams by initiating a flow instability, which we have observed in sinusoidally oscillating, electroosmotic channel flows. As the effect occurs within an oscillating electroosmotic flow, we refer to it here as an electrokinetic instability (EKI). The rapid stretching and folding of material lines associated with this instability can be used to stir fluid streams with Reynolds numbers of order unity, based on channel depth and rms electroosmotic velocity. This paper presents a preliminary description of the EKI and the design and fabrication of two micromixing devices capable of rapidly stirring two fluid streams using this flow phenomenon. A high-resolution CCD camera is used to record the stirring and diffusion of fluorescein from an initially unmixed configuration. Integration of fluorescence intensity over measurement volumes (voxels) provides a measure of the degree to which two streams are mixed to within the length scales of the voxels. Ensemble-averaged probability density functions and power spectra of the instantaneous spatial intensity profiles are used to quantify the mixing processes. Two-dimensional spectral bandwidths of the mixing images are initially anisotropic for the unmixed configuration, broaden as the stirring associated with the EKI rapidly stretches and folds material lines (adding high spatial frequencies to the concentration field), and then narrow to a relatively isotropic spectrum at the well-mixed conditions.

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We have developed an acrylic microfluidic device that sequentially couples liquid-phase isoelectric focusing (IEF) and free solution capillary electrophoresis (CE). Rapid separation (<1 min) and preconcentration (73x) of species were achieved in the initial IEF dimension. Using full-field fluorescence imaging, we observed nondispersive mobilization velocities on the order of 20 microm/s during characterization of the IEF step. This transport behavior allowed controlled electrokinetic mobilization of focused sample bands to a channel junction, where voltage switching was used to repeatedly inject effluent from the IEF dimension into an ampholyte-based CE separation. This second dimension was capable of analyzing all fluid volumes of interest from the IEF dimension, as IEF was 'parked' during each CE analysis and refocused prior to additional CE analyses. Investigation of each dimension of the integrated system showed time-dependent species displacement and band-broadening behavior consistent with IEF and CE, respectively. The peak capacity of the 2D system was approximately 1300. A comprehensive 2D analysis of a fluid volume spanning 15% of the total IEF channel length was completed in less than 5 min.

In random solid solutions the atomic-scale structure, i.e., the nature of the near-neighbor (nn) environment, is not well understood because of the fact that standard diffraction techniques average the structure over distances which are large on the scale of a lattice constant. One consequence of this lack of microscopic information is that calculations of the properties of solid solutions have often relied on simple approximations. One of the most used of these models is the virtual-crystal approximation (VCA) 1 which assumes that all atoms occupy the average lattice positions defined by the x-ray lattice constants. With use of the VCA, properties of the alloy, such as the electronic band structure, can be calculated whether or not the alloy lattice constant varies linearly with composition between those of the end members, i.e., follows Vegard's Law. 2 Similarly for dilute alloys, the assumption that the impurity-host distance is equal to the host-host distance is often used to calculate alloy properties, even those which may depend very sensitive-(to be published), Paper No. IAEA-CN-41/A-3. 6 G. Becker et al., to be publishedo ly on distance, e.g., the magnetic properties and the NMR and ESR spectra. However, the validity of this assumption, namely, an average distance or equal impurity and host distances, has never been systematically addressed with experimental measurements.We have used extended x-ray-absorption fine structure (EXAFS) to address these issues in random solid solutions since this technique is well suited to the study of local bonding, especially the determination of nn distances relative to a well-defined standard. As a result EXAFS has been used successfully to study other issues in alloys. These include studies of dilute binary metal alloy systems 3 where the main issues addressed were local clustering or chemical order, such as Guinier-Preston zones, and deviations from the continuum elastic theory. Other EXAFS studies of ternary alloys 4 " 6 have indicated that the nn distances do differ from the average, but the main emphasis was on other issues and so these studies were not performed over a wide In random solid solutions of Gaix In x As, the Ga-As and In-As near-neighbor distances change by only 0.04 Aasi varies from 0.01 to 0.99, despite the fact that this alloy accurately follows Vegard's law, with a change in average near-neighbor spacing of 0.17 A. This result contradicts the underlying assumption of the virtual-crystal approximation. Nonetheless, the cation sublattice approaches a virtual crystal with a broadened single distribution of second-neighbor distances, whereas the anion sublattice exhibits a bimodal anion-anion second-neighbor distribution.1412

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