Motivation
Rapid mixing of macromolecular solutions presents a significant challenge to chip-based molecular diagnostics. Many biochemical sensing techniques such as immunoassays and hybridization analyses require the rapid, homogeneous mixing of macromolecular solutions, such as DNA or globular proteins. Bioanalytical microsystems contend with the characteristically low Reynolds (Re) numbers of microflows and the low diffusion coefficients of macromolecules in achieving rapid mixing.
Project Description
We have observed that microchannel liquid flows subjected to alternating electric fields exhibit, under certain electric field strengths, frequencies, and geometries, a flow instability capable of rapidly stirring micro- and nanoliter volume solutions. Here we present two micromixers: a prototype device (fabricated from PDMS) shown in Figure 1 and a more robust, 2nd generation device (fabricated from glass) shown in Figure 2. Both micromixers demonstrate the electrokinetic instability (EKI) as a rapid stirring mechanism. We have performed a preliminary characterization of mixer performance using fluorescein dilution experiments. In these experiments, a high-resolution CCD camera is used to capture the rapid dispersion of dyed and undyed buffers as a result of EKI stirring. Sample movies of both the PDMS and Glass Micromixer in operation are shown below.
(a)
(Mouseover figure to begin movie)
(b)
Figure 1. (a) Schematic of PDMS EKI micromixer. Channel network fabricated from molded PDMS and sealed with a glass coverslide. The channels have a nominal width and depth of 1 mm by 300 mm. Fluids A and B are introduced through input ports 1 and 2 and hydrodynamically advected towards output port 4. Platinum electrodes inserted into ports 3 and 4 conduct a high voltage sine wave effecting EKI stirring along the channel extending from ports 3 to 4. (b) Flow visualization of EKI stirring. Area visualized (3 x 1 mm) is half-way downstream of channel intersection and port 4. The frequency and applied voltage were 10 Hz and 1 kV, while the electrode spacing was 9 mm.
(a)
(Mouseover figure to begin movie)
(b)
Figure 2. (a) Schematic of Glass EKI micromixer. Channel network fabricated from wet-etched Borofloat substrates. The width and depth of the microchannels is 300 by 100 mm, respectively. A syringe pump is used to advect fluids A and B to the central square mixing chamber. Platinum electrodes inserted into ports 3 and 4 provide the AC excitation. Here the EKI is confined to the square mixing chamber (1 x 1 mm) and to fluid channel regions extending two channel widths from the chamber. (b) Flow visualization of EKI stirring within the mixing chamber. Flow is from left to right. Here the applied voltage and frequency were 4 kV and 5 Hz. Initially distinct buffer streams entering the mixing chamber are stirred as a result of the EKI. The near-uniform intensity profile of the output stream is indicative of fairly well-stirred fluid exiting the mixing chamber.
References
1.) Oddy, M.H., Santiago, J.G., and Mikkelsen, J.C., "Electrokinetic Instability Micromixing," Analytical Chemistry, Vol. 73, No. 24, 5822-5832, 2001.