Study of electro-osmosis in microchips

• Main Authors: Hsiao-Ping Chen, 陳曉蘋
• Other Authors: Shau-Chun Wang
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/13873502854904890208

博士 === 國立中正大學 === 化學所 === 96 === An efficient mixing and pumping device using ac electro-osmosis driven by field- induced polarization at high frequency by non-contact electrodes is developed. The device consists of three circular reservoirs (3 mm in diameter) connected by two 1 × 1 mm channels and electrodes are outside of the mixing and pumping unit. The mechanism uses the external field to charge the surface capacitively. The charging and mixing are enhanced at tailor-designed channel corners by exploiting the high normal fields at geometric singularities. The induced channel surface dielectric polarization and the resulting electric counter-ion double layer produce an effective Zeta potential in excess of 1V and an electro-osmotic slip velocity at 1 cm/s and larger, both 1-2 order of magnitude larger than dc electro-osmosis. The polarization is non-uniform at the corners due to field leakage to the dielectric substrate and the inhomogeneous slip velocity produces intense mixing vortices that effectively homogenize solutes in 30 s, in contrast to hour-long mixing by pure diffusion. This ac induced electro-osmotic pump has a net flow with a maximum pumping throughput of 1 micro-liter/s toward the side channel. The non-contact electrodes used at high frequency can minimize electrode bubble generation and contaminants from electrochemical reactions at voltages beyond 1 V for electrolytes. Polarization over the entire channel surface, quadratic scaling with respect to the field and high voltage at high frequency without electrode bubble generation are the reasons why the current pump is superior to earlier dc and ac EO pumps. A transient 10^6-fold concentration of double-layer counterions by a high-intensity electric field is demonstrated at the exit pole of a mm-sized conducting nanoporous granule that permits ion permeation. The phenomenon is attributed to a unique counterion screening dynamics that transforms half of the surface field into a converging one toward the ejecting pole. The surface conduction flux then funnels a large upstream electro-osmotic convective counterion flux into the injecting hemisphere toward the zero-dimensional gate of the ejecting hemisphere to produce the super concentration. As the concentrated counterion is ejected into the electroneutral bulk electrolyte, it attracts co-ions and produce a corresponding concentration of the co-ions. A novel microstirring strategy is applied to accelerate the digestion rate of the substrate catalyzed by sol-gel encapsulated enzyme. An ac nonlinear electrokinetic vortex flow is used to stir the solution in a microfluidic reaction chamber to reduce the diffusion length between the immobilized enzyme and substrate in the solution. High-intensity nonlinear electroosmotic microvortices are generated around a small (1.2 mm) conductive ion exchange granule when ac electric fields (133 V/cm) are applied. Coupling between these microvortices and the on-and-off electrophoretic motion of the granule in low frequency (0.1 Hz) ac fields produces chaotic stream lines to stir substrate molecules sufficiently. Within a 5-min digestion period, the catalytic reaction rate of immobilized trypsin increases almost 30-fold with adequate reproducibility (15%) due to sufficient stirring action through the introduction of the nonlinear electrokinetic vortices. In contrast, low-frequency ac electroosmotic flow without the granule, provides limited stirring action and increases the reaction rate approximately 9-fold with barely acceptable reproducibility (30%). Dye molecules are used to characterize the increases in solute diffusivity in the reaction reservoir in which sol-gel particles are placed, with and without the presence of granule, and compared with the static case. The solute diffusivity enhancement data show respective increases of ~30 and ~8 times, with and without the presence of granule. These numbers are consistent with the ratios of the enhanced reaction rate. In capillary electrophoresis, effective optical signal quality improvement is obtained when high frequency (>100 Hz) external pulse fields modulate analyte velocities with synchronous lock-in detection. However, the pulse frequency is constrained under a critical value corresponding to the time required for the bulk viscous flow, which arises due to viscous momentum diffusion from the electro-osmotic slip in the Debye layer, to reach steady-state. By solving the momentum diffusion equation for transient bulk flow in the micro-channel, we show that this set-in time to steady-state and hence, the upper limit for the pulse frequency is dependent on the characteristic diffusion length scale and the channel geometry; for cylindrical capillaries, the set-in time is one half of that for rectangular slot channels. From the estimation of the set-in time and hence the upper frequency modulation limit, we propose that the half width/the radii of planar/cylindrical channels does not exceed 100/140 micro-meter such that there is a finite working bandwidth range above 100 Hz and below the upper limit in order for flicker noise to be effectively suppressed.