Research On Sample Electrophoresis Transport Characteristics In Microchannel Turns

In microfluidic electrophoresis chips, the geometry of the separation microchannel has an important impact on sample electrophoresis transport characteristics. MicroChannel turns contribute to extending the separation length and reducing the disturbance of external pressure gradient. However, sample bands become skewed and distorted after microchannel turns. This stretching of the sample band as it traverses a turn is known as the racetrack effect, and it has the power to drastically reduce the separation efficiency, perhaps even results in the bankruptcy of separation analysis. Theory has shown that the main factor responsible for racetrack effect is the width of the microchannel turn. Here, the sample electrophoresis transport characteristics in microchannel turns with different constriction ratios are conducted to optimize the dimensional parameter of these narrowed turns. The objective of this study is to providing guidance in designing the microfiuidic electrophoresis chip.In this dissertation, the distribution of electric field, velocity field and sample concentration field are solved numerically with coordinate transform and finite volume method, and effects of constriction ratio on sample transport velocity and band broadening are presented. Based on numerical computation, glass microfiuidic chips are fabricated, and experiments using microfluid fluorescence visualization technology are conducted to study sample electrophoresis transport characteristics in microchannel turns, with the aim to verify results of numerical computation.Results of numerical computation in separation microchannels with a width of 80μm show that, ratio of average velocity in inner wall to that in outer wall is 1.04 in the microchannel turn with 31.25% constriction ratio, and velocity shape is plug-shaped and average velocity value in the microchannel turn is maximized. While higher or lower constriction ratios result in deviation from plug shape and decrease of average velocity value; Sample band broadening after the microchannel turn with 31.25% constriction ratio is only 1.28% of that after a constant turn, and turn-induced dispersion is minimized. Experimental results also indicate that electrophoresis transport velocity of the sample band in the microchannel turn and band broadening caused by racetrack effect agree very well with results of numerical computation, not only qualitatively but also quantitatively after the normalization.Based on numerical and experimental results, for separation channels with a width of 80/ttn, 31% is the optimal constriction ratio to reduce band broadening by introducing narrowed turns. As the constriction ratio further decreases, constriction and expansion of the channel geometry will induce a parabolic pressure flow. Band broadening caused by the pressure flow overcomes the reduction of the racetrack effect, which leads to an overall increase of the sample band width.