Contraction-expansion Microchannel Research Articles

Flow characteristics and mixing performance of electrokinetically driven non-Newtonian fluid in contraction–expansion microchannel

A numerical investigation is performed into the flow characteristics and mixing performance of electrokinetically driven non-Newtonian fluid in a contraction–expansion microchannel. In the study, the rheological behavior of the fluid is characterized using a power-law model. The results show that the volumetric flow rate reduces as the flow behavior index increases, and thus an improved mixing performance is obtained. Furthermore, it is shown that for all considered values of the flow behavior index, the mixing performance can be enhanced by increasing the ratio of the main channel width to the contraction channel width, extending the length of the contraction channel, assigning a smaller value to the nondimensional Debye–Huckel parameter, and applying an appropriate electric field strength. Finally, it is shown that although the mixing efficiency reduces with a reducing flow behavior index, an acceptable mixing performance can still be obtained given an appropriate specification of the flow conditions and geometry parameters.

Single-molecule DNA dynamics in tapered contraction-expansion microchannels under electrophoresis

We investigated the dynamics of single DNA molecules driven by the electrophoretic force in several tapered contraction-expansion microchannels. Under high localized electric-field gradients, fast transition between the stretching and compression of DNA molecules was achieved. Numerically, a combination of the finite element method and the coarse-grained Brownian dynamics simulation was used to capture the dynamics of single DNA molecules simplified as freely-draining bead-spring wormlike chains. A generalized predictor-corrector time marching scheme was proposed in this work. It was found that the initial conformation, the initial center-of-mass location, and the electric-field strength are three major factors affecting the DNA dynamics. The forced relaxation due to the reverse compression in the expansion zone can speed the relaxation of DNA molecules compared with the free relaxation in the bulk. We have also simulated DNA dynamics in different contraction-expansion microchannels by changing the length or the small-end width of the contraction zone (with other geometrical lengths fixed). Decreasing the small-end width can provide higher DNA stretching due to both increased Deborah number and increased accumulated strain. Increasing the length of the contraction zone, on the other hand, only slightly increases the accumulated strain, while greatly decreases the Deborah number, causing a decrease in DNA stretching. Experimentally, DNA molecules were gradually stretched in the contraction zone and then were quickly compressed back within a short distance outside the contraction zone. DNA chains in different initial configurations demonstrate different behaviors in contraction-expansion microchannels. The Brownian dynamics simulation results are in qualitative agreement with the experimental observations.

Simulations of IEF in microchannel with variable cross‐sectional area

This study develops a 1-D mass transport model to describe the electrophoresis transport behavior within a microchannel with a variable cross-sectional area. Utilizing three different numerical schemes, simulations are performed to investigate the IEF of proteins in ampholyte-based pH gradients within both a planar microchannel and a contraction-expansion microchannel, respectively. The simulation results obtained using the modified 1-D mass transport model and the finite-volume method (FVM) for the IEF separation of a single protein sample in a ten-ampholyte-based pH gradient within a planar microchannel are consistent with those presented by Shim et al. [Electrophoresis 2007, 28, 572-586] using a 2-D FVM scheme. In addition, the Courant-Friedrichs-Lewy number insensitive conservation element and solution element (CNI-CESE) method is found to be both more robust and more computationally efficient than the conventional CESE scheme when modeling IEF phenomena within a contraction-expansion microchannel. In modeling the IEF separation of four sample ampholytes in a 20-ampholtye-based pH gradient within a contraction-expansion microchannel, the results obtained using the CNI-CESE scheme are in good agreement with those published in literature. Moreover, the simulations can be performed significantly faster with the new 1-D model and the CNI-CESE scheme. Finally, the results obtained using the modified 1-D mass transport model and the CNI-CESE scheme demonstrate that the concentration of the focused test sample and the resolution of the pH gradient within the microchannel increase as the number of ampholytes used to accomplish the IEF separation process is increased.

Effects of Ampholyte Dissociation Constants on Protein Separation in On-Chip Isoelectric Focusing

Numerical simulations are presented for ampholyte-based isoelectric focusing in 2D microgeometries. In this study, model proteins are focused in the presence of 25 biprotic ampholytes under an applied electric field. Each protein is considered as a simple polypeptide having ten charge states, while the biprotic ampholytes are selected to generate a shallow pH range of 6 to 9. Straight and contraction-expansion microchannels are considered here, and a nominal electric field of 300 V/cm is maintained for separation of proteins. Six distinct values of deltapKs between 1 and 3.5 are investigated for ampholytes to form pH profiles in a 1 cm long microchannel. Simulation results show that relatively larger values of deltapK(deltapK > 3) are required to form stepless pH profiles in the system. The peak heights and differential resolutions of focused proteins are much higher for lower values of deltapK for which a stepped pH profile is evident. For each protein, the time it takes for the two edges of a peak to merge increases linearly with deltapK, while the focusing time goes up exponentially with increasing deltapK. Both merging and focusing times of protein are higher for contraction-expansion microchannel than those of straight microchannel. For a particular value of deltapK, the contracted "Zoom" region of contraction-expansion channel is able to form more tightly focused bands than the expanded region.