Chih- Chang

The hydrodynamic focusing effect inside rectangular microchannels

This paper presents a theoretical and experimental investigation into the hydrodynamic focusing effect in rectangular microchannels. Two theoretical models for two-dimensional hydrodynamic focusing are proposed. The first model predicts the width of the focused stream in symmetric hydrodynamic focusing in microchannels of various aspect ratios. The second model predicts the location and the width of the focused stream in asymmetric hydrodynamic focusing in microchannels with a low or high aspect ratio. In both models, the theoretical results are shown to be in good agreement with the experimental data. Hence, the models provide a useful means of performing a theoretical analysis of flow control in microfluidic devices using hydrodynamic focusing effects. The ability of the proposed models to control the focused stream within a micro flow cytometer is verified in a series of experimental trials performed using polystyrene microparticles with a diameter of 20 µm. The experimental data show that the width of the focused stream can be reduced to the same order of magnitude as that of the particle size. Furthermore, it is shown that the microparticles can be successfully hydrodynamically focused and switched to the desired outlet port of the cytometer. Hence, the models presented in this study provide sufficient control to support cell/particle counting and sorting applications.

Three-dimensional hydrodynamic focusing in two-layer polydimethylsiloxane (PDMS) microchannels

In this work, we designed and fabricated a three-dimensional hydrodynamic focusing microfluidic device. The device comprises a two-layer PDMS microchannel structure. There are four inlet ports and one outlet port. The fluids are all injected by syringe pumps. A sample flow stream was first vertically constrained into a narrow stream, and then horizontally focused on one small core region from a cross-section perspective, which is useful for cell/particle counting. We showed the numerical and experimental images of the focused stream shape from a cross-section perspective; experimental images were captured using a confocal fluorescence microscope. We also investigated the effect of channel aspect ratio on the vertical focusing effect using CFD simulations. The results showed that the sample flow can be focused successfully in the lower aspect ratio of the main channel (slightly greater than 0.5) in our design. Furthermore, the effect of the Reynolds number on the vertical focusing effect was also investigated. The numerical results showed that the rectangular-like shape of the focused stream from the cross-section perspective was deformed as the Reynolds number was high due to stronger secondary flows produced in the vertical focusing unit. This phenomenon was also demonstrated experimentally. The device only works well at low Reynolds numbers (approximately less than 5). The device can be integrated into an on-chip flow cytometer.

Computational analysis of electrokinetically driven flow mixing in microchannels with patterned blocks

Electroosmotic flow in microchannels is restricted to low Reynolds number regimes characterized by extremely weak inertia forces and laminar flow. Consequently, the mixing of different species occurs primarily through diffusion, and hence cannot readily be achieved within a short mixing channel. The current study presents a numerical investigation of electrokinetically driven flow mixing in microchannels with various numbers of incorporated patterned rectangular blocks. Furthermore, a novel approach is introduced which patterns heterogeneous surfaces on the upper faces of these rectangular blocks in order to enhance species mixing. The simulation results confirm that the introduction of rectangular blocks within the mixing channel slightly enhances species mixing by constricting the bulk flow, hence creating a stronger diffusion effect. However, it is noted that a large number of blocks and hence a long mixing channel are required if a complete mixing of the species is to be obtained. The results also indicate that patterning heterogeneous upper surfaces on the rectangular blocks is an effective means of enhancing the species mixing. It is shown that increasing the magnitude of the heterogeneous surface zeta potential enables a reduction in the mixing channel length and an improved degree of mixing efficiency.

A particle tracking method for analyzing chaotic electroosmotic flow mixing in 3D microchannels with patterned charged surfaces

This paper presents a numerical simulation investigation into electroosmotic flow mixing in three-dimensional microchannels with patterned non-uniform surface zeta potentials. Three types of micromixers are investigated, namely a straight diagonal strip mixer (i.e. the non-uniform surface zeta potential is applied along straight, diagonal strips on the lower wall of the mixing channel), a staggered asymmetric herringbone strip mixer and a straight diagonal/symmetric herringbone strip mixer. A particle tracing algorithm is used to visualize and evaluate the mixing performance of the various mixers. The particle trajectories and Poincare maps of the various mixers are calculated from the three-dimensional flow fields. The surface charge patterns on the lower walls of the microchannels induce electroosmotic chaotic advection in the low Reynolds number flow regime, and hence enhance the passive mixing effect in the microfluidic devices. A quantitative measure of the mixing performance based on Shannon entropy is employed to quantify the mixing of two miscible fluids. The results show that the mixing efficiency increases as the magnitude of the heterogeneous zeta potential ratio (|ζR|) is increased, but decreases as the aspect ratio (H/W) is increased. The mixing efficiency of the straight diagonal strip mixer with a length ratio of l/W = 0.5 is slightly higher than that obtained from the same mixer with l/W = 1.0. Finally, the staggered asymmetric herringbone strip mixer with θ = 45°, ζR = −1, l/W = 0.5 and H/W = 0.2 provides the optimal mixing performance of all the mixers presented in this study.

A perspective on streaming current in silica nanofluidic channels: Poisson–Boltzmann model versus Poisson–Nernst–Planck model

Choi and Kim [J. Colloid Interface Sci. 333 (2009) 672] proposed a new wall boundary condition for zeta-potential and surface charge density to describe the electrokinetic flow-induced currents in silica nanofluidic channels using the Poisson-Boltzmann (PB) model and the Poisson-Nernst-Planck (PNP) model. They showed that the results from the PNP model are in close agreement with the experimental data reported by van der Heyden et al. [Phys. Rev. Lett. 95 (2005) 116104]. In this paper, a theoretical model based on the PB model incorporating their proposed boundary condition is presented, which does not necessitate highly expensive computational effort. The results from our proposed model are shown to be in agreement with their numerical results of the PNP model. The present model also addresses the importance of the electrical resistance of reservoirs or the position of the electrodes for the measurement of the streaming current. Further, we point out that there is a misinterpretation in a comparison between their numerical results and those of van der Heyden et al.s experiments. Finally, we conclude that the experimental data still cannot be predicted accurately by their proposed boundary condition and model, especially for the electrolyte concentration C(0)<10(-3)M.

End effects on electro-osmotic flows in micro-channels

Although the micro-channels of typical lab-on-a-chip micro-fluidic devices are connected to reservoirs, existing analyses of the flow physics within the micro-channels frequently ignore the end effects induced by these reservoirs. Accordingly, this study presents an analytical and numerical investigation into electro-osmotic flow (EOF) which takes into account the end effects associated with the inlets and outlets of the micro-channels. The results indicate that the EOF contractions and expansions which occur in the inlet and outlet regions, respectively, induce streamwise pressure gradients, which result in non-flat EOF velocity profiles within the micro-channel. Furthermore, it is proven theoretically that this pressure gradient eventually vanishes once the micro-channel becomes sufficiently long that the end effects no longer exert an influence on the flow. An empirical relation between the entrance length and Reynolds number is established via parametric studies.

Chaotic mixing in electro-osmotic flows driven by spatiotemporal surface charge modulation

This paper presents an investigation into chaotic mixing in an electro-osmotic flow through a microchannel. In the mixing system, the continuous throughput flow has the form of a pluglike electro-osmotic flow induced by a permanent surface charge on the wall surface, while electro-osmotic flows contributed by spatiotemporal surface charge variations act as a perturbed flow. The spatiotemporal surface charge variations are achieved using the field-effect control method. The analyses consider two different spatiotemporal surface charge modulation schemes, designated as “MS I” and “MS II,” respectively. It is shown that both modulation schemes prompt the crossing of the flow streamlines at different instances in time and produce a chaotic mixing effect. Utilizing the thin double layer assumption, the study commences by solving the biharmonic equation for the electro-osmotic flow fields analytically. The mixing phenomena induced by the two modulation schemes are then analyzed using the Lagrangian particle tra...

Hydrodynamic Focusing Effect on Two- Unmixed-Fluid in Microchannels

This study performs a theoretical investigation into the parallel flow of two unmixed fluids subjected to a hydrodynamic focusing effect in microchannels with a rectangular cross-section. The width of the focused stream or the position of the interface between the two unmixed fluids can be controlled by adjusting the relative volumetric flow rates of the side and center streams. This study derives an analytical solution for the velocity profile of the fluid flow within the microchannel for the case of laminar, steady state fully developed flow, in which the interface between the two fluids is planar. This analytical solution is then used to develop a simple theoretical model with which to predict the width of the focused stream for a given volumetric flow rate ratio, dynamic viscosity ratio of the two fluids, and microchannel aspect ratio.