Ruey-Jen Yang

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.

Analysis of electroosmotic flow with step change in zeta potential

The term electroosmotic flow refers to the bulk flow of an aqueous solution induced by the application of the electric field to the zeta potential. The characteristics of EOF in a microchannel depend upon the nature of the zeta potential, i.e., whether it is uniform or nonuniform. In this study, the full Navier-Stokes equation and the Nernst-Planck equation are used to model the change in EOF characteristics that occur when a step change in zeta potential is applied. It is found that the thickness of the electrical double layer gradually increases downstream from the location at which the zeta potential is increased. The results indicate that a step change in zeta potential causes a significant variation in the velocity profile and in the pressure distribution.

Electrokinetically driven active micro-mixers utilizing zeta potential variation induced by field effect

This paper presents a new electrokinetically driven active micro-mixer which uses localized capacitance effects to induce zeta potential variations along the surface of silica-based microchannels. The mixer is fabricated by etching bulk flow and shielding electrode channels into glass substrates and then depositing Au/Cr thin films within the latter to form capacitor electrodes, which establish localized zeta potential variations near the electrical double layer (EDL) region of the electroosmotic flow (EOF) within the microchannels. The potential variations induce flow velocity changes within a homogeneous fluid and a rapid mixing effect if an alternating electric field is provided. The current experimental data confirm that the fluid velocity can be actively controlled by using the capacitance effect of the buried shielding electrodes to vary the zeta potential along the channel walls. While compared with commonly used planar electrodes across the microchannels, the buried shielding electrodes prevent current leakage caused by bad bonding and allow direct optical observation during operation. It also shows that the buried shielding electrodes can significantly induce the field effect, resulting in higher variations of zeta potential. Computational fluid dynamic simulations are also used to study the fluid characteristics of the developed active mixers. The numerical and experimental results demonstrate that the developed microfluidic device permits a high degree of control over the fluid flow and an efficient mixing effect. Moreover, the developed device could be used as a pumping device as well. The development of the active electrokinetically driven micro-mixer could be crucial for micro-total-analysis-systems.

An investigation into curved and moving boundary treatments in the lattice Boltzmann method

Curved boundary treatments provide a means of improving the computational accuracy of the conventional stair-shaped approximation used in lattice Boltzmann (LB) simulations. Furthermore, curved boundary treatments can be extended to the modeling of moving boundary problems simply by adding a momentum term to the bounced distribution functions at the solid surface. This study commences by investigating three conventional interpolating treatments for curved boundaries in LB problems, namely the Filippova and Hanel (FH) model [O. Filippova, D. Hanel, Grid refinement for lattice-BGK models, J. Comput. Phys. 147 (1998) 219-228], Bouzidis model [M. Bouzidi, M. Firdaouss, P. Lallemand, Momentum transfer of a Boltzmann-lattice fluid with boundaries, Phys. Fluids 13(11) (2001) 3452-3459], and Yus model [D. Yu, R. Mei, W. Shyy, A Unified Boundary Treatment in Lattice Boltzmann Method, AIAA 2003-0953, New York, 2003]. Previous investigations have indicated that the interpolations would break the mass conservation at the boundaries, since the inaccuracy in evaluation of the momentum transfer at boundary leads to a net mass flux. Based on this reason, a concept of the interpolation-free treatment for modeling the curved and moving boundary conditions is proposed to overcome the drawback of these interpolation-based curved boundary treatments. In present study, two interpolation-free models are then proposed, namely on-site interpolation-free (OSIF) and composite interpolation-free (CPIF) models. These proposed models are initially applied to simulate the flow in the channels containing a stationary square block positioned at various locations along the longitudinal axis. The simulations results are then compared with those obtained using the three conventional interpolating treatments. The interpolation-free models are then applied to the case of moving boundary problems in which a square block and a cylindrical block, respectively, move with a constant speed along a channel containing stationary flows. To test the Galilean effect of the proposed CPIF model, a Couette flow past the stationary square/cylinder block with the moving top/bottom walls is simulated. Overall, the numerical results show that the proposed interpolation-free curved treatment models significantly improve the accuracy of the mass flux computation near the solid surface, and thus enhance the accuracy of the momentum interaction at the moving boundaries.

A nanochannel-based concentrator utilizing the concentration polarization effect

This study develops a microfluidic device comprising two microchannels connected by a nanochannel in which the sample is concentrated via an ionic depletion effect. Using Rhodamine 6G dye for visualization purposes, it is shown that through an appropriate manipulation of the external potentials applied to the reservoirs of the device, an ionic depletion region with an electrical conductivity around 50 times lower than that of the buffer solution can be induced on the anodic side of the nanochannel. Furthermore, via an appropriate time‐based switching of the external electrical potentials, the sample species can be concentrated close to the conductivity ratio within an interval of 1 min. Finally, it is shown that by varying the configuration of the electrical potentials applied to the device reservoirs, the concentrated sample can be delivered either to a single outlet reservoir or to multiple reservoirs.

Electrokinetic energy conversion efficiency in ion-selective nanopores

Taking hydrodynamic slippage into account, we derived an exact expression for the figure of merit to evaluate the electrokinetic energy conversion efficiency and power of an ion-selective nanopore. In the absence of slip, and when the ratio of the radius and the Gouy-Chapmann length (a/λGC) is approximately 10, the highest efficiency for an electrolyte consisting of simple monovalent ions was predicted to be approximately 9.7%. While this efficiency is low, it can be greatly improved to a potentially practical efficiency (>40%) when the slip ratio (b/a) is greater than 0.7.

Variable-volume-injection methods using electrokinetic focusing on microfluidic chips

This paper adopts a physical model and a numerical simulation approach to study electrokinetic focusing injection on microfluidic chips. The model reflects the principal material transport mechanisms such as electrokinetic migration, ionic concentration, fluid flow, and diffusion. The current study also involves the design and testing of various injection systems used to deliver a sample plug. A novel double-cross injection system has been developed which uses electrokinetic focusing to achieve variable-volume injection of the sample plug. The injection technique uses a unique sequence of loading steps with different electric potential distributions and potential magnitudes within the various channels to effectuate a virtual valve. The proposed design combines several functions of traditional sample plug injection systems on a single microfluidic chip.

Electrokinetic behaviour of overlapped electric double layers in nanofluidic channels

The ionic concentration variation arising in overlapped electric double layers in nanochannels presents different electrokinetic behaviour from that in microchannels. Using a modified concentration approach, the results reveal that the surface charge density is insensitive to the concentration in the low-salt regime, and a nearly constant streaming conductance is predicted. A scaling analysis confirms that the streaming potential also has a constant value because of length scale effects, but not due to the accumulated charge in the double layer.