Multi-stream Laminar Flow Research Articles

Quasi-Optical Terahertz Microfluidic Devices for Chemical Sensing and Imaging.

We first review the development of a frequency domain quasi-optical terahertz (THz) chemical sensing and imaging platform consisting of a quartz-based microfluidic subsystem in our previous work. We then report the application of this platform to sensing and characterizing of several selected liquid chemical samples from 570–630 GHz. THz sensing of chemical mixtures including isopropylalcohol-water (IPA-H2O) mixtures and acetonitrile-water (ACN-H2O) mixtures have been successfully demonstrated and the results have shown completely different hydrogen bond dynamics detected in different mixture systems. In addition, the developed platform has been applied to study molecule diffusion at the interface between adjacent liquids in the multi-stream laminar flow inside the microfluidic subsystem. The reported THz microfluidic platform promises real-time and label-free chemical/biological sensing and imaging with extremely broad bandwidth, high spectral resolution, and high spatial resolution.

Modulating patterns of two-phase flow with electric fields

This paper describes the use of electro-hydrodynamic actuation to control the transition between three major flow patterns of an aqueous-oil Newtonian flow in a microchannel: droplets, beads-on-a-string (BOAS), and multi-stream laminar flow. We observed interesting transitional flow patterns between droplets and BOAS as the electric field was modulated. The ability to control flow patterns of a two-phase fluid in a microchannel adds to the microfluidic tool box and improves our understanding of this interesting fluid behavior.

High Throughput Assay of Diffusion through Cx43 Gap Junction Channels with a Microfluidic Chip

This paper describes a microfluidic-based assay capable of measuring gap-junction mediated dye diffusion in cultured cells. The technique exploits multistream laminar flow to selectively expose cells to different environments, enabling continuous loading of cells in one compartment while monitoring, in real time, dye diffusion into cells of a neighboring compartment. A simple one-dimensional diffusion model fit to the data extracted the diffusion coefficient of four different dyes, 5-(6)-carboxyfluorescein, 5-chloromethylfluorescein, Oregon green 488 carboxylic acid, and calcein. Different inhibitors were assayed for their ability to reduce dye coupling. The chip can screen multiple inhibitors in parallel in the same cell preparation, demonstrating its potential for high throughput. The technique provides a convenient method to measure gap junction mediated diffusion and a screen for drugs that affect gap junction communication.

Spatially Resolved Measurement of Dissolved Oxygen in Multistream Microfluidic Devices

Dissolved oxygen (DO) is an important parameter with significant effect on cellular development and function. Micron-scale laminar flow and hydrodynamic focusing provide ideal tools for the generation of controlled chemical micro-environments and their application as stimuli to cells. In this paper, we demonstrate the generation and characterization of multistream laminar flow and hydrodynamically focused sample streams with defined dissolved oxygen concentrations on chip. A solid-state oxygen sensor layer was integrated into PDMS-based microchannels and calibrated. Several combinations of sample and buffer streams with concentrations ranging from 0 to 34 mg/l DO were generated and measured for up to three independent parallel flow streams. In addition, diffusion-based stream broadening measured with the sensor was used to determine the coefficient of diffusion of O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in the flow medium. The devices have the potential to provide novel insights into cell biology and improve the relevance of in vitro cell assays.

Numerical study of the effect of the channel and electrode geometry on the performance of microfluidic fuel cells

Using COMSOL Multiphysics 3.5, 3D numerical models of different microfluidic fuel cells have been developed in this paper to determine the effect of different modifications which have been implemented in the microfluidic fuel cell since its advent. These modifications include the channel geometry aspect ratio and electrode configuration, the third flow between the anolyte and catholyte in the channel (i.e., multi-stream laminar flow), and multiple periodically placed inlets. To be consistent with the convention, the output power of the device is normalized by the electrode surface area; however, the power density calculations are also performed through normalization by the device volume. It is shown that the latter method is more realistic and providing more information from the design point of view since the ultimate goal in designing the microfluidic fuel cell is to fabricate a compact, yet powerful device. Finally, a novel design of the microfluidic fuel cell with a tapered channel is suggested and compared to the non-tapered geometry through the polarization curves. The steps which have been taken in COMSOL to obtain these polarization curves are clearly and thoroughly explained. The Butler–Volmer equation was implemented to incorporate for the electrochemical reactions at the electrodes. The “Conductive Media DC” module, in COMSOL, is used to model the electric fields within the fuel cell. The concentration distributions of the reactant species are obtained using the “Incompressible Navier–Stokes” and “Convection and Diffusion” modules. Solving these equations together predicts the current density for given cell voltage values. The results demonstrate the cell voltage losses due to activation, ohmic and concentration overpotentials. It is shown that for a fixed value of the cell voltage (say 0.45 V), the fuel cell with multiple periodically placed inlets has the highest fuel utilization (i.e., 62.3%); while the “Simple square” geometry depicts 13.8% fuel utilization at this potential. Thus, the multiple-inlets design is particularly suitable for low-voltage applications which require high current. Also, the results of the tapered geometry proposed in this paper show that tapering the channel enhances the polarization curve comparing to the square cross-section geometry with extended electrodes. In essence, the fuel utilization of the “Extended square” geometry is increased from 15.4% to 57.6% by tapering the channel. This is due to the fact that the mixing region growth rate is restricted in the tapered geometry, and hence the electrodes on the top and bottom walls of the channel can be more extended toward the centre of the channel before the crossover occurs.

Generation of complex concentration profiles by partial diffusive mixing in multi-stream laminar flow

This paper proposes novel microfluidic concentration gradient generator (CGG) devices that are capable of constructing complex profiles of chemical concentrations by laterally combining the constituent profiles (e.g., linear and bell-shaped) generated in simple Y- or psi-shaped mixers. While the majority of currently existing CGG devices are based on complete mixing of chemical species, our design harnesses partial diffusive mixing in multi-stream laminar flow, and hence, features simple network structures and enhanced device reliability. An iterative simulation approach that incorporates our previous system-level models of CGG networks is developed to locate best-matched combinations of geometrical and operating parameters (e.g., inlet flow rates and inlet sample concentrations) for the device design. Microfluidic CGG chips are fabricated and experimentally characterized using optimal layout and operating conditions selected by the design process. The experimental results not only serve as a benchmark for model verification but also establish the feasibility of concentration gradient generation based on partial mixing for a variety of microfluidic applications.

Characterization of microfluidic fuel cell based on multiple laminar flow

This paper discusses a membraneless microfluidic fuel cell using the multi-stream laminar flow to keep the fuel and oxidant streams separated when they are flowing at a low Reynolds number state. The system we used consists of a microfluidic channel with three inlets. The fuel, oxidant, and the electrolyte solution enter the channel from the inlets and continue to flow in parallel without turbulent mixing (diffusive exchange occurs across the interface at a micro scale). During the working process of the fuel cell, controllable output potential and current of the fuel cell can be obtained and studied by changing the flow rate of the stream between the fuel and the oxidant. Results indicate that this novel design can be used in microscopic scale power source systems such as the lab-on-chip control systems because it is easy to integrate with other micro functional systems.

Investigation of heterogeneous electrochemical processes using multi-stream laminar flow in a microchannel

A multi-component microfluidic electrochemical cell is shown to be a useful analytical tool for probing complex coupled processes in electrolytic systems. We recently reported an enzymatic signal amplification phenomenon that may provide increased sensitivity when detecting bio-analytes (M. S. Hasenbank, E. Fu and P. Yager, Langmuir, 2006, 22, 7451-7453), but to fully harness this method requires an improved understanding of the underlying electrochemical and chemical processes. We use spatial control of electrolyte streams on patterned conductive substrates in a microfluidic platform to elucidate the coupling of homogeneous chemical steps to heterogeneous electrochemical charge transfer processes. Because the gold surface was observable using SPR imaging, electrochemical phenomena could be monitored optically in real time. Based on these and additional results, we propose a mechanism for the novel amplification phenomenon that involves direct electron transfer between surface-immobilized enzyme molecules and the gold surface. This improved understanding of the underlying mechanism should enable the future implementation of this phenomenon in signal amplification schemes for highly sensitive lab-on-a-chip biosensors.

Gravity-induced reorientation of the interface between two liquids of different densities flowing laminarly through a microchannel

This paper experimentally quantifies the reorientation of the liquid-liquid interface between fluids of different densities flowing side-by-side in pressure-driven laminar flow in microchannels. A gravity-induced pressure mismatch at the interface will gradually drive the denser fluid to occupy the lower portion of the microchannel. The rate of this process is expected to depend on the interplay of viscous forces--which tend to dominate at the microscale-and inertial and gravitational forces. A correlation that relates the position of such a liquid-liquid interface to physical variables and channel dimensions was derived. The extent of reorientation of the streams was then related to two dimensionless numbers: Fr, the square root of the ratio of inertial to gravitational forces; and Re/Fr2, the ratio of gravitational to viscous forces. Further analysis showed that the reorientation of the streams depends only on the gravitational and viscous forces, but not inertia. The quantitative description of the position of the interface between liquids of different densities described in this paper aids in the rational design of the rapidly growing number of microchemical systems that utilize multistream laminar flow for performing spatially resolved chemistry and biology inside microfluidic channels.

Microfluidic fuel cell based on laminar flow

This paper discusses a novel microfluidic fuel cell concept that utilizes the occurrence of multi-stream laminar flow at the microscale to keep the fuel and oxidant streams separated yet in diffusional contact. The system consists of a Y-shaped microfluidic channel in which two liquid streams containing fuel and oxidant merge and continue to flow laminarly in parallel between two catalyst-covered electrodes on opposing walls without turbulent mixing. Preliminary results indicate that this novel fuel cell concept may lead to the development of efficient room temperature power sources of microscopic dimensions that are comparable or better in performance than conventional polymer–electrolyte-membrane based microfuel cells that typically operate between 60 and 80 ◦ C. © 2004 Elsevier B.V. All rights reserved.

Pressure-Sensitive Microfluidic Gates Fabricated by Patterning Surface Free Energies Inside Microchannels

A series of pressure-sensitive microfluidic gates to regulate liquid flow have been successfully fabricated by patterning surface free energies inside microchannels using self-assembled monolayers in combination with either multistream laminar flow or photolithography. The designs are based on the principles of surface-directed liquid flow previously reported. Aqueous liquids, including protein solutions, are confined to the hydrophilic pathways (or the most hydrophilic pathway) under spontaneous flow conditions and flow into the hydrophobic regions or the less hydrophilic pathways when pressures exceed critical values. A programmable pressure-sensitive liquid delivery device is demonstrated. We have also investigated the initial rate of liquid flow in surface-patterned microchannels under spontaneous flow conditions from both analytical and experimental approaches. The methods described here provide an alternative to the conventional approaches to control liquid flow in the fast-developing field of micro...

Principles of surface-directed liquid flow in microfluidic channels.

To direct liquid flow inside microchannels, surface free energies were patterned by use of self-assembled monolayers (SAMs) in combination with either multistream laminar flow or photolithography. For the photolithographic method, two photocleavable SAMs were designed and synthesized. Carboxylic acid-terminated monolayers were obtained by photodeprotection, which was confirmed by contact angle and X-ray photoelectron spectroscopy. Using either of these patterning methods, we show that aqueous liquids flow only along the hydrophilic pathways when the pressure is maintained below a critical value; the liquids are referred to as being confined by virtual walls. Several principles of liquid flow in surface-patterned channels were derived analytically and verified experimentally. These principles include the maximum pressure that virtual walls can withstand, the critical width of the hydrophilic pathway that can support spontaneous flow, the smallest width of the liquid streams under an external pressure, the critical radius of curvature of turns that can be introduced into the hydrophilic pathway without liquid crossing the hydrophilic-hydrophobic boundary, and the minimal distance for two liquid streams to remain separated under the maximum pressure. Experimental results are in good agreement with the analytical predictions.

Surface-directed liquid flow inside microchannels.

Self-assembled monolayer chemistry was used in combination with either multistream laminar flow or photolithography to pattern surface free energies inside microchannel networks. Aqueous liquids introduced into these patterned channels are confined to the hydrophilic pathways, provided the pressure is maintained below a critical value. The maximum pressure is determined by the surface free energy of the liquid, the advancing contact angle of the liquid on the hydrophobic regions, and the channel depth. Surface-directed liquid flow was used to create pressure-sensitive switches inside channel networks. The ability to confine liquid flow inside microchannels with only two physical walls is expected to be useful in applications where a large gas-liquid interface is critical, as demonstrated here by a gas-liquid reaction.