Flow In Microfluidic Channels Research Articles

Design Considerations for a Microfluidic Device to Quantify the Platelet Adhesion to Collagen at Physiological Shear Rates

Accurate assessment of blood platelet function is essential in understanding thrombus formation which plays a central role in cardiovascular disease. Parallel plate flow chambers have been widely used as they allow for platelet adhesion on a collagen surface at physiologically relevant fluid mechanical forces. Standard parallel plate flow chambers typically need several milliliters of blood, which is substantially more than can be obtained from small animals. We designed, fabricated, and assessed the functionality of a microfluidic channel with a width of 500 microm and a height of 50 microm in which a wall shear rate of 1000 s(-1) can be achieved with a flow rate of 15 microL/min. The velocity distribution in the microchannel predicted from the equations of motion was compared to experimentally measured velocities of fluorescent beads. This analysis showed that the motion of beads was quite similar to the predicted motion. Adhesion of platelets from whole blood at a shear rate of 1000 s(-1) onto a collagen surface using the microfluidic flow channel was qualitatively similar to platelet adhesion observed with a standard sized parallel plate flow chamber. After 5 min flow the surface coverage of platelets in the microfluidic device was about 55% while in a traditional size flow chamber the surface coverage was about 75%. This suggests that the microfluidic flow chamber can be used to quantify platelet adhesion for system where only very small amounts of blood are available.

Generation and manipulation of “smart” droplets

We report the generation and manipulation of electrorheological (ER) droplets that exhibit the giant ER effect. The experiments were carried out on specially designed microfluidic chips, in which the ER droplets were generated by using the microfluidic flow-focusing approach. Both the size and formation rate of these droplets can be controlled through digitally applied electrical signals. The principle of droplet manipulation is based on the electrical responsiveness of ER droplets and hence the denotation of “smart” when the electrical signals can be triggered by sensing/control devices. Due to the unique characteristics of the GER effect, the smart droplets can deform and even stop the microfluidic channel flow under an applied electric field. The pressure difference induced by the smart droplets inside the micro-channel is controllable by varying the field strengths, droplet sizes and particle concentrations in the GER suspension. By trapping and timed release of smart droplets in different micro-branch channels, we demonstrate that the smart droplets generated upstream cannot only be stored or displayed in the desired downstream channel(s) and thereby offer the potential of micro-droplet display, but also be useful in counting, flow directing and sorting the desired number of passive droplets sandwiched between two smart droplets. Such capabilities of smart droplets will enable the programmable control of discrete processes in bio-analysis, chemical reactions, digital microfluidics, and digital droplet display.

Modelling small-patterned neuronal networks coupled to microelectrode arrays

Cultured neurons coupled to planar substrates which exhibit ‘well-defined’ two-dimensional network architectures can provide valuable insights into cell-to-cell communication, network dynamics versus topology, and basic mechanisms of synaptic plasticity and learning. In the literature several approaches were presented to drive neuronal growth, such as surface modification by silane chemistry, photolithographic techniques, microcontact printing, microfluidic channel flow patterning, microdrop patterning, etc. This work presents a computational model fit for reproducing and explaining the dynamics exhibited by small-patterned neuronal networks coupled to microelectrode arrays (MEAs). The model is based on the concept of meta-neuron, i.e., a small spatially confined number of actual neurons which perform single macroscopic functions. Each meta-neuron is characterized by a detailed morphology, and the membrane channels are modelled by simple Hodgkin–Huxley and passive kinetics. The two main findings that emerge from the simulations can be summarized as follows: (i) the increasing complexity of meta-neuron morphology reflects the variations of the network dynamics as a function of network development; (ii) the dynamics displayed by the patterned neuronal networks considered can be explained by hypothesizing the presence of several short- and a few long-term distance interactions among small assemblies of neurons (i.e., meta-neurons).

Enabling a microfluidic immunoassay for the developing world by integration of on-card dry reagent storage

As part of an effort to create a point-of-care diagnostic system for the developing world, we present a microfluidic flow-through membrane immunoassay with on-card dry reagent storage. By preserving reagent function, the storage and reconstitution of anhydrous reagents enables the devices to remain viable in challenging, unregulated environmental conditions. The assay takes place on a disposable laminate card containing both a porous membrane patterned with capture molecules and a fibrous pad containing an anhydrous analyte label. To conduct the assay, the card is placed in an external pumping and imaging instrument capable of delivering sample and rehydrated reagent to the assay membrane at controlled flow rates to generate quantitative results. Using the malarial antigen Plasmodium falciparum histidine-rich protein II (PfHRP2) as a model, we demonstrate selection of dry storage conditions, characterization of reagent rehydration, and execution of an automated on-card assay. Gold-antibody conjugates dried in a variety of sugar matrices were shown to retain 80-96% of their activity after 60 days of storage at elevated temperatures, and the release profile of the reconstituted reagent was characterized under flow in microfluidic channels. The system gave a detection limit in the sub-nanomolar range in under nine minutes, showing the potential to expand into quantitative, multi-analyte analysis of human blood samples.

Fabrication of microfluidic devices based on glass–PDMS–glass technology

We describe an alternative technology for the fabrication of microfluidic devices having a glass–PDMS–glass sandwich configuration. The advantage of this technique relies on the use of a patterned PDMS layer as both a bonding layer and a working layer for lateral fluid confinements. Conventional lithography, etch and lift off techniques can be easily applied to both glass substrates, which enhances patterning capabilities of top and bottom surfaces of microchannels. The detailed fabrication process is presented and discussed. The performance of the fabricated devices is also illustrated through the measurement of electro-osmotic flows in microfluidic channels coated with different materials.

Using three-phase flow of immiscible liquids to prevent coalescence of droplets in microfluidic channels: criteria to identify the third liquid and validation with protein crystallization.

This manuscript describes the effect of interfacial tensions on three-phase liquid-liquid-liquid flow in microfluidic channels and the use of this flow to prevent microfluidic plugs from coalescing. One problem in using microfluidic plugs as microreactors is the coalescence of adjacent plugs caused by the relative motion of plugs during flow. Here, coalescence of reagent plugs was eliminated by using plugs of a third immiscible liquid as spacers to separate adjacent reagent plugs. This work tested the requirements of interfacial tensions for plugs of a third liquid to be effective spacers. Two candidates satisfying the requirements were identified, and one of these liquids was used in the crystallization of protein human Tdp1 to demonstrate its compatibility with protein crystallization in plugs. This method for identifying immiscible liquids for use as a spacer will also be useful for applications involving manipulation of large arrays of droplets in microfluidic channels.

Diagnosis of transient electrokinetic flow in microfluidic channels

Measuring the electro-osmotic velocity distributions in microchannels is usually performed for steady-state electrokinetic flows. Characterizing time-dependent electrokinetic flows is of importance to the development of microfluidic devices such as rapid capillary electrophoretic separation system, ac pumps, novel micromixers, etc. In this paper, we use a micron-resolution particle image velocimetry (micro-PIV) based phase locking technique with an ordinary PIV charge coupled device (CCD) camera to carry out an experimental study of the transient electrokinetic flow in microchannels by synchronizing different trigger signals for the laser, CCD camera, and in-house designed high-voltage switch. With the transient micro-PIV technique, we further propose a method to decouple the particle electrophoretic velocity from the micro-PIV measured velocity and to determine the zeta potential of the channel wall. The time evolution of the full-field, electro-osmotic velocity distributions in both open- and closed-end rectangular microchannels is obtained. Using the slip velocity approach and the measured channel zeta potential, the theoretical predictions of the transient electro-osmotic flow in the open- and closed-end microchannels are obtained, and they are found to be in good agreement with the experimental results.

Droplet coalescence by geometrically mediated flow in microfluidic channels

Microfluidic flow is geometrically mediated at a trifurcating junction allowing periodically formed, equally spaced out emulsion droplets to redistribute and fuse consistently. This is achieved by controlling the ratio between the droplet transport time across the trifurcating junction and the drainage time of the fluid volume separating the droplets t r/t d. Three different microfluidic trifurcation geometries have been designed and compared for their droplet fusion efficiencies. Fusion of up to six droplets has been observed in these devices. The fusion of two droplets occurs when t r/t d is equal to 1.25 and the number of fused droplets increases with t r/t d. When the junction length (d) is 216 μm fusion of 2–6 six droplets are possible however when the junction length is increased to 360 μm fusion of only two droplets is observed.

Effect of finite reservoir size on electroosmotic flow in microchannels

In electrokinetically driven microfluidic applications, reservoirs are indispensable and have finite sizes. During operation processes, as the liquid level in reservoirs keeps changing as time elapses, a backpressure is generated. Thus, the flow in microfluidic channels actually exhibits a combination of the electroosmotic flow and the time-dependent induced backpressure-driven flow. In this paper, a model is presented to describe the effect of the finite reservoir size on electroosmotic flow in a rectangular microchannel. Important parameters that describe the effect of finite reservoir size on flow characteristics are discussed. A new concept termed as “effective pumping period” is introduced to characterize the reservoir size effect. The proposed model identifies the mechanisms of the finite-reservoir size effects and is verified by experiment using the micro-PIV technique. The results reported in this study can be used for facilitating the design of microfluidic devices.

A Microfluidic Channel Flow Cell for Electrochemical ESR

The design, fabrication, and characterization of microfluidic channel flow devices for in situ simultaneous hydrodynamic electrochemical ESR is reported. The microelectrochemical reactors consist of gold film electrodes situated within rectangular ducts of height 350 microm and widths in the range 500-2000 microm. The small dimensions of the channels result in minimal dielectric loss when centralized within a cylindrical TE011 resonant cavity, leading to a high level of sensitivity. This is demonstrated by using the one-electron oxidation of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) in acetonitrile as a model system, wherein the ESR spectra obtained for the corresponding stable radical cation are of a high signal-to-noise ratio. Signal intensity is measured as a function of flow rate for this system, and the behavior is validated by means of 3-dimensional numerical modeling of the hydrodynamic flow profile.

Electrocoalescence of drops synchronized by size-dependent flow in microfluidic channels

The use of microfluidic devices to control drops of water in a carrier oil is a promising means of performing biological and chemical assays. An essential requirement for this is the controlled coalescence of pairs of drops to mix reagents together. We show that this can be accomplished through electrocoalescence of drops synchronized by size-dependent flow in microfluidic channels. Smaller drops move faster due to the Poiseuille flow, allowing pairs of surfactant-stabilized drops to be brought into contact where they are coalesced with an electric field. We apply this method to an enzyme assay to measure enzyme kinetic constants.

Pressure drops for droplet flows in microfluidic channels

Designing on-chip microfluidic pumps requires that the dependence of pressure drop across the device on the fluid flow rate be known. This relationship can be complicated for the case of two-phase droplet type flows. We characterized the flow-pressure relationship for the single-file flow of water drops in oil in microfluidic channels of rectangular cross sections. The pressure for such droplet flows was always larger—sometimes over 50% larger—than that for corresponding single-phase flows of the continuous phase. This is in spite of the fact that the water drops had a substantially lower viscosity than the continuous phase oil. The excess pressure was found to correlate reasonably well with the size of the drops relative to the size of the channels. This correlation for the excess pressure, as well as a correlation presented here for the size of drops in microchannels, should provide convenient guidelines in designing microfluidic devices for two-phase flows.

Surface-Directed Boundary Flow in Microfluidic Channels

Channel geometry combined with surface chemistry enables a stable liquid boundary flow to be attained along the surfaces of a 12 microm diameter hydrophilic glass fiber in a closed semi-elliptical channel. Surface free energies and triangular corners formed by PDMS/glass fiber or OTS/glass fiber surfaces are shown to be responsible for the experimentally observed wetting phenomena and formation of liquid boundary layers that are 20-50 microm wide and 12 microm high. Viewing this stream through a 20 microm slit results in a virtual optical window with a 5 pL liquid volume suitable for cell counting and pathogen detection. The geometry that leads to the boundary layer is a closed channel that forms triangular corners where glass fiber and the OTS coated glass slide or PDMS touch. The contact angles and surfaces direct positioning of the fluid next to the fiber. Preferential wetting of corner regions initiates the boundary flow, while the elliptical cross-section of the channel stabilizes the microfluidic flow. The Young-Laplace equation, solved using fluid dynamic simulation software, shows contact angles that exceed 105 degrees will direct the aqueous fluid to a boundary layer next to a hydrophilic fiber with a contact angle of 5 degrees. We believe this is the first time that an explanation has been offered for the case of a boundary layer formation in a closed channel directed by a triangular geometry with two hydrophobic wetting edges adjacent to a hydrophilic surface.

Joule heating and its effects on electroosmotic flow in microfluidic channels

Joule heating is resulted from inevitable volumetric heating when an electric field is applied across conducting media and it would impose limitations to the performance of electrokinetic microfluidic devices. In this paper, the Joule heating and its effects on electroosmotic flow (EOF) in PDMS microfluidic channels are reported. 3D numerical simulation of the Joule heating induced temperature field and its effects on the EOF and electrophoretic transport of solutes in microchannels was performed. Experiments were carried out to verify the proposed models and the developed numerical code. A Rhodamine B based thermometry technique was used to measure the solution temperature distributions in PDMS microfluidic channels. The micro particle image velocimetry (micro-PIV) technique was used to characterize the velocity profiles of the EOF under the influence of Joule heating. The numerical simulations were compared with the experimental results, and reasonable agreement is found. Both the numerical simulations and the experimental results show that the presence of the Joule heating causes the EOF velocity to deviate from its normal ‘‘plug-like’’ profile; the EOF velocity exhibits a concaved shape in the hydrodynamically developing region and a convex (parabolic) pattern in the fully developed region.

In situ and Online Monitoring of Hydrodynamic Flow Profiles in Microfluidic Channels Based upon Microelectrochemistry: Optimization of Electrode Locations

Herein, we extend our previous approach concerning the reconstruction of profiles of pressure-driven hydrodynamic flow in microfluidic channels based on current measurements at band electrode(s) [see the preceding paper ChemPhysChem 2005, 6, 1581]. We address the central issue of optimization of geometrical parameters describing the electrode(s) assembly (a single band and two bands working in generator-collector mode) within the channel flow cell to enhance the speed and precision of the flow profile reconstruction method.

Molecular dynamics simulations of oscillatory flows in microfluidic channels

In this paper we apply the direct non-equilibrium molecular dynamics technique to oscillatory flows of fluids in microscopic channels. Initially, we show that the microscopic simulations resemble the macroscopic predictions based on the Navier–Stokes equation very well for large channel width, high density and low temperature. Further simulations for high temperature and low density show that the non-slip boundary condition traditionally used in the macroscopic equation is greatly compromised when the fluid–wall interactions are the same as the fluid–fluid interactions. Simulations of a system with very narrow channel width confirm earlier findings of Poiseuille flow, namely, that the velocity profiles are modulated. We find that these modulations cannot be explained by the local area density model.

Optofluidic control using photothermal nanoparticles

Photothermal metallic nanoparticles have attracted significant attention owing to their energy-conversion properties. Here, we introduce an optofluidic application based on a direct optical-to-hydrodynamic energy conversion using suspended photothermal nanoparticles near the liquid-air interface. Using light beams with submilliwatt power, we can drive and guide liquid flow in microfluidic channels to transport biomolecules and living cells at controlled speeds and directions. Previously, a variety of methods for controlling microscale liquid flow have been developed owing to the increasing interest for microfluidics-based biochemical analysis systems. However, our method dispenses with the need for complex pump and valve devices, surface chemistry and electrode patterning, or any other further effort towards substrate fabrication. Instead, our optofluidic control method will allow the fabrication of all-optical large-scale integrated microfluidic circuits for biomolecular and cellular processing without any physical valve or mechanical pumping device.

Microfluidic integration of porous photonic crystal nanolasers for chemical sensing

We have recently developed planar photonic crystal nanolasers based on porous cavity designs. High-quality factor cavities confine light within the pores of the photonic crystal and, thus, our lasers are ideally suited for the investigation of nanoscale interactions between light and matter. We have demonstrated the operation of photonic crystal lasers within different chemical solutions, embedded them into silicone microfluidic flow channels, and were able to detect refractive index changes as small as /spl Delta/n=0.005. We predict that our porous nanolasers can detect refractive index changes as small as /spl Delta/n=8.23/spl middot/10/sup -4/.

DNA Molecules in Microfluidic Oscillatory Flow

The conformation and dynamics of a single DNA molecule undergoing oscillatory pressure-driven flow in microfluidic channels is studied using Brownian dynamics simulations, accounting for hydrodynamic interactions between segments in the bulk and between the chain and the walls. Oscillatory flow provides a scenario under which the polymers may remain in the channel for an indefinite amount of time as they are stretched and migrate away from the channel walls. We show that by controlling the chain length, flow rate and oscillatory flow frequency, we are able to manipulate the chain extension and the chain migration from the channel walls. The chain stretch and the chain depletion layer thickness near the wall are found to increase as the Weissenberg number increases and as the oscillatory frequency decreases.

Electroosmotic flow in microchannels with arbitrary geometry and arbitrary distribution of wall charge

General solutions are developed for direct current (DC) and alternating current (AC) electroosmotic flows in microfluidic channels with arbitrary cross-sectional geometry and arbitrary distribution of wall charge (zeta potential). The applied AC electric field can also be of arbitrary waveform. By proposing a nondimensional time scale ϖ defined as the ratio of the diffusion time of momentum across the electric double-layer thickness to the period of the applied electric field, we demonstrate analytically that the Helmholtz–Smoluchowski electroosmotic velocity is an appropriate slip condition for AC electroosmotic flows in typical microfluidic applications. With this slip condition approach, electroosmotic flows in rectangular and asymmetric trapezoidal microchannels with nonuniform wall charge, as examples, are investigated. The unknown constants in the proposed general solutions are numerically determined with a least-squares method through matching the boundary conditions. We find that the wall charge affects significantly the electroosmotic flow while the channel geometry does not. Moreover, the flow feature is characterized by another nondimensional time scale Ω defined as the ratio of the diffusion time of momentum across the channel hydraulic radius to the period of the applied electric field. The onset of phase shift between AC electroosmotic velocity and applied electric field is also examined analytically.