Flow In Microfluidic Channels Research Articles

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.

Characterization of DNA hybridization kinetics in a microfluidic flow channel

In this investigation we report on the influence of volumetric flow rate, flow velocity, complementary DNA concentration, height of a microfluidic flow channel and time on DNA hybridization kinetics. A syringe pump was used to drive Cy3-labeled target DNA through a polydimethylsiloxane (PDMS) microfluidic flow channel to hybridize with immobilized DNA from the West Nile Virus. We demonstrate that a reduction of channel height, while keeping a fixed volumetric flow rate or a fixed flow velocity, enhances mass transport of target DNA to the capture probes. Compared to a passive hybridization, the DNA hybridization in the microfluidic flow channel generates higher fluorescence intensities for lower concentration of target DNA during the same fixed period of time. Within a fixed 2 min time period the fastest DNA hybridization at a 50 pM concentration of target DNA is achieved with a continuous flow of target DNA at the highest flow rate and the lowest channel height.

Velocity-independent microfluidic flow cytometry.

Pressure-driven flow in microfluidic channels is characterized by a distribution of velocities. This distribution makes it difficult to implement conventional flow cytometry data analysis. We have demonstrated a method to measure velocity as an independent parameter when performing microfluidic flow cytometry. This method allows velocity-independent analysis of particles such as beads or cells, and allows flow cytometry analysis of extended objects, such as long DNA molecules. It allows accurate flow cytometry in transient and nonuniform flows. This general measurement method could be used in the future to measure the velocity of particles in a variety of existing microfluidic devices without the need for changes in their design.

Passive electrophoresis in microchannels using liquid junction potentials.

The formation of the liquid junction potential (LJP) is a well-studied phenomenon that occurs in the presence of ionic concentration gradients. Although the LJP has been well characterized, its impact has generally been overlooked in microfluidic applications. The characteristics of flow in microfluidic channels cause this phenomenon to be particularly important, both as a source of deviation from anticipated results and as a tool capable of being harnessed to perform useful tasks. It is demonstrated that LJPs formed in microchannels can induce appreciable electrophoretic transport of charged species without the use of electrodes or an external power supply. This process is demonstrated in an H-filter (an H-shaped microfluidic channel used to bring two fluids into contact allowing extraction of diffusing species from one stream to another) by generating junction potentials between two flowing streams containing different concentrations of strong electrolytes and observing the mass transport of the charged dye fluorescein between those streams. It is shown that the LJP can be controlled to either accelerate or decelerate mass transport across a fluid interface in the absence of an interposed membrane. A preliminary mathematical description of the phenomena is offered to support the hypothesis that the observed mass transport is a result of the LJP. Possible practical microfluidic applications of electrophoretic transport without electrodes are discussed.

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.

Effect of analyte adsorption on the electroosmotic flow in microfluidic channels.

The predictability and constancy over time of the electroosmotic flow in microchannels is an important consideration in microfluidic devices. A common cause for alteration of the flow is the adsorption of analytes to channel walls, for example, during capillary electrophoresis of proteins. It is shown that certain experimental data, published by Towns and Regnier (Towns, J. K; Regnier, F. E. Anal. Chem. 1992, 64, 2473-2478.), on the anomalous elution times for proteins in capillary electrophoresis can be explained using a simple model for analyte adsorption that uses a result first reported by Anderson and Idol (Anderson, J. L.; Idol, W. K Chem. Eng. Commun. 1985, 38, 93-106.) on the electroosmotic flux in capillaries with axial variations in zeta-potential. It is suggested that it might be possible to use such a model to dynamically correct for altered elution times in capillary electrophoretic devices.

Polymer-Based Packaging Platform for Hybrid Microfluidic Systems

A polymer-based packaging platform for creating hybrid microfluidic systems is presented. Polydimethylsiloxane (PDMS) is cast into an acrylic mold frame with suspended elements that are removed after curing to form chip cavities, inlet and outlet ports, microchannels, and reservoirs. The packaging approach enables the integration of off-the-shelf components such as pumps and valves with glass microfluidic devices, electronic chips, sample reservoirs, and flow channels. A particle pre-concentration module with a glass capture chip and integrated micropump is shown as an example. A pneumatically driven microfluidic pumping module is also shown. Custom microfluidic interconnects for interfacing to micro-scale fluidic systems are presented. The connectors are capable of withstanding more than 1000 psi and allow microdevices to be rapidly connected to macroscopic devices and systems, without the use of tools.

Velocity measurement of particulate flow in microfluidic channels using single point confocal fluorescence detection.

This article presents a non-invasive, optical technique for measuring particulate flow within microfluidic channels. Confocal fluorescence detection is used to probe single fluorescently labeled microspheres (0.93 microm diameter) passing through a focused laser beam at a variety of flow rates (50 nL min(-1)-8 microL min(-1)). Simple statistical methods are subsequently used to investigate the resulting fluorescence bursts and generate velocity data for the flowing particles. Fluid manipulation is achieved by hydrodynamically pumping fluid through microchannels (150 microm wide and 50 microm deep) structured in a polydimethylsiloxane (PDMS) substrate. The mean fluorescence burst frequency is shown to be directly proportional to flow speed. Furthermore, the Poisson recurrence time and width of recovered autocorrelation curves is demonstrated to be inversely proportional to flow speed. The component-based confocal fluorescence detection system is simple and can be applied to a diversity of planar chip systems. In addition, velocity measurement only involves interrogation of the fluidic system at a single point along the flow stream, as opposed to more normal multiple-point measurements.

Formation of natural pH gradients in a microfluidic device under flow conditions: model and experimental validation.

A new isoelectric focusing technique has been developed that incorporates natural pH gradient formation in microfluidic channels under flowing conditions. In conjunction, a one-dimensional finite difference model has been developed that solves a system of algebraic-ordinary differential equations that describe the phenomena occurring in the system, including hydrolysis at the electrodes, buffering effects of weak acids and bases, and mass transport due to both diffusion and electrophoresis. A quantitative, noninvasive, optically based method of monitoring pH gradient formation is presented, and the experimental data generated by this method are found to be in good agreement with model predictions. In addition, the model provides a theoretical explanation for initially unexpected experimental results. Model predictions are also shown to match well with experimental results of microfluidic isoelectric focusing of a single protein species. Accounting for the nonuniform velocity profile, characteristic of pressure-driven flow in microfluidic channels, is found to improve predictions of dynamic pH changes close to the electrodes and overall time required to reach steady state, but to reduce the accuracy of dynamic pH change predictions in other regions of the channel.

A Novel Instrument for Studying the Flow Behaviour of Erythrocytes through Microchannels Simulating Human Blood Capillaries

A novel instrument has been developed to study the microrheology of erythrocytes as they flow through channels of dimensions similar to human blood capillaries. The channels are produced in silicon substrates using microengineering technology. Accurately defined, physiological driving pressures and temperatures are employed whilst precise, real-time image processing allows individual cells to be monitored continuously during their transit. The instrument characterises each cell in a sample of ca. 1000 in terms of its volume and flow velocity profile during its transit through a channel. The unique representation of the data in volume/velocity space provides new insights into the microrheological behaviour of blood. The image processing and subsequent data analysis enable the system to reject anomalous events such as multiple cell transits, thereby ensuring integrity of the resulting data. By employing an array of microfluidic flow channels we can integrate a number of different but precise and highly reproducible channel sizes and geometries within one array, thereby allowing multiple, concurrent, isobaric measurements on one sample. As an illustration of the performance of the system, volume/velocity data sets recorded in a microfluidic device incorporating multiple channels of 100 μm length and individual widths ranging between 3.0 and 4.0 μm are presented.