Mixing In Microchannels Research Articles

Electroosmotic mixing in microchannels

Mixing is an essential, yet challenging, process step for many Lab on a Chip (LOC) applications. This paper presents a method of mixing for microfluidic devices that relies upon electroosmotic flow. In physical tests and in computer simulations, we periodically vary the electric field with time to mix two aqueous solutions. Good mixing is shown to occur when the electroosmotic flow at the two inlets pulse out of phase, the Strouhal number is on the order of 1, and the pulse volumes are on the order of the intersection volume.

Electrohydrodynamic mixing in microchannels

AbstractElectrohydrodynamic flow generated by the motion of charge carriers in an electric field was investigated for rapid mixing in microchannel devices. A “tee” geometry of channels ranging from 40 to 110 μm in width and 30 to 60 μm in depth was used to contact two miscible fluids, such as alcohols. A fluorescent dye (Rhodamine B) was used to quantify mixing. A potential difference ranging from 0 to 900 V was applied between two electrodes separated by a distance of 450 μm, resulting in field strengths ranging from 0 to 2 × 106 V/m. Results show that when no electric field is applied, mixing of the two streams is driven by diffusion and is, therefore, slow. The mixing length, at which the concentration across the channel becomes essentially uniform, decrease significantly as the applied voltage was increased. There seems to be a threshold of applied voltage below which mixing is not enhanced by the electric field under the conditions studied. In strong electric fields, the mixing length was less than 150 μm, while in the absence of an electric field, the mixing length was greater than 5,000 μm.

Numerical investigation of mixing in microchannels with patterned grooves

Mixing in microchannels with patterned grooves was studied numerically by CFD simulations and particle tracking technique. Point location, velocity interpolation and a fourth-order adaptive Runge–Kutta integration scheme were applied in the particle tracking algorithms. Using these algorithms, Poincaré maps were calculated from the 3D velocity field exported from a CFD package for microfluidics (MemCFD™). For small aspect ratio (α = 0.05) grooves, the results showed that there was no significant irregularity in the Poincaré map, and indicated little chaotic effect. For high aspect ratio (α = 0.30) grooves, the flow pattern became more jumbled, but there was no apparent evidence that indicated the flow was chaotic, for Reynolds numbers up to five. However, Poincaré maps could still be used to evaluate the performance of this type of micro-mixer. The particle trajectories recorded in the Poincaré maps were circular-like patterns. By counting the number of dots to form one circle in the Poincaré maps, the length of the channel to enable one recirculation could be calculated. The results indicated that this length had an exponential relation to the aspect ratio of grooves, and it was independent of the flow velocity. The CFD simulation showed that the transverse motion could fold and stretch fluids to increase their interfacial area. The results showed that micro-mixers with patterned grooves caused rotation of the fluid streams. This rotation can reorient the folding in the depth direction, and can enhance passive mixing in microfluidic devices with shallow channels.

Efficient spatial-temporal chaotic mixing in microchannels

A chaotic micro mixer with multiple side channels is designed and investigated, in which fluid can be stirred by pumps through the side channels. By stretching and folding fluid in the main and side channels, chaotic mixing can be achieved. A simple mathematic model is derived to understand the movement of particles in the microchannel. Spatial trajectories of fluid particles are projected to Poincaré sections by mapping. The route from the quasi-period to chaos is revealed to be destruction of KAM curves and shrinkage of the quasi-periodic areas. Lyapunov exponents (LE) are used as the mixing index and the criteria to evaluate the chaotic behavior of the system. We found that LE is closely related to the amplitude and frequency of stirring and can be used to optimize our design and operation. From the relationship of LE and striation thickness, the minimal mixing length required can be estimated, which is much shorter than that needed in passive mixer design.

Electro-hydrodynamic micro-fluidic mixer.

Fluid mixing in microchannels is needed for many applications ranging from bio-arrays to micro-reactors, but is typically difficult to achieve. A simple geometry micro-mixer is proposed based on the electro-hydrodynamic (EHD) force present when the fluids to be mixed have different electrical properties and are subjected to an electric field. The electrodes are arranged so that the electric field is perpendicular to the interface between the two fluids, creating a transversal secondary flow. The technique is demonstrated experimentally using the flow of two liquids with identical viscosity and density, but different electrical properties. The volume flow rate and average velocity are 0.26 microl s(-1) and 4.2 mm s(-1), respectively, corresponding to a Reynolds number Re= 0.0174. The effect of a continuous (DC) electric field and two alternating (AC)- sinusoidal and square - electric fields is explored. At the appropriate parameter values, very good mixing takes place in less than 0.1 s, over a very short distance (within a fraction of the width 250 microm of the electrodes).

Visualization of convective mixing in microchannel by fluorescence imaging**Selectedpaper from the Fourth International Symposium on Particle Image Velocimetry(Göttingen, 2001).

A novel measurement technique using fluorescent dye in combination withmicroresolution particle image velocimetry (micro-PIV) has been devised toinvestigate convective mixing in microspace. Tris(bipyridine)ruthenium(II), whosefluorescent intensity when excited by ultraviolet light is strongly temperaturedependent, was applied to the bottom surface of a cover glass, that served as theupper boundary surface of a flow channel. This set-up thus realized atwo-dimensional temperature measurement of the microflow channel. Aspatial resolution of 5 µm × 5 µm and a temperature resolution of 0.26 Kwere achieved by using a cooled CCD camera and a 10× objective lensof a microscope. Pure water at differing temperatures was injected intoopposite inlets of a T-shaped microchannel bound by cover glass and PDMS.The mixing process in the junction area was visualized by the presenttemperature and the micro-PIV techniques. The convective heat flux wascalculated from measurement of velocity and temperature and compared to theheat conduction. It is found that the heat flux due to conduction waslarger than that due to convection, thus it is noted that heat conductionmay be an important factor in the design process of microfluidic devices.

A magnetic microstirrer and array for microfluidic mixing

We report the development of a micromachined magnetic-bar micromixer for microscale fluid mixing in biological laboratory-on-a-chip applications. The mixer design is inspired by large scale magnetic bar mixers. A rotating magnetic field causes a single magnetic bar or an array of them to rotate rapidly within a fluid environment. A fabrication process of the magnetic bar mixer is developed. Results of fluid mixing in micro channels and chambers are investigated using experimental means and computer-aided fluid simulation.

Optimizing layout of obstacles for enhanced mixing in microchannels

To improve mixing, obstacles have been placed in the channel to try to disruptflow and reduce the diffusion path. The disruption to flow velocity field alters theflow direction from one fluid to another. In this way, convection may occur toenhance the mixing. Ideally, properly designed geometric parameters,such as layout and number of obstacles, improve the mixing performancewithout increasing the pressure drop. In this work, CoventorWareTM, acommercial computational fluid dynamics tool for microfluidics was used to studythe mixing of two liquids in a ‘Y’ channel. The results indicate that asymmetriclayout of the obstacle has more effect on the mixing than the number of obstacles.Placing obstacles in the microchannels is a novel method for mixing inmicrofluidic devices, and the results can provide useful information in the designof these devices.

Using patterned substrates to promote mixing in microchannels.

Using a lattice Boltzmann model for fluid dynamics, we investigate the flow and phase behavior of a binary fluid moving over a patterned substrate within a microchannel. The binary fluid consists of two immiscible components, A and B, and this liquid is subjected to a Poiseuille flow. The substrate is decorated with a checkerboard pattern of A- and B-like patches. Through a coupling of hydrodynamics and thermodynamics, each component is driven to flow from the nonwettable domains to wettable regions. As a consequence, the A and B fluids undergo extensive mixing within the microchannels. We investigate how the degree of mixing depends on the size of the patches, the velocity of the imposed flow field, and the characteristics of the fluid. The results provide guidelines for creating localized "mixing stations" within microfluidic devices. The findings also reveal how a combination of imposed flow fields and surface patterning can be exploited to control the phase behavior of complex fluids.

Visualization of Convective Mixing in Microchannel by Fluorescence Imaging.

A two-dimensional temperature measurement technique was developed using fluorescence in order to investigate convective mixing phenomena in a microspace. The present study focused on the spatial and temperature resolution using a fluorescence dye, i.e., Ru(bpy)2+3, whose flutrescence intensity is strongly dependent on temperature, yielding a precise measurement in the microspace. The temperature resolution obtained from this study showed ± 0.26 K in the range from 297 K to 334 K at the spatial resolution of 5×5 μm. The present work utilized a T-shaped microchannel with two inlets, which was fabricated with silicone elastamer using the replica molding technique. Velocity measurements in the junction area of the microchannel were performed by a micro-resolution particle image velocimetry. It was observed that the conduction heat flux was more dominant than the convection heat flux when two fluids at different temperature merged together in the junction area. The measurement techniques in microspace accomplished in the present study have an ability to visualize and to investigate mass and thermal diffusion in the microchannel, which will contribute significantly to develop the microdevices.