CiteULike: tathabhatt's library [113 articles]

A Theoretical and Experimental Study of Charge and Discharge Cycles in a Storage Vessel for Adsorbed Natural Gas

Moises Bastos-Neto — 2005-12-21T10:29:12-00:00

Adsorption, Vol. 11, No. 2. (March 2005), pp. 147-157.

This study presents experimental data of storage and delivery tests of methane on activated carbon carried out in a prototype vessel at pressures up to 40 atm. Adsorption equilibrium data at high pressure were measured using a gravimetric apparatus. Experimental data obtained from the storage/delivery tests are compared to those obtained from process simulation using a dynamic model. The simulation model was run using the measured equilibrium data as input parameters. A good agreement was observed between experimental and simulated results. Histories of pressure and stored mass were satisfactorily well predicted. Despite heat effects, not precisely taken into account in the model, there was a reasonably good agreement between simulation and experiment for the average temperature inside the vessel.

Optically driven viscous micropump using a rotating microdisk

Shoji Maruo — 2008-03-14T02:38:37-00:00

Applied Physics Letters, Vol. 91, No. 8. (2007)

An optically driven micropump using viscous drag exerted on a rotating disk microrotor was developed. The disk microrotor (diameter of 10 µm), which has three columns as targets for the optical trap, is confined to a U-shaped microchannel. To pump fluid, the disk microrotor is rotated by a time-shared optical trapping technique. The flow field inside the U-shaped microchannel was analyzed using finite element method (FEM) based on the Navier-Stokes equation. The optimized micropump was fabricated using a two-photon microfabrication technique. The flow rate of the micropump agreed with simulation result obtained by FEM analysis.

A semi-discrete and non-local crystal plasticity model for nanocrystalline metals

DH Warner — 2008-03-12T04:31:48-00:00

Scripta Materialia, Vol. 54, No. 7. (April 2006), pp. 1397-1402.

The application of traditional continuum crystal plasticity models to nanocrystalline metals is discussed and compared to a novel semi-discrete crystal plasticity model. The new model exhibits larger stress heterogeneities and greater amounts of intragranular plasticity.

Solids Movement in Rotary Kilns in the Slumping Regime: Model Using a Control Plane Parallel to the Steepest Descent

Luis Gonzalez — 2008-03-11T21:41:54-00:00

Particle & Particle Systems Characterization, Vol. 22, No. 2. (2005), pp. 119-132.

A new approach is proposed to model the bulk movement of solids in rotary drums operating at low rotation speeds, in slumping and rolling regimes. The model yields an equation similar to Saeman's equation, but which is valid also for the slumping regime and active area in the rolling regime.The model was developed for constant depth using a control surface containing the steepest descent direction, so that any contribution from sliding particles to flow rate can be neglected. By considering an appropriate virtual kiln, the model is extended to the more general, variable depth situation.

A new DPIV proceeding algorithm and its application in particle motion study in a rotary drum

Zhixiao Zhang — 2008-03-11T21:39:40-00:00

Journal of Thermal Science, Vol. 11, No. 2. (5 May 2002), pp. 186-192.

Abstract A new digital particle image velocimetry (DPIV) proceeding method, namely, consecutive motion vector estimation algorithm, which fully employs the global information of sequential gray images and the physics property of flow field to get the precise velocity vector at each pixel, is proposed. Therefore it enlarges the application range of DPIV, especially for high-speed flow, large velocity gradient flow and rotational flow. The new method is applied to study the particle motion qualitatively in a partially filled rotary drum in this work. The results indicate that the newly developed algorithm describes the real flow field of granular motion in the transverse plane of rotary drum more accurately than the conventional cross-correlation method, which is adopted here as the comparative one. It can be concluded from DPIV analysis that the burden bed in rotary drum consists of two distinct regions, the active layer and the plug flow region. Moreover, the boundary of these two regions is an arc and the rotational speed plays an important role in the particles flow field.

Residence time distribution and material flow studies in a rotary kiln

P Sai — 2008-03-11T21:38:49-00:00

Metallurgical and Materials Transactions B, Vol. 21, No. 6. (14 December 1990), pp. 1005-1011.

Abstract Experiments were conducted in a rotary kiln containing ilmenite particles to study the residence time distribution (RTD) of low-density particles, holdup, and bed depth profile. The variables include feed rate of solids, slope and rotational speed of the kiln, type and size of the tracer, and dam height. Correlations are presented for mean residence time, dispersion number, holdup, and steady-state throughput of solids in terms of the process variables. A simple method is proposed to estimate the dam height that gives rise to a flat profile of solids bed along the length of the kiln.

The Transient Response of Granular Flows in an Inclined Rotating Cylinder

RJ Spurling — 2008-03-11T21:27:20-00:00

Chemical Engineering Research and Design, Vol. 79, No. A1. (January 2001), pp. 51-61.

This paper reports the results of an experimental and theoretical study to investigate the transient response of the granular flow through a laboratory scale inclined rotating cylinder to large step changes in one of three variables: (i) mass feed rate, (ii) rotation speed or (iii) axis inclination. Experimental measurements are reported for a range of operating conditions, sizes of step and a number of cylinder geometries. A mechanistic model for the transient response is derived based on a published steady state model. The dynamic model parameters are the cylinder radius, length, discharge dam height, axis inclination and rotation speed, the granular feed rate, and the bulk density and dynamic angle of repose of the granular material. The model has no adjustable parameters, making it useful for scale-up. The model takes the form of a non-linear partial differential equation, equivalent to a one dimensional unsteady diffusion equation with variable coefficients, and solution has been obtained numerically. Good agreement is found between the model and experiment both in the range of cases considered in the current study, and also for published experimental work, for which the cylinder size and granular material properties differ substantially from those of this work.

Flow of materials in rotary kilns used for sponge iron manufacture: Part III. Effect of ring formation within the kiln

Amit Chatterjee — 2008-03-11T21:20:25-00:00

Metallurgical and Materials Transactions B, Vol. 14, No. 3. (1983), pp. 393-399.

Abstract The formation of accretions or ’rings’ in rotary kilns used for the manufacture of directly reduced iron (sponge iron) affects the residence time of the charge, kiln hold-up, and the kiln output to a great extent. In this part of the work, the effect of ring formation was simulated at room temperature in a scaled-down model of a rotary kiln by inserting conical dams in the shape of frustrums of cones through the feed end of the kiln. The influence of such conical dams on various operating parameters was evaluated.

Flow of materials in rotary kilns used for sponge iron manufacture: Part I. Effect of some operational variables

Amit Chatterjee — 2008-03-11T21:16:15-00:00

Metallurgical and Materials Transactions B, Vol. 14, No. 3. (1983), pp. 375-381.

Abstract Looking forward to the need of developing coal-based sponge iron technology in India, a country having no significant resources of either coking coal or natural gas, the Research and Development Division of the Tata Iron and Steel Company Limited (TISCO) set up a rotary kiln based direct reduction pilot plant in 1975. In this pilot plant, a totally indigenous technology for production of sponge iron has been developed in which non-coking coal is essentially used as the reductant. For easy scaling up of the TISCO Direct Reduction (TDR) process to units of 300 to 400 tpd capacity, it was thought necessary to explore some of the fundamental aspects of material flow in a rotary kiln. This was carried out by studying the flow of materials in room temperature models. The work was divided into two parts: first, a study of the influence of several operating parameters,viz., rotational speed, inclination of the kiln, effect of circular dams at the feed and the exit ends of the kiln,etc., and second, an investigation of the extent of segregation of a mixture of solids in a rotating kiln. The highlights of the experimental results dealing mainly with the effect of various kiln operating variables on the filling degree profile of the charge in the kiln are presented.

Flow of materials in rotary kilns used for sponge iron manufacture: Part II. Effect of kiln geometry

Amit Chatterjee — 2008-03-11T21:14:47-00:00

Metallurgical and Materials Transactions B, Vol. 14, No. 3. (1983), pp. 383-392.

Abstract The present work identifies the basic features of burden movement in a rotary kiln. The cold model study was conducted with iron ore as the feed material to determine the influence of length to diameter ratio (L/D) of a rotary kiln on the filling degree, hold-up, and residence time of the charge. An empirical equation correlating different operating variables has been derived on the basis of the experimental results. The influence of individual parameters under different conditions on the residence time and back spillage has also been evaluated.

Granular dynamics of inelastic spheres in Couette flow

Cliff Lun — 2008-03-11T20:42:46-00:00

Physics of Fluids, Vol. 8, No. 11. (1996), pp. 2868-2883.

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A simple kinetic theory for granular flow of binary mixtures of smooth, inelastic, spherical particles

M Farrell — 2008-03-11T20:41:37-00:00

Acta Mechanica, Vol. 63, No. 1. (29 November 1986), pp. 45-60.

Summary Following the approach of the kinetic theory for mixtures of dense gases, the general conservation equations for the rapid flow of a binary mixture of smooth, inelastic, spherical granular particles are derived. Explicit constitutive relations for stress and rate of energy dissipation are obtained by making simple approximations for the particle velocity distribution functions. These approximations are appropriate for cases where collisional interactions are the dominant mechanism for momentum and energy exchange in the system. The theory is applied to the case of simple shear flow. In general, the theory predicts that stresses decrease with increasing concentration of the small particles and decreasing diameter ratio of small to large particles. Theoretical predictions of stresses are compared with experimental results and reasonable agreement is found.

Numerical simulation of inelastic frictional spheres in simple shear flow

CKK Lun — 2008-03-11T20:39:36-00:00

Journal of Fluid Mechanics, Vol. 258 (1994), pp. 335-353.

A numerical program is developed to simulate an assembly of inelastic frictional spheres inside a control volume undergoing rapid shearing motion induced by the top and bottom moving periodic boundaries. A sticking–sliding collision model is used to emulate binary collisions of real particles. After the flow has reached a steady state, ensemble averages of macroscopic properties such as translational and rotational granular temperatures, and kinetic and collisional stresses at different solids concentrations are obtained. The present results are compared with previous theoretical, numerical and experimental works, and favourable agreement is found among them. The simulation results show that the stresses are anisotropic and decrease with decreasing coefficient of restitution and increasing friction coefficient. At high solids fraction, above about 0.5, there exists a critical concentration where the layering effects of particles, the formation of high-density microstructures and the increase in correlation of particle velocities are the major causes of abrupt changes in flow properties.

The effects of an impact velocity dependent coefficient of restitution on stresses developed by sheared granular materials

C Lun — 2008-03-11T20:36:32-00:00

Acta Mechanica, Vol. 63, No. 1. (29 November 1986), pp. 15-44.

Summary Following the granular flow kinetic theory of Lun, Savage, Jeffrey and Chepurniy, a moment method is used to obtain the approximate form for the single particle velocity distribution function for the case of smooth, slightly inelastic, uniform spherical particles in which the coefficient of restitutione depends upon the particle impact velocity. Constitutive equations for stress are derived and the theory is applied to the case of a simple shear flow. Theoretical predictions of stresses are compared with experimental results. The effect of the impact velocity dependente is to cause the stresses to vary with the shear rate raised to a power less than two; this is consistent with the experimental observations. On the basis of the present theory and comparisons with experimental data it is concluded that theoretical models which include both surface friction and an impact velocity dependente will lead to improved agreement between the theoretical predictions and the measurements.

Kinetic theory for granular flow of dense, slightly inelastic, slightly rough spheres

CKK Lun — 2008-03-11T20:34:31-00:00

Journal of Fluid Mechanics, Vol. 233 (1991), pp. 539-559.

A general set of conservation equations and constitutive integrals for the dynamic properties of the rapid flow of a granular material consisting of slightly inelastic and slightly rough spherical particles is derived by following an approach used in the kinetic theory of dense gases. By taking moments of the translational and rotational particle velocities in the general transport moment equation and making the Enskog approximation, the singlet velocity distribution function is determined. As a result, the constitutive relations and coefficients such as stresses, energy fluxes, rates of translational and rotational energy interchanges, shear viscosity, spin viscosity, bulk viscosity and ‘thermal’ conductivities are obtained. The present theory incorporates the kinetic as well as the collisional contributions for stresses and energy fluxes. Thus, it is appropriate for dilute as well as dense concentrations of solids. For the case of simple shear flow, there is favourable agreement between the theoretical predictions of stresses and both the experimental measurements and the results from computer simulations.

Particle size segregation in inclined chute flow of dry cohesionless granular solids

SB Savage — 2008-03-11T20:29:14-00:00

Journal of Fluid Mechanics, Vol. 189 (1988), pp. 311-335.

If granular materials comprising particles of identical material but different sizes are sheared in the presence of a gravitational field, the particles are segregated according to size. The small particles fall to the bottom and the larger ones drift to the top of the sheared layer. In an attempt to isolate and study some of the essential segregation mechanisms, the paper considers a simplified problem involving the steady two-dimensional flow of a binary mixture of small and large spherical particles flowing down a roughened inclined chute. The flow is assumed to take place in layers that are in motion relative to one another as a result of the mean shear. For relatively slow flows, it is proposed that there are two main mechanisms responsible for the transfer of particles between layers. The first mechanism, termed the ‘random fluctuating sieve’, is a gravity-induced, size-dependent, void-filling mechanism. The probability of capture of a particle in one layer by a randomly generated void space in the underlying layer is calculated as a function of the relative motion of the two layers. The second, termed the ‘squeeze expulsion’ mechanism, is due to imbalances in contact forces on an individual particle which squeeze it out of its own layer into an adjacent one. It is assumed that this mechanism is not size preferential and that there is no inherent preferential direction for the layer transfer. This second physical mechanism in particular was proposed on the basis of observations of video recordings that were played back at slow speed. Since the magnitude of its contribution is determined by the satisfaction of overall mass conservation, the exact physical nature of the mechanism is of less importance. By combining these two proposed mechanisms the net percolation velocity of each species is obtained. The mass conservation equation for fines is solved by the method of characteristics to obtain the development of concentration profiles with downstream distance. Although the theory involves a number of empirical constants, their magnitude can be estimated with a fair degree of accuracy. A solution for the limiting case of dilute concentration of fine particles and a more general solution for arbitrary concentrations are presented. The analyses are compared with experiments which measured the development of concentration profiles during the flow of a binary mixture of coarse and fine particles down a roughened inclined chute. Reasonable agreement is found between the measured and predicted concentration profiles and the distance required for the complete separation of fine from coarse particles. doi:10.1017/S002211208800103X

Segregation in binary mixtures

M Larcher — 2008-03-11T20:18:33-00:00

Geophysical Research Abstracts, Vol. 8 (2006), 09240.

To investigate the vertical structure of free-surface liquid-granular flows, it is of particular interest to be able to materialise steady flow conditions. In the recent past, a thorough activity has been devoted to the rheology of flowing mixtures of well-sorted granular particles and water (Armanini et al., 2005). In this contribution, we present a preliminary analysis of the evolution, in time and space, of bimodal mixtures in quasiuniform condition. In accordance with experimental evidence given by Savage & Lun (1988), Gray & Hutter (1997) and Gray (2001), segregation phenomena of particles of different sizes have been observed, with the formation of layers with the largest particles on top and the smaller at the bottom. This phenomenon has been investigated with the support of a newly developed imaging technique, based on the development of findings by Spinewine et al. (2003), able to tackle non-uniformly sized granular materials (within IMPACT European Project, 2001-2004). Moreover, special devices were built to allow width-average and local measurements of the distribution of the relative, and absolute, solid-concentration of the two size fractions. Furthermore, data from sidewall image and local measurements allow also to infer some information about three dimensional effects. The larger sediments employed are PVC particles with an equivalent-spherical diameter equal to 3.7 mm, also utilized for the experiments with a single size-fraction, while the smaller are quite uniform plastic spheres having a mean diameter of 0.9 mm. Both materials have a density of about 1570 kg/m3. In the experiments, a mixture of water and sediments was introduced steadily in the upstream part of the flume in well-mixed conditions, with a quite uniform distribution of both granulometric classes throughout the flow depth. Setup, initial and boundary conditions have been designed to have flows running over the rigid rough pavement of the flume, with no deposition phenomena. A progressive particle segregation was observed in downstream direction, with the larger particles migrating upwards and the smaller ones downwards. The distance measured experimentally at which segregation is almost complete was compared with the values predicted by the theory of Vallance & Savage (2000). Theoretical results can be considered results in good accordance with experimental evidence.

Kinetic theories for granular flow: inelastic particles in Couette flow and slightly inelastic particles in a general flow field

CKK Lun — 2008-03-11T20:08:05-00:00

Journal of Fluid Mechanics, Vol. 140 (1984), pp. 223-256.

The flow of an idealized granular material consisting of uniform smooth, but nelastic, spherical particles is studied using statistical methods analogous to those used in the kinetic theory of gases. Two theories are developed: one for the Couette flow of particles having arbitrary coefficients of restitution (inelastic particles) and a second for the general flow of particles with coefficients of restitution near 1 (slightly inelastic particles). The study of inelastic particles in Couette flow follows the method of Savage & Jeffrey (1981) and uses an ad hoc distribution function to describe the collisions between particles. The results of this first analysis are compared with other theories of granular flow, with the Chapman-Enskog dense-gas theory, and with experiments. The theory agrees moderately well with experimental data and it is found that the asymptotic analysis of Jenkins & Savage (1983), which was developed for slightly inelastic particles, surprisingly gives results similar to the first theory even for highly inelastic particles. Therefore the ‘nearly elastic’ approximation is pursued as a second theory using an approach that is closer to the established methods of Chapman-Enskog gas theory. The new approach which determines the collisional distribution functions by a rational approximation scheme, is applicable to general flowfields, not just simple shear. It incorporates kinetic as well as collisional contributions to the constitutive equations for stress and energy flux and is thus appropriate for dilute as well as dense concentrations of solids. When the collisional contributions are dominant, it predicts stresses similar to the first analysis for the simple shear case.

Granular flow in a rotating cylindrical drum

T Elperin — 2008-03-11T20:03:43-00:00

Europhysics Letters, Vol. 42, No. 6. (15 June 1998), pp. 619-623.

Flow of granular material in a partially filled rotating cylinder is studied using a boundary layer approach. Simple equations are derived to describe granular dynamics in a transverse plane. A small parameter is recognized, and the derived equations are solved analytically in the first-order approximation of the small parameter. Cascading layer thickness, mean velocity along the layer and profile of the free surface are determined in a closed analytical form.

Granular flow behaviour in the transverse plane of a partially filled rotating cylinder

AA Boateng — 2008-03-11T19:58:23-00:00

Journal of Fluid Mechanics, Vol. 330 (January 1997), pp. 233-249.

Material flow in partially filled rotating cylinders (rotary kilns) is encountered in many practical applications of material processing, for example incineration, calcination, grain drying, etc. The flow behaviour in the cross-section is important to other transport mechanisms such as mixing and energy distribution within the bed material. The paper describes an experimental study which was carried out with the objective of understanding and improving our predictive capabilities of the rheological behaviour of granular materials in rotary cylinders. Measurement techniques similar to that used in chute flows have been employed to measure flow characteristics, e.g. particle velocities, granular temperature, and solid concentration (in the shear layer developed between the free surface and the bulk of the bed) for different materials having a wide range of coefficients of restitution. The results of the experiments provide the necessary assumptions, constraints, and data for granular flows in partially filled rotating cylinders.

The Rotary Kiln: An Investigation of Bed Heat Transfer in the Transverse Plane

S Dhanjal — 2004-12-28T16:39:24-00:00

Metallurgical and Materials Transactions B, Vol. 35, No. 6., 1059.

Chaotic Mixing in a Steady Flow in a Microchannel

Claire Simonnet — 2008-03-11T19:37:05-00:00

Physical Review Letters, Vol. 94, No. 13. (2005)

We report experiments on mixing of a passively advected fluorescent dye in a low Reynolds number flow in a microscopic channel. The channel is a chain of repeating segments with a custom designed profile that generates a steady three-dimensional flow with stretching and folding, and chaotic mixing. A few statistical characteristics of mixing in the flow are studied and are all found to agree with theoretical and experimental results for the flows in the Batchelor regime of mixing that are chaotic in time. The proposed microchannel provides fast and efficient mixing and is simple to fabricate.

Geometrical optimization of helical flow in grooved micromixers

Scott Lynn — 2008-03-11T19:33:20-00:00

Lab on a Chip, No. 7. (2007), pp. 580-587.

Owing to the enhancement of surface effects at the micro-scale, patterned grooves on a microchannel floor remain a powerful method to induce helical flows within a pressure driven system. Although there have been a number of numerical studies on geometrical effects concerning fluid mixing within the staggered herringbone mixer, all have focused mainly on the groove angle and depth, two factors that contribute greatly to the magnitude of helical flow. Here we present a new geometrical factor that significantly affects the generation of helical flow over patterned grooves. By varying the ratio of the length of the grooves to the neighboring ridges, helical flow can be optimized for a given groove depth and channel aspect ratio, with up to 50% increases in transverse flow possible. A thorough numerical study of over 700 cases details the magnitude of helical flow over unsymmetrical patterned grooves in a slanted groove micro-mixer, where the optimized parameters for the slanted groove mixer can be translated to the staggered herringbone mixer. The optimized groove geometries are shown to have a large dependence on the channel aspect ratio, the groove depth ratio, and the ridge length.

Chaotic mixing using periodic and aperiodic sequences of mixing protocols in a micromixer

Tae Kang — 2008-03-11T19:28:52-00:00

Microfluidics and Nanofluidics

Abstract We conducted a numerical study on mixing in a barrier embedded micromixer with an emphasis on the effect of periodic and aperiodic sequences of mixing protocols on mixing performance. A mapping method was employed to investigate mixing in various sequences, enabling us to qualitatively observe the progress of mixing and also to quantify both the rate and the final state of mixing. First, we introduce the design concept of the four mixing protocols and the route to achieve chaotic mixing of the mixer. Then, several periodic sequences consisting of the four mixing protocols are used to investigate the mixing performance depending on the sequence. Chaotic mixing was observed, but with different mixing rates and different final mixing states significantly influenced by the specific sequence of mixing protocols and inertia. As for the effect of inertia, the higher the Reynolds number the larger the rotational motion of the fluid leading to faster mixing. We found that a sequence showing the best mixing performance at a certain Reynolds number is not always superior to other sequences in a different Reynolds number regime. A properly chosen aperiodic sequence results in faster and more uniform mixing than periodic sequences.

Quantification of chaotic strength and mixing in a micro fluidic system

Ho Kim — 2008-03-11T19:26:50-00:00

Journal of Micromechanics and Microengineering, Vol. 17, No. 11. (1 November 2007), pp. 2197-2210.

Comparative studies of five different techniques commonly employed to identify the chaotic strength and mixing efficiency in micro fluidic systems are presented to demonstrate the competitive advantages and shortcomings of each method. The ‘chaotic electroosmotic stirrer’ of Qian and Bau (2002 Anal. Chem. 74 3616–25) is utilized as the benchmark case due to its well-defined flow kinematics. Lagrangian particle tracking methods are utilized to study particle dispersion in the conceptual device using spectral element and fourth-order Runge–Kutta discretizations in space and time, respectively. Stirring efficiency is predicted using the stirring index based on the box counting method, and Poincar´e sections are utilized to identify the chaotic and regular regions under various actuation conditions. Finite time Lyapunov exponents are calculated to quantify the chaotic strength, while the probability density function of the stretching field is utilized as an alternative method to demonstrate the statistical analysis of chaotic and partially chaotic cases. Mixing index inverse, based on the standard deviation of scalar species distribution, is utilized as a metric to quantify the mixing efficiency. Series of numerical simulations are performed by varying the Peclet number (Pe) at fixed kinematic conditions. The mixing time (tm) is characterized as a function of the Pe number, and tm ∝ ln(Pe) scaling is demonstrated for fully chaotic cases, while tm ∝ Peα scaling with α ≈ 0.33 and α = 0.5 are observed for partially chaotic and regular cases, respectively. Employing the aforementioned techniques, optimum kinematic conditions and the actuation frequency of the stirrer that result in the highest mixing/stirring efficiency are identified.

Chaotic mixing in microfluidic devices driven by oscillatory cross flow

Jr Phelan — 2008-03-11T19:16:10-00:00

Physics of Fluids, Vol. 20, No. 2. (2008)

Shear-induced particle migration in one-, two-, and three-dimensional flows

C Gao — 2008-03-11T19:14:44-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 2. (2008)

We investigate the segregation resulting from the competition between advection and shear-induced migration of suspensions in steady open flows. Herringbone channels form a concentration profile deviating from the particle focusing found in straight channels. Transients can result from a buckling instability during the onset of migration when particle-depleted fluid is injected into particle-rich fluid. In chaotic flows, the better mixing found at low bulk volume fraction is not seen at higher bulk volume fraction. Thus, the ability of static mixers to reduce the effects of shear-induced migration is significantly limited.

Multivortex micromixing

Arjun Sudarsan — 2008-03-11T03:43:00-00:00

Proceedings of the National Academy of Sciences, Vol. 103, No. 19. (9 May 2006), pp. 7228-7233.

The ability to mix liquids in microchannel networks is fundamentally important in the design of nearly every miniaturized chemical and biochemical analysis system. Here, we show that enhanced micromixing can be achieved in topologically simple and easily fabricated planar 2D microchannels by simply introducing curvature and changes in width in a prescribed manner. This goal is accomplished by harnessing a synergistic combination of (i) Dean vortices that arise in the vertical plane of curved channels as a consequence of an interplay between inertial, centrifugal, and viscous effects, and (ii) expansion vortices that arise in the horizontal plane due to an abrupt increase in a conduit's cross-sectional area. We characterize these effects by using confocal microscopy of aqueous fluorescent dye streams and by observing binding interactions between an intercalating dye and double-stranded DNA. These mixing approaches are versatile and scalable and can be straightforwardly integrated as generic components in a variety of lab-on-a-chip systems. 10.1073/pnas.0507976103

Microscale flow visualization

Sinton — 2006-05-18T17:45:40-00:00

Microfluidics and Nanofluidics, Vol. 1, No. 1. (November 2004), pp. 2-21.

Microfluidics-a review

P Gravesen — 2007-02-12T18:58:36-00:00

Journal of Micromechanics and Microengineering, Vol. 3, No. 4. (1993), pp. 168-182.

An overview is given of research activities in the field of fluid components or systems built with microfabrication technologies. This review focuses on the fluidic behaviour of the various devices, such as valves, pumps and flow sensors as well as the possibilities and pitfalls related to the modelling of these devices using simple flow theory. Finally, a number of microfluidic systems are described and comments on future trends are given.

Departure from Navier-Stokes hydrodynamics in confined liquids

Karl Travis — 2006-03-27T09:30:15-00:00

Physical Review E (Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics), Vol. 55, No. 4. (1997), pp. 4288-4295.

In this work we use nonequilibrium molecular dynamics (NEMD) to simulate an atomic liquid undergoing gravity-fed flow down a narrow channel. We compare the simulation results against the predictions of classical Navier-Stokes theory for two different channel widths. For a channel width of 5.1 molecular diameters, we find that the velocity profile deviates significantly from the hydrodynamic prediction. The shape of this velocity profile is found to be independent of the applied field (pressure gradient). We find that the heat flux profile does not agree with the cubic profile predicted by Navier-Stokes hydrodynamics, but shows significant oscillations located about one molecular diameter from the walls. This result differs from the earlier work of Todd and Evans [B. D. Todd and D. J. Evans, J. Chem. Phys. 103, 9804 (1995)], in which an assumption of a purely quadratic velocity profile resulted in very weak oscillations in the heat flux. We find that in narrow channels the viscosity cannot be described by a linear, local constitutive relation. However, classical Navier-Stokes behavior is approached for a channel width of > ~ 10 molecular diameters.

Microfluidics: Fluid physics at the nanoliter scale

Todd Squires — 2006-01-16T15:14:37-00:00

Reviews of Modern Physics, Vol. 77, No. 3. (2005)

Microfabricated integrated circuits revolutionized computation by vastly reducing the space, labor, and time required for calculations. Microfluidic systems hold similar promise for the large-scale automation of chemistry and biology, suggesting the possibility of numerous experiments performed rapidly and in parallel, while consuming little reagent. While it is too early to tell whether such a vision will be realized, significant progress has been achieved, and various applications of significant scientific and practical interest have been developed. Here a review of the physics of small volumes (nanoliters) of fluids is presented, as parametrized by a series of dimensionless numbers expressing the relative importance of various physical phenomena. Specifically, this review explores the Reynolds number Re, addressing inertial effects; the Péclet number Pe, which concerns convective and diffusive transport; the capillary number Ca expressing the importance of interfacial tension; the Deborah, Weissenberg, and elasticity numbers De, Wi, and El, describing elastic effects due to deformable microstructural elements like polymers; the Grashof and Rayleigh numbers Gr and Ra, describing density-driven flows; and the Knudsen number, describing the importance of noncontinuum molecular effects. Furthermore, the long-range nature of viscous flows and the small device dimensions inherent in microfluidics mean that the influence of boundaries is typically significant. A variety of strategies have been developed to manipulate fluids by exploiting boundary effects; among these are electrokinetic effects, acoustic streaming, and fluid-structure interactions. The goal is to describe the physics behind the rich variety of fluid phenomena occurring on the nanoliter scale using simple scaling arguments, with the hopes of developing an intuitive sense for this occasionally counterintuitive world.

Patterning Flows Using Grooved Surfaces

AD Stroock — 2008-03-11T03:17:03-00:00

Anal. Chem., Vol. 74, No. 20. (15 October 2002), pp. 5306-5312.

Abstract: Through a simple analytical description we quantify how pressure-driven flows over grooved surfaces develop transverse components, which, for shallow grooves, can be modeled with simple anisotropic effective boundary conditions. Helical recirculation results in channels or capillaries with grooved walls. An experimental validation of our model is presented. Our analysis provides a workable guide for the design of 3D flows with simple patterns of grooved regions, e.g., to control the position of streams in the cross section of a channel or to promote mixing. Potential applications in microfluidics are outlined.

The centrifugal microfluidic Bio-Disk platform

Jens Ducrée — 2007-12-11T16:35:45-00:00

Journal of Micromechanics and Microengineering, Vol. 17, No. 7. (2007), pp. S103-S115.

This paper reviews the centrifugal 'Bio-Disk' platform which is based on rotationally controlled, multi-scale liquid handling to fully integrate and automate complex analysis and synthesis protocols in the life sciences. The platform offers the crucial ingredients for a rapid development of applications: a coherent library of fluidic unit operations, a device technology for actuation, liquid interfacing and detection as well as a developer toolbox providing experimental testing, rapid prototyping and simulation capabilities. Various applications in the fields of life science, in vitro diagnostics and micro-process engineering are demonstrated.

Artificial cilia for active micro-fluidic mixing (DOI: 10.1039/b717681c)

J den Toonder — 2008-03-11T02:40:02-00:00

Lab on Chip (2008)

In lab-on-chip devices, on which complete (bio-)chemical analysis laboratories are miniaturized and integrated, it is essential to manipulate fluids in sub-millimetre channels and sub-microlitre chambers. A special challenge in these small micro-fluidic systems is to create good mixing flows, since it is almost impossible to generate turbulence. We propose an active micro-fluidic mixing concept inspired by nature, namely by micro-organisms that swim through a liquid by oscillating microscopic hairs, cilia, that cover their surface. We have fabricated artificial cilia consisting of electro-statically actuated polymer structures, and have integrated these in a micro-fluidic channel. Flow visualization experiments show that the cilia can generate substantial fluid velocities, up to 0.6 mm s−1. In addition, very efficient mixing is obtained using specially designed geometrical cilia configurations in a micro-channel. Since the artificial cilia can be actively controlled using electrical signals, they have exciting applications in micro-fluidic devices.

Micromixers—a review

Nam-Trung Nguyen — 2007-12-28T23:53:09-00:00

Journal of Micromechanics and Microengineering, Vol. 15, No. 2. (2005), pp. R1-R16.

This review reports the progress on the recent development of micromixers. The review first presents the different micromixer types and designs. Micromixers in this review are categorized as passive micromixers and active micromixers. Due to the simple fabrication technology and the easy implementation in a complex microfluidic system, passive micromixers will be the focus of this review. Next, the review discusses the operation points of the micromixers based on characteristic dimensionless numbers such as Reynolds number Re, Peclet number Pe, and in dynamic cases the Strouhal number St. The fabrication technologies for different mixer types are also analysed. Quantification techniques for evaluation of the performance of micromixers are discussed. Finally, the review addresses typical applications of micromixers.

Active micromixer based on artificial cilia

Vinayak Khatavkar — 2008-03-11T02:27:50-00:00

Physics of Fluids, Vol. 19, No. 8. (2007)

We propose a design for an active micromixer that is inspired by the motion of ciliated micro-organisms occurring in nature. The conceptual design consists of an array of individually addressable artificial cilia in the form of microactuators covering the channel wall. The microactuators can be set into motion by an external stimulus such as an electric or a magnetic field, inducing either a primary or secondary motion in the surrounding fluid. To validate the concept and to help to design the precise mixer configuration, we developed a computational fluid-structure model. This model is based on a fictitious domain method that couples the microactuator motion to the concomitant fluid flow, fully capturing the mutual fluid-structure interactions. The simulated flow patterns resulting from the motion of single and multiple actuated elements (in a microchannel filled with a Newtonian fluid) under the action of a time-periodic forcing function are analyzed using dynamical systems theory to quantify the mixing efficiency. The results show that with a proper actuation scheme, two microactuators placed on the same wall of a microchannel can indeed induce effective mixing by chaotic advection; their distance should be small, but collisions should be avoided, and they can be actuated in a rather broad regime around 90° out of phase. Placing actuators on opposite walls also induces exponential stretching in the fluid, but if their length is relatively small, of the order of 20% of the channel height, mixing effectiveness is higher when they are arranged on the same wall.

A passive planar micromixer with obstructions for mixing at low Reynolds numbers

Bhagat — 2008-03-11T02:27:50-00:00

Journal of Micromechanics and Microengineering, Vol. 17, No. 5. (2007), pp. 1017-1024.

Control and detection of chemical reactions in microfluidic systems

Andrew Demello — 2008-03-11T02:27:50-00:00

Nature, Vol. 442, No. 7101. (2006), pp. 394-402.

Microfluidic systems for chemical kinetics that rely on chaotic mixing in droplets

Michelle Bringer — 2008-03-11T02:27:50-00:00

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol. 362, No. 1818. (2004), pp. 1087-1104.

This paper reviews work on a microfluidic system that relies on chaotic advection to rapidly mix multiple reagents isolated in droplets (plugs). Using a combination of turns and straight sections, winding microfluidic channels create unsteady fluid flows that rapidly mix the multiple reagents contained within plugs. The scaling of mixing for a range of channel widths, flow velocities and diffusion coefficients has been investigated. Due to rapid mixing, low sample consumption and transport of reagents with no dispersion, the system is particularly appropriate for chemical kinetics and biochemical assays. The mixing occurs by chaotic advection and is rapid (sub-millisecond), allowing for an accurate description of fast reaction kinetics. In addition, mixing has been characterized and explicitly incorporated into the kinetic model.

Formation of Droplets and Mixing in Multiphase Microfluidics at Low Values of the Reynolds and the Capillary Numbers

JD Tice — 2008-03-11T02:27:50-00:00

Langmuir, Vol. 19, No. 22. (2003), pp. 9127-9133.

Abstract: This paper reports an experimental characterization of a simple method for rapid formation of droplets, or plugs, of multiple aqueous reagents without bringing reagents into contact prior to mixing. Droplet-based microfluidics offers a simple method of achieving rapid mixing and transport with no dispersion. In addition, this paper shows that organic dyes at high concentrations should not be used for the visualization of flow patterns and mixing of aqueous plugs in multiphase flows in this system (fluorinated carrier fluid and PDMS microchannels). It reports an inorganic dye that can be used instead. This work focuses on mixing in plugs moving through straight channels. It demonstrates that, when traveling through straight microchannels, mixing within plugs by steady recirculating flow is highly sensitive to the initial distribution of the aqueous reagents established by the eddy flow at the tip of the forming plug (twirling). The results also show how plugs with proper distribution of the aqueous reagents could be formed in order to achieve optimal mixing of the reagents in this system.

A Microfluidic System for Controlling Reaction Networks in Time

Helen Song — 2008-03-11T02:27:50-00:00

Angewandte Chemie, Vol. 115, No. 7. (2003), pp. 792-796.

No Abstract

Chaotic mixer for microchannels.

AD Stroock — 2008-03-11T02:27:50-00:00

Science, Vol. 295, No. 5555. (2002), pp. 647-651.

It is difficult to mix solutions in microchannels. Under typical operating conditions, flows in these channels are laminar-the spontaneous fluctuations of velocity that tend to homogenize fluids in turbulent flows are absent, and molecular diffusion across the channels is slow. We present a passive method for mixing streams of steady pressure-driven flows in microchannels at low Reynolds number. Using this method, the length of the channel required for mixing grows only logarithmically with the P\éclet number, and hydrodynamic dispersion along the channel is reduced relative to that in a simple, smooth channel. This method uses bas-relief structures on the floor of the channel that are easily fabricated with commonly used methods of planar lithography.

A well-mixed, polymer-based microbioreactor with integrated optical measurements

Zhiyu Zhang — 2008-03-11T02:27:50-00:00

Biotechnology and Bioengineering, Vol. 93, No. 2. (2006), pp. 286-296.

We describe a 150 µL microbioreactor fabricated in poly(methylmethacrylate) (PMMA) and poly(dimethylsiloxane) (PDMS) to cultivate microbial cell cultures. Mixing is achieved by a small magnetic stir bar and fluorescent sensors are integrated for on-line measurement of pH and dissolved oxygen. Optical transmission measurements are used for cell density. The body of the reactor is poly(methylmethacrylate) with a thin layer of poly (dimethylsiloxane) for aeration, oxygen diffuses through this gas-permeable membrane into the microbioreactor to support metabolism of bacterial cells. Mixing in the reactor is characterized by observation of mixing of dyes and computational fluid dynamics simulations. The oxygenation is described in terms of measured KLa values for microbioreactor, 20-75/h corresponding to increasing stirring speed 200-800 rpm. Escherichia coli cell growth in the microbioreactor is demonstrated and the growth behavior is benchmarked with conventional bench-scale bioreactors, flasks and tubes. Batch culture experiments with Saccharomyces cerevisiae further demonstrate the reproducibility and flexibility of the microbioreactor system. \\copyright 2005 Wiley Periodicals, Inc.

Patterning of flow and mixing in rotating radial microchannels

Jens Ducrée — 2008-03-11T02:27:50-00:00

Microfluidics and Nanofluidics, Vol. 2, No. 2. (2006), pp. 97-105.

Abstract  We demonstrate how the speed of mixing under laminar conditions can be appreciably enhanced in concurrent centrifugal flows through straight, low-aspect-ratio microchannels pointing in radial direction in the plane of rotation. The convective mixing is driven by the inhomogeneous distribution of the velocity-dependent Coriolis pseudo force and the interaction of the so-induced transverse currents with the side walls. By investigating the key impact parameters, which are the geometry of the channels and the speed of rotation, it is shown that the contact surface between two laminar flows can be folded to shorten mixing times by up to two orders of magnitude!

The origins and the future of microfluidics

George Whitesides — 2008-03-11T02:27:50-00:00

Nature, Vol. 442, No. 7101. (2006), pp. 368-373.

Introduction: mixing in microfluidics

Julio Ottino — 2008-03-11T02:27:49-00:00

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol. 362, No. 1818. (2004), pp. 923-935.

In this paper we briefly review the main issues associated with mixing at the microscale and introduce the papers comprising the Theme Issue.

An Optimized Micromixer with Patterned Grooves

Yanghua Tang — 2008-03-11T02:05:24-00:00

MEMS, NANO and Smart Systems, 2004. ICMENS 2004. Proceedings. 2004 International Conference on (2004), pp. 300-305.

With the development of microfluidic systems, there is a growing interest in micro scale laminar flow mixing. In this work, the fluid rotating angle and mixing efficiency in a micromixer with patterned grooves are studied as a function of the dimensions of the microstructure by numerical simulation. We found that mixing efficiency does not always increase with higher fluid stream rotation in the microchannel. High groove aspect ratios are not advantageous to fluid rotation. Experiments on mixture of two fluids were done on a micromixer fabricated in PDMS by replica molding. An 85% mixing efficiency was obtained in a 30mm long mixing channel with two dyed liquids.

Numerical investigation of mixing in microchannels with patterned grooves

H Wang — 2008-03-11T01:51:39-00:00

Journal of Micromechanics and Microengineering, Vol. 13, No. 6. (2003), pp. 801-808.

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, Poincare maps were calculated from the 3D velocity field exported from a CFD package for microfluidics (MemCFD). For small aspect ratio (a = 0.05) grooves, the results showed that there was no significant irregularity in the Poincare map, and indicated little chaotic effect. For high aspect ratio (a = 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, Poincare maps could still be used to evaluate the performance of this type of micro-mixer. The particle trajectories recorded in the Poincare maps were circular-like patterns. By counting the number of dots to form one circle in the Poincare 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.

Radial Segregation in a 2d Drum: An Experimental Analysis

F Cantelaube — 2008-03-07T20:28:10-00:00

Europhysics Letters, Vol. 30, No. 3. (20 April 1995), pp. 133-138.

We report experimental results on radial segregation in a 2d rotating drum half-filled by a mixture of small and large disks. This segregation is present both in the intermittent (avalanches) and in the continuous flow regimes. The kinetics of the process are analysed and appear to be very fast. Some results on a frrst quantitative analysis of the trapping process-which is the basic mechanism of segregation-within the surface flow are presented.