Introduction: mixing in microfluidics

Micro Magnetic Gyromixer for Speeding up Reactions in Droplets

We report a novel micro magnetic gyromixer designed for accelerating mixing hence reactions in droplets. The gyromixer is fabricated with magnetite-PDMS composite using soft lithography. The mixer spins and balances itself on the droplet surface through the gyroscopic effect, rapidly homogenizing the enclosed reagents by stretching and folding internal fluid streamlines to enhance mixing. We examined the capability of the gyromixer for improving biochemical reactions in droplets by monitoring the biotin-streptavidin binding as a linker in a quantum dot fluorescence resonant energy transfer (QD-FRET) sensing system. The remotely controlled gyromixer exhibits high flexibility and potential for integration in a variety of droplet-based miniaturized total analysis systems (μTAS) to reduce turnaround times.

Quantifying mixing in viscously unstable porous media flows

Viscous fingering is a well-known hydrodynamic instability that sets in when a less viscous fluid displaces a more viscous fluid. When the two fluids are miscible, viscous fingering introduces disorder in the velocity field and exerts a fundamental control on the rate at which the fluids mix. Here we analyze the characteristic signature of the mixing process in viscously unstable flows, by means of high-resolution numerical simulations using a computational strategy that is stable for arbitrary viscosity ratios. We propose a reduced-order model of mixing, which, in the spirit of turbulence modeling and in contrast with previous approaches, recognizes the fundamental role played by the mechanical dissipation rate. The proposed model captures the nontrivial interplay between channeling and creation of interfacial area as a result of viscous fingering.

Microfluidic devices: useful tools for bioprocess intensification

The dawn of the new millennium saw a trend towards the dedicated use of microfluidic devices for process intensification in biotechnology. As the last decade went by, it became evident that this pattern was not a short-lived fad, since the deliverables related to this field of research have been consistently piling-up. The application of process intensification in biotechnology is therefore seemingly catching up with the trend already observed in the chemical engineering area, where the use of microfluidic devices has already been upgraded to production scale. The goal of the present work is therefore to provide an updated overview of the developments centered on the use of microfluidic devices for process intensification in biotechnology. Within such scope, particular focus will be given to different designs, configurations and modes of operation of microreactors, but reference to similar features regarding microfluidic devices in downstream processing will not be overlooked. Engineering considerations and fluid dynamics issues, namely related to the characterization of flow in microchannels, promotion of micromixing and predictive tools, will also be addressed, as well as reflection on the analytics required to take full advantage of the possibilities provided by microfluidic devices in process intensification. Strategies developed to ease the implementation of experimental set-ups anchored in the use of microfluidic devices will be briefly tackled. Finally, realistic considerations on the current advantages and limitation on the use of microfluidic devices for process intensification, as well as prospective near future developments in the field, will be presented.

Computational Study of pH-sensitive Hydrogel-based Microfluidic Flow Controllers

This computational study investigates the sensing and actuating behavior of a pH-sensitive hydrogel-based microfluidic flow controller. This hydrogel-based flow controller has inherent advantage in its unique stimuli-sensitive properties, removing the need for an external power supply. The predicted swelling behavior the hydrogel is validated with steady-state and transient experiments. We then demonstrate how the model is implemented to study the sensing and actuating behavior of hydrogels for different microfluidic flow channel/hydrogel configurations: e.g., for flow in a T-junction with single and multiple hydrogels. In short, the results suggest that the response of the hydrogel-based flow controller is slow. Therefore, two strategies to improve the response rate of the hydrogels are proposed and demonstrated. Finally, we highlight that the model can be extended to include other stimuli-responsive hydrogels such as thermo-, electric-, and glucose-sensitive hydrogels.

Acoustic microstreaming increases the efficiency of reverse transcription reactions comprising single-cell quantities of RNA

Correlating gene expression with behavior at the single-cell level is difficult, largely because the small amount of available mRNA (<1 pg) degrades before it can be reverse transcribed into a more stable cDNA copy. This study tested the capacity for a novel acoustic microstreaming method ("micromixing"), which stirs fluid at microliter scales, to improve cDNA yields from reverse transcription (RT) reactions comprising single-cell quantities of RNA. Micromixing significantly decreased the number of qPCR cycles to detect cDNA representing mRNA for hypoxanthine phosphoribosyl-transferase (Hprt) and nuclear receptor-related 1 (Nurr1) by ~9 and ~15 cycles, respectively. The improvement was equivalent to performing RT with 10- to 100-fold more cDNA in the absence of micromixing. Micromixing enabled reliable detection of the otherwise undetectable, low-abundance transcript, Nurr1. It was most effective when RNA concentrations were low (0.1-1 pg/µL, a "single-cell equivalent") but had lesser effects at higher RNA concentrations (~1 ng/µL). This was supported by imaging experiments showing that micromixing improved mixing of a low concentration (20 pg/µL) of fluorescence-labeled RNA but not a higher concentration (1 ng/µL). We conclude that micromixing significantly increases RT yields obtainable from single-cell quantities of RNA.

Increasing cDNA yields from single-cell quantities of mRNA in standard laboratory reverse transcriptase reactions using acoustic microstreaming

Correlating gene expression with cell behavior is ideally done at the single-cell level. However, this is not easily achieved because the small amount of labile mRNA present in a single cell (1-5% of 1-50 pg total RNA, or 0.01-2.5 pg mRNA, per cell) mostly degrades before it can be reverse transcribed into a stable cDNA copy. For example, using standard laboratory reagents and hardware, only a small number of genes can be qualitatively assessed per cell. One way to increase the efficiency of standard laboratory reverse transcriptase (RT) reactions (i.e. standard reagents in microliter volumes) comprising single-cell amounts of mRNA would be to more rapidly mix the reagents so the mRNA can be converted to cDNA before it degrades. However this is not trivial because at microliter scales liquid flow is laminar, i.e. currently available methods of mixing (i.e. shaking, vortexing and trituration) fail to produce sufficient chaotic motion to effectively mix reagents. To solve this problem, micro-scale mixing techniques have to be used. A number of microfluidic-based mixing technologies have been developed which successfully increase RT reaction yields. However, microfluidics technologies require specialized hardware that is relatively expensive and not yet widely available. A cheaper, more convenient solution is desirable. The main objective of this study is to demonstrate how application of a novel "micromixing" technique to standard laboratory RT reactions comprising single-cell quantities of mRNA significantly increases their cDNA yields. We find cDNA yields increase by approximately 10-100-fold, which enables: greater numbers of genes to be analyzed per cell; more quantitative analysis of gene expression; and better detection of low-abundance genes in single cells. The micromixing is based on acoustic microstreaming, a phenomenon where sound waves propagating around a small obstacle create a mean flow near the obstacle. We have developed an acoustic microstreaming-based device ("micromixer") with a key simplification; acoustic microstreaming can be achieved at audio frequencies by ensuring the system has a liquid-air interface with a small radius of curvature. The meniscus of a microliter volume of solution in a tube provides an appropriately small radius of curvature. The use of audio frequencies means that the hardware can be inexpensive and versatile, and nucleic acids and other biochemical reagents are not damaged like they can be with standard laboratory sonicators.

Deciding whether to go with the flow: evaluating the merits of flow reactors for synthesis

The fine chemicals and pharmaceutical industries are transforming how their products are manufactured, where economically favorable, from traditional batchwise processes to continuous flow. This evolution is impacting synthetic chemistry on all scales-from the laboratory to full production. This Review discusses the relative merits of batch and micro flow reactors for performing synthetic chemistry in the laboratory.

A multiplexed immunoassay system based upon reciprocating centrifugal microfluidics

A novel, centrifugal disk-based micro-total analysis system (μTAS) for low cost and high throughput semi-automated immunoassay processing was developed. A key innovation in the disposable immunoassay disk design is in a fluidic structure that enables very efficient micro-mixing based on a reciprocating mechanism in which centrifugal acceleration acting upon a liquid element first generates and stores pneumatic energy that is then released by a reduction of the centrifugal acceleration, resulting in a reversal of direction of flow of the liquid. Through an alternating sequence of high and low centrifugal acceleration, the system reciprocates the flow of liquid within the disk to maximize incubation/hybridization efficiency between antibodies and antigen macromolecules during the incubation/hybridization stage of the assay. The described reciprocating mechanism results in a reduction in processing time and reagent consumption by one order of magnitude.

Computational design of mixers and pumps for microfluidic systems, based on electrochemically-active conducting polymers

We present a theoretical description of the propagation of composition waves along a strip of electrochemically-active conducting polymer, upon electrochemical stimulation. We develop an efficient solution of the electro-neutral Nernst-Plank equations in 2-D for electromigration and diffusional transport in the solution based on an extension of the methods of Scharfetter and Gummel [D. L. Scharfetter and H. K. Gummel, IEEE Trans. Electron Devices, 1969, ED16, 64-77.] and of Cohen and Cooley [H. Cohen and J. W. Cooley, Biophys. J., 1965, 5, 145-162.], and demonstrate important effects of the geometry of the cell. Under some circumstances, waves reflecting back from the end of the strip are predicted. We then demonstrate theoretically how such waves, associated as they are with expansion of the polymer, could be employed to enhance mixing or induce pumping in microfluidic systems.

Wavefront velocity oscillations of carbon-nanotube-guided thermopower waves: nanoscale alternating current sources

The nonlinear coupling between exothermic chemical reactions and a nanowire or nanotube with large axial heat conduction results in a self-propagating thermal wave guided along the nanoconduit. The resulting reaction wave induces a concomitant thermopower wave of high power density (>7 kW/kg), resulting in an electrical current along the same direction. We develop the theory of such waves and analyze them experimentally, showing that for certain values of the chemical reaction kinetics and thermal parameters, oscillating wavefront velocities are possible. We demonstrate such oscillations experimentally using a cyclotrimethylene-trinitramine/multiwalled carbon nanotube system, which produces frequencies in the range of 400 to 5000 Hz. The propagation velocity oscillations and the frequency dispersion are well-described by Fourier's law with an Arrhenius source term accounting for reaction and a linear heat exchange with the nanotube scaffold. The frequencies are in agreement with oscillations in the voltage generated by the reaction. These thermopower oscillations may enable new types of nanoscale power and signal processing sources.

Controlling mass transport in microfluidic devices

Microfluidic platforms offer exquisite capabilities in controlling mass transport for biological studies. In this review, we focus on recent developments in manipulating chemical concentrations at the microscale. Some techniques prevent or accelerate mixing, whereas others shape the concentration gradients of chemical and biological molecules. We also highlight several in vitro biological studies in the areas of organ engineering, cancer, and blood coagulation that have benefited from accurate control of mass transfer.

Streaming by leaky surface acoustic waves

Acoustic streaming, the generation of mean flow by dissipating acoustic waves, provides a promising method for flow pumping in microfluidic devices. In recent years, several groups have been experimenting with acoustic streaming induced by leaky surface waves: (Rayleigh) surface waves excited in a piezoelectric solid interact with a small volume of fluid where they generate acoustic waves and, as result of the viscous dissipation of these waves, a mean flow. We discuss the computation of the corresponding Lagrangian mean flow, which controls the trajectories of fluid particles and hence the mixing properties of the flows generated by this method. The problem is formulated using the averaged vorticity equation which extracts the dominant balance between wave dissipation and mean-flow dissipation. Particular attention is paid to the thin boundary layer that forms at the solid/liquid interface, where the flow is best computed using matched asymptotics. This leads to an explicit expression for a slip velocity, which includes the effect of the oscillations of the boundary. The Lagrangian mean flow is naturally separated into three contributions: an interior-driven Eulerian mean flow, a boundary-driven Eulerian mean flow and the Stokes drift. A scale analysis indicates that the latter two contributions can be neglected in devices much larger than the acoustic wavelength but need to be taken into account in smaller devices. A simple two-dimensional model of mean flow generation by surface acoustic waves is discussed as an illustration.

Microfluidics in inorganic chemistry

The application of microfluidics in chemistry has gained significant importance in the recent years. Miniaturized chemistry platforms provide controlled fluid transport, rapid chemical reactions, and cost-saving advantages over conventional reactors. The advantages of microfluidics have been clearly established in the field of analytical and bioanalytical sciences and in the field of organic synthesis. It is less true in the field of inorganic chemistry and materials science; however in inorganic chemistry it has mostly been used for the separation and selective extraction of metal ions. Microfluidics has been used in materials science mainly for the improvement of nanoparticle synthesis, namely metal, metal oxide, and semiconductor nanoparticles. Microfluidic devices can also be used for the formulation of more advanced and sophisticated inorganic materials or hybrids.

Dynamics of two trapped Brownian particles: Shear-induced cross-correlations

The dynamics of two Brownian particles trapped by two neighboring harmonic potentials in a linear shear flow is investigated. The positional correlation functions in this system are calculated analytically and analyzed as a function of the shear rate and the trap distance. Shear-induced cross-correlations between particle fluctuations along orthogonal directions in the shear plane are found. They are linear in the shear rate, asymmetric in time, and occur for one particle as well as between both particles. Moreover, the shear rate enters as a quadratic correction to the well-known correlations of random displacements along parallel spatial directions. The correlation functions depend on the orientation of the connection vector between the potential minima with respect to the flow direction. As a consequence, the inter-particle cross-correlations between orthogonal fluctuations can have zero, one or two local extrema as a function of time. Possible experiments for detecting these predicted correlations are described.

Optimal frequency for microfluidic mixing across a fluid interface

A new analytical tool for determining the optimum frequency for a micromixing strategy to mix two fluids across their interface is presented. The frequency dependence of the flux is characterized in terms of a Fourier transform related to the apparatus geometry. Illustrative microfluidic mixing examples based on electromagnetic forcing and fluid pumping strategies are presented.

Degradation studies of transparent conductive electrodes on electroactive poly(vinylidene fluoride) for uric acid measurements

Biochemical analysis of physiological fluids using, for example, lab-on-a-chip devices requires accurate mixing of two or more fluids. This mixing can be assisted by acoustic microagitation using a piezoelectric material, such as the β-phase of poly(vinylidene fluoride) (β-PVDF). If the analysis is performed using optical absorption spectroscopy and β-PVDF is located in the optical path, the material and its conductive electrodes must be transparent. Moreover, if, to improve the transmission of the ultrasonic waves to the fluids, the piezoelectric transducer is placed inside the fluidic structures, its degradation must be assessed. In this paper, we report on the degradation properties of transparent conductive oxides, namely, indium tin oxide (ITO) and aluminum-doped zinc oxide, when they are used as electrodes for providing acoustic microagitation. The latter promotes mixing of chemicals involved in the measurement of uric acid concentration in physiological fluids. The results are compared with those for aluminum electrodes. We find that β-PVDF samples with ITO electrodes do not degrade either with or without acoustic microagitation.

Abatement of mixing in shear-free elongationally unstable viscoelastic microflows

The addition of minute amounts of chemically inert polyacrylamide polymer to liquids results in large instabilities under steady electro-osmotic pumping through 2 : 1 constrictions, demonstrating that laminar flow conditions can be broken in electro-osmotic flow of viscoelastic material. By excluding shear and imposing symmetry we create a platform where only elongational viscoelastic instabilities, and diffusion, affect mixing. In contrast to earlier studies with significant shear that found up to orders of magnitude increase in mixing we find that inclusion of polymers excites large viscoelastic instabilities yet mixing is reduced relative to polymer-free liquids. The absolute decrease in mixing we find is consistent with the understanding that adding polymer increases viscosity while viscoelastic flows progress towards elastic turbulence, a type of mild (Batchelor) turbulence, and indicates that electro-osmotic pumped devices are an ideal platform for studying viscoelastic instabilities without supplementary factors.

Biological microdevice with fluidic acoustic streaming for measuring uric acid in human saliva

The healthcare system requires new devices for a rapid monitoring of a patient in order to improve the diagnosis and treatment of various diseases. Accordingly, new biomedical devices are being developed. In this paper, a fully-integrated biological microdevice for uric acid analysis in human saliva is presented. It is based on optical spectrophotometric measurements and incorporates a mixture system based on acoustic streaming, that enhances the fluids reaction due to both heating and agitation generated by this effect. Acoustic streaming is provided by a piezoelectric beta-PVDF film deposited underneath the microfluidic die of the device. Further, it incorporates the electronics for the detection, readout, data processing and signal actuation. Experimental results proved that acoustic streaming based on this piezoelectric polymer is advantageous and reduces in 55% the time required to obtain the analysis results.

Dynamics of a trapped Brownian particle in shear flows

The Brownian motion of a particle in a harmonic potential, which is simultaneously exposed either to a linear shear flow or to a plane Poiseuille flow is investigated. In the shear plane of both flows the probability distribution of the particle becomes anisotropic and the dynamics is changed in a characteristic manner compared to a trapped particle in a quiescent fluid. The particle distribution takes either an elliptical or a parachute shape or a superposition of both depending on the mean particle position in the shear plane. Simultaneously, shear-induced cross-correlations between particle fluctuations along orthogonal directions in the shear plane are found. They are asymmetric in time. In Poiseuille flow thermal particle fluctuations perpendicular to the flow direction in the shear plane induce a shift of the particle's mean position away from the potential minimum. Two complementary methods are suggested to measure shear-induced cross-correlations between particle fluctuations along orthogonal directions.

Chaotic mixing in microchannels via low frequency switching transverse electroosmotic flow generated on integrated microelectrodes

In this paper we present a numerical and experimental investigation of a chaotic mixer in a microchannel via low frequency switching transverse electroosmotic flow. By applying a low frequency, square-wave electric field to a pair of parallel electrodes placed at the bottom of the channel, a complex 3D spatial and time-dependence flow was generated to stretch and fold the fluid. This significantly enhanced the mixing effect. The mixing mechanism was first investigated by numerical and experimental analysis. The effects of operational parameters such as flow rate, frequency, and amplitude of the applied voltage have also been investigated. It is found that the best mixing performance is achieved when the frequency is around 1 Hz, and the required mixing length is about 1.5 mm for the case of applied electric potential 5 V peak-to-peak and flow rate 75 microL h(-1). The mixing performance was significantly enhanced when the applied electric potential increased or the flow rate of fluids decreased.

Extensional instability in electro-osmotic microflows of polymer solutions

Fluid transport in microfluidic systems typically is laminar due to the low Reynolds number characteristic of the flow. The inclusion of suspended polymers imparts elasticity to fluids, allowing instabilities to be excited when substantial polymer stretching occurs. For high molecular weight polymer chains we find that flow velocities achievable by standard electro-osmotic pumping are sufficient to excite extensional instabilities in dilute polymer solutions. We observe a dependence in measured fluctuations on polymer concentration which plateaus at a threshold corresponding to the onset of significant molecular crowding in macromolecular solutions; plateauing occurs well below the overlap concentration. Our results show that electro-osmotic flows of complex fluids are disturbed from the steady regime, suggesting potential for enhanced mixing and requiring care in modeling the flow of complex liquids such as biopolymer suspensions.

Chaotic micromixing in open wells using audio-frequency acoustic microstreaming

Mixing fluids for biochemical assays is problematic when volumes are very small (on the order of the 10 microL typical of single drops), which has inspired the development of many micromixing devices. In this paper, we show that micromixing is possible in the simple open wells of standard laboratory consumables using appropriate acoustic frequencies that can be applied using cheap, conventional audio components. Earlier work has shown that the phenomenon of acoustic microstreaming can mix fluids, provided that bubbles are introduced into a specially designed microchamber or that high-frequency surface acoustic wave devices are constructed. We demonstrate a key simplification: acoustic micromixing at audio frequencies by ensuring the system has a liquid-air interface with a small radius of curvature. The meniscus of a drop in a small well provided an appropriately small radius, and so an introduced bubble was not necessary. Microstreaming showed improvement over diffusion-based mixing by 1-2 orders of magnitude. Furthermore, significant improvements are attainable through the utilization of chaotic mixing principles, whereby alternating fluid flow patterns are created by applying, in sequence, two different acoustic frequencies to a drop of liquid in an open well.

Cavitation microstreaming and stress fields created by microbubbles

Cavitation microstreaming plays a role in the therapeutic action of microbubbles driven by ultrasound, such as the sonoporative and sonothrombolytic phenomena. Microscopic particle-image velocimetry experiments are presented. Results show that many different microstreaming patterns are possible around a microbubble when it is on a surface, albeit for microbubbles much larger than used in clinical practice. Each pattern is associated with a particular oscillation mode of the bubble, and changing between patterns is achieved by changing the sound frequency. Each microstreaming pattern also generates different shear stress and stretch/compression distributions in the vicinity of a bubble on a wall. Analysis of the micro-PIV results also shows that ultrasound-driven microstreaming flows around bubbles are feasible mechanisms for mixing therapeutic agents into the surrounding blood, as well as assisting sonoporative delivery of molecules across cell membranes. Patterns show significant variations around the bubble, suggesting sonoporation may be either enhanced or inhibited in different zones across a cellular surface. Thus, alternating the patterns may result in improved sonoporation and sonothrombolysis. The clear and reproducible delineation of microstreaming patterns based on driving frequency makes frequency-based pattern alternation a feasible alternative to the clinically less desirable practice of increasing sound pressure for equivalent sonoporative or sonothrombolytic effect. Surface divergence is proposed as a measure relevant to sonoporation.

Applications of micromixing technology

This review presents an application of micromixer technologies, which have driven a number of critical research trends over the past few decades, particularly for chemical and biological fields. Micromixer technologies in this review are categorized according to their applications: (1) chemical applications, including chemical synthesis, polymerization, and extraction; (2) biological applications, including DNA analysis, biological screening enzyme assays, protein folding; and (3) detection/analysis of chemical or biochemical content combined with NMR, FTIR, or Raman spectroscopies. In the chemical application, crystallization, extraction, polymerization, and organic synthesis have been reported, not only for laboratory studies, but also for industrial applications. Microscale techniques are used in chemical synthesis to develop microreactors. In clinical medicine and biological studies, microfluidic systems have been widely applied to the identification of biochemical products, diagnosis, drug discovery, and investigation of disease symptoms. The biological and biochemical applications also include enzyme assays, biological screening assays, cell lysis, protein folding, and biological analytical assays. Nondestructive analytical/detection methods have yielded a number of benefits to chemical and biochemical processes. In this chapter, we introduce analytical methods those are frequently integrated into micromixing technologies, such as NMR, FT-IR, and Raman spectroscopies. From the study of micromixers, we discovered that the Re number and mixing time depends on the specific application, and we clustered micromixers in various applications according to the Re number and mixing performance (mixing time). We expect that this clustering will be helpful in designing of micromixers for specific applications.

Design and fabrication of piezoelectric microactuators based on β-poly (vinylidene fluoride) films for microfluidic applications

This paper reports a fabrication method for producing piezoelectric poly (vinylidene fluoride) films in their electroactive β-phase that features controlled thickness, smooth and flat surface, and high transparency. These piezoelectric films are suitable for being used as integrated microactuators, such as piezoelectric pumps and/or mixers, in microfluidic applications. Their actuation circuit design is also reported. ATR-FTIR, UV-VIS transmittance spectroscopy and SEM techniques were used for calculating the β-phase content, for determining the transparency and for evaluating the morphology of the produced β-PVDF films, respectively. β-PVDF films with a thickness of about 25 µm were deposited by spin-coating. It was concluded that the processing parameter that mostly affect the films quality was their drying temperature. Indeed, the drying temperature of 30 °C proved to be the most suitable for obtaining non-porous and transparent films, with a β-phase content of approximately 75%.

Electrothermally driven flows in ac electrowetting

Mixing within sessile drops can be enhanced by generating internal flow patterns using ac electrowetting. While for low ac frequencies, the flow patterns have been attributed to oscillations of the drop surface, we provide here the driving mechanism of the hitherto unexplained high-frequency flows. We show that: (1) the electric field in the liquid bulk becomes important, leading to energy dissipation due to Joule heating and a temperature increase of several degrees Celsius, and (2) the fluid flow at these frequencies is generated by electrothermal effect, i.e., gradients in temperature give rise to gradients in conductivity and permittivity, the electric field acting on these inhomogeneities induces an electrical body force that generates the flow. We solved numerically the equations for the electric, temperature and flow fields. The temperature is obtained from a convection-diffusion equation where Joule heating is introduced as a source term. From the solution of the electric field and the temperature, we compute the electrical force that acts as a body force in Stokes equations. Our numerical results agree with previous experimental observations.

Direct measurement of shear-induced cross-correlations of Brownian motion

Shear-induced cross-correlations of particle fluctuations perpendicular and along streamlines are investigated experimentally and theoretically. Direct measurements of the Brownian motion of micron-sized beads, held by optical tweezers in a shear-flow cell, show a strong time asymmetry in the cross-correlation, which is caused by the non-normal amplification of fluctuations. Complementary measurements on the single particle probability distribution substantiate this behavior and both results are consistent with a Langevin model. In addition, a shear-induced anticorrelation between orthogonal random displacements of two trapped and hydrodynamically interacting particles is detected, having one or two extrema in time, depending on the positions of the particles.

Inertial microfluidics

Despite the common wisdom that inertia does not contribute to microfluidic phenomena, recent work has shown a variety of useful effects that depend on fluid inertia for applications in enhanced mixing, particle separation, and bioparticle focusing. Due to the robust, fault-tolerant physical effects employed and high rates of operation, inertial microfluidic systems are poised to have a critical impact on high-throughput separation applications in environmental cleanup and physiological fluids processing, as well as bioparticle focusing applications in clinical diagnostics. In this review I will discuss the recent accelerated progress in developing prototype inertial microfluidic systems for a variety of applications and attempt to clarify the fundamental fluid dynamic effects that are being exploited. Finally, since this a nascent area of research, I will suggest some future promising directions exploiting fluid inertia on the microscale.

Microchemical systems for continuous-flow synthesis

Microchemical systems have evolved rapidly over the last decade with extensive chemistry applications. Such systems enable discovery and development of synthetic routes while simultaneously providing increased understanding of underlying pathways and kinetics. We review basic trends and aspects of microsystems as they relate to continuous-flow microchemical synthesis. Key literature reviews are summarized and principles governing different microchemical operations discussed. Current trends and limitations of microfabrication, micromixing, chemical synthesis in microreactors, continuous-flow separations, multi-step synthesis, and integration of analytics are delineated. We conclude by summarizing the major challenges and outlook related to these topics.

Molecular dynamics simulations of polymeric fluids in narrow channels: methods to enhance mixing

Mixing of shear thinning polymeric fluids in long channels with patterned boundary conditions is studied through molecular dynamics simulations. Patterned wettability was shown to induce spatially varying slip lengths at the channel walls which in turn induce mixing in the fluid. To quantify the amount of mixing for different wave lengths of patterns, transverse velocity profiles were evaluated. The transverse velocity profiles from the molecular dynamics simulations were then compared with predictions from continuum modeling and good quantitative agreement was found. Offsetting the pattern was shown to produce better mixing in the center of the channel. Transverse flow is found to increase when the radius of gyration of the chains is smaller than the pattern length. We also implement an oscillating (time dependent) body force and find that the transverse flow increases significantly. However, we do not find an increase in transverse flow with frequency of the oscillation as predicted from continuum modeling and we postulate reasons for this behavior.

Reciprocating flow-based centrifugal microfluidics mixer

Proper mixing of reagents is of paramount importance for an efficient chemical reaction. While on a large scale there are many good solutions for quantitative mixing of reagents, as of today, efficient and inexpensive fluid mixing in the nanoliter and microliter volume range is still a challenge. Complete, i.e., quantitative mixing is of special importance in any small-scale analytical application because the scarcity of analytes and the low volume of the reagents demand efficient utilization of all available reaction components. In this paper we demonstrate the design and fabrication of a novel centrifugal force-based unit for fast mixing of fluids in the nanoliter to microliter volume range. The device consists of a number of chambers (including two loading chambers, one pressure chamber, and one mixing chamber) that are connected through a network of microchannels, and is made by bonding a slab of polydimethylsiloxane (PDMS) to a glass slide. The PDMS slab was cast using a SU-8 master mold fabricated by a two-level photolithography process. This microfluidic mixer exploits centrifugal force and pneumatic pressure to reciprocate the flow of fluid samples in order to minimize the amount of sample and the time of mixing. The process of mixing was monitored by utilizing the planar laser induced fluorescence (PLIF) technique. A time series of high resolution images of the mixing chamber were analyzed for the spatial distribution of light intensities as the two fluids (suspension of red fluorescent particles and water) mixed. Histograms of the fluorescent emissions within the mixing chamber during different stages of the mixing process were created to quantify the level of mixing of the mixing fluids. The results suggest that quantitative mixing was achieved in less than 3 min. This device can be employed as a stand alone mixing unit or may be integrated into a disk-based microfluidic system where, in addition to mixing, several other sample preparation steps may be included.

Stirring and mixing of liquids using acoustic radiation force

The possibility of using acoustic radiation force in standing waves for stirring and mixing small volumes of liquids is theoretically analyzed. The principle of stirring considered in this paper is based on moving the microparticles suspended in a standing acoustic wave by changing the frequency so that one standing wave mode is replaced by the other, with differently positioned minima of potential energy. The period-average transient dynamics of solid microparticles and gas microbubbles is considered, and simple analytical solutions are obtained for the case of standing waves of variable amplitude. It is shown that bubbles can be moved from one equilibrium position to another two to three orders of magnitude faster than solid particles. For example, radiation force in a standing acoustic wave field may induce movement of microbubbles with a speed of the order of a few m/s at a frequency of 1 MHz and ultrasound pressure amplitude of 100 kPa, whereas the speed of rigid particles does not exceed 1 cms under the same conditions. The stirring effect can be additionally enhanced due to the fact that the bubbles that are larger and smaller than the resonant bubbles move in opposite directions. Possible applications of the analyzed stirring mechanism, such as in microarrays, are discussed.