Miniaturized laboratory-on-a-chip systems have been extensively developed over the past decade as promising tools for a wide range of applications, specifically in chemical synthesis and biomedical diagnostics. Droplet-based microfluidic systems have become ubiquitous in such applications by providing essential tools to perform rapid as well as high throughput measurements on small volumes of fluids. Thus far, the majority of the research endeavors have been focused on liquid-liquid systems for generating microscale drops (typically water in oil). Droplets generated in liquid-liquid microfluidic systems tend to be very uniform in size, and due to high surface area to volume ratio of micro-droplets, heat and mass transfer occurs at higher rates as compared to continuous-flow microfluidics. Generation of droplets in a gaseous medium, on the other hand, have been widely used in applications that involve open environment liquid spraying, such as ink-jet printers. However, usually in such applications there is no control over either the size or frequency of the generated droplets, and as a result droplets formed in these systems are widely distributed in size. Here we demonstrate an alternative scheme for controlled generation of liquid droplets in a microfluidic chip using a high speed gas stream. We have incorporated the inertial effect of a high-speed gaseous medium with the flow-focusing geometry, fabricated in a PDMS chip, in order to generate droplets with controlled size. Flow regimes involved in this scheme may be divided in three main regions i.e. co-flow, jetting, and dripping among which only dripping regime is capable of producing distinct aqueous droplets in the channel. It should be noted that poor surface conditions and high gas flow rates may result in generation of satellite droplets together with the main droplet in the dripping region, which substantially affects the monodispersity of the droplets. The generated drops were collected thereafter and it is shown that monodisperse droplets with known size ranging from 50 μm to 100 μm in diameter can be achieved within the dripping flow regime. We believe this method offers beneficial opportunities for the next generation of Lab-on-a-chip devices in which the introduction of a gaseous medium is required, namely oxidation, detection of airborne particles, and formation of micro-particles and micro-gels. Furthermore, the high speed droplets generated in this method represent the basis for a new approach based on droplet pair collisions for fast efficient micromixing which provides a significant development in modern LOC and mTAS devices.

In this paper, the three-dimensional (3D) structures of a micellar solution flow in the curvilinear microchannel have been investigated by means of confocal micro particle image velocimetry (PIV). The working fluid is aqueous solution of CTAC/NaSal (cetyltrimethylammonium / Sodium Salysilate). As the flow rate increases, the flow gradually gets into the irregular motion. It is found that the inside flow seems not completely chaotic, but in a manner of oscillation. To be specific, the flow nonlinearity grows as the flow rate increases, the inside flow shows different structures near the wall region and in the bulk due to the elongation of viscoelastic surfactant. Typically, two sub-streams were twisted together, and their flow directions change at the locations where the signs of geometric curvature change. The oscillation stripes represented the area of high extensional stress in the viscoelastic fluid, and were further identified by using polarized high-speed camera. Moreover, statistics shows that the viscoelastic flow field inside the curved microchannel shares the main features of elastic turbulence.

A new microfluidic device for fast and high throughput microparticle focusing is reported. The particle focusing is based on the combination of inertial lift force effect and centrifugal force effect generated in a microchannel with a series of repeated sharp corners on one side of the channel wall. The inertial lift force effect induces two focused particles streams in the microchannel, and the centrifugal force generated at the sharp corner structures tends to drive the particles laterally away from the corner. With the use of a series of the repeated, sharp corner structures, a single and highly focused particle stream was achieved near the straight channel wall at a wide range of flow rates. In comparison to other hydrodynamic particle focusing methods, this method is less sensitive to the flow rate and can work at a higher flow rate (high throughput). With its simple structure and operation, and high throughput, this method can be potentially used in microparticle focusing processes in a variety of lab-on-a chip applications.

Acoustic focusing and separations is a growing field of research since it is an efficient and continuous method for particle manipulation in microfluidic systems. Using microfabrication, microfluidic devices driven by an acoustic resonator were used to focus various microparticle suspensions. By simple tuning of frequency, amplitude, and channel geometry, controllable focusing patterns and alignments were obtained. This approach afforded the separation of particles of contrasting sizes, shapes, densities, porosities, and compressibilities. In this study we present the method for the fabrication of these lab-on-chip devices and report on their performance in the manipulation of microsized particles.

Acoustophoresis which means separation of particles and cells using acoustic waves is becoming an intensive research subject. The method is based on inducing an ultrasonic compression standing wave inside a microchannel. A finite element approach is used to model the acoustic and electro-mechanical behavior of the piezoelectric material, the micro-channel geometry as well as the fluid inside the channel. The choices of silicon and PDMS materials are investigated as the chip materials for the resonator. A separation channel geometry which is commonly used in the literature is implemented in this study and the fluid flow inside the microchannel geometry is simulated using computational fluid dynamics. The acoustic field inside the fluid channel is also be simulated using the finite element method. For the separation process to be successful micro-particles of different diameter groups should end up in different channels of the micro-separator. In order to simulate real life scenarios, each particle size group have a size distribution within themselves. For realistic simulation results the particles will be released into the micro separator from a different starting locations (starting location distribution). The results of this Monte-Carlo based finite element simulation approach will be compared with the reported experimental results.

Capacitive microsensor is a widely used detection technique for the analysis and assays in biochemistry, biomedicine, environmental monitoring and force measurement in MEMS. In this study, a liquid-metal based capacitive microsensor is proposed. A pair of high aspect ratio polydimethylsiloxane (PDMS) microchannels were fabricated symmetrically and vertically to both sides of detection channel. These two microchannels were filled with room temperature liquid-metal to form two electrodes. These two liquid-metal microchannels worked as two polar plates of a micro parallel-plate capacitor with the detection channel going between them. When the fluid changes in the detection channel, the capacitance of the micro parallel-plate capacitor will also change. Thus, by measuring the change in the capacitance of the micro capacitor, the fluid in the detection channel can be therefore detected. The capacitive microsensor can provide a wide range of applications in microfluidic technologies, such as detection of droplets, cells, particles or biological macromolecules. For better understanding of the capacitive microsensor, theoretical model was developed to reveal the working principle. To evaluate the capacitive microsensor, experiments were also performed to measure the microparticle concentration in deionized water by measuring its dielectric constant.

Suspension flow has an important role in various applications such as paint, material and pharmaceutical industries. Settling is considered as a resisting phenomenon in the processes dealing with suspensions. Using nanoparticles as an additive to micro-particulates has been studied in limited studies. This work presents an experimental investigation to assess the effectiveness of nanoparticles in reduction of suspension settling. Microscopic imaging and transmission measurement were used to analyze the stability factors in a container. Transmission analysis revealed that presence of nanoparticles in the suspension, decreased the sedimentation rate. Microscopy showed that the settling rate decreased after adding nanoparticles. This is attributed to the repulsive forces between microparticles caused by the halos of nanoparticle. Highly charged nanoparticles segregate to region near negligibly charged microspheres because of their repulsive Coulombic interactions. Therefore, microparticles exhibit an effective charge in presence of nanoparticles. Obtained results indicate that the nanoparticles with appropriate volume fraction can stabilize suspension of microparticles and can successfully minimize the settling rate. Proposed technique can be also implemented in other applications such as heat pipes and heat transfer devices.

Quantitative measurements are obtained for DNA flows in planar abrupt 90° micro-bends. Micro-particle image velocimetry (μPIV) is utilized to characterize flow fields including the secondary flow region in the inner, upstream corner of the bend where vortex growth is observed. The presence and strength of the elastic vortex is quantified at high resolution for increasing Reynolds and Weissenberg number (9.9 × 10 −7 < Re < 4.2 × 10 −3 , 0.42 < Wi < 42). Velocity fields of a Newtonian fluid (water) in the micro-bend do not contain a secondary flow. Understanding the viscoelastic flow behavior of complex biological fluids anticipated for use in biomedical microdevices is critical for the advancement of optimized designs comprised of numerous spatially varying canonical microfluidic components.

In the recent years, emulsification technologies that generate droplets individually have attracted a great deal attention in various fields, e.g., for chemicals, cosmetics, foods, and pharmaceuticals. Such drop-by-drop emulsification technologies include membrane emulsification using microporous membranes and microchannel (MC) emulsification, among others. The authors developed MC emulsification chips, consisting of parallel microgrooves or compactly arranged straight through-holes. Using this MC emulsification technique, the authors have evaluated the formulation a two-phase system consisting of size-controlled O/W emulsions loaded with bioactive molecules, such as β-carotene or γ-oryzanol, PUFAs or polyphenols. The MC emulsification process enabled the production of β-carotene-loaded O/W emulsions with average droplet size ( d av ) of 27.6 μm and coefficient of variation (CV) of 2.3% and γ-oryzanol-loaded droplets with d av of 28.8 μm and CV of 3.8%. The highly monodisperse O/W emulsions were physically stable during up 4 months storage in darkness at 5 °C. In addition, we investigated the formation characteristics of O/W emulsion droplets in the presence of electrolyte by MC emulsification using differently charged surfactants. Droplet formation was conducted by pressurizing a dispersed phase (refined soybean oil) through the MC silicon chip into a continuous phase containing 1.0 wt% of sodium dodecyl sulfate (SDS) or polyoxyethylene (20) sorbitan monolaurate (Tween 20), and an electrolyte (NaCl) (0–1.0 mol/L). Monodisperse O/W emulsions with an d av of 26 μm and a CV below 5% were produced when the NaCl concentration was lower than a threshold level that is 0.3 mol/L for SDS and 0.5 mol/L for Tween 20. The authors also developed a large MC emulsification device including a newly designed asymmetric MC array chip to realize the mass production of uniformly sized droplets on a liter per hour scale, so that satisfying the minimum droplet productivity needed for industrial-scale production. The large MC emulsification device has a potential droplet productivity exceeding several tons per year, which could satisfy a minimum industrial-scale production of monodisperse microdispersions containing emulsion droplets, microparticles, and microcapsules loaded with bioactive compounds. Such systems have as continuously increasing potential application in the formulation of functional foods, providing a good opportunity to improve the solubility of bioactive compounds, so that increasing their bioavailability.

Temperature management in microsystems is a technical problem with an increasing importance: although the power consumption of integrated circuits is not increasing, due to further miniaturization the local power density is still increasing. Moreover, in the near future more and more micro components will be integrated in flexible system-in-foil (SIF) packages. These packages can contain ultra-thin (8–50 micron) flexible embedded silicon chips combined with polymer electronics, optical systems and microfluidic channels e.g. for point-of-care diagnostics. However, the low thermal conductivity of the polymeric package is aggravating the heat management problem. The life span of micro components, but also the performance of some micro components, like (O)LEDs, can be strongly temperature dependent. Therefore an adequate temperature control is required. The thermal management problems can potentially be addressed by embedding micro-channels containing a flowing cooling medium in close proximity and preferably directly underneath the electronic circuit. However, many applications do not allow for external pumps and therefore pumping needs to be integrated in these channels as well. In this paper some promising integrated micro pumping techniques, like AC-electro osmosis and ferrofluidic pumping, will be described and discussed. The multi-physics modeling approach will be presented and the numerical results will be analyzed and compared with flow fields that are measured by 3D astigmatism micro particle tracking velocimetry (3D micro-PTV).

Observation of biological samples is a common issue in many applications relative to marine environment and in the field of water treatments. In particular, the ability to detect the presence of bacteria such as Criptosporidium and Giardia Lamblia in freshwater contributes to prevent people from critical diseases even in developed countries. The main challenge in this field is to analyze a large enough volume of biological sample to make it representative of the selected environment, while characterizing the species of interest whose size is often many order of magnitude smaller. In order to obtain detailed information of the observed species, the magnification of the visualization systems — often optical microscopes — should fit the size of the objects, involving a restrained field of view. As a result; the rate of analysis is lowered and the characterization of the samples time-consuming. To tackle this issue, an increase of the flow rate is possible by focusing of particles in the observation field of view. Such technique allows for increasing the overall flow rate, inversely related to the sampling time. In this article a new 3D hydrofocusing device is presented. A square section glass capillary (400×400μm 2 ) used as the nozzle is inserted into a square section glass channel (2×2mm 2 ). The inlets are fixed on a custom device that enables the sheath flow to completely wrap the sample flow after the injection. The 3D position of the particles used as representative substitutes of the biological species has been measured thanks to digital holographic microscopy and their distribution in cross-sections 2mm and 30mm downstream the injection nozzle compared to numerical simulations. A successful match of the location has been observed. The carrier fluid used in the experiments was water seeded with 27–45μm diameter neutrally buoyant particles. Several flow rates have been tested for both samples and sheath flow to investigate the shape of the focused stream line and to validate the prototype design. A maximum constriction — ratio between the part of the cross-sections where particles are present with and without focusing sheath flow — of 47% has been observed confirming the potentiality of the technique.

Analysis of environmental changes has become a great issue nowadays with the increase of pollution and global warming. Surveys about growing populations of species and apparition of new parasites are more and more relevant to preserve our environment and populations needs. The ability to observe for instance the presence of bacteria such as Criptosporidium and Giardia Lamblia in freshwater contributes to prevent people from diseases even in developed countries. The main issue in the observation of such species is that the size of the biological sample should be large enough to be representative of its surrounding environment, meaning that the amount of the scanned sample is several orders superior to the size of the micro species of interest. A major limitation for optical instruments in performing this kind of analysis is that the magnification required should fit the size of the particles, severely constraining the field of view. As a result, the rate of analysis is often so slow to be compatible with the large samples to analyze. One way to increase the rate of analysis is to manipulate particles in order to focus them in the field of view of the visualization system. In this article we use acoustic standing waves to achieve this goal by directing particles in the observation region. A 1mm × 1mm square section glass channel, excited by a piezo transducer is used. The carrier fluid is water while several types of particles, both synthetic and biological are tested. The dynamic behaviour of particles under the acoustic field has been investigated and the 3D position of each particle is reported. In order to increase the depth of investigation, we used a Digital Holographic Microscope. This technology permit to scan a larger volume of fluid and also to calculate the 3D position of each detected particle flowing in the channel. A focusing streamline of as thin as 1/20 of the channel cross section has been successfully achieved.

In microfluidic devices, electrokinetic phenomena can be engaged to manipulate liquids, particles, or cells. In general, electrokinetic phenomena are related to the existence of electrical double layers (EDL), which are present e.g. where solid walls are in contact with electrolytes. Due to surface charges of the solid, ions of opposite charge are attracted and lead to a thin electrically non-neutral zone — the EDL. The application of an electrical field, consequently, leads to forces onto the fluid inside the EDL, capable of driving a flow within the microchannel. This particular flow is termed electroosmotic. If further the electrodes are placed inside the microchannel, due to small distances, strong inhomogeneous electrical fields can be induced, which in turn cause complex flow fields. This is true despite low applied electrode potentials of a few Volts. Numerical simulations of various authors have shown the potential for pumping and mixing of such complex flows in microchannels with internal electrodes. We focus on the electroosmotic flow within rectangular microchannels of 100 × 200 μm cross section. In each microchannel eight pairs of electrodes are placed onto the top and bottom glass covers, whereas the spatial offset of the electrodes is chosen differently in each of the microchannels. Due to the limited optical access through the glass covers, only the two velocity components which are tangential to the glass cover (i.e. the x– and y–components) are accessible by means of the micro–particle–image velocimetry (μPIV). The expected electroosmotic flow, however, features vortices which cannot be recognized within this x–y velocity plane. Therefore, a number of two–dimensional, two–component velocity fields is measured at different heights of the microchannel. From an integration of the continuity equation, subsequently, the third velocity component in the z–direction, which is normal to the glass cover, can be determined at reasonable accuracy. A further difficulty arises from the fact that the microparticles, used for μPIV, are not electrically neutral. Hence, in addition to drag forces from the electroosmotic flow, they experience electrophoretic forces. Hence, the particle movement appears to be a superposition of electroosmotic and electrophoretic effects.

Experimental studies of blood microflows in rectangular biocompatible polymer microchannels measured using micro-particle image velocimetry are reported. The data processing methods, data collection methods, and choice of channel material are demonstrated to impact the velocity profile measurements obtained. Results show that the use of red blood cells as tracer particles creates a large depth of correlation which can approach the size of the vessel itself and decrease the accuracy of the method. It is shown that changing the amount of overlap in the post-processing parameters affects the results by nearly 10%. The velocity profile is studied as a function of the flow rate of the blood, the hematocrit, or percentage of red blood cells, the shape of the channel, and the channel material. The results highlighted here show that the best processing options include pre-processing, the use of fluorescent tracer particles instead of the red blood cells themselves as tracers give a more accurate prediction of the profile, and the use of silicone as the channel material more closely mimics the behavior of physiology. Acrylic biocompatible polymer channels are shown to give a more parabolic profile at lower levels of hematocrit, while silicone biocompatible polymer channels give a velocity profile that looks more like in vivo flow studies.

In this study, a cross-sectional capacitance is measured in a microchannel with 12 multi-layered electrodes. The cross-sectional capacitance of polystyrene particles and deionized water is measured at upstream and downstream microchannel. Based on the measured cross-sectional capacitance, the particle volume fraction is calculated at three area measurements with different electrode pair. The three area measurements are at the adjacent pair, horizontal pair and vertical pair. The particle volume fraction at the adjacent pair area is almost same and higher at upstream and downstream cross-section. While at the vertical and horizontal pair areas, the particle volume fraction increased from upstream to downstream cross-section around 1.0–9.3% at electrode pair near the walls whilst decreased 0.4–0.96% at electrode pair near the center streamlines. It shows the particles are more attracted to migrate to the equilibrium position at near to the walls. The particles migration induced by inertial lift forces is reflected to Serge-Silberberg phenomenon where the particles are distributed and transverse from the center streamlines of fluid flowing to the area near the walls.

The purpose of the work presented this paper is to design a model to study experimentally and numerically a micro-Couette blood flow to obtain a constant and controlled shear rate that is a suitable environment for analysis of Red Blood Cell (RBC) aggregation. Due to the simplicity of the flow conditions, aggregate size can be related to the constant shear rate applied. This Couette flow is created by the motion of a second fluid that entrains the blood. The experimental work is coupled with 3D numerical simulations performed using a research computational fluid dynamics solver, Nek5000, based on the spectral element method, while the experiments are conducted using a micro-particle image velocimetry system. Two models of microchannels, with different dimensions, 150 × 33 μm and 170 × 64 μm, are fabricated in the laboratory using standard photolithography methods. The design of the channel is based on several parameters determined by the simulations. A Newtonian model is tested numerically and experimentally. Blood is then tested experimentally to be compared to the simulation results. We find that using a velocity ratio of 4 between the two Newtonian fluids, we create a flow where one third of the channel thickness is filled with the fluid destined to be blood. In the blood experiments, the velocity profile in this layer is approximately linear, resulting in the desired controlled conditions for the study of RBC aggregation.

The motion of a neutrally buoyant spherical micro particle suspended in a liquid inside a ‘U’ shaped micro channel is detected using particle tracking velocimetry. The ratio of the particle diameter to the hydraulic diameter of the channel is found to be 0.1. Poiseuille velocity profile is observed inside the micro channel and the spherical particle is convected with the Poiseuille velocity field. The magnitude of the lag factor is determined experimentally and is compared with the values reported in existing literature. The effect of other forces and interactions on the particle is quantitatively studied. It was found that effects of Brownian, Van der Waals and electrostatic forces do not significantly contribute to the velocity of the particle.

Micro particle image velocimetry forms the basis of modern microscale flow measurement techniques. However, the motion of tracking particles that have been introduced into the fluid to measure flow velocity in micro fluidic flows can be affected by a number of different body forces that may not affect the carrier fluid. Under these conditions, molecular tagging velocity (MTV) has a distinct advantage in resolving fluid motion. A MTV approach, based on the photobleaching mechanism, is presented that allows any desired pattern of flow markers to be written into the region-of-interest to track the flow. This allows standard particle master shadowgraphy algorithms to be used to track groups of tagged molecules in the flow resolving two components of the bulk fluid motion. The MTV techniques developed so far for microfluidic application provide only a one dimensional flow measurement, typically along the axial direction perpendicular to the tagged region. Other tagging methods include the use of a grid and a structured mask for macro-scale and micro-scale flows respectively. One of the limitations of these approaches is that velocity information for only a predefined pattern is available. The MTV technique presented here is capable of resolving two dimensional flow velocity information with the limitations associated with the application of a mask. The salient feature of the presented approach is the use of a laser scanner which allows unrestricted, repeatable and accurate movement of the write laser within the field-of-view. The temporal displacement of the tagged region is captured onto an imaging device. The proof of concept of the technique is presented in the current work. A known flow velocity through a curved section of a 750 μm wide serpentine channel is measured using the designed system. The tagged regions are in the form of dark dots which advect with the flow. The temporal and spatial evolution of these tagged regions is tracked to reveal the flow information. The aim of this technique is to perform velocity measurements in a dielectrophoretic flow of a mixture of nanoparticles and the caged fluorescent dye, which may show different flow behavior, possibly in two opposite directions due to the charge on them.

Optical propulsion via laser source is a relatively new and non-contact tool for manipulation of microscopic objects. The method is based on the radiation pressure of light photons on the micron sized particles. Applications of the technique mainly cover microscopic separation, purification and cellular studies. Due to high power intensity of laser beams, absorption of light may result in heating and damage of objects to be manipulated. In addition, the difference between heated and cold zones can lead to a naturally driven flow around the objects. So precisely controlled conditions should be set up to avoid thermal effects. In this work, a theoretical study is conducted to investigate the thermal effects on a particulate flow in a mini-channel. Thermal effects are studied as a result of light absorption either in carrier fluid or dispersed phase. A continuous laser beam is considered as a radiation heat source and wave propagation and interaction with suspension of microspheres in water is simulated. The main objective of mentioned simulation is to determine the conditions of pure hydrodynamic laser-particle interaction. Especially the criteria for properties of particles and beam parameters are presented. Derivation of limits is based on the maximum allowable temperature increase and the magnitude of thermal induced flow. The results are crucial for design and selection of elements in microscopic separation by laser beam.

A flow control method is presented that employ liquid and gas jets to enhance heat and mass transfer in micro domains. By introducing pressure disturbances, mixing can be significantly enhanced through the promotion of early transition to a turbulent flow. Since heat transfer mechanisms are closely linked to flow characteristics, the heat transfer coefficient can be significantly enhanced with rigorous mixing. The flow field of water around a low aspect ratio micro circular pillar of diameter 150 μm entrenched inside a 225 μm high by 1500 μm wide microchannel with active flow control was studied and its effect on mixing is discussed. A steady control jet emanating from a 25 μm slit on the pillar was introduced to induce favorable disturbances to the flow in order to modify the flow field, promote turbulence, and increase large-scale mixing. Micro particle image velocimetry ( μPIV ) was employed to quantify the flow field, the spanwise vorticity, and the turbulent kinetic energy ( TKE ) in the microchannel. Flow regimes (i.e., steady, transition from quasi-steady to unsteady, and unsteady flow) were elucidated. The turbulent kinetic energy was shown to significantly increase with the controlled jet, and therefore, significantly enhance mixing at the micro scale.