Increasing the mixing rate of fluid droplets within modern lab-on-a-chip technology is being increasingly stressed by members of the biological and chemical communities that wish to study reaction kinetics and mechanisms. Faster mixing allows for a more complete analysis of a wider range of chemical and biological reactions, and due to this technologies ability to help better characterize these reactions, there is an increasingly large number of demands for chips that can fully mix droplets in an order of microseconds. High speed mixing like this is difficult to characterize using standard microscopy. This article looks at how to achieve high-speed droplet collisions using flow focusing geometry, and how these collisions are visualized using a novel laser-imaging system. Microchannel droplet mixing is plagued with severe limitations due to the restricted microchannel lengths between elements within the chips, as well as strong surface tension forces involved and low Reynolds numbers that are found in all microchannel flow. These characteristics further reduce the rate of mixing, and thus the rate of diffusion, between the two colliding droplets. Previous attempts at mixing droplets in microchannels stemmed from two methods; hydrodynamic flow focusing, and chaotic advection mixing, this later approach leading to the advent of inertial-based droplet micromixer technology. Both of these techniques rely on rapidly decreasing the diffusion length scales between the two mixing droplets through the creation of striations. Generating these droplets while developing a method for quantifying high-speed diffusion mixing is the main focus of this article. For this study, liquid droplets are formed in a flow-focusing configuration using a high speed gaseous flow and collided at angled Y-junctions in the microchannel fabricated in PDMS. In order to determine the instantaneous rates of mixing within the internal droplet flow during the collision process, an experimental setup using epi-fluorescence microscopy in conjunction with high speed laser-induced fluorescence was developed. This setup is capable of tracking and recording fluorophore concentration within the droplets during the collision and coalescence process in multiple images. The droplets are distinguished using differing concentrations of fluorophore which correspond to differing levels of fluorescent intensity while the colliding droplets remain unmixed, and consistent levels of fluorescent intensity when the colliding droplets are fully mixed. These images are then processed using a statistical analysis software to determine both the diffusion length scale as well as the quasi-instantaneous rate of mixing at all times during the collision process.

We present a method to quantify and enhance separation of binary cells mixture in the microfluidic device using high frequency dielectrophoresis (>20 MHz). At these frequencies, the DEP response depends primarily on the dielectric properties of the cytoplasm. In order to achieve efficient separation, there must be a difference in the intrinsic dielectric properties of the populations to be sorted. For algae cells, the shift in high frequency dielectrophoresis response during lipid accumulation can be used as a basis of separation. We defined a separability parameter based on the expected difference in the dielectrophoresis responses of the algae cells. Chlamydomonas reinhardtii cells were cultured in regular media and then the same cells were cultured under nitrogenfree conditions to accumulate neutral (non-polar) lipids. Separability of microalgae cells with different lipid content via high frequency dielectrophoresis were investigated by a thin needle shaped electrodes patterned by standard photolithographic and wet etching procedures. Experimental separability factors were measured by estimation of relative lipid content with BODIPY 505/515 fluorescence dye and calculating the area-weighted intensity average of fluorescent images. Theoretical separability parameter was calculated using analytical analysis of single shell model by MATLAB. Theoretical and experimental separability parameters, as tools to determine the optimal separation method, were calculated for microalgae cells with different lipid content. This objective function was maximized in the range of 35–45 MHz for C. reinhardtii cells after 21 days of lipid accumulation in a static separation. In order to design a continuous cell sorter device, the theoretical separation factor was maximized based on differences in the magnitude or the direction of the DEP force.

Lab-on-a-chip (LOAC) devices are emerging technologies that aim to perform all of the laboratory functions of traditional diagnostic tests on single microchips. Microarrays are one promising type of LOAC device that consist of an array of droplets for testing tens to thousands of samples simultaneously. Microarrays are commonly used in gene sequencing, pathogen detection, determining microbial resistances, and conducting enzyme-linked immunosorbent assays (ELISAs). As droplets in these arrays dry, the majority of material within the droplet is deposited around the periphery. This phenomenon is referred to as the coffee stain effect. The non-uniform depositions left by this effect can result in variation of fluorescence intensity measurements in automated vision systems. A means of producing more uniform particle depositions for the microscopy analysis would allow for more accurate test results. One promising method for suppression of the coffee stain effect involves the use of electrowetting on dielectric (EWOD). EWOD devices apply an electrokinetic force at the three-phase contact line to manipulate the shape of a droplet interface. The Mugele group has already begun investigating EWOD’s effects on the coffee stain effect and found that an AC voltage applied to droplets on EWOD devices can suppress the coffee stain effect and produce smaller, more uniform droplet deposition patterns. This work presents (i) a method to characterize the deposition pattern left by a desiccated droplet as a function of radial position and (ii) a discussion of the microfabrication technique used to create devices to perform EWOD assisted desiccation for both AC and DC voltages.

We report on a novel microfluidic strategy for building monodisperse asymmetric vesicles with customized composition, size, and interfacial properties at high-throughput. The microfluidic device encompasses a triangular post region and two flow-focusing regions. The major steps involved in the vesicle building process include: (1) forming highly uniform water emulsion templates in the inner-leaflet lipid solution, (2) replacing the inner-leaflet lipid solution with the outer-leaflet lipid solution, (3) creating water-in-oil-in-water double emulsions, and (4) extracting the excess outer-leaflet lipid solution from the double emulsions. Bilayer membrane asymmetry and unilamellarity are confirmed using a fluorescence quenching assay and quantitative measurements of fluorescent intensities. This method addresses many of the deficiencies found in existing technologies, and yields asymmetries as high as 95%. The asymmetric vesicles built using this strategy hold the potential to serve as model systems to investigate fundamental problems in membrane biology.

This study focused on the diffusion and mixing phenomena investigated by using luminol chemiluminescence (CL) to estimate the local chemical reaction rate in the T-junction microchannel. Generally, the degree of mixing in microchannel is calculated by the deviation of the obtained concentration profiles from the uniform concentration profile by using fluorescence technique. Thus, the degree of mixing is a macroscopic estimate for the whole microchannel, which is inappropriate for understanding the diffusion and mixing phenomena in the mixing layer. In this study, the luminol CL reaction is applied to visualize the local chemical reaction and to estimate the local diffusion and mixing phenomena at an interface between two liquids in microchannel. Luminol emits blue chemiluminescence when it reacts with the hydrogen peroxide at the mixing layer. Experiments were carried out on the T-junction microchannel with 200 microns in width and 50 microns in depth casted in the PDMS chip. The chemiluminescence intensity profiles clearly show the mixing layer at an interface between two liquids. The experimental results are compared with the results of numerical simulation that involves solving the mass transport equations including the chemical reaction term. By calibrating CL intensity to the chemical reaction rate estimated by the numerical simulation, the local chemical reaction profile can be quantitatively estimated from the CL intensity profile.

This paper summarized our recent works of the laser-based measurement techniques for investigating micro- and nano-scale transport phenomena. Micron-resolution particle image velocimetry combined with the laser induced fluorescence (LIF) technique was developed for analyzing velocity and ion concentration distributions simultaneously. The measurement system was based upon a confocal microscopy to realize the depth-resolution of approximately 2 μm, and this technique was applied to liquid-liquid mixing flows, gas-liquid two-phase flows and gas permeation phenomena through membranes. To evaluate the electrostatic potential at solid-liquid interface (i.e., zeta-potential), the LIF technique was advanced with the evanescent wave illumination. Fluorescent dye within approximately 100 nm from a microchannel wall was irradiated. This technique was applied to microdevices with a surface modification pattern, and the zeta-potential distribution was successfully visualized. Two proposed techniques will contribute to novel applications related to microscale multiphase flows or electrokinetics.

We report a microfluidic platform for sizing and separation of DNA called PicoSep that is among the most sensitive to date, requiring only yoctomoles of DNA (10 −24 moles) and picoliters of sample (<10 pL). The cohesive integration of cylindrical illumination confocal spectroscopy based (CICS) single molecule counting with free solution hydrodynamic separation increases detection sensitivity >10 3 -fold over traditional laser induced fluorescence (LIF) and vastly improves quantification accuracy. Separation is performed via single molecule free solution hydrodynamic separation (SML-FSHS). SML-FSHS relies on the wall exclusion mechanism to separate DNA as it is driven down buffer filled microchannel. High separation efficiency (plate number = 10 5 – 10 6 ) and high sizing resolution (37 bp – 2.1 kbp) are obtained across a wide dynamic range of DNA (100 bp – 27 kbp) in a single separation. Quantitative accuracy up to the limits imposed by molecular shot noise is achieved, and a limit of detection <20 molecules is demonstrated. Through the development of PicoSep, we have significantly reduced the cost and complexity of single molecule instrumentation while achieving sizing and quantification performance that surpasses capillary electrophoresis (CE). Unlike CE, no viscous sieving matrices, high voltage power supplies, or capillary wall coatings are necessary, making devices inexpensive, simple to fabricate, and easy to incorporate into lab on a chip systems.

An experimental investigation of water flow in a rectangular T-channel with inner dimensions of 20 × 20 mm (inlet channel) and 20 × 40 mm (outlet mixing channel) has been conducted. The inlet Reynolds number Re , based on inlet hydraulic diameter, ranged from Re 1 = 90 to 775. Inlet flow conditions were asymmetric, and inlet Re ratios of Re 1 / Re 2 = 1.24, 1.65 and 2.45 were obtained by varying volumetric flow rate. Dynamical conditions and T-channel geometry are directly applicable to microscale mixing. Planar laser induced fluorescence (LIF) was used to characterize flow regimes and behaviors, including periodicity, in the inlet and outlet channels. Two distinct flow regimes were identified and characterized for asymmetric inlet Re . For low inlet Re , Re 1 ≤ 150, and all Re 1 / Re 2 , flows were steady. For higher Re and all Re 1 / Re 2 , a jet flow regime, characterized by two counter rotating vortices and increasingly turbulent at higher Re dominated the flow in the junction. Qualitative mixing characteristics for all flow regimes, based on LIF visualizations in the outlet channel, are also presented.

The flow patterns and concentration distribution in a passive scaled-up micromixer is investigated experimentally over 5 ≤ Re ≤ 100. Four meandering elements serve to enlarge species interface area through the formation of Dean vortices, while 36 slanted grooved structures enhance secondary flow by promoting helical flow. The alternating direction of the curved channel segments and slanted grooves alternated the direction of secondary flow, increasing mixing. Flow visualization provides flow pattern information as seen from the top view, while induced fluorescence provides concentration distribution information at the outlet along the center plane. At low Re (Re = 5), species appear nearly horizontally stacked at the outlet, with a curved interface. As the Reynolds number increases, the angle the species interface makes with the horizontal direction increases due to the increased strength in rotational flow. At higher Re, the formation of Dean vortices is apparent.

Interfacial phenomena due to surface forces are important in microfluidic devices with their relatively large surface areas and small volumes. Most experimental studies of interfacial transport estimate flow velocities from the motion of tracer particles less than 1 μm in diameter, and assume that the particle displacements over a known interval are due to the fluid velocity field. This talk discusses some of our research on measuring fluid velocities in Poiseuille and electrokinetically driven flows over the first ∼0.5 μm next to the solid wall from the motion of an ensemble of O (10 5 ) fluorescent spheres as small as 100 nm illuminated by evanescent waves. Because the evanescent-wave intensity decays exponentially with wall-normal distance, the particle-wall separation can be determined from the brightness of each particle image, and used to estimate the steady-state distribution of the tracers near the fluid-solid interface. Recently, evanescent-wave illumination has been combined with fluorescence thermometry, where water temperature fields are estimated from changes in the intensity of the emissions from aqueous solutions of temperature-sensitive fluorophores. The Poiseuille flow of a fluorescein solution through a heated channel at a Reynolds number of 8.3 was illuminated with evanescent waves to obtain the solution temperature at an average distance of 74 nm from the wall. The temperature results obtained over three different regions were in good agreement with numerical predictions of the wall surface temperature, even in the presence of temperature differences exceeding 15 °C over the 1 mm width of the channel.

An experimental investigation of water flow in a T-shaped channel with rectangular cross section (20 × 20 mm inlet ID and 20 × 40 mm outlet ID) has been conducted for a Reynolds number Re range of 56 to 422, based on inlet diameter. Dynamical conditions and the T-channel geometry of the current study are applicable to the microscale. This study supports a large body of numerical work, and resolution and the interrogation region are extended beyond previous experimental studies. Laser induced fluorescence (LIF) and particle imaging velocimetry (PIV) are used to characterize flow behaviors over the broad range of Re where realistic T-channels operate. Scalar structures previously unresolved in the literature are presented. Special attention is paid to the unsteady flow regimes that develop at moderate Re , which significantly impact mixing but are not yet well characterized or understood. An unsteady symmetric topology, which develops at higher Re and negatively impacts mixing, is presented, and mechanisms behind the wide range of mixing qualities predicted for this regime are explained. An optimal Re operating range is identified based on multiple experimental trials.

Efficient water management is crucial for the good performances of proton-exchange membrane fuel cells (PEMFCs). The geometric and physical characteristics of the components of a PEMFC as well as operating conditions have an impact on the transport of water through the porous transport layer (PTL) and the two-phase flow regimes in the microchannels. One parameter of importance is the local temperature, which affects properties such as surface tension and is coupled with phase change. Indeed, a temperature difference of about 5K is expected across the PTL, with spatial variations due to the geometry of the flow field plate. We present preliminary results obtained with a first experimental setup for the ex-situ characterization of two-phase flow regimes in the flow channels. Water is pushed through the PTL, which is sandwiched between a porous metal foam and the flow field plate. The air flow rate, temperature and humidity can be controlled. The cell can be heated up by applying an electrical current through the metal foam. A transparent window is located on top of the flow channel. The two-phase flow within the micro-channels is visualized using a high-speed camera and laser-induced fluorescence. Preliminary results obtained under isothermal conditions at room temperature show that different two-phase flow regimes occur in the channels depending on the operating conditions, in good qualitative agreement with data from the literature. Eventually, a new visualization cell is presented that is expected to correct the flaws of the previous design and will allow a better thermal control. It will be possible to adjust the temperature gradient and the mean temperature in order to observe their impact on two-phase flow regimes for different types of PTL and flow rates. The results will provide a better understanding of water transport in PEMFC and benchmark data for the validation of numerical models.

Efficient mixing at the microscale remains a formidable engineering challenge. Recent advancement and proliferation of Lab-on-a-Chip (LOC) and Micro Total Analysis Systems (μTAS) has demanded accelerated development and demonstration of novel micromixers as successful mixing is critical to device performance. Passive techniques such as chaotic advection and shear thinning as well as active methods utilizing electric fields show great promise at meeting these requirements. A new droplet-based mixing technique currently being developed aims at improving micromixer rates passively by increasing the Reynolds number in the microchannel. High speed gaseous flows with Reynolds numbers from 1 to 300 are used to detach and transport discrete droplets to a collision zone where droplet interaction and subsequent mixing is achieved under highly inertial conditions. The design utilizes variants of the standard T-junction arrangement for both the detachment and collision process. A fluorescing and non-fluorescing droplet pair are brought into contact in a collision zone and allowed to interact with relative velocities in the 0.1 to 5m/s range. Mixing rates are quantified using an optical based measurement technique that examines temporal changes in droplet intensity as mixing progresses. Both the detachment and collision processes are captured using a high speed camera capable of frame rates in excess of 10MHz. Experimental results are obtained for different collision zone geometry arrangements and microchannel aspect ratios to assess mixing performance. A description and sufficient explanation of the optical measurement techniques used to quantify mixing rates is provided, including limitations and shortcomings of this simplified approach. Analytical models are developed to gain better understanding of the key physical mechanisms driving droplet mixing and experimental results are correlated against this order of magnitude model. Based on these results, recommendations are made for potential design improvements and issues are addressed concerning mixing using two-phase gas/liquids flows.

A capillary electrophoresis microchip system with a confocal LIF detector was successfully constructed. RBPhOH, a new fluorescence dye synthesized from rhodamine B, was first investigated in a glass microchip for the selective and sensitive determination of copper ion by LIF method. The influences of the pH value, concentration of running buffer, detection voltage, and sample loading time on the response of LIF detector were studied. RBPhOH dissolved in 20 mM pH 6.0 Tris containing 10% acetonitrile could detect copper ions with a detection limit of 1.34×10 − 7 mol L −1 (S/N=3), an order of magnitude lower than those of conductivity detectors.

Microfluidics has experienced a significant increase in research activities in recent years with a wide range of applications emerging, such as micro heat exchangers, energy conversion devices, microreactors, lab-on-chip devices and micro total chemical analysis systems (μTAS). Efforts to enhance or extend the performance of single phase microfluidic devices are met by two-phase flow systems [1, 2]. Essential for the design and control of microfluidic systems is the understanding of the fluid/hydrodynamic behavior, especially pressure drop correlations. These are well established for single phase flow, however, analytical correlations for two-phase flow only reflect experimentally obtained values within an accuracy of ± 50% [3, 4]. The present study illustrates the effect of two-phase flow regimes on the pressure drop. Experimental measurement data is put into relation of calculated values based on established correlations of Lockhart-Martinelli with Chisholm modifications for macroscopic flows [5, 6] and Mishima-Hibiki modifications for microscale flows [7]. Further, the experimental pressure drop data is superimposed onto two-phase flow maps to identify apparent correlations of pressure drop abnormalities and flow regimes. The experiments were conducted in a square microchannel with a width of 200 μm. Optical access is guaranteed by an anodically bonded glass plate on a MEMS fabricated silicon chip. Superficial velocities range from 0.01 m/s to 1 m/s for the gas flow and from 0.0001 m/s to 1 m/s for the liquid flow with water as liquid feed and CO 2 as gas. The analysis of the flow regimes was performed by imaging the distinct flow regimes by laser induced fluorescence microscopy, employing Rhodamine B as the photosensitive dye. The pressure drop was synchronically recorded with a 200 mbar, 2.5 bar and 25 bar differential pressure transmitter and the data was exported via a LabView based software environment, see Figure 1. Figure 2 illustrates the experimentally obtained pressure drop in comparison to the calculated values based on the Lockhard-Martinelli correlation with the Chisholm modification and the Mishima-Hibiki modification. For both cases the predications underestimate the two-phase pressure drop by more than 50%. Nevertheless, the regression of the experimental data has an offset of linear nature. Two-phase flow is assigned to flow regime maps of bubbly, wedging, slug or annular flow defined by superficial gas and liquid velocities. In Figure 3 the pressure drop is plotted as a surface over the corresponding flow regime map. Transition lines indicate a change of flow regimes enclosing an area of an anticline in the pressure data. In the direct comparison between the calculated and the measured values, the two surfaces show a distinct deviation. Especially, the anticline of the experimental data is not explained by the analytical correlations. Figure 4 depicts the findings of Figure 3 at a constant superficial velocity of 0.0232 m/s. The dominant influence of the flow regimes on the pressure drop becomes apparent, especially in the wedging flow regime. The evident deviation of two-phase flow correlations for the pressure drop is based on omitting the influence of the flow regimes. In conclusion, the study reveals a strong divergence of pressure drop measurements in microscale two-phase flow from established correlations of Lockhart-Martinelli and recognized modifications. In reference to [8, 9], an analytical model incorporating the flow regimes and, hence, predicting the precise pressure drop would be of great benefit for hydrodynamic considerations in microfluidics.

Because nanochannels act as lenses that can focus electric fields on a chip, they can concentrate and filter ions, molecules and nanocolloids by ion selectivity, electrodeless dielectrophoresis and surface hybridization. However, before fabricated nanochannels can lead to the next-generation of chip-scale dialysis membranes, preconcentrators for mass spectrometers, ion sieves, molecular sensors, solar/fuel cell membranes etc, the anomalous dc I-V characteristics and ac impedance of such nanochannels must be understood. Curious phenomena such as limiting and over-limiting currents, rectification, hysteretic cyclic voltametry, Warburg impedance, channel cross-talk etc will be studied with fabricated nanochannels, confocal microscopy and fluorescence correlation spectroscopy and analyzed mathematically with fundamental Nernst-Planck ion transport theories, Guoy-Chapman equilibrium electrolyte theories and new non-equilibrium extended polarization theories.

Rapid mixing is essential in the micro-fluidic systems. The purpose of this study is to develop the compact micro-mixer by switching the electroosmosis flow on the simple cross channel. Micro-mixers have been developed in a variety of ways; by the parallel and serial lamination, and the chaotic advection on the passive micro-mixers. On the active micro-mixers, the external disturbance effects such as pressure, electrohydrodynamics and acoustic are applied to enhance the mixing. However, the mixing process occurs at the mixing channel located at the downstream of junction or the mixing chamber, thus the mixing length is required to achieve a homogeneous mixing. In this study, the new concept of the mixing process, a batch processing mixing, is applied to the micro-mixer. The mixing and the transport processes are separated by switching the electroosmosis flow. On the mixing phase, the oscillating reversed flow induces the mixing at the junction of the cross channel. After the mixing process, the liquid mixture flows out through the outlet channel. The mixing is completed at the junction, thus the outlet mixing-channel is not necessary for the batch processing mixing. Experiments are carried out on the PDMS micro cross-channel chip. Channel is 200μm in width and 50μm in depth. The concentration field is measured by the fluorescence technique. The electrical field in the micro channel is generated by the high voltage power supply and is switched by the relay circuit controlled by the D/A converter. The mixing degree reaches about 90% within 0.9mm length of the outlet channel in the case of 500V applied voltage with the frequency of 1.41Hz. On the batch processing mixing, however, the reversed flow occurred at the mixing phase, the flow rate of the liquid mixture becomes small than that in the steady state voltage pattern. To increase the flow rate while keeping the same flow pattern, the electroosmotic velocity and the switching frequency are increased to keep the same Strouhal number. In the case of 900V applied voltage with 2.55Hz, the flow rate becomes 1.8 times keeping the same mixing degree. The batch processing mixing is effective for the compacting the micro-mixer.

Water management in proton exchange membrane fuel cells (PEMFCs) is critical in efficient operation of fuel cells during normal operation as well as purge and start-up conditions. Insufficient membrane hydration impedes the flow of protons and an overabundance of water obstructs the flow of reactants in the gas diffusion layer (GDL) and in gas distribution channels. These two extremes of water content in PEMFCs significantly reduce performance and efficiency, causing material degradation and potential failure. Visualization and quantitative measurement of water content in PEMFCs lead to greater comprehension of water distribution and transport processes. A wide variety of imaging techniques have been employed in literature to reveal water distribution and transport processes on both macroscale and microscale. The presented techniques utilize visible, infrared, X-rays, fluorescence microscopy, nuclear magnetic resonance (NMR), and neutron radiography to visualize water, measure temperature distributions, and quantify water content. Each imaging technique has intrinsic advantages, disadvantages, and limitations for water detection and will be discussed. A critical evaluation of these techniques and their suitability for visualization of specific components of PEMFC are also discussed.

We have developed a method to detect non-fluorescent analytes on standard microfluidic chip platforms equipped with fluorescence detection. We leverage isotachophoresis (ITP) to electrophoretically segregate both analytes and fluorescent species termed mobility markers into distinct zones. The fluorescent marker zones bound analyte zones, so that gaps in the fluorescent signal indicate the presence and concentration of analytes. We here demonstrate separation and indirect detection of amino acids, serine and phenylalanine and organic acids, acetic acid and phenylpropionic acid (∼10 μM) using this technique. We also present an indirect detection of the environmental toxin phenol [1][2] (∼10 μM) using two mobility markers. We show preliminary numerical simulation results that provide useful guidelines in design and optimization of our indirect detection assay.

We propose a simple method to implement three-dimensional (3D) hydrodynamic focusing with a circular micro-channel for the sample flow. We utilized soft lithography technique and a cylindrical object such as an optical fiber to construct a 3D hydrodynamic focusing master mold made of photoresist and the fiber. The 3D microfluidic focusing device can be duplicated by polydimethylsiloxance (PDMS) casting. When peeling the PDMS cast from the master, there is a crack along the circular channel. However, this crack can be recovered from the thermal bonding of the PDMS cast and a PDMS substrate. We applied this technique to fabricate 3D flow-focusing devices for a microfluidic fluorescence detection chip and a micro-emulsion chip. In the case of microfluidic fluorescence detection, the sample flow can form a small and stable cylindrical flow by controlling the flow rates of sheath flow and sample flow. This allows fluorescent particles to be focused at the center of the channel and to be excited uniformly by laser. Experimental results show that the coefficients of variation for 3D hydrodynamic focusing is 4 to 10 times lower than those of 2D hydrodynamic focusing. As a consequence, the 3D focusing chip provides advantages of stable sample flow and highly uniform detection signals. The micro emulsion chip with 3D hydrodynamic focusing can avoid the wetting problem of two immiscible fluids and successfully produce emulsion droplets.