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This study extends our previous work on droplet generation in microfluidic T-junction generators to include dynamic interfacial tension effects created by the presence of surfactants. In Paper I [T. Glawdel, C. Elbuken, and C. L. Ren, Phys. Rev. E 85, 016322 (2012)], we presented experimental findings regarding the formation process in the squeezing-to-transition regime, and in Paper II [T. Glawdel, C. Elbuken, and C. L. Ren, Phys. Rev. E 85, 016323 (2012)] we developed a theoretical model that describes the performance of T-junction generators without surfactants. Here we study dynamic interfacial tension effects for two surfactants, one with a small molecular weight that adsorbs quickly, and the other with a large molecular weight that adsorbs slowly. Using the force balance developed in Paper II we extract the dynamic interfacial tension from high speed videos obtained during experiments. We then develop a theoretical model to predict the dynamic interfacial tension in microfluidic T-junction generators as a function of the surfactant properties, flow conditions, and generator design. This model is then incorporated into the overall model for generator performance to effectively predict the size of droplets produced when surfactants are present.

Temperature gradient focusing (TGF) is a counterflow gradient focusing technique, which utilizes a temperature gradient across a microchannel or capillary to separate analytes. With an appropriate buffer, the temperature gradient creates a gradient in both the electric field and electrophoretic velocity. Combined with a bulk counter flow, ionic species concentrate at a unique point where the total velocity sums to zero and separate from each other. Scanning TGF uses varying bulk flow so that a large number of analytes that have large differences in electrophoretic mobility can be sequentially focused and passed by a single detection point. Up to now, scanning TGF examples have been performed using a linear temperature gradient which has limitations in improving peak capacity and resolution at the same time. In this work, we develop a bilinear temperature gradient along the separation channel that improves both peak capacity and separation resolution simultaneously. The temperature profile along the channel consists of a very sharp gradient used to preconcentrate the sample followed by a shallow gradient that increases separation resolution. A specialized design is developed for the heaters to achieve the bilinear profile using both analytical and numerical modeling. The heaters are integrated onto a hybrid PDMS/glass chip fabricated using conventional sputtering and soft-lithography techniques. Separation performance is characterized by separating several different dyes and amino acids that have close electrophoretic mobilities. Experiments show a dramatic improvement in peak capacity and resolution in comparison to the standard linear temperature gradient.

This is the second part of a two-part study on the generation of droplets at a microfluidic T-junction operating in the transition regime. In the preceding paper [Phys. Rev. E 85, 016322 (2012)], we presented our experimental observations of droplet formation and decomposed the process into three sequential stages defined as the lag, filling, and necking stages. Here we develop a model that describes the performance of microfluidic T-junction generators working in the squeezing to transition regimes where confinement of the droplet dominates the formation process. The model incorporates a detailed geometric description of the drop shape during the formation process combined with a force balance and necking criteria to define the droplet size, production rate, and spacing. The model inherently captures the influence of the intersection geometry, including the channel width ratio and height-to-width ratio, capillary number, and flow ratio, on the performance of the generator. The model is validated by comparing it to speed videos of the formation process for several T-junction geometries across a range of capillary numbers and viscosity ratios.

This is the first part of a two-part study on the generation of droplets at a microfluidic T-junction operating in the transition regime where confinement of the droplet creates a large squeezing pressure that influences droplet formation. In this regime, the operation of the T-junction depends on the geometry of the intersection (height-to-width ratio, inlet width ratio), capillary number, flow ratio, and viscosity ratio of the two phases. Here in paper I we presented our experimental observations through the analysis of high-speed videos of the droplet formation process. Various parameters are tracked during the formation cycle such as the shape of the droplet (penetration depth and neck), interdroplet spacing, production rate, and flow of both phases across several T-junction designs and flow conditions. Generally, the formation process is defined by a two-stage model consisting of an initial filling stage followed by a necking stage. However, video evidence suggests the inclusion of a third stage, which we term the lag stage, at the beginning of the formation process that accounts for the retraction of the interface back into the injection channel after detachment. Based on the observations made in this paper, a model is developed to describe the formation process in paper II, which can be used to understand the design and operation of T-junction generators in the transition regime.

When droplets enter a junction they sort to the channel with the highest flow rate at that instant. Transport is regulated by a discrete time-delayed feedback that results in a highly periodic behavior where specific patterns can continue to cycle indefinitely. Between these highly ordered regimes are chaotic structures where no pattern is evident. Here we develop a model that describes droplet sorting under various asymmetries: branch geometry (length, cross-section), droplet resistance and pressures. First, a model is developed based on the continuum assumption and then, with the assistance of numerical simulations, a discrete model is derived to predict the length and composition of the sorting pattern. Furthermore we derive all unique sequences that are possible for a given distribution and develop a preliminary estimation of why chaotic regimes form. The model is validated by comparing it to numerical simulations and results from microfluidic experiments in PDMS chips with good agreement.

The study of yeast cell morphology requires consistent identification of cell cycle phases based on cell bud size. A computer-based image processing algorithm is designed to automatically classify microscopic images of yeast cells in a microfluidic channel environment. The images were enhanced to reduce background noise, and a robust segmentation algorithm is developed to extract geometrical features including compactness, axis ratio, and bud size. The features are then used for classification, and the accuracy of various machine-learning classifiers is compared. The linear support vector machine, distance-based classification, and k-nearest-neighbor algorithm were the classifiers used in this experiment. The performance of the system under various illumination and focusing conditions were also tested. The results suggest it is possible to automatically classify yeast cells based on their morphological characteristics with noisy and low-contrast images.

The electrokinetic transport phenomena are to be numerically studied based on cross-linked microchannel networks, which have been commonly employed for on-chip capillary electrophoresis applications. Applied potential field, flow field and concentration field should be solved to predict the species transport process under electrokinetic flows. Together with the well-designed channel geometry, a detailed physical model was firstly formulated through a series of governing equations and corresponding boundary/initial conditions, which was briefly re-presented from our previous publications. The emphasis of current work was to justify the simplest non-dimensional scheme and identify the most beneficial parameters so that an effective and simplified non-dimensional model was developed for numerical studies.Copyright © 2010 by ASME

Capillary isoelectric focusing (CIEF) is a high-resolution capillary electrophoresis (CE) technique for separating zwitterionic biomolecules, such as proteins and peptides. In this method, by generating a stable pH gradient along the length of the capillary and under the influence of a constant electric field, samples can be separated according to their different isoelectric points (pI). For identifying the focused zones in CIEF, the whole column imaging detection (WCID) is more reliable than any other single point detection methods since it avoids the need of focused peak mobilization, presenting several advantages such as lower detection time, minimized peak dispersion and consequently higher resolution. Capillary-based IEF-WCID has been invented by Convergent Bioscience Inc. (iCE280 analyzer) for separation of proteins and biomarkers [1–2]. In the iCE280 analyzer, hollow fibers are glued to the capillary to separate electrolytes from the samples and a metal slit with a 65 μm opening is glued to the top of the capillary to improve detection sensitivity by blocking stray light. However, this method has several limitations because of the use of capillary such as low throughput, difficulty to be integrated with other separation modes and low detection sensitivity.

This paper presents a cost-effective and scalable method for real-time detection of microdroplets in microchannels. Sensing electrodes are manufactured using standard microfabrication techniques. Commercial capacitive sensors are used for detection. The functional and simple hardware ensures portability and yields a signal-to-noise ratio of 4 for electrodes covered with a passivation layer. Experimental results for detection of aqueous droplets in silicone oil are presented.Copyright © 2010 by ASME

One of the most commonly used microfluidic droplet generator designs is the T-junction as shown in Figure 1. In this design, the dispersed phase microchannel, usually containing an aqueous solution, perpendicularly intersects the main channel, which contains the continuous phase (oil). The T-junction design has been widely adopted because of its simplicity and the superior control it offers over droplet size. Two primary operational regimes have been identified where breakup is dominated by confinement of the emerging droplet within the microchannel, known as the squeezing regime (Ca0.02). The operational mode of the T-junction generator is primarily characterized by the Capillary number (Ca = μU / γ), which describes the competition between the aforementioned forces. In between the two regimes exists the transition regime where both confinement and shear forces are important.

PDMS (Polydimethylsiloxane) is widely used as a microfluidic chip material for various applications due to its desirable properties [1, 2]. However PDMS has several drawbacks that limit its utilization in a number of microfluidic applications [1–4]. Properties such as the hydrophobic nature, sample absorption, and low electrokinetic properties (low zeta potential) are some issues that must be considered before using PDMS for numerous applications [3]. In many PDMS based chips electroosmotic pumping is used for fluid flow and sample transport along the microchannel networks. Simplicity of implementation in microfluidic chips, fast response time, and the plug-like velocity profile are the major advantages of electroosmotic flow compared to other fluid pumping techniques [2]. This type of flow utilizes the formation of electric double layer (EDL) in microchannels and the movement of ions under an applied external electric field. Thus, the surface properties of the channel material and liquid properties (ionic concentration, pH, and viscosity) play major roles in electroosmotic pumping for different solutions in microchannels.Copyright © 2010 by ASME

Surface tension driven flow is used in numerous microfluidic applications. It is considered a passive pumping technique which doesn’t require any external energy, aside from the interfacial surface energy between the fluid and walls. Thus, it is preferred in applications where the goal is fluid and sample transport. In many applications PDMS (Polydimethylsiloxane) is the most adapted material for chip manufacturing in microfluidics. PDMS has several aspects that make it favorable for microfluidic applications. Ease of chip fabrication, cost effectiveness, chemical stability, and good optical properties are features offered by PDMS and desirable for microfluidics. On the other hand, PDMS has some shortcomings. One of importance is that PDMS is naturally hydrophobic. For this reason it is hard to achieve surface tension flow in native PDMS for various fluids used in microfluidics. Thus, native PDMS must be treated to get hydrophilic surface properties. The most used method for altering PDMS properties to a hydrophilic state is by plasma treatment. This treatment has several aspects where it enhances the attachment of PDMS to substrates, it alters the surface from a hydrophobic to a hydrophilic state, and it increases the electrokinetic properties of PDMS. As a result, after plasma treatment surface tension pumping can be achieved in PDMS, unlike native PDMS. However, plasma treatment is not permanent due to the diffusion of non-cured PDMS species to the surface of microchannels, as is well documented in the literature. The change of plasma treated PDMS with time will affect both the electrokinetic and surface tension driven flow. To our knowledge, researchers have quantitatively documented the time effect on plasma treated PDMS microchannels (aging of PDMS) for electrokinetic flow, but not for surface tension driven flow. Therefore, a quantitative examination of the time effect on surface tension driven flow for plasma treated PDMS gives valuable information on both regaining the hydrophobic properties in PDMS and changes in the passive flow conditions. In this work a quantitative study on the hydrophobic recovery for oxygen-plasma treated PDMS and its effects on surface tension flow was examined. The study was performed with a quantitative flow visualization technique (micro particle image velocimetry). It was found that the aging of PDMS will strongly affect surface tension flow of water based solutions in PDMS microchannels. This study gives important information on the effectiveness of surface tension driven flow for oxygen plasma treated PDMS microchannels.Copyright © 2010 by ASME

A new, simple method was reported to prepare PDMS membranes with micrometer size pores for microfluidic chip applications. The pores were formed by adding polystyrene and toluene into PDMS prepolymer solution prior to spin-coating and curing. The resulting PDMS membrane has a thickness of around 10 microm and macropores with a diameter ranging from 1 to 2 microm measured using scanning electron microscope (SEM) imaging. This PDMS membrane was validated by integrating it with PDMS microfluidic chips for protein separation using isoelectric focusing mechanism coupled with whole channel imaging detection (IEF-WCID). It has been shown that five standard pI markers and a mixture of two proteins, myoglobin and beta-lactoglobulin, can be separated using these chips. The results indicated that this macroporous PDMS membrane can replace the dialysis membrane in PDMS chips for the IEF-WCID technique. The preparation method of macroporous PDMS membrane may be potentially applied in other fields of microfluidic chips.

This study presents a microfluidic system that incorporates electroosmotic pumps, a concentration gradient generator and a fish cell line (rainbow trout gill) to perform toxicity testing on fish cells seeded in the system. The system consists of three mechanical components: (1) a toxicity testing chip containing a microfluidic gradient generator which creates a linear concentration distribution of toxicant in a cell test chamber, (2) an electroosmotic (EO) pump chip that controls the flow rate and operation of the toxicity chip, and (3) indirect reservoirs that connect the two chips allowing for the toxicant solution to be pumped separately from the electroosmotic pump solution. The flow rate and stability of the EO pumps was measured and tested by monitoring the gradient generator using fluorescence microscopy. Furthermore, a lethality test was performed with this system setup using a rainbow trout gill cell line (RTgill-W1) as the test cells and sodium dodecyl sulfate as a model toxicant. A gradient of sodium dodecyl sulfate, from 0 to 50 microg mL(-1), was applied for 1 hr to the attached cells, and the results were quantified using a Live/Dead cell assay. This work is a preliminary study on the application of EO pumps in a living cell assay, with the potential to use the pumps in portable water quality testing devices with RTgill-W1 cells as the biosensors.

A poly(dimethylsiloxane)(PDMS)/SU-8/quartz hybrid chip was developed and applied in the isoelectric focusing (IEF) of proteins with ultraviolet (UV) absorbance-based whole-channel imaging detection (UV-WCID). Each hybrid chip was made of three layers: a PDMS flat top substrate, a bottom quartz substrate and a middle layer of SU-8 photoresist. The SU-8 serves two purposes: it contains the microchannel used for IEF separation, and acts as an optical slit that absorbs UV light below 300 nm improving detection sensitivity in WCID. The novel hybrid design demonstrates a two to three times improvement in sensitivity over a comparable PDMS/PDMS design. In addition, the hybrid chip exhibits increased heat dissipation due to the superior thermal conductivity of the bottom quartz substrate allowing for larger electric fields to be used in separations. The hybrid design with IEF-UV-WCID was successful in resolving a complicated sample, hemoglobin control, with high fidelity.

Rhodamine B based fluorescence thermometry is commonly used in microfluidics to measure fluid temperatures in microchannels. Notable absorption of Rhodamine B into PDMS channel walls, however, causes difficulties in obtaining accurate temperature measurements due to a steady increase in the overall fluorescent signal. A simple and effective technique is reported that removes the fluorescent signal from absorbed Rhodamine B dye by means of photobleaching with a high intensity light source before taking images for thermometry analysis. The temperature field at the convergence of hot and cold streams in a Y-channel fabricated in PDMS/glass microfluidic chip is studied to demonstrate the execution of the photobleaching technique.

Simple-structured, well-functioned disposable poly(dimethylsiloxane) (PDMS) microchips were developed for capillary isoelectric focusing with whole column imaging detection (CIEF-WCID). Side-by-side comparison of the developed microchips with well-established commercial capillary cartridges demonstrated that the disposable microchips have comparable performance as well as advantages such as absence of lens effect and possibility of high-aspect-ratio accompanied with a dramatic reduction in cost.

A poly(dimethylsiloxane) microfluidic chip-based cartridge is developed and reported here for protein analysis using isoelectic focusing (IEF)-whole-channel imaging detection (WCID) technology. In this design, commercial dialysis membranes are integrated to separate electrolytes and samples and to reduce undesired pressure-driven flow. Fused-silica capillaries are also incorporated in this design for sample injection and channel surface preconditioning. This structure is equivalent to that of a commercial fused-silica capillary-based cartridge for adapting to an IEF analyzer (iCE280 analyzer) to perform IEF-WCID. The successful integration of dialysis membranes into a microfluidic chip significantly improves IEF repeatability by eliminating undesired pressure-driven hydrodynamics and also makes sample injection much easier than that using the first-generation chip as reported recently. In this study, two microfluidic chips with a 100-microm-high, 100-microm-wide and a 200-microm-high, 50-microm-wide microchannel, respectively, were applied for qualitative and quantitative analysis of proteins. The mixture containing six pI markers with a pH range of 3-10 was successfully separated using IEF-WCID. The pH gradient exhibited a good linearity by plotting the pI value versus peak position, and the correlation coefficient reached 0.9994 and 0.9995 separately for the two chips. The separation of more complicated human hemoglobin control sample containing HbA, HbF, HbS, and HbC was also achieved. Additionally, for the quantitative analysis, a good linearity of IEF peak value versus myoglobin concentration in the range of 20-100 microg/mL was obtained.

Design, simulation, fabrication and characterization of polymeric photo-thermal microgrippers are presented. The microgrippers were fabricated using a single layer of SU-8. Thermal actuation was achieved in a non-contact manner by focusing laser beam on the grippers from a remote source. Finite element simulations of the microgrippers are presented to investigate the temperature distribution, finger deflection and induced stress. The experimental results demonstrate that the polymeric microgripper with a length of 900 mum can achieve a deflection of 22 mum. The microgrippers also feature high repeatability and biocompatibility, therefore they can be used for microassembly and biomanipulation applications specifically for single cell manipulation.

This study aims to develop an experimental method for determining the focused-sample stream width in microfluidic chips. The focused-sample stream width in the junction of a cross-linked microchannel is determined by the distance between the two streamlines dividing the focusing sheath streams and the sample stream, which should be insensitive to the sample properties. However, the commonly used FWHM intensity method is dependent on the dye properties and cannot be used to estimate the sample focusing effects very accurately. The proposed method is based upon the fact that the cross-stream concentration profile of the sample stream in a region downstream of the junction matches the Gaussian distribution. Therefore, the width of the focused sample stream can be estimated from the channel widths and the variance and maximum concentration of the experimentally measured concentration profile. To validate this method, fluorescein dye has been used to obtain the concentration profile. It has been shown that the proposed method is independent of the fluorescent dye properties. Comparison between the proposed method, FWHM method, and some reported analytical methods also validates the proposed method.

Photo-thermally actuated polymer (SU-8) microgrippers were designed, simulated and characterized. The microgrippers were actuated by thermal expansion of compliant polymer parts. The required heat was transferred to the devices by laser absorption. The microgrippers were made of a single SU-8 layer and dyed before the experiments for enhanced laser absorption. Finite element simulation results were used to predict the working range of the grippers. It has been demonstrated that using a simple design 22 μm of deflection can be achieved for a micro-gripper of approximately 900 μm long. The gripping experiments have demonstrated the successful operation of polymeric photo-thermal microgrippers.Copyright © 2008 by ASME

A poly(dimethylsiloxane) (PDMS) microfluidic chip-based cartridge was fabricated by sandwiching commercial dialysis membrane and inserting fused-silica capillary into the end of channel according to the principle and structure of a commercial fused-silicon capillary-based cartridge, which can adapt to an IEF analyzer for isoelectric focusing with whole channel imaging detection (IEF-WCID). The novel design of sandwiching membrane in this chip not only eliminated the unfavorable hydrodynamic pressure, leading to poor IEF reproducibility, but made the sample injection much easy. Thus the reproducibility of analysis was very good. The prepared microfluidic chips were applied for qualitative and quantitative analysis of proteins. The six pI markers in the range of 3–10 were separated by IEF under the optimized conditions. The pH gradients exhibited good linear by plotting the pI versus peak position, and the correlation coefficient reached to 0.9994 and 0.9995. The separation of more complicated human hemoglobin control and myoglobin sample could be achieved. By comparison with the separation efficiency obtained on the microfluidic chip and commercial cartridge, the results were similar, which indicates the capillary cartridge may be replaced with the cost-efficient PDMS microfluidic chip. It is anticipated the high throughput analysis can be easily performed on this microfluidic chip patterned multi-channels. The techniques of capillary electrophoresis (CE) have been extensively explored for the chip-based separation. Isoelectric focusing (IEF) as one of high-resolution CE techniques has been widely applied for the separation of zwitterionic biomolecules, such as proteins and peptides. After the samples were focused at their corresponding pIs, the focused zones were mobilized to pass through the detection point for obtaining an electropherogram. This single-point detection imposes extensive restriction for chip-based IEF because a mobilization process requires additional time and lowers resolution and reproducibility of the separation [1]. An alternative is whole-column imaging detection developed by Pawliszyn et al [2] is an ideal detection method for IEF because no mobilization is required, which avoids the disadvantages as mentioned previously. Most microfluidic systems could be fabricated in glass/silicon or polymers in which the channels are defined using photolithography and micromachining. Mao and Pawliszyn [3] have developed a method for IEF on an etched quartz chip following whole-channel imaging detection (WCID). Ren et al [4] presented an integrated WCID system on glass microfluidic chip. However, these materials have some disadvantages such as expensive and fragile and so on. An attractive alternative for fabrication of microfluidic devices is using poly(dimethylsiloxane) (PDMS) as material, which has unique properties such as nontoxic, optical transparent down to 280 nm, elastomeric, hydrophobic surface chemistry Yao et al. [5] designed the glass/PDMS microchip integrated whole-column fluorescence imaging detection for IEF of R-phycoerythrin. Our preliminary studies have successfully developed a PDMS chip-based cartridge for IEF-WCID. It is due to hydrodynamic flow between two reservoirs that the focused zones were mobilized, thus gave poor reproducibility and difficulty in sample infusion. As membranes have been integrated into microchips for microdialysis, protein digestion, solid-phase extraction, desalting, pumping and so on, it could minimize hydrodynamic flow by using membranes as a filter. Although a simple PDMS chip-based cartridge has been successfully fabricated in our labs according to the principle of commercial capillary-based cartridge, it is difficult to introduce the sample into channel for IEF-WCID. As the vacuum was applied in one end of channel for infusing of solution into channel, the lifetime of this chip-based cartridge is shortened. Additionally, the hydrodynamic flow is occurred due to the different heights of anolyte and catholyte in two reservoirs, respectively. The IEF separation was deteriorated by the infusion of anolyte or catholyte, thus leading to poor reproducibility of IEF-WCID analysis. Similar to the hollow fiber in the commercial capillary-based cartridge in which it is aimed to separate the sample in the capillary and electrolytes in the reservoirs, porous membrane was integrated into PDMS chips for decrease of hydrodynamic flow [6]. As a result, integration of dialysis membrane is considered into the design of our new chip-based cartridge. Up to now, many approaches have been described to integrate membranes into glass/quartz or polymeric microfluidic chips. A simple method is direct incorporation by gluing or clamping commercial flat membranes. A major problem of this method is sealing, otherwise, a phenomenon of leakage around the membranes is always occurred due to the capillary force. A novel approach of sandwiching dialysis membrane was developed as schematically indicated in Figure 1. After optimizing IEF conditions, the separation of pI markers was performed on the obtained PDMS microfluidic chip. As exhibited in Figure 2a, six pI markers could be well separated on the PDMS chips patterned the channel of 100 μm deep, 100 μm wide by IEF-WCID. All the peaks were sharp and symmetric, indicating that both EOF and analytes adsorption were completely suppressed by the dynamic coating of PVP. The plots of peak position versus pI of these pI markers suggested good linearity of pH gradient (as shown in Figure 2b). The linear correlation coefficient was 0.9995 (n = 6). As expected to the capillary-based cartridge, the PDMS microfluidic chips could be applied for qualitative and quantitative analysis of proteins. Figure 3a exhibited that human hemoglobin control AFSC contains four known isoforms (HbA, HbF, HbS and HbC) mixed with two pI marker 6.14 and 8.18 were well separated on the PDMS chip by IEF-WCID, indicating the strong separation ability of chip similar to the commercial capillary-based cartridge. According to the linearity of pH gradient, these four isoforms with the pIs of 7.0, 7.1, 7.3 and 7.5, respectively, could be detected. An unknown isoform in human hemoglobin control marked asterisk in Figure 3A observed besides the definite four isoforms A, F, S and C. The myoglobin from horse heart contains two isoforms, whose pIs are 6.8 and 7.2, respectively. It can be seen from Figure 3b that these two isoforms were separated on PDMS chip by IEF-WCID. The peak 1 and 2 could be assigned to the two isoforms according to their pI. The pI of unknown peak marked asterisk could be measured to 6.25.Copyright © 2008 by ASME

A number of microfluidic applications require precise thermal control where in-channel temperature measurements are necessary during the prototyping stage. Rhodamine B based laser induced fluorescence is a common technique used to obtain high resolution measurement of the fluid temperature field. However, PDMS has a tendency to absorb small hydrophobic dyes, such as Rhodamine B, which results in a steady increase in the overall fluorescent signal. This increase in light intensity causes a significant problem that must be overcome to obtain reliable temperature measurements. In this work a simple technique is described to remove the fluorescent signal originating from absorbed Rhodamine B dye particles that does not require surface modification or any significant alterations to the experimental setup. Instead a high power light source is used to photobleach the particles prior to taking images for thermometry analysis. Herein we demonstrate the technique with a conventional fluorescence microscope and a 100W mercury arc lamp and study the temperature field at the intersection of a Y-channel PDMS/glass chip where hot and cold streams merge.Copyright © 2008 by ASME

Temperature control is key to microfluidic-based Lab-on-a-Chip devices for a variety of applications such as polymerase chain reaction for DNA amplification and isoelectric focusing for protein separation where pH gradients are thermally generated. The most widely used temperature measurement method involves the mixing of the buffer solution with a fluorescent dye, which has a temperature-dependent fluorescent intensity. The temperature distribution in the liquid can be obtained by monitoring the fluorescent intensity distribution in the channel. However, this method can not be easily applied to polymer-made microfluidic chips because of dye absorption and penetration into polymer chips, electrophoresis of dye which causes artificial temperature gradients, and inevitable photobleaching of fluorescent dye. Therefore, a novel method is developed and presented here for temperature measurement by utilizing photobleaching of fluorescent dye. This method includes two novel contributions: i) a specially developed model for converting temperature-dependent photobleaching speed distribution to temperature distribution, and ii) an introduction of a thin polydimethylsiloxane (PDMS) layer with saturated Rhodamine B for solving the above-mentioned dye diffusion and electrophoresis problems. In this new method, a thin PDMS layer saturated with Rhodamine B is bonded with another PDMS layer with microchannels instead of mixing the dye with the buffer solution. Therefore, the problems associated with dye diffusion into PDMS chips and electrophoresis when an electrical field is applied to channels are avoided. The developed theory is validated by comparing the experimentally measured temperature distribution with numerical predicted results. The theory and its validation will be presented and discussed.Copyright © 2008 by ASME

A numerical model has been developed and is used to study the loading and dispensing processes in on-chip cross-linked microchannels. The electrokinetic transport characteristics and the roles of species’ electrophoretic mobilities and diffusion coefficients on the electrokinetic flow are revealed. A study is also performed on an implementation of multi-stage injection. The study of conventional one-step injection and separation is performed and helps construct a distinct understanding of the processes. Species movement and sample plug development with diffusion are examined; results include concentration profiles and contour plots over a range of injection and separation time. Real-time monitoring of different species’ movements is performed for injection guidance. Some limitations of the separation process are presented with potential solutions, such as the removable tail effect and exceptional quick diffusion. Using innovative dynamic control, efforts are made to control the flow and species transport for improved sample plugs, which is key to achieving excellent electrophoretic separation. Through a series of multi-step injection schemes, four typical sample plugs are produced with specific attributes such as reduced dispersion leakage, desirable sample plug size, enhanced shape, etc. Comparisons of conventional and the proposed methods are performed. Typical resulting sample plugs are evaluated using the two developed parameters of resolution and detectability for numerically simulated separation processes. Depending on requirements, one can generate some specific sample plugs through this multi-step dynamic injection method. The resulting understanding will assist in the design of microfluidic devices for separation by providing insight into the process influences and controls and by identifying areas for further research.Copyright © 2008 by ASME

A novel method is presented for on-chip temperature measurements using a poly(dimethylsiloxane) (PDMS) thin film dissolved with Rhodamine B dye. This thin film is sandwiched between two glass substrates (one of which is 150 microm thick) and bonded to a microchannel molded in a PDMS substrate. Whole-chip (liquid and substrate) temperature measurements can be obtained via fluorescent intensity visualization. For verification purposes, the thin film was tested with a tapered microchannel subjected to Joule heating, with resulting axial temperature gradients comparing well with numerical simulations. Errors induced by the definite film thickness are discussed and accounted for during experimental and analytical analysis. Alternative validation using the traditional in-channel Rhodamine B injection method was also attempted. The thin film has several advantages over traditional methods. First, false intensity readings due to adsorption and absorption of Rhodamine B into PDMS channels are eliminated. Second, whole-chip temperature measurements are possible. Third, separation of working liquid from Rhodamine B dye prevents possible electrophoresis effects.

A novel method for in-situ temperature measurements of microfluidic devices using thin-film poly(dimethylsiloxane) (PDMS) saturated with Rhodamine B dye is reported. Rhodamine B, a dye with temperature dependent fluorescent intensity, is frequently injected into the working fluid for on-chip temperature field visualization of glass and silicon based microfluidic devices. However, such a visualization method results in unreliable temperature measurements due to high absorption and adsorption for polymeric devices such as PDMS. Thus, an inexpensive temperature measurement technique is developed in which a thin PDMS layer (∼30 μm) is fabricated and submersed for several days into a Rhodamine B solution. To prevent backward diffusion of the dye into the working fluid during operation, a glass barrier (∼150 μm) is bonded between the thin film and the PDMS mold containing the microchannel design of interest. Temperature measurements are made by utilizing standard method of measuring changes in the normalized fluorescent intensity. For verification purposes, a new calibration curve is developed and the thin film is tested with a microchannel subjected to joule heating. The resulting temperature field along the axial direction of the channel for different applied powers compares well with numerical simulations. Analysis of dye intensities before and after experiments provides temperature deviation estimates due to photobleaching. Errors in temperature measurement due to the film thickness are discussed.Copyright © 2008 by ASME

A new continuous perfusion cell culture chip is studied that utilizes electroosmotic pumping to control fluid flow. Electroosmotic flow is not typically used for living cells due to the inherently high electric fields that may harm cells. Problems associated with EOF and cells are solved by incorporating electroosmotic pumps (EO pumps) which generate an induced pressure driven flow in regions with cells. Several advantages of EO pumps include pulse free flow, quick flow control and precise movement of minute volumes of fluid. An ion exchange system consisting of photopolymerized salt bridges are used to separate the media from the electrode reservoirs. However, the high salt concentration in cell culture medium creates significant problems for EO pumps such as decreased flow rate due to low zeta potential, increased electrolysis due to high current draw, significant joule heating, bubble formation and polarization. Attempts to solve these problems with the proposed microfluidic chip are discussed. The pumps are characterized to determine the flow rate for applied currents. Preliminary results with rainbow trout gill cells show that pump can be operated for 5hrs without harming the cells. The work presented here discusses the design and development of the system to this point.Copyright © 2007 by ASME

A complete mathematical model is developed for application to simulate the unsteady two-step on-chip sample injection and separation processes in microfluidic devices. The origin and applicability of the slip-wall velocity boundary condition is discussed. Due to electrophoresis effect, migration influence of every species is considered in the model and then solved for separation analysis. The model is non-dimensionalized in a unique manner to reveal effects of some key fundamental parameters: the Reynolds-Schmidt number, electrophoretic mobility of sample species, applied potentials, etc. In particular, the influence of ReSci is examined over the commonly encountered range and the effect of electrophoretic mobilities on separation is investigated for three different types of samples. Results include center-line concentration profiles as well as concentration contour plots over a range of nondimensional time (less than 400). Resolution is defined and employed to evaluate the separation results. The magnitude of calculated separation resolution (around 2.0) is comparable to experimental results. Through parametric studies, the characteristics of both injection and separation are revealed numerically and well understood for future effective control and innovative chip design.Copyright © 2007 by ASME

A Y-channel design is proposed to find the zeta potential for PDMS and PDMS/glass microchannels with the current monitoring technique. The major advantages of this design are four fold: i) the displacement process can be quickly repeated many times increasing the amount of measurements for a single fluid; ii) start and end times are well defined since there is minimal diffusion at the liquid/liquid interface; iii) overall preparation and operation are simplified; and iv) errors caused by undesirable pressure driven flow (Laplace and head differences) and changes in pH due to electrolysis are suppressed. The zeta potential found from current monitoring experiments with the new Y-channel design gave similar results when compared to results found in literature using a conventional straight channel design. Modifications on the slope calculation method were proposed and results agreed with the total length calculation method within 6.8% variation of the zeta potential values averaged for all experiments compared to the total length method. Using the new design, the zeta potential of a number of commonly used buffer solutions for protein and DNA application were tested in PDMS and PDMS/glass hybrid channels.Copyright © 2007 by ASME

The development of Lab-on-a-Chip devices with integrated bio-analysis functions requires a complex network of microfluidic transport and processes. Many of these functions call for the isolation or separation of specific bio-particles or cells. The design of a miniaturized cell-sorting device for handheld operation must follow the strict parameters associated with Lab-on-a-Chip technology. The limitations include applied voltage, high efficiency of cell-separation, repeatability, size, flow control, and cost, among others. Currently used designs have achieved successful levels of cell-isolation. However, further improvements in the microfluidic chip design are important for incorporation into larger systems. This study evaluates specific design modifications that contribute to the reduction of required applied potential aiming for developing portable devices, improved operation reliability by minimizing induced pressure disturbance when electrokinetic pumping is employed and incorporating online filters to reduce channel blockage, and improved flow control by incorporating directing streams achieving dynamic sorting and counting. The chip designs fabricated in glass and polymeric materials include asymmetric channel widths for sample focusing, nonuniform channel depth for minimizing induced pressure disturbance, directing streams to assist particle flow control, and online filters for reducing channel blockage. Fluorescence-based visualization of electrokinetic focusing, flow field phenomena, and dynamic cell-sorting demonstrate the advantages of the chip design. Numerical simulations in COMSOL are validated by the experimental data and used to investigate the effects of channel geometry and fluid properties on the flow field.Copyright © 2007 by ASME

Precise design and operational control of the polymerase chain reaction process is key to the performance of on-chip DNA analysis. This research is dedicated to understanding the fluid flow and heat transfer mechanisms occurring in continuous flow PCR chips from the engineering point of view. In this work, a 3-dimensional model was developed to simulate the electrical potential field, the flow field, and the temperature field in an electroosmosis-based continuous flow PCR chip. On the basis of the simultaneous solution to this model, the effects of the channel/chip size, the chip material, and the applied voltage difference on the temperature distribution and control are discussed in detail. The importance of each heat transfer mechanism for different situations is also discussed. It was found that if a larger chip thickness or a material with a lower heat conductivity was used, the temperature in the microfluidic PCR chip would decrease dramatically. The effects of the applied electrical field strength and flow velocity on the temperature distribution, however, are negligible for microchannels with a small cross-sectional area. With bigger channels, the flow direction will affect the temperature distribution in the channel because heat convection will dominate heat transfer.

IEF is a high-resolution separation method taking place in a medium with continuous pH gradients, which can be set up by applying electrical field to the liquid in a diverging microchannel. The axial variation of the channel cross-sectional area will induce nonuniform Joule heating and set up temperature gradient, which will generate pH gradient when proper medium is used. In order to operationally control the thermally generated pH gradients, fundamental understanding of heat transfer phenomena in microfluidic chips with diverging microchannels must be improved. In this paper, two 3-D numerical models are presented to study heat transfer in diverging microchannels, with static and moving liquid, respectively. Through simulation, the temperature distribution for the entire chip has been revealed, including both liquid and solid regions. The model for the static liquid scenario has been compared with published results for validation. Parametric studies have showed that the channel geometry has significant effects on the peak temperature location, and the electrical conductivity of the medium and the wall boundary convection have effects on the generated temperature gradients and thus the generated pH gradients. The solution to the continuous flow model, where the medium convection is considered, shows that liquid convection has significant effects on temperature distribution and the peak temperature location.

Studies of the sample transport in a microchannel with the electrical conductivity gradient are critical to develop techniques for on-chip sample transport control. A numerical model presented in this paper, consisting of the electrical potential equation, full Navier–Stokes equation and species conservation equation, is used to simulate sample transport in a microchannel with the consideration of the conductivity gradient. There are two situations studied here, sample pumping (where sample separation is minimized by employing a high-conductivity buffer in the sample region), and sample stacking (where sample separation is expedited by using a low-conductivity buffer as the sample carrier). The effects of applied electrical potential, sample diffusion coefficient and the ratio of conductivity of the driving buffer over the sample carrying buffer are investigated by using the developed model.

Electroosmotic flow with solution displacement in microchannels is often encountered in many lab-on-a-chip devices where washing procedures are designed. In order to investigate the detailed flow structures for a displacement process between two different electrolyte solutions, a three-dimensional numerical model is developed in this paper. KCl solution and LaCl3 solution are mainly used as sample solutions. A 2cm long straight microchannl with a rectangle cross sectional area (height of 100μm and width of 200 μm) was employed in this study. The governing equations of applied electrical field, flow field and concentration field were numerical solved with SIMPLE algorithm, which are based on a finite control volume scheme. The observed flow structures such as back flow in the center of the channel and distortion of plug-like electroosmotic velocity profile are discussed in detail. It is found that the distortion in flow field is due to the induced pressure gradient, which is resulted from the nonuniformity of electroosmotic mobilities and electrical conductivities of two solutions. Finally, the displacement between DIUF water and LaCl3 solution is briefly studied.Copyright © 2005 by ASME

There has been tremendous interest in developing micro technologies towards the integration and automation of Biochips or Lab-on-a-Chip devices due to their wide range of applications in environmental, chemical and biomedical engineering fields. The laminar flow nature in microfluidic devices offers opportunities to microfabricate the desired structures inside microchannels and pattern culturing medium inside microchannels. However, no analysis tools are available to provide optimized configurations for control the flow for microfabrication. Therefore, the goal of this study is to develop a numerical model to study transport phenomena in a cross-linked microchannels aiming to explore an optimized configuration for the microfabrication of specific desired features inside microchannel networks through investigating the effects of controlling parameters on the multistream flow. In this study, electroosmotic flow with induced pressure-driven flow will be employed. This model consists of a set of equations describing the applied potential field, flow field and concentration field in such geometries. The effects of various operational parameters are investigated based on the simultaneous solution to this model, to explore optimized configurations for flow and mass transport control in crossing linked microchannels.Copyright © 2005 by ASME

Development of nano-devices for various applications has drawn great attention recently driven by the need of miniaturizing the devices for the integration and automation of Biochips or Lab-on-a-Chip devices. Fundamental understanding of transport phenomena in nanofluidic channels is critical for systematic design and precise control of such devices. The goal of this study is to develop a theoretical model to study electroosmotic flow in nanochannels. Instead of using the Boltzmann distribution, the conservation condition of ion number and the Nernst equation are used in this new model to find the ionic concentration field in the nanochannels. A correct boundary condition for the concentration field at the wall of the channel is developed and the symmetry condition of the potential field at the center of the nanochannel is applied to this model. The ionic concentration field, electrical potential field and flow field are obtained by numerically solving this model. Comparisons of area-average velocity between the numerical simulations and experimental results reported in literature are provided.Copyright © 2005 by ASME

A common application in microfluidic devices is on-chip capillary electrophoresis (CE). In this process, sample species are transported by electroosmotic flow and separated based on their electrophoretic mobilities. Separated analytes are typically detected using laser-induced fluorescence. It has been found that the sample shape and size, which is critical to the later detection processes or the quality of other analytical techniques, depends on many parameters, such as the sample diffusion coefficient, the applied voltages, and the electrical conductivity difference between sample and buffer. The conductivity difference can alter the electric field strength, which is the driving force behind both the electroosmotic bulk flow and the electrophoretic velocity of individual species. Therefore, the manipulation technique is required to consider the transport processes with conductivity differences. A numerical model presented in this paper is used to simulate the sample transport process with the consideration of conductivity gradient in order to develop the sample manipulation techniques. There are two situations studied here, which are sample pumping (where bulk transport is increased and analyte separation is delayed using a relatively high conductivity sample), and sample stacking (where bulk transport is decreased and analyte separation is expedited using a relatively low conductivity sample). The effects of applied electrical potential, sample diffusion coefficient and the extent of conductivity difference on the sample control are investigated through the developed model. The simulation results show that the sample transport with the consideration of conductivity gradient differs significantly from that of uniform conductivity case.Copyright © 2004 by ASME

This work presents a robust method for whole chip temperature mapping in microfluidic devices using a photostable fluorescent-polymer thin film that can be incorporated during the bonding stage. Temperature measurements are based on the ratio of two bands in the fluorescence spectrum of N,N-bis(2,5-di-tertbutylphenyl)-3,4,9,10-perylenedi carboximide (BTBP) dye. Spectral bands were carefully chosen to minimize errors caused by photobleaching of the dye which results in a perceived drift in the temperature with time. The improvements result in a useful working time for this type of measurement to >20 h. We achieve a temperature resolution