Bubble dispenser in microfluidic devices

Foam Generation Through a Single Pore with Rectangular Cross-Section: Hysteretic Behavior and Geometric Limitation of the Volume Fraction of Bubbles

We study foam production and destabilization through a flow-focusing geometry, namely a single pore of rectangular cross-section, by coinjecting gas and liquid at constant pressure, Pg, and constant flow rate, Qw. We observe that bubble production results from a Rayleigh-Plateau destabilization of the internal gas thread that occurs at the pore neck when its width becomes comparable to the height of the rectangular-section channel. Using a simple model and numerical approach, we (i) predict the shape of the gas jet and its stability range as a function of flow parameters and device geometry, which we successfully compare with our experimental results, and (ii) demonstrate the existence of a critical local pressure drop at the pore neck that determines whether or not a stable gas flow can form. We thus show that bubble foam generation exhibits hysteretic behavior due to hydrodynamic feedback and demonstrate that there is a maximum bubble volume fraction that the generated foam cannot exceed, the value of which is fixed by the geometry. Our results suggest that the foam collapse observed in porous media when the fractional gas flow becomes too large may result from hydrodynamic feedback inhibiting foam generation and not necessarily from coalescence between bubbles, as is usually claimed.

Experimental studies on droplet characteristics in a microfluidic flow focusing droplet generator: effect of continuous phase on droplet encapsulation

The efficacy of droplet-based microfluidic assays depends on droplet size, pattern, generation rate, etc. The size of the droplet is affected by numerous variables as flow rate ratio, viscosity ratio, microchannel geometry, surfactants, nature of fluids and other dimensionless numbers. This work reports rigorous analysis and optimization of the behavior of droplets with change in flow rate ratio and viscosity ratio in a flow-focusing device. Droplets were produced for different flow rate ratios maintaining a constant aqueous phase and varying the continuous phase, to have capillary numbers ranging from 0.01 to 0.1. It was observed that the droplet size decreased with the increase in flow rate ratio, and vice versa. It was noted that as the viscosity ratio was increased, the dispersed phase elongated before the complete breakup and long droplets were formed in the microchannel. Smaller droplets were formed for lower viscosity ratios with a combination of higher flow rate ratios. An empirical relation has been developed to predict the droplet length in terms of capillary number and flow rate ratio for different viscosity ratios. In addition, microparticle encapsulation in individual droplets was attempted to realize the effect of flow rate of the continuous phase for various flow rate ratios on encapsulation efficiency.

Nanoparticle-Coated Microbubbles for Combined Ultrasound Imaging and Drug Delivery

Biomedical microbubbles stabilized by a coating of magnetic or drug-containing nanoparticles show great potential for theranostics applications. Nanoparticle-coated microbubbles can be made to be stable, to be echogenic, and to release the cargo of drug-containing nanoparticles with an ultrasound trigger. This Article reviews the design principles of nanoparticle-coated microbubbles for ultrasound imaging and drug delivery, with a particular focus on the physical chemistry of nanoparticle-coated interfaces; the formation, stability, and dynamics of nanoparticle-coated bubbles; and the conditions for controlled nanoparticle release in ultrasound. The emerging understanding of the modes of nanoparticle expulsion and of the transport of expelled material by microbubble-induced flow is paving the way toward more efficient nanoparticle-mediated drug delivery. This Article highlights the knowledge gap that still remains to be addressed before we can control these phenomena.

Self-assembly of droplets in three-dimensional microchannels

Self-assembly of droplets guided by microfluidic channels have potential applications ranging from high throughput assays to materials synthesis, but such demonstrations have been limited primarily to two-dimensional (2D) assembly of droplets in planar microfluidic devices. We demonstrated the use of three-dimensional (3D) microchannels to self-assemble droplets into ordered 2D and 3D arrays by designing microchannels with axial gradients in height and controlling the volume fraction of the droplets in the channel. In contrast to previous demonstrations, ordered 2D arrays of droplets were assembled at low volume fractions of the dispersed phase. Interestingly, we found that the self-assembly of droplets in microchannels was highly path dependent. The assembly of droplets was governed by transitions in the cross-sectional shapes of the microchannel, not the final geometry of the chamber for the assembly of droplet, which is a hitherto rarely explored phenomenon. The assembled droplets were used as templates for the fabrication of millimeter scale, anisotropic hydrogel fibers with ordered pore sizes (∼250 μm). These demonstrations suggested that 3D microchannels would be a viable platform for the manipulation of droplets, and applicable for the continuous synthesis of complex materials with 3D morphologies.

Bubble-bubble pinch-off in symmetric and asymmetric microfluidic expansion channels for ordered foam generation

By incorporating the techniques of geometrically mediated splitting and bubble-bubble breakup, the present work offers a novel microfluidic foam generation system via production of segregated, mono- or bidisperse bubbles at capacities exceeding 10 000 bubbles per second. Bubble-bubble pinch-off is precise at high capillary numbers (Ca > 0.065), generating monodisperse or bidisperse daughter bubbles for a symmetric or an asymmetric expansion respectively. Bi- or tridisperse foam is produced as pinch-off perfectly alternates such that the system contains twice the number of fragmented bubbles as intact bubbles. A relationship between the upstream bubble extension and the capillary number demarcates the different regimes of pinch-off defined with respect to frequency and precision: non-splitting, irregular, polydisperse, and monodisperse (or bidisperse for an asymmetric expansion). For tridisperse foam generation via a fixed asymmetric expansion geometry, the wall bubble confinement can be tuned to adjust the pinch-off accuracy in order to access a spectrum of fragmented bubble size ratios. The simplicity in operating and characterizing our system will enable studies on dynamic bubble interactions and ordered, wet foam applications.

Highly pressurized partially miscible liquid-liquid flow in a micro-T-junction. I. Experimental observations

This is the first part of a two-part study on a partially miscible liquid-liquid flow (liquid carbon dioxide and deionized water) which is highly pressurized and confined in a microfluidic T-junction. Our main focuses are to understand the flow regimes as a result of varying flow conditions and investigate the characteristics of drop flow distinct from coflow, with a capillary number, Ca_{c}, that is calculated based on the continuous liquid, ranging from 10^{-3} to 10^{-2} (10^{-4} for coflow). Here in part I, we present our experimental observation of drop formation cycle by tracking drop length, spacing, frequency, and after-generation speed using high-speed video and image analysis. The drop flow is chronologically composed of a stagnating and filling stage, an elongating and squeezing stage, and a truncating stage. The common "necking" time during the elongating and squeezing stage (with Ca_{c}∼10^{-3}) for the truncation of the dispersed liquid stream is extended, and the truncation point is subsequently shifted downstream from the T-junction corner. This temporal postponement effect modifies the scaling function reported in the literature for droplet formation with two immiscible fluids. Our experimental measurements also demonstrate the drop speed immediately following their generations can be approximated by the mean velocity from averaging the total flow rate over the channel cross section. Further justifications of the quantitative analysis by considering the mass transfer at the interface of the two partially miscible fluids are provided in part II.

Acoustofluidic control of bubble size in microfluidic flow-focusing configuration

This paper reports a method to control the bubble size generated in a microfluidic flow-focusing configuration. With an ultrasonic transducer, we induce acoustic streaming using a forward moving, oscillating gas-liquid interface. The induced streaming substantially affects the formation process of gas bubbles. The oscillating interface acts as a pump that increases the gas flow rate significantly and forms a larger bubble. This method is applicable to a wide range of gas pressure from 30 to 90 kPa and flow rate from 380 to 2700 μL h(-1). The bubble size can be tuned repeatedly with the response time on the order of seconds. We believe that this method will enhance the capability of a microfluidic bubble generator to produce a tunable bubble size.

A bubble-based microfluidic gas sensor for gas chromatographs

We report a new proof-of-concept bubble-based gas sensor for a gas chromatography system, which utilizes the unique relationship between the diameters of the produced bubbles with the gas types and mixture ratios as a sensing element. The bubble-based gas sensor consists of gas and liquid channels as well as a nozzle to produce gas bubbles through a micro-structure. It utilizes custom-developed software and an optical camera to statistically analyze the diameters of the produced bubbles in flow. The fabricated gas sensor showed that five types of gases (CO2, He, H2, N2, and CH4) produced (1) unique volumes of 0.44, 0.74, 1.03, 1.28, and 1.42 nL (0%, 68%, 134%, 191%, and 223% higher than that of CO2) and (2) characteristic linear expansion coefficients (slope) of 1.38, 2.93, 3.45, 5.06, and 5.44 nL/(kPa (μL s(-1))(-1)). The gas sensor also demonstrated that (3) different gas mixture ratios of CO2 : N2 (100 : 0, 80 : 20, 50 : 50, 20 : 80 and 0 : 100) generated characteristic bubble diameters of 48.95, 77.99, 71.00, 78.53 and 99.50 μm, resulting in a linear coefficient of 10.26 μm (μL s(-1))(-1). It (4) successfully identified an injection (0.01 μL) of pentane (C5) into a continuous carrier gas stream of helium (He) by monitoring bubble diameters and creating a chromatogram and demonstrated (5) the output stability within only 5.60% variation in 67 tests over a month.

Bubbles and foams in microfluidics

Microfluidics offers great tools to produce highly-controlled dispersions of gas into liquid, from isolated bubbles to organized microfoams. Potential technological applications are manifold, from novel materials to scaffolds for tissue engineering or enhanced oil recovery. More fundamentally, microfluidics makes it possible to investigate the physics of complex systems such as foams at scales where the capillary forces become dominant, in model experiments involving few well-controlled parameters. In this context, this review does not have the ambition to detail in a comprehensive manner all the techniques and applications involving bubbles and foams in microfluidics. Rather, it focuses on particular consequences of working at the microscale, under confinement, and hopes to provide insight into the physics of such systems. The first part of this work focuses on bubbles, and more precisely on (i) bubble generation, where the confinement can suppress capillary instabilities while inertial effects may play a role, and (ii) bubble dynamics, paying special attention to the lubrication film between bubble and wall and the influence of confinement. The second part addresses the formation and dynamics of microfoams, emphasizing structural differences from macroscopic foams and the influence of the confinement.

Formation and dynamics of partially wetting droplets in square microchannels

Droplet motion and dynamic wetting transitions are experimentally investigated over a wide range of viscosities and flow rates in square microchannels

Theoretical investigation of a technique to produce microbubbles by a microfluidic T junction

A microfluidic technique is proposed to produce microbubbles. A gaseous stream is injected through a T junction into a channel transporting a liquid current. The gas adheres to a hydrophobic strip printed on the channel surface. When the gas and liquid flow rates are set appropriately, a gaseous rivulet flows over that strip. The rivulet breaks up downstream due to a capillary pearling instability, which leads to a monodisperse collection of microbubbles that can be much smaller than the channel size. The physics of the process is theoretically investigated, using both full numerical simulation of the Navier-Stokes equations and a linear stability analysis of an infinite gaseous rivulet driven by a coflowing liquid stream. This stability analysis allows one to determine a necessary condition to get this effect in a T junction device. It also provides reasonably good predictions for the size of the produced microbubbles as obtained from numerical experiments.

Dripping and jetting in microfluidic multiphase flows applied to particle and fiber synthesis

Dripping and jetting regimes in microfluidic multiphase flows have been investigated extensively, and this review summarizes the main observations and physical understandings in this field to date for three common device geometries: coaxial, flow-focusing and T-junction. The format of the presentation allows for simple and direct comparison of the different conditions for drop and jet formation, as well as the relative ease and utility of forming either drops or jets among the three geometries. The emphasis is on the use of drops and jets as templates for microparticle and microfiber syntheses, and a description is given of the more common methods of solidification and strategies for achieving complex multicomponent microparticles and microfibers.

Liquid Flooded Flow-Focusing Microfluidic Device for in situ Generation of Monodisperse Microbubbles

Current microbubble-based ultrasound contrast agents are administered intravenously resulting in large losses of contrast agent, systemic distribution, and strict requirements for microbubble longevity and diameter size. Instead we propose in situ production of microbubbles directly within the vasculature to avoid these limitations. Flow focusing microfluidic devices (FFMDs) are a promising technology for enabling in situ production as they can produce microbubbles with precisely controlled diameters in real-time. While the microfluidic chips are small, the addition of inlets and interconnects to supply the gas and liquid phase greatly increases the footprint of these devices preventing the miniaturization of FFMDs to sizes compatible with medium and small vessels. To overcome this challenge, we introduce a new method for supplying the liquid (shell) phase to an FFMD that eliminates bulky interconnects. A pressurized liquid-filled chamber is coupled to the liquid inlets of an FFMD, which we term a flooded FFMD. The microbubble diameter and production rate of flooded FFMDs were measured optically over a range of gas pressures and liquid flow rates. The smallest FFMD manufactured measured 14.5 × 2.8 × 2.3 mm. A minimum microbubble diameter of 8.1 ± 0.3 μm was achieved at a production rate of 450,000 microbubbles/s (MB/s). This represents a significant improvement with respect to any previously reported result. The flooded design also simplifies parallelization and production rates of up to 670,000 MB/s were achieved using a parallelized version of the flooded FFMD. In addition, an intravascular ultrasound (IVUS) catheter was coupled to the flooded FFMD to produce an integrated ultrasound contrast imaging device. B-mode and IVUS images of microbubbles produced from a flooded FFMD in a gelatin phantom vessel were acquired to demonstrate the potential of in situ microbubble production and real-time imaging. Microbubble production rates of 222,000 MB/s from a flooded FFMD within the vessel lumen provided a 23 dB increase in B-mode contrast. Overall, the flooded design is a critical contribution towards the long- term goal of utilizing in situ produced microbubbles for contrast enhanced ultrasound imaging of, and drug delivery to, the vasculature.

CO(2) dissolution in water using long serpentine microchannels

The evolution of carbon dioxide bubbles dissolving in water is experimentally examined using long microchannels. We study the coupling between bubble hydrodynamics and dissolution in confined geometries. The gas impregnation process in liquid produces significant flow rearrangements. Depending on the initial volumetric liquid fraction, three operating regimes are identified, namely saturating, coalescing, and dissolving. The morphological and dynamical transition from segmented to dilute bubbly flows is investigated. Tracking individual bubbles along the flow direction is used to calculate the temporal evolution of the liquid volumetric fraction and the average flow velocity near reference bubbles over long distances. This method allows us to empirically establish the functional relationship between bubble size and velocity. Finally, we examine the implication of this relationship during the coalescing flow regime, which limits the efficiency of the dissolution process.

Thermocapillary migration in small-scale temperature gradients: application to optofluidic drop dispensing

We experimentally investigate the thermocapillary migration induced by local laser heating of the advancing front of a growing droplet confined in a microfluidic channel. When heating implies an effective increase in interfacial tension, the laser behaves as a "soft door" whose stiffness can be tuned via the optical parameters (beam power and waist). The light-driven thermocapillary velocity of a growing droplet, which opposes the basic flow, is characterized for different types of fluid injection, either pressure or flow rate driven, and various channel aspect ratios. Measurements are interpreted using a simplified model for the temperature gradient at the interface, based on a purely diffusive, three-layer system. Considering the mean temperature gradient, we demonstrate that the classical large-scale temperature gradient behavior is retrieved in the opposite case when the thermal gradient length scale is smaller than the droplet size. We also demonstrate that the thermocapillary velocity is proportional to the smallest droplet curvature imposed by the channel confinement. This suggests that the thermocapillary velocity is in fact proportional to the mean temperature gradient and to the largest interface curvature radius, which both coincide with the imposed one and the spherical droplet radius in large-scale and unconfined situations. Furthermore, as used surfactant concentrations are largely above the critical micelle concentration, we propose a possible explanation, relying on state-of-the-art considerations on high-concentration surfactant-covered interfaces for the observed effective increase in interfacial tension with temperature. We also propose a mechanism for explaining the blocking effect at the scaling-law level. This mechanism involves the temporal evolution of hydrodynamic and thermocapillary forces, based on experimental observations. We finally show that this optocapillary interaction with a microfluidic droplet generator allows for controlling either the flow rate (valve) or the droplet size (sampler), depending on the imposed fluid injection conditions.

Characterization of acoustic droplet formation in a microfluidic flow-focusing device

Local control of droplet formation with acoustic actuation in a microfluidic flow-focusing device is investigated, and the effects of acoustic voltage, frequency, flow-rate ratio, fluid viscosity, and flow vorticity are characterized. Acoustic actuation is provided to affect droplet breakup in the squeezing regime by imposing periodic oscillation to the fluid-fluid interface and, therefore, a periodic change in its curvature at the cross-junction of the device. Time reduction is observed for the three key stages of droplet breakup in the squeezing regime: dispersed phase flow-front advancement into the orifice, pressure buildup upstream and within the orifice together with liquid inflation downstream, and finally the thinning and pinch-off of the liquid thread. It is found that acoustic actuation has less of an effect on droplet size for the continuous phase with a higher viscosity due to the restrained interfacial vibration under a high shear stress environment. Periodic velocity flow fields within the dispersed phase at different phases of one oscillation cycle are calculated based on the results from phase-averaged microresolution-particle-image velocimetry (μPIV). The oscillation paths for the points of maximum vorticities of phase-averaged velocity components are traced, which reveals that the motion is mainly along the y direction.

Dissolution of carbon dioxide bubbles and microfluidic multiphase flows

We experimentally study the dissolution of carbon dioxide bubbles into common liquids (water, ethanol, and methanol) using microfluidic devices. Elongated bubbles are individually produced using a hydrodynamic focusing section into a compact microchannel. The initial bubble size is determined based on the fluid volumetric flow rates of injection and the channel geometry. By contrast, the bubble dissolution rate is found to depend on the inlet gas pressure and the fluid pair composition. For short periods of time after the fluids initial contact, the bubble length decreases linearly with time. We show that the initial rate of bubble shrinkage is proportional to the ratio of the diffusion coefficient and the Henry's law constant associated with each fluid pair. Our study shows the possibility to rapidly impregnate liquids with CO(2) over short distances using microfluidic technology.

Design and Preliminary Testing of a Novel Dual-Chambered Syringe

Intravenous catheterization is the most common invasive medical procedure today and is designed to introduce medication directly into the blood stream. Common practice is to administer medicine with one syringe, followed by a saline flush to clear the line of any residual medication. The risk of infection due to the introduction of bacteria in the catheter hub is increased with the number of times the hub is accessed. In addition, the two-step process adds millions of nursing hours per year and is prone to error. The goal of this effort was to design and test a dual-chamber syringe that could be reliably used for both dispensing medicine and the saline flush, and be produced at a low cost. The syringe has a novel dual-chamber design with a proximal chamber for medicine and a distal chamber that contains saline. The saline chamber has a fixed volume when the handle is locked into position, which allows the handle to control the variable volume of the medicine chamber. Between the two chambers is a plunger that surrounds the small channel (which is an extension of the distal chamber) that separates the saline from the medicine. When the distal chamber is unlocked, the handle controls the volume of the saline chamber. By this mechanism, the syringe is able inject the medicine followed by the saline flush with a single access to the catheter hub. The smooth operation of the device relies on a locking mechanism to control the rear plunger and volume of the distal saline chamber, and a bubble plug residing in the small channel between the chambers that prevents mixing of the medicine and saline fluids. The bubble plug is held in place by a balance of forces that depend on geometric variables and fluid properties. The chosen design prevents mixing of the two fluids during the operation of the device, which was experimentally validated with mass spectrometry. The dual-chamber syringe has successfully achieved the design goal of a single syringe for the two-step catheter procedure of dispensing medicine and a saline flush. This novel design will reduce the potential for catheter-based infection, medical errors, medical waste, and clinician time. Preliminary test results indicate that this innovation can significantly improve the safety and efficiency of catheter-based administration of medicine.

Bubble formation dynamics in various flow-focusing microdevices

The aim of this study is to investigate three types of gas-liquid micromixer geometries, including a cross-shape and two converging shape channels for the bubble formation in different liquids. The bubble shape, size, and formation mechanism were investigated under various experimental conditions such as the flow rates of two phases, physical properties of the liquid, and mixer geometries. A micro particle image velocimetry technique and a high-speed camera were used to characterize and quantify gas-liquid flows. It was revealed that the bubble formation, in particular the bubble size, depends on the geometry of the mixing section between two phases. A correlation gathering numerous experimental data was elaborated for the estimation of the bubble size. The influence of different parameters such as the flow rate ratio between two phases, surface tension, and liquid viscosity is well taken into consideration on the basis of the understanding of the bubble formation mechanism at the microscale. This paper marks an original improvement in the domain where no flow field characterizations or correlations were established in flow-focusing devices.

Formation of miscible fluid microstructures by hydrodynamic focusing in plane geometries

We experimentally investigate the flow structures formed when two miscible fluids that have large viscosity contrasts are injected and hydrodynamically focused in plane microchannels. Parallel viscous flows composed of a central stream surrounded by symmetric sheath streams are examined as a function of the flow rates, fluid viscosities, and rates of molecular diffusion. We study miscible interfacial morphologies and show a route for manipulating viscous flow-segregation processes in plane microsystems. The diffusion layer at the boundary of an ensheathed fluid grows as function of the distance downstream and depends on the Péclet number. In particular, we observe diffusion-enhanced viscous ensheathing processes. In the presence of a constriction, we investigate the formation of a lubricated viscous thread in the converging flow and also the buckling morphologies of the thread in the diverging flow. This study, relevant to multifluid flow between a "thick" material and a "thin" solvent, demonstrates the possibility to further control steady and oscillatory miscible fluid microstructures.

Experimental observations of the squeezing-to-dripping transition in T-shaped microfluidic junctions

An experimental study of droplet breakup in T-shaped microfluidic junctions is presented in which the capillary number and flow rate ratio are varied over a wide range for several different viscosity ratios and several different ratios of the inlet channel widths. The range of conditions corresponds to the region in which both the squeezing pressure that arises when the emerging interface obstructs the channel and the viscous shear stress on the emerging interface strongly influence the process. In this regime, the droplet volume depends on the capillary number, the flow rate ratio, and the ratio of inlet channel widths, which controls the degree of confinement of the droplets. The viscosity ratio influences the droplet volume only when the viscosities are similar. When there is a large viscosity contrast in which the dispersed-phase liquid is at least 50 times smaller than the continuous-phase liquid, the resulting size is independent of the viscosity ratio and no transition to a purely squeezing regime appears. In this case, both the droplet volume and the droplet production frequency obey power-law behavior with the capillary number, consistent with expectations based on mass conservation of the dispersed-phase liquid. Finally, scaling arguments are presented that result in predicted droplet volumes that depend on the capillary number, flow rate ratio, and width ratio in a qualitatively similar way to that observed in experiments.

Droplet microfluidics

Droplet-based microfluidic systems have been shown to be compatible with many chemical and biological reagents and capable of performing a variety of "digital fluidic" operations that can be rendered programmable and reconfigurable. This platform has dimensional scaling benefits that have enabled controlled and rapid mixing of fluids in the droplet reactors, resulting in decreased reaction times. This, coupled with the precise generation and repeatability of droplet operations, has made the droplet-based microfluidic system a potent high throughput platform for biomedical research and applications. In addition to being used as microreactors ranging from the nano- to femtoliter range; droplet-based systems have also been used to directly synthesize particles and encapsulate many biological entities for biomedicine and biotechnology applications. This review will focus on the various droplet operations, as well as the numerous applications of the system. Due to advantages unique to droplet-based systems, this technology has the potential to provide novel solutions to today's biomedical engineering challenges for advanced diagnostics and therapeutics.

Swirling of viscous fluid threads in microchannels

Viscous threads that are swept along in the flow of a less viscous miscible liquid can break up into viscous swirls. We experimentally investigate the evolution of miscible threads that flow off center in microchannels. Thin threads near the walls of a straight square channel become unstable to shear-induced disturbances. The amplification of the undulations transverse to the flow direction ultimately causes the threads to break up and form an array of individual viscous swirls, the miscible counterparts of droplets. This swirling instability provides a means for passively producing discrete diffusive microstructures in a continuous flow regime.

Micro air bubble manipulation by electrowetting on dielectric (EWOD): transporting, splitting, merging and eliminating of bubbles

This paper describes various manipulations of micro air bubbles using electrowetting on dielectric (EWOD): transporting, splitting, merging and eliminating. First, in order to understand the response of bubbles to EWOD, the contact angle modulation is measured in a capped air bubble and confirmed to be in good agreement with the Lippmann-Young equation until saturation. Based on the contact angle measurement, testing devices for the bubble manipulations are designed and fabricated. Sequential activations of patterned electrodes generate continuous bubble transportations. Bubble splitting is successfully realized by activating a single electrode positioned in the middle of bubble base. However, it is found that there are criteria that make splitting possible only in certain conditions. For successful splitting, smaller channel gap, larger bubble size, wider splitting electrode and/or larger contact angle changes by EWOD are preferred. These criteria are verified by a series of experiments as well as a static analysis. Bubble merging is achieved by moving bubbles towards each other in two different channel configurations: (1) channel I, where bubbles are in contact with the bottom channel plate only, and (2) channel II, where bubbles in contact with the top as well as bottom channel plates. Furthermore, eliminating a bubble to the ambient air is accomplished. All the bubble manipulation techniques may provide a versatile integrated platform not only to manipulate micro objects by utilizing micro bubbles as micro carriers, but also to enable a discrete bubble-based gas analysis system.

Folding of viscous threads in diverging microchannels

We study the folding instability of a viscous thread surrounded by a less viscous miscible liquid flowing from a square to a diverging microchannel. Because of the change in the flow introduced by the diverging channel, the viscous thread minimizes viscous dissipation by oscillating to form bends rather than by simply dilating. The folding frequency and the thread diameter can be related to the volume flow rates and thus to the characteristic shear rate. Diffusive mixing at the boundary of the thread can significantly modify the folding flow morphologies. This microfluidic system enables us to control the bending of the thread and to enhance mixing between liquids having significantly different viscosities.