Generation Of Uniform Droplets Research Articles

Breakup behavior of a molten metal jet

The breakup behavior of a molten metal jet into a still gas was studied numerically. Droplet formation was modeled by imposing a sinusoidal waveform perturbation or an amplitude-modulated waveform perturbation. The effect of the temperature on the jet breakup behavior was examined by modeling the liquid metal properties, including density, viscosity, and surface tension, as a function of the temperature. The process by which a molten metal jet was ejected from an orifice exit was modeled to include the wetting of the molten metal on the orifice surface at the gas interface using a dynamic contact angle (θD). The effects of the oscillation amplitude (A=0.10–0.30), the Strouhal number (St=0.20–0.50) and the Weber number (We=11.63–129.19) were studied. The imposition of a periodic perturbation yielded linear or nonlinear breakup behavior in the molten metal jet. The conditions found to be optimal for the continuous generation of uniform droplets were identified by optimizing the uniformity of the main droplet, the regular distance between main droplets, the presence of satellite droplets, and the coalescence of the droplets due to surface instabilities and hydrodynamic interactions. The effects of the modulated amplitude (B) and the frequency ratio (N) on the coalescence and separation of neighboring droplets were examined.

Effect of wetting behavior on generation of uniform aluminum droplets obtained by pneumatic drop-on-demand technique

Micro metal droplet is the basic building block of three-dimensional metal parts fabricated by micro droplet deposition manufacturing (MDDM) technique. In this paper, the effect of wetting behavior between liquid metal and spray nozzle on the generation of micro aluminum droplets produced by pneumatic drop-on-demand (DOD) technique was investigated by simulation and experiment. A finite element model of liquid–gas flow was established based on the improved level set method (LSM). Then the generation of micro liquid aluminum droplets under different wetting conditions was simulated. A series of spraying experiments were also performed on micro droplet deposition experiment platform. The results show that the generation and flight of micro aluminum droplets are influenced by wetting condition between liquid metal and the nozzle surface significantly. Additionally, the effect of wetting behavior on the droplet size was analyzed to achieve the smallest building block. It was found that the droplet radius decreased with the increase of contact angle exponentially, which agreed with the numerical calculation and experiment results. On this basis, a wettability criterion was proposed for selecting nozzle materials. These works would be helpful for the processing optimization and equipment improvement of MDDM technique.

Membranes with tailored wettability properties for the generation of uniform emulsion droplets with high efficiency

Membrane emulsification is a promising technology for the production of micro–nano particles, which is able to compete with the conventional mechanical emulsification processes. The production of emulsions with narrow droplet size distribution at dispersed phase fluxes (productivity) sufficiently high to make the process suitable for industrial application is still a considerable challenge. The interfacial tension between the dispersed phase and the membrane pore wall is a crucial parameter to maintain droplets shape while enhancing productivity. In the present paper, a membrane thickness with asymmetric properties in terms of wettability between external and internal sides has been tested in the preparation of W/O emulsions. The membrane surface wettability modification was obtained by adsorption of hydrophobic macromolecules on the lumen side of hydrophilic membrane. Lipase was used as a model macromolecule. W/O emulsion droplets with smaller droplets size have been produced with lipase-loaded membrane compared with the unmodified hydrophilic membrane. High dispersed phase flux of 30Lh−1m−2 was also obtained with a significant increase of process productivity compared to the use of hydrophobic membranes. These results show that membrane–protein interaction can be used to functionalize opportunely the membrane for membrane emulsification application reducing emulsification time and increasing dispersed phase flux without modifying the control on droplets properties in terms of size and size distribution.

A highly parallel microfluidic droplet method enabling single-molecule counting for digital enzyme detection.

Although digital detection of nucleic acids has been achieved by amplification of single templates in uniform microfluidic droplets and widely used for genetic analysis, droplet-based digital detection of proteins has rarely been reported, largely due to the lack of an efficient target amplification method for protein in droplets. Here, we report a key step towards digital detection of proteins using a highly parallel microfluidic droplet approach for single enzyme molecule detection in picoliter droplets via enzyme catalyzed signal amplification. An integrated microfluidic chip was designed for high throughput uniform droplet generation, monolayer droplet collection, incubation, detection, and release. Single β-galatosidase (β-Gal) molecules and the fluorogenic substrate fluorescein di-β-D-galactopyranoside were injected from two separated inlets to form uniform 20 μm droplets in fluorinated oil at a frequency of 6.6 kHz. About 200 000 droplets were captured as a monolayer in a capture well on-chip for subsequent imaging detection. A series of β-Gal solutions at different concentrations were analyzed at the single-molecule level. With no enzyme present, no droplets were found to fluoresce, while brightly fluorescent droplets were observed under single-enzyme molecule conditions. Droplet fluorescence intensity distribution analysis showed that the distribution of enzyme molecules under single-molecule conditions matched well with theoretical prediction, further proving the feasibility of detecting single enzyme molecules in emulsion droplets. Moreover, the population of fluorescent droplets increased as the β-Gal concentration increased. Based on a digital counting method, the measured concentrations of the enzyme were found to match well with input enzyme concentration, establishing the accuracy of the digital detection method for the quantification of β-Gal enzyme molecules. The capability of highly parallel detection of single enzyme molecules in uniform picoliter droplets paves the way to microdroplet based digital detection of proteins.

Parallel generation of uniform fine droplets at hundreds of kilohertz in a flow-focusing module.

Droplet-based microfluidic systems enable a variety of biomedical applications from point-of-care diagnostics with third world implications, to targeted therapeutics alongside medical ultrasound, to molecular screening and genetic testing. Though these systems maintain the key advantage of precise control of the size and composition of the droplet as compared to conventional methods of production, the low rates at which droplets are produced limits translation beyond the laboratory setting. As well, previous attempts to scale up shear-based microfluidic systems focused on increasing the volumetric throughput and formed large droplets, negating many practical applications of emulsions such as site-specific therapeutics. We present the operation of a parallel module with eight flow-focusing orifices in the dripping regime of droplet formation for the generation of uniform fine droplets at rates in the hundreds of kilohertz. Elevating the capillary number to access dripping, generation of monodisperse droplets of liquid perfluoropentane in the parallel module exceeded 3.69 × 10(5) droplets per second, or 1.33 × 10(9) droplets per hour, at a mean diameter of 9.8 μm. Our microfluidic method offers a novel means to amass uniform fine droplets in practical amounts, for instance, to satisfy clinical needs, with the potential for modification to form massive amounts of more complex droplets.

Parallel generation of uniform fine droplets at hundreds of kilohertz in a flow-focusing module

Parallel generation of uniform fine droplets at hundreds of kilohertz in a flow-focusing module

Study on the mechanism of droplet formation in T-junction microchannel

In the present paper, the mechanism of droplet formation in immiscible liquid/liquid T-junction system has been studied numerically and analytically. Different droplet stream patterns such as long slugs, small spheres, stable stream connected to a jet and finally parallel flow are observed, which are conventionally called squeezing, dripping and jetting regimes appearing under appropriate numerical setups. We emphasize that the effects of surface force and viscous force in this system alternatively dominate the droplets stream, and the periodically varied pressure and local velocity ensure the generation of uniform droplets. In particular, a two-dimensional lubrication analysis is applied to the confined continuous flow between dispersed emerging phase and the channel wall, which verifies that the pressure buildup upstream facilitate the long slugs formation. The numerical results and theoretical analysis hold a good agreement with the experimental measurements.

Development of Adjustable Signal Generation System for Uniform Droplets Spray

The generation of uniform droplets in the size of hundred microns was described in this paper. An experimental device capable of producing uniform droplet has been developed. For the purpose of adjusting the signal generator for both continuous droplets spray and drop-on-demand, the piezoelectric driving signal was set up with a PC and software of Labview. With the signal generation system, Flexible signal adjustment appears to be more suitable than simple signal generator to generate uniform droplets.

Predication and measurement of deflected trajectory and temperature history of uniform metal droplets in microstructures fabrication

The deposition of metal droplets has been attracted wide interests for potential applying in microstructures fabrication. A uniform droplet generator with image acquiring system and droplet temperature measuring system was established. A coupling analysis method on combination of droplets dynamic and thermal models was proposed. Effect of experimental parameters on deflected trajectory and the temperature history of the uniform droplets was investigated. The measured trajectory and temperature history agreed well with the predicted results, indicating the validity of the proposed analysis method. The predicted results with different experimental parameters showed that the deflected trajectories and thermal histories of the droplets with diameters close to 100 μm or less were strongly dependent on their sizes. Small droplets had a narrow deposition length for obtaining parts with good metallurgical bonding. The predicted results also showed that it was effective in controlling the deflected trajectories and thermal histories of droplets by adjusting the pressure when it was low. At last, micro alloy tubes with good metallurgical bonding were obtained using 50 μm diameter nozzle when the experimental parameters were chosen according to theoretical models. The deposited results show that proper parameters for microstructures deposition can be obtained according to the proposed analysis method.

Microvalve Enabled Digital Microfluidic Systems for High-Performance Biochemical and Genetic Analysis

Microfluidic devices offer unparalleled capability for digital microfluidic automation of sample processing and complex assay protocols in medical diagnostic and research applications. In our own work, monolithic membrane valves have enabled the creation of two platforms that precisely manipulate discrete, nanoliter-scale volumes of sample. The digital microfluidic Automaton uses two-dimensional microvalve arrays to combinatorially process nanoliter-scale sample volumes. This programmable system enables rapid integration of diverse assay protocols using a universal processing architecture. Microfabricated emulsion generator array (MEGA) devices integrate actively controlled 3-microvalve pumps to enable on-demand generation of uniform droplets for statistical encapsulation of microbeads and cells. A MEGA device containing 96 channels confers the capability of generating up to 3.4 × 10(6) nanoliter-volume droplets per hour for ultrahigh-throughput detection of rare mutations in a vast background of normal genotypes. These novel digital microfluidic platforms offer significant enhancements in throughput, sensitivity, and programmability for automated sample processing and analysis.

Breakup control of a liquid jet by disturbance manipulation

In this paper, we study the breakup of a circular jet subject to a manipulated disturbance applied by an actuator (here a piezoelectric) in order to control droplets sizes and patterns over a wide range. According to previous studies, breakup of a jet is strongly affected by the characteristics of the disturbance. When the breakup is not periodic and repeatable, it is described as an irregular or random breakup. On the other hand, over a frequency range for the disturbance, the breakup can be made repeatable, though not necessarily uniform, in which the droplets pinch off the stream at a constant rate. In this case, the breakup is called regular, although satellites or secondary droplets might form among the main droplets. In most applications, a very narrow frequency range (much narrower than the regular breakup frequency range) is used for operation, since it is the only range where uniform droplets form. This work demonstrates that by eliminating secondary droplets or satellites, the entire regular breakup regime becomes available for desirable operation, i.e., uniform droplet generation. The regular breakup regime is specified experimentally and an additional disturbance is added to the driving disturbance in order to eliminate the nonuniform droplets. This method provides us with droplets of the desired size when they cannot be produced with a single frequency disturbance alone. The results also demonstrate that different frequencies for the additional disturbance produce different patterns for the droplets sizes.

Agarose droplet microfluidics for highly parallel and efficient single molecule emulsion PCR

An agarose droplet method was developed for highly parallel and efficient single molecule emulsion PCR. The method capitalizes on the unique thermoresponsive sol-gel switching property of agarose for highly efficient DNA amplification and amplicon trapping. Uniform agarose solution droplets generated via a microfluidic chip serve as robust and inert nanolitre PCR reactors for single copy DNA molecule amplification. After PCR, agarose droplets are gelated to form agarose beads, trapping all amplicons in each reactor to maintain the monoclonality of each droplet. This method does not require cocapsulation of primer labeled microbeads, allows high throughput generation of uniform droplets and enables high PCR efficiency, making it a promising platform for many single copy genetic studies.

Generation of Monodisperse Mesoporous Silica Microspheres with Controllable Size and Surface Morphology in a Microfluidic Device

AbstractA one‐step in situ method, termed microfluidic diffusion‐induced self‐assembly, for the synthesis of monodisperse ordered mesoporous silica microspheres, is reported. The method combines microfluidic generation of uniform droplets and subsequent in situ rapid solvent diffusion‐induced self‐assembly within the microfluidic channel. The mesoporous silica microspheres prepared in this way reveal well‐ordered 2D hexagonal mesostructures with unprecedented corrugated surface morphology of disordered mesopores that are larger than 15 nm. It is speculated that the formation of an interfacial subphase and rapid diffusion of solvent to oil are attributed to the formation of the unique surface morphology. It is also shown that the surface morphology and the particle size of the mesoporous silica microspheres can be systematically controlled by adjusting fluidic conditions.

Cover Picture: Generation of Monodisperse Mesoporous Silica Microspheres with Controllable Size and Surface Morphology in a Microfluidic Device (Adv. Funct. Mater. 24/2008)

AbstractA one‐step in situ method, termed microfluidic diffusion‐induced self‐assembly, for the synthesis of monodisperse ordered mesoporous silica microspheres, is reported. The method combines microfluidic generation of uniform droplets and subsequent in situ rapid solvent diffusion‐induced self‐assembly within the microfluidic channel. The mesoporous silica microspheres prepared in this way reveal well‐ordered 2D hexagonal mesostructures with unprecedented corrugated surface morphology of disordered mesopores that are larger than 15 nm. It is speculated that the formation of an interfacial subphase and rapid diffusion of solvent to oil are attributed to the formation of the unique surface morphology. It is also shown that the surface morphology and the particle size of the mesoporous silica microspheres can be systematically controlled by adjusting fluidic conditions.

Generation of Uniform Droplets through the Breakup of a Liquid Sheet using a Novel Microfluidic Device

Generation of Uniform Droplets through the Breakup of a Liquid Sheet using a Novel Microfluidic Device

Generation of uniform Fine Droplets Under Spindle Mode in Electrohydrodynamic Atomization

A series of experiments were conducted to generate fine liquid(water) drops through the electrohydrodynamic atomization process. The atomization mode depended on flow rate and DC voltage input. For water, having electric conductivity larger than 10(sup)-7S/m, the spindle mode turned out to be the only mode to generate uniform-size drops within the range of 30-450 microns that have wide applications. Within this mode, both the uniformity and the fineness of drops were improved at an optimum voltage input for a given flow rate. This optimum voltage increased with increasing of the liquid flow rate. Another important parameter considered was the nozzle material with different electric conductivity and liquid wettability. A stainless-steel nozzle (the material with high electric conductivity and high liquid wettability) and a silica nozzle (the electrically non-conducting material with low liquid wettability) were tested and compared; and more uniform drops could be obtained with the silica nozzle.

Technique for Producing Capillaries with Reproducible Orifice Diameters for Uniform Droplet Generation

A procedure is described for fabricating capillary orifice tips useful for the generation of uniform droplets. These generator tips operate at relatively low applied pressure, are relatively resistant to clogging, and incorporate orifice diameters reproducible to 1%. The method involves implanting a fused-silica capillary within a glass sleeve. When heated, the glass and fused silica wet to form a tight seal with no cracking caused by differences in thermal expansion. This technique permits the routine production of capillary tips with fixed orifice and outer diameters.

Influence of Pulse Signal Spike and Liquid Characteristics on Performance of Uniform-Droplet Generator

The performance of a pulsed-chamber, uniform-droplet generator was enhanced through the development of a digital pulse generator system. A trapezoidal pulse signal, variable in terms of peak voltage, rise time, dwell time, and fall time, actuated a piezoelectric disk to force droplets through a precision orifice. Variable pulse control enabled precise delivery of either paraffinic oil or water droplets. The effects of each pulse variable on droplet size and velocity were examined using a phase-Doppler particle analyzer and a photographic technique. Droplet size increased with increased voltage, rise time, dwell time, fall time, and integrated area under the pulse curve. Air bubbles in the chamber did not prevent droplet delivery, except for certain combinations of bubble size and location. Dwell time was statistically the most important variable affecting oil droplet size and delivery. Liquid reservoir height was most important for water. Increased pulse voltage tended to increase droplet size and velocity because of increased impulse and momentum.

On the genesis of droplet stream microspeed dispersions

Mechanisms that govern uniform droplet generation have been investigated. Droplet formation of viscous low vapor pressure fluids has been achieved by imposing sinusoidal pressure disturbances on a capillary stream. The capillary stream breakup and subsequent droplet propagation took place in a vacuum so that there were no significant interactions with the surrounding atmosphere. Microfluctuations of the droplet speeds have been examined after the droplets have traveled approximately 30 000 droplet diameters, and can vary from 1×10−6 to 1×10−4 times the average droplet speed depending on forcing conditions. Growth rate measurements were made of the streams radial disturbance prior to breakup, and were found to be intrinsically related to the measurements of the droplet stream’s speed microfluctuations. From this relation, it is suggested that studies of the characteristics of the droplet stream gives information about the capillary wave instabilities, which lead to droplet formation. A model that describes the microfluctuations is developed and is in excellent agreement with the experimentally obtained values. The model suggests that the source of droplet stream microspeed dispersions is the growth of a noise disturbance that modulates the otherwise controlled periodic pressure perturbation, which initiates droplet breakup.

A versatile system for stable generation of uniform droplets

A system for the production of liquid droplets of uniform size and velocity is described. The precise control of the operating fluid provided by this system allows the generator to function in a droplet-on-demand, burst, or continuous mode of operation. The basic component of the uniform droplet generator is a radially contracting piezoelectric cylinder which forces liquid through a glass-capillary nozzle. Droplet diameters ranging from 5 to 500 μm have been produced by appropriate selection of nozzle aperture and operating mode. Further control of size, velocity, and interdroplet spacing is exercised through the voltage- and fluid-control elements of the system. In addition to its ability to operate in any orientation, the droplet generator is designed with a small cross section and a water cooling feature for operation in coflowing, elevated-temperature environments. The uniform-droplet-generation system has exhibited long-term stability in operations ranging from a single isolated droplet to a stream of calibration-standard droplets separated by center-to-center spacings of less than two diameters. Its capabilities in controlling droplet development and dynamics make it an attractive candidate for use in a wide range of liquid-droplet applications.