Liquid-liquid Segmented Flow Research Articles

Actively homogenizing fluid distribution and slug length of liquid-liquid segmented flow in parallelized microchannels

Liquid-liquid segmented flow offers the unique characteristics of high heat and mass transfer combined with a narrow residence time distribution and thus shows great potential in process intensification of biphasic applications. Capacity increase of this technology can only be achieved by numbering-up, if scale-related advantages are to be preserved. This implies homogeneous residence times and volume flows in all parallel channels. Conventionally this is achieved by static fluid distribution concepts with high pressure drops. This contribution develops a microfluidic valve based on temperature-controlled viscosity adjustments. It can be used to actively compensate for manufacturing tolerances in parallelized micro reactors and thus requires a lower pressure drop than static distribution concepts to achieve a homogeneous fluid distribution. The distributor concept is combined with sensors and actuators for flow patterns and an automated parallelization concept for homogenizing flow patterns of liquid-liquid slug flows in micro channels is presented.

Catalyst-free biphasic oxidation of Thiophenes in continuous-flow

A mild and practical oxidation process of thiophene derivatives in continuous-flow has been developed. The oxidant phase consists of mCPBA and acetonitrile, which formed an immiscible liquid-liquid segmented flow with the fuel phase. In the oxidation of thiophenes, high yields of sulfone could be realized with only 10 min of residence time in the microreactor. The oxidant phase can be separated with the fuel phase with a continuous membrane separator, rendering it a clean and potentially scalable method to facilitate fuel refining.

A device for continuous and flexible adjustment of liquid-liquid slug size in micro-channels

In liquid-liquid segmented flow slug size determines the interfacial area as well as the intensity of Taylor vortices and is thus an important operational parameter, decisive for mass transfer performance of biphasic micro-reactors and -extractors. Resulting slug sizes conventionally depend on material properties, flow and geometric parameters of the mixing point of the liquids. To evade tedious material specific design of the mixing point, modular devices, which allow active adjustment of developing slug sizes, are desirable. Such a slug generator is presented in this study. By altering the mixing point geometry during operation, it allows continuous manipulation of the slug formation process and thus slug size. The slug generation process in this device is investigated experimentally and with help of CFD simulations in order to identify its geometric influences. The findings lead to an optimized device design, whose capability to generate slugs of adjustable size and with low dispersity is demonstrated experimentally.

Rapid synthesis of [Au25(Cys)18] nanoclusters via carbon monoxide in microfluidic liquid-liquid segmented flow system and their antimicrobial performance

Atomically precise thiolate-gold nanoclusters with well-defined structures attract attention for use in various applications. However, most of the recently reported synthetic methods rely on prolonged synthesis times (a few hours to days) in order to produce high purity materials with a single cluster size. Such extended synthesis times make these processes ill-suited for adaptation to industrial scale production with continuous flow. In this work, an improved method for the synthesis of thiolated Au25 nanoclusters is presented utilising a microfluidic system and CO-mediated reduction. The optimized system, based on a coiled flow inverter with inner diameter of 1 mm operating at 80 °C and 500 kPa took only 3 min for the synthesis of atomically precise cysteine-capped [Au25(Cys)18] nanoclusters, as characterized by ultraviolet–visible spectroscopy and electrospray ionization mass spectrometry. The productivity of the system was increased by using higher reactant concentrations which led to a throughput of 0.9 gAu per day, without changing the reaction time or affecting the product purity. The Au nanoclusters were used as photobactericidal enhancement materials. In antimicrobial testing against S. aureus, encapsulation of the Au nanoclusters into crystal violet impregnated silicone showed high photobactericidal activity (~1.7 log reduction in viable bacteria) upon 6 h illumination of white light at ~312 lx, while crystal violet did not show significant photobactericidal activity on its own.

In-flow detection of ultra-small magnetic particles by an integrated giant magnetic impedance sensor

We have designed and fabricated a microfluidic system made of glass and polydimethylsiloxane. A micro-magnetometer has been integrated to the system. This sensor is made of a giant magneto-impedance wire known to have very high magnetic sensitivity at room temperature. A liquid-liquid segmented multiphase flow was generated in the channel using a Y-shaped inlet junction. The dispersed phase plugs contained superparamagnetic iron oxide (20 nm) nanoparticles at a molar concentration of 230 mmol/l. We have shown both theoretically and experimentally that in-flow detection of these nanoparticles is performed by the microsystem for concentration as small as 5.47 × 10−9 mol. These performances show that it is conceivable to use this system for ex-vivo analysis of blood samples where superparamagnetic iron oxide nanoparticles, initially used as magnetic contrast agents, could be functionalized for biomarkers fishing. It opens new perspectives in the context of personalized medicine.

Trace analysis of mercury(ii) ions using aptamer-modified Au/Ag core–shell nanoparticles and SERS spectroscopy in a microdroplet channel

We report the rapid and highly sensitive trace analysis of mercury(ii) ions in water using a surface-enhanced Raman scattering (SERS)-based microdroplet sensor. Aptamer-modified Au/Ag core-shell nanoparticles have been fabricated and utilized as highly functional sensing probes. All detection processes for the reaction between mercury(II) ions and aptamer-modified nanoparticles were performed in a specially designed microdroplet channel. Small water droplets that included sample reagents were separated from each other by an oil phase that continuously flowed along the channel. This two-phase liquid-liquid segmented flow system prevented the adsorption of aggregated colloids to the channel walls due to localized reagents within encapsulated droplets. The result was reduced residence time distributions. The limit of detection (LOD) of mercury(II) ions in water was determined by the SERS-based microdroplet sensor to be below 10 pM, which is three orders below the EPA-defined maximum contaminant level. This combination of a SERS-based microfluidic sensor with aptamer-based functional nanoprobes can be used for in-the-field sensing platforms, due to its size and simplicity.

Surface modification of droplet polymeric microfluidic devices for the stable and continuous generation of aqueous droplets.

Droplet microfluidics performed in poly(methyl methacrylate) (PMMA) microfluidic devices resulted in significant wall wetting by water droplets formed in a liquid-liquid segmented flow when using a hydrophobic carrier fluid such as perfluorotripropylamine (FC-3283). This wall wetting led to water droplets with nonuniform sizes that were often trapped on the wall surfaces, leading to unstable and poorly controlled liquid-liquid segmented flow. To circumvent this problem, we developed a two-step procedure to hydrophobically modify the surfaces of PMMA and other thermoplastic materials commonly used to make microfluidic devices. The surface-modification route involved the introduction of hydroxyl groups by oxygen plasma treatment of the polymer surface followed by a solution-phase reaction with heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane dissolved in fluorocarbon solvent FC-3283. This procedure was found to be useful for the modification of PMMA and other thermoplastic surfaces, including polycyclic olefin copolymer (COC) and polycarbonate (PC). Angle-resolved X-ray photoelectron spectroscopy indicated that the fluorination of these polymers took place with high surface selectivity. This procedure was used to modify the surface of a PMMA droplet microfluidic device (DMFD) and was shown to be useful in reducing the wetting problem during the generation of aqueous droplets in a perfluorotripropylamine (FC-3283) carrier fluid and could generate stable segmented flows for hours of operation. In the case of PMMA DMFD, oxygen plasma treatment was carried out after the PMMA cover plate was thermally fusion bonded to the PMMA microfluidic chip. Because the appended chemistry to the channel wall created a hydrophobic surface, it will accommodate the use of other carrier fluids that are hydrophobic as well, such as hexadecane or mineral oils.

Mechanized determination of n-octanol/water partition constants using liquid—liquid segmented flow extraction

A simple and rapid method for the determination of partition constants in the n-octanol/water system is presented. The substance under study is dissolved in n-octanol or water, and injected into a stream of the corresponding phase. Liquid-liquid segmented flow is utilized for equilibration of the two phases and after equilibration a fraction of the aqueous phase is separated with the aid of a hydrophilic membrane. The amount of substance in the aqueous phase is measured spectrophotometrically. The measurements are repeated at a small number of different phase flow ratios and the results obtained are plotted against these ratios. From slope and intercept for the regression line through these points the conditional partition constant, D', can be calculated. A simplified procedure using only two measurements, one of which is obtained with the flow rate of organic phase set to 0, can also be used. Partition constants in the range 0.48 < D < 873 (-0.32 < log D < 2.94) have been determined using this system. The precision, measured as relative standard deviation (RSD) is below 15%, corresponding to a standard deviation of +/- 0.1 log unit or less over the range of measurement. Measured values correspond to literature values within 0.07 log units. The time needed for a determination is about 15 min per substance, in many cases even shorter and the sample consumption is, in most cases, less than 1 mg.

Continuous flow extraction of indium with bis(2-ethylhexyl)phosphoric acid in 4-methylpentane-2-one coupled on-line with flame atomic absorption spectrometry

Extraction in liquid-liquid segmented flow is used for preconcentration of indium from dilute nitric acid solutions into bis(2-ethylhexyl)phosphoric acid dissolved in 4-methylpentane-2-one. The extraction setup is coupled on-line with flame spectrometry to give a fully mechanized system. The detection limit of the method is 0.03 mg l −1, the calibration plot is linear up to 1.75 mg l −1. Repeatability is 1.5% RSD measured at 1 mg l −1. Sample throughput is 60 h −1.

Extraction rate in liquid-liquid segmented flow injection analysis

The extraction mechanism in liquid-liquid segmented flow-injection systems is investigated. A film is formed on the tubing wall by the phase with the highest affinity to the tubing material. This film surrounds the segments of the other phase. To a great extent the extraction takes place at the interface of this film. The extraction rate is influenced by the segment length, the inner diameter of the tubing and the flow velocity. Short segments, small inner diameter and high linear flow velocity lead to a high extraction rate. These findings indicate that miniaturization of the flow system will lead to faster extraction and to decreased sample-zone broadening.

On-line liquid-liquid extraction in a segmented flow directly coupled to on-column injection into a gas chromatograph

A mechanized system for extractive sample workup for gas chromatography coupled on-line to an on-column injector is described. Extraction is performed in a liquid-liquid segmented flow in a glass coil internally coated with a hydrophobic layer. After extraction the phases are separated with the aid of a hydrophobic membrane supported by a screen coated with Teflon. The organic phase is fed to a loop injector. The system is closed from the atmosphere and was proven to provide rapid and precise workup of seawater samples of the determination of the halocarbons chloroform, bromodichloromethane, dibromochloromethane, bromoform, trichloroethylene, tetrachloroethylene, carbon tetrachloride, and 1,1,1-trichloroethane. Compared to manual extraction, smaller volumes of sample and organic phase are needed. Reliability was tested during a 3-week cruise when 350 seawater samples were processed without system failure. Concentrations down to the picogram-per-liter level in water can be determined, using injection volumes up to 130 /sup +/L.