We perform experimental studies of droplet breakup in microfluidic T-junctions in a range of capillary numbers lying between 4×10−4 and 2×10−1 and for two viscosity ratios of the fluids forming the dispersed and continuous phases. The present paper extends the range of capillary numbers explored by previous investigators by two orders of magnitude. We single out two different regimes of breakup. In a first regime, a gap exists between the droplet and the wall before breakup occurs. In this case, the breakup process agrees well with the analytical theory of Leshansky and Pismen [Phys. Fluids 21, 023303 (2009)]. In a second regime, droplets keep obstructing the T-junction before breakup. Using physical arguments, we introduce a critical droplet extension for describing the breakup process in this case.

http://absimage.aps.org/image/DFD09/MWS_DFD09-2009-000196.pdf

Droplet break-up in microfluidic T-junctions at small capillary numbers

Advances in the area of sample preparation are significant and have been growing significantly in recent years. This initial step of the analysis is essential and must be carried out properly, consisting of a complicated procedure with multiple stages. Consequently, it corresponds to a potential source of errors and will determine, at the end of the process, either a satisfactory result or a fail. One of the advances in this field includes the miniaturization of extraction techniques based on the conventional sample preparation procedures such as liquid-liquid extraction and solid-phase extraction. These modern techniques have gained prominence in the face of traditional methods since they minimize the consumption of organic solvents and the sample volume. As another feature, it is possible to reuse the sorbents, and its coupling to chromatographic systems might be automated. The review will emphasize the main techniques based on liquid-phase microextraction, as well as those based upon the use of sorbents. The first group includes currently popular techniques such as single drop microextraction, hollow fiber liquid-phase microextraction, and dispersive liquid-liquid microextraction. In the second group, solid-phase microextraction techniques such as in-tube solid-phase microextraction, stir bar sorptive extraction, dispersive solid-phase extraction, dispersive micro solid-phase microextraction, and microextraction by packed sorbent are highlighted. These approaches, in common, aim the determination of analytes at low concentrations in complex matrices. This article describes some characteristics, recent advances, and trends on miniaturized sample preparation techniques, as well as their current applications in food, environmental, and bioanalysis fields.

Recent advances and trends in miniaturized sample preparation techniques

In two decades of development, impressive strides have been made for automating basic laboratory operations in droplet-based microfluidics, allowing the emergence of a new form of high-throughput screening and experimentation in nanoliter to femtoliter volumes. Despite advancements in droplet storage, manipulation, and analysis, the field has not yet been widely adapted for many high-throughput screening (HTS) applications. Broad adoption and commercial development of these techniques require robust implementation of strategies for the stable storage, chemical containment, generation of libraries, sample tracking, and chemical analysis of these small samples. We discuss these challenges for implementing droplet HTS and highlight key strategies that have begun to address these concerns. Recent advances in the field leave us optimistic about the future prospects of this rapidly developing technology.

High-throughput screening by droplet microfluidics: perspective into key challenges and future prospects.

Dimensionless analysis on liquid-liquid flow patterns and scaling law on slug hydrodynamics in cross-junction microchannels

Multiphase flow in microfluidics: From droplets and bubbles to the encapsulated structures.

During the last few decades, microfluidic liquid–liquid extractors have been developed to address the need for separating solutes in analytical chemistry and efficiently recovering products in microfluidic reactors. This review classifies the various microfluidic liquid–liquid extractors into three major groups based on their flow arrangement: stop-flow microfluidic extractors (MEs), cocurrent MEs, and countercurrent MEs. Each group is further classified into several subcategories based on flow pattern and/or working principle. The review focuses on how to establish these three groups of microfluidic liquid–liquid extractors, including the difficulties and corresponding solutions for establishing these MEs, as well as their advantages and disadvantages. The review ends with conclusions and the outlook of the field.

Review of Microfluidic Liquid–Liquid Extractors

High-throughput extraction and separation of Ce(III) and Pr(III) using a chaotic advection microextractor

A highly efficient microextractor with large fluxes was designed based on oscillation to improve its extraction performance. The flow patterns and mass transfer performance of the microextractor were experimentally investigated. The influence of Reynolds number on these metrics of a liquid-liquid system was discussed. From the flow visualization experiment, three different flow patterns were observed: stratified flow, single oscillating flow, and full oscillating flow. It was found that the mass transfer performance was closely related to the conversion of the flow patterns. In the stratified flow, the extraction efficiency decreased with an increase in Reynolds number, while in the single oscillating flow and fully oscillating flow regimes, the extraction efficiency increased with increasing Reynolds number. When the flux of the aqueous phase was 45 mL/min with a corresponding residence time of 0.027 s, an extraction efficiency of 98.9% was achieved. Furthermore, the overall volumetric mass transfer coefficient of the oscillating extractor was about two to three orders of magnitude higher than that of traditional liquid-liquid macro-scale extractors and one magnitude higher than that of other liquid-liquid micro-scale extractors. The high efficiency, high throughput, and short residence time gives the oscillating extractor a competitive advantage over other liquid-liquid extractors.

Intensification of liquid-liquid mass transfer by oscillation in a high-throughput microextractor

This article presents a continuous counter-current extraction column with a high throughput that relies on a modified Taylor–Couette vortex flow in a centrifugal force field. The extraction column consists of a top settler, bottom settler, stationary outer cylinder connecting the two settlers, and rotating inner cylinder with a spiral guide channel. The organic and aqueous phases flow continuously and counter-currently through the annulus between the outer and inner cylinders. The high-speed inner cylinder breaks up the organic phase into small droplets in the continuous aqueous phase, and three flow patterns, i.e., a disordered droplet flow, banded droplet flow, and full droplet flow, can be formed at different rotating speeds. The spiral rotating inner cylinder was proved to facilitate the formation of the banded droplet flow, which enhances the mass transfer and increases the operating flexibility. A high back-extraction efficiency of approximately 95.6% was achieved using a spiral rotating inner cylinder of 25 cm in contact height with a rotating speed of 800 rpm, even at a high total throughput of up to 7.3 L h−1 cm−2.

Continuous counter-current centrifugal extraction column with high throughput using a spiral inner cylinder

Although microextraction is much more efficient than conventional macroextraction, its practical application has been limited by low throughputs and difficulties in constructing robust countercurrent microextraction (CCME) systems. In this work, a robust CCME process was established based on a novel passive microextractor with four units without any moving parts. The passive microextractor has internal recirculation and can efficiently mix two immiscible liquids. The hydraulic characteristics as well as the extraction and back-extraction performance of the passive CCME were investigated experimentally. The recovery efficiencies of the passive CCME were 1.43–1.68 times larger than the best values achieved using cocurrent extraction. Furthermore, the total throughput of the passive CCME developed in this work was about one to three orders of magnitude higher than that of other passive CCME systems reported in the literature. Therefore, a robust CCME process with high throughputs has been successfully constructed, which may promote the application of passive CCME in a wide variety of fields.

Tingliang Xie

Papers

Review of Microfluidic Liquid–Liquid Extractors

High-throughput extraction and separation of Ce(III) and Pr(III) using a chaotic advection microextractor

Intensification of liquid-liquid mass transfer by oscillation in a high-throughput microextractor

Continuous counter-current centrifugal extraction column with high throughput using a spiral inner cylinder

High-throughput countercurrent microextraction in passive mode