Gas-liquid-liquid three-phase flow pattern and pressure drop in a microfluidic chip: similarities with gas-liquid/liquid-liquid flows

Dynamics and controllability of droplet fusion under gas-liquid-liquid three-phase flow in a microfluidic reactor

Gas–liquid–liquid three-phase flow systems have unique advantages of controlling reagent manipulation and improving reaction performance. However, there remains a lack of insight into the dynamics and controllability of water droplet fusion assisted by gas bubbles, particularly scaling laws for use in the design and operation of complex multiphase flow processes. In the present work, a microfluidic reactor with three T-junctions was employed to sequentially generate gas bubbles and then fuse two dispersed water droplets. The formation of the dispersed phase was divided into multiple stages, and the bubble/droplet size was correlated with operating parameters. The formation of the second dispersed droplet at the third T-junction was accompanied by the fusion of the two dispersed water droplets that were formed. It revealed a two-stage process (i.e. drainage and fusion) for the two droplets to fuse while becoming mature by breaking-up with the secondary water supply stream. In addition, a droplet contact model was employed to understand the influence on the process stability and uniformity of the merged/fused droplets by varying the surfactant concentration (in oil), the viscosity of the water phase, and the flow rates of different fluids. The study provides a deeper understanding of the droplet fusion characteristics on gas–liquid–liquid three-phase flow in microreactors for a wide range of applications.

Gas–liquid–liquid three-phase flow systems have unique advantages of controlling reagent manipulation and improving reaction performance.

Manipulation of gas-liquid-liquid systems in continuous flow microreactors for efficient reaction processes

Gas-liquid-liquid flow in microreactors holds great potential towards process intensification of operation in multiphase systems, particularly by a precise control over the three-phase contact patterns and the associated mass transfer enhancement. This work reviews the manipulation of gas-liquid-liquid three-phase flow in microreactors for carrying out efficient reaction processes, including gas-liquid-liquid reactions with catalysts residing in either liquid phase, coupling of a gas-liquid reaction with the liquid-liquid extraction, inert gas assisted liquid-liquid reactions and particle synthesis under three-phase flow. Microreactors are shown to be able to provide well-defined flow patterns and enhanced gas-liquid/liquid-liquid mass transfer rates towards the optimized system performance. The interplay between hydrodynamics and mass transfer, as well as its influence on the overall microreactor system performance is discussed. Meanwhile, future perspectives regarding the scale-up of gas-liquid-liquid microreactors in order to meet the industrial needs and their potential applications especially in biobased chemicals and fuels synthesis are further addressed.

Thermographic characterization of thin liquid film formation and evaporation in microchannels

The science of transport in microchannels has greatly benefited applications ranging from micro-mixing, chemical synthesis and biological analysis to compact and efficient energy devices. One of the most critical and intricate phenomena in this field of science is the dynamics of thin liquid film formation during the flow of liquid and gas/vapor mixtures. These films can form in microseconds and be less than a micrometer thick, while dominating thermal transport in phase-change processes. Here, we report the captured details of these phenomena using a new measurement technique with unprecedented spatial and temporal resolutions of 20 μm and 100 μs, respectively. Thin films with thicknesses ranging from 1 to 20 μm forming around elongated bubbles over a capillary number range of 0.025 to 0.1 are characterized. The measurements suggest that these films thermally develop and evaporate at timescales in the order of 1-10 ms, two orders of magnitude longer than their formation timescale. The formation, reflow and evaporation of the liquid film constitute a complex dynamic involving variations of the film thickness over the periphery of a rectangular channel, leading to a thicker liquid film feeding (through lateral capillary wicking) a much thinner rapidly evaporating film. As a result, the thinner film dictates the rate of surface heat transfer while the thicker film determines the duration of thin film evaporation. A modified Bretherton model provides the best fit to the experimental results.

Liter-scale production of uniform gas bubbles via parallelization of flow-focusing generators

Microscale gas bubbles have demonstrated enormous utility as versatile templates for the synthesis of functional materials in medicine, ultra-lightweight materials and acoustic metamaterials. In many of these applications, high uniformity of the size of the gas bubbles is critical to achieve the desired properties and functionality. While microfluidics have been used with success to create gas bubbles that have a uniformity not achievable using conventional methods, the inherently low volumetric flow rate of microfluidics has limited its use in most applications. Parallelization of liquid droplet generators, in which many droplet generators are incorporated onto a single chip, has shown great promise for the large scale production of monodisperse liquid emulsion droplets. However, the scale-up of monodisperse gas bubbles using such an approach has remained a challenge because of possible coupling between parallel bubbles generators and feedback effects from the downstream channels. In this report, we systematically investigate the effect of factors such as viscosity of the continuous phase, capillary number, and gas pressure as well as the channel uniformity on the size distribution of gas bubbles in a parallelized microfluidic device. We show that, by optimizing the flow conditions, a device with 400 parallel flow focusing generators on a footprint of 5 × 5 cm² can be used to generate gas bubbles with a coefficient of variation of less than 5% at a production rate of approximately 1 L h^-1. Our results suggest that the optimization of flow conditions using a device with a small number (e.g., 8) of parallel FFGs can facilitate large-scale bubble production.

A bubble- and clogging-free microfluidic particle separation platform with multi-filtration

Microfiltration is a compelling method to separate particles based on their distinct size and deformability. However, this approach is prone to clogging after processing a certain number of particles and forming bubbles in the separation procedure, which often leads to malfunctioning of devices. In this work, we report a bubble-free and clogging-free microfluidic particle separation platform with high throughput. The platform features an integrated bidirectional micropump, a hydrophilic microporous filtration membrane and a hydrophobic porous degassing membrane. The bidirectional micropump enables the fluid to flow back and forth repeatedly, which flushes the filtration membrane and clears the filtration micropores for further filtration, and to flow forward to implement multi-filtration. The hydrophobic porous membrane on top of the separation channel removes air bubbles forming in the separation channel, improving the separation efficiency and operational reliability. The microbead mixture and undiluted whole blood were separated using the microfluidic chip. After 5 cycles of reverse flushing and forward re-filtration, a 2857-fold enrichment ratio and an 89.8% recovery rate of 10 μm microbeads were achieved for microbead separation with 99.9% removal efficiency of 2 μm microbeads. After 8 cycles, white blood cells were effectively separated from whole blood with a 396-fold enrichment ratio and a 70.6% recovery rate at a throughput of 39.1 μl min^-1, demonstrating that the platform can potentially be used in biomedical applications.

Ultra-thin liquid film extraction based on a gas–liquid–liquid double emulsion in a microchannel device

A gas–liquid–liquid double emulsion with ultra-thin liquid film is proposed for the mass transfer enhancement of an extreme phase ratio system.