Drag-induced breakup mechanism for droplet generation in dripping within flow focusing microfluidics

Droplet-Based Microfluidics as a Platform to Design Food-Grade Delivery Systems Based on the Entrapped Compound Type

Microfluidic technology has emerged as a powerful tool for several applications, including chemistry, physics, biology, and engineering. Due to the laminar regime, droplet-based microfluidics enable the development of diverse delivery systems based on food-grade emulsions, such as multiple emulsions, microgels, microcapsules, solid lipid microparticles, and giant liposomes. Additionally, by precisely manipulating fluids on the low-energy-demand micrometer scale, it becomes possible to control the size, shape, and dispersity of generated droplets, which makes microfluidic emulsification an excellent approach for tailoring delivery system properties based on the nature of the entrapped compounds. Thus, this review points out the most current advances in droplet-based microfluidic processes, which successfully use food-grade emulsions to develop simple and complex delivery systems. In this context, we summarized the principles of droplet-based microfluidics, introducing the most common microdevice geometries, the materials used in the manufacture, and the forces involved in the different droplet-generation processes into the microchannels. Subsequently, the encapsulated compound type, classified as lipophilic or hydrophilic functional compounds, was used as a starting point to present current advances in delivery systems using food-grade emulsions and their assembly using microfluidic technologies. Finally, we discuss the limitations and perspectives of scale-up in droplet-based microfluidic approaches, including the challenges that have limited the transition of microfluidic processes from the lab-scale to the industrial-scale.

Dynamic Behaviours of Monodisperse Double Emulsion Formation in a Tri-Axial Capillary Device

We investigated experimentally, analytically, and numerically the formation process of double emulsion formations under a dripping regime in a tri-axial co-flow capillary device. The results show that mismatches of core and shell droplets under a given flow condition can be captured both experimentally and numerically. We propose a semi-analytical model using the match ratio between the pinch-off length of the shell droplet and the product of the core growth rate and its pinch-off time. The mismatch issue can be avoided if the match ratio is lower than unity. We considered a model with the wall effect to predict the size of the matched double emulsion. The model shows slight deviations with experimental data if the Reynolds number of the continuous phase is lower than 0.06 but asymptotically approaches good agreement if the Reynolds number increases from 0.06 to 0.14. The numerical simulation generally agrees with the experiments under various flow conditions.

Scaling up the throughput of microfluidic droplet-based materials synthesis: A review of recent progress and outlook

The last two decades have witnessed tremendous progress in the development of microfluidic chips that generate micrometer- and nanometer-scale materials. These chips allow precise control over composition, structure, and particle uniformity not achievable using conventional methods. These microfluidic-generated materials have demonstrated enormous potential for applications in medicine, agriculture, food processing, acoustic, and optical meta-materials, and more. However, because the basis of these chips' performance is their precise control of fluid flows at the micrometer scale, their operation is limited to the inherently low throughputs dictated by the physics of multiphasic flows in micro-channels. This limitation on throughput results in material production rates that are too low for most practical applications. In recent years, however, significant progress has been made to tackle this challenge by designing microchip architectures that incorporate multiple microfluidic devices onto single chips. These devices can be operated in parallel to increase throughput while retaining the benefits of microfluidic particle generation. In this review, we will highlight recent work in this area and share our perspective on the key unsolved challenges and opportunities in this field.

Generation and Dynamics of Janus Droplets in Shear-Thinning Fluid Flow in a Double Y-Type Microchannel

Droplets composed of two different materials, or Janus droplets, have diverse applications, including microfluidic digital laboratory systems, DNA chips, and self-assembly systems. A three-dimensional computational study of Janus droplet formation in a double Y-type microfluidic device filled with a shear-thinning fluid is performed by using the multiphaseInterDyMFoam solver of the OpenFOAM, based on a finite-volume method. The bi-phase volume-of-fluid method is adopted to track the interface with an adaptive dynamic mesh refinement for moving interfaces. The formation of Janus droplets in the shear-thinning fluid is characterized in five different states of tubbing, jetting, intermediate, dripping and unstable dripping in a multiphase microsystem under various flow conditions. The formation mechanism of Janus droplets is understood by analyzing the influencing factors, including the flow rates of the continuous phase and of the dispersed phase, surface tension, and non-Newtonian rheological parameters. Studies have found that the formation of the Janus droplets and their sizes are related to the flow rate at the inlet under low capillary numbers. The rheological parameters of shear-thinning fluid have a significant impact on the size of Janus droplets and their formation mechanism. As the apparent viscosity increases, the frequency of Janus droplet formation increases, while the droplet volume decreases. Compared with Newtonian fluid, the Janus droplet is more readily generated in shear-thinning fluid due to the interlay of diminishing viscous force, surface tension, and pressure drop.

High-Throughput Aqueous Two-Phase System Droplet Generation by Oil-Free Passive Microfluidics

Aqueous two-phase system (ATPS) droplet generation has significant potential in biological and medical applications because of its excellent biocompatibility. However, the ultralow interfacial tension of ATPS makes droplet generation extremely challenging when compared with the conventional water-in-oil (W/O) system. In this paper, we passively produced ATPS droplets with a wide range of droplet size and high production rate without the involvement of an oil phase and external forces. For the first time, we reported important information of the flow rate and capillary (Ca) number for passive, oil-free ATPS droplet generation. It was found that the range of Ca numbers of the continuous phase under the jetting flow regime is 0.3-1.7, as compared to less than 0.1 in the W/O system, indicating the ultralow interfacial tension in ATPS. In addition, we successfully generated ATPS droplets with a radius as small as 7 μm at the maximum frequency up to 300 Hz, which has not been achieved in previous studies. The size and generation frequency of ATPS droplets can be controlled independently by adjusting the inlet pressures and corresponding flow rates. We found that the droplet size is correlated with the pressure and flow rate ratios with the power-law exponents of 0.8 and 0.2, respectively.

A hand-held, power-free microfluidic device for monodisperse droplet generation

Here we develop a microfluidic device to generate monodispersion sub-nanoliter size droplets. Our system reaches steady state within 3 s after the flow starts and generates 100,000 droplets in 28 s with high size consistency (CV < 8%). This low cost device is composed with a microfluidic chip, 2 tubings, a collection vial, a syringe and a station; and is in the size of an iPad Mini (4” × 6” × 3/4”). In this system, all incoming reagents share the same pressure drop across the fluidic passage to generator droplets. A single source negative pressure is applied to the fluids to create the flow by a vacuum at the exit end of the device. The vacuum is generated on-site by pulling the plunger of a syringe. The position of the plunger before and after pulling determines the degree of vacuum. A fixture is used to hold the plunger after it is pulled to maintain its vacuum. Although this system loses vacuum gradually as the liquid filling in, it maintains a flow rates with the changes less than 10% and droplet sizes changes less than 2% during the course of generating 150,000 droplets. The pressure drop across the chip, the flow rates of all reagents, the droplet size and generation frequency are predictable, programmable, and reproducible. This device is designed for generating droplets for single cell genome profiling application but can be also used for digital PCR or other droplet-based applications.