Microdroplet Generation with Dilute Surfactant Concentration in a Modified T-Junction Device

Droplet Microfluidic Devices: Working Principles, Fabrication Methods, and Scale-Up Applications

Compared with the conventional emulsification method, droplets generated within microfluidic devices exhibit distinct advantages such as precise control of fluids, exceptional monodispersity, uniform morphology, flexible manipulation, and narrow size distribution. These inherent benefits, including intrinsic safety, excellent heat and mass transfer capabilities, and large surface-to-volume ratio, have led to the widespread applications of droplet-based microfluidics across diverse fields, encompassing chemical engineering, particle synthesis, biological detection, diagnostics, emulsion preparation, and pharmaceuticals. However, despite its promising potential for versatile applications, the practical utilization of this technology in commercial and industrial is extremely limited to the inherently low production rates achievable within a single microchannel. Over the past two decades, droplet-based microfluidics has evolved significantly, considerably transitioning from a proof-of-concept stage to industrialization. And now there is a growing trend towards translating academic research into commercial and industrial applications, primarily driven by the burgeoning demands of various fields. This paper comprehensively reviews recent advancements in droplet-based microfluidics, covering the fundamental working principles and the critical aspect of scale-up integration from working principles to scale-up integration. Based on the existing scale-up strategies, the paper also outlines the future research directions, identifies the potential opportunities, and addresses the typical unsolved challenges.

Determination of Dynamic Interfacial Tension during the Generation of Tiny Droplets in the Liquid-Liquid Jetting Flow Regime

Liquid-liquid dispersion coupled with droplet formation and mass transfer of surfactants is one of the most typical phenomena in many chemical processes. As in every aspect of this process, the interfacial tension variation caused by the unsaturated adsorption of surfactants on the droplet surface plays an important role. This article focuses on microdroplet formation and the dynamic interfacial behavior of surfactants in the jetting regime. In a capillary embedded step T-junction device, controllable preparation of monodisperse droplets is achieved, and a correlation for predicting droplet sizes is established. A method for measuring the dynamic interfacial tension is provided. Mass transfer coefficients are then calculated for Tween 20 during the droplet formation process by a semiempirical correlation. The results indicate that dynamic interfacial tensions are lower than those obtained when the surfactant is adsorbed to equilibrium. Based on the tip-streaming phenomenon, mass transfer coefficients for Tween 20 can reach up to ∼10^-3 m/s, higher than those obtained in coaxial microfluidic devices. All the preliminary results shed light on the nature of droplet formation and will be of significance for application in industrial apparatuses.

New Electrochemical Flow-Cell Configuration Integrated into a Three-Dimensional Microfluidic Platform: Improving Analytical Application in the Presence of Air Bubbles

A newly configured electrochemical flow cell to be used for (end-channel) amperometric detection in a microfluidic device is presented. The design was assembled to place the reference electrode in a separated compartment, isolated from the flow in the microchannel, while the working and counter electrodes remain in direct contact with both compartments. Moreover, a three-dimensional coil-shaped microfluidic device was fabricated using a nonconventional protocol. Both devices working in association enabled us to solve the drawback caused by the discrete injection when the automatic micropipette was used. The high performance of the proposed electrochemical flow cell was demonstrated after in situ modifying the surface of the platinum working electrode with surfactant (e.g., using Tween 20 at 0.10%). As the reference electrode remained out of contact with the flowing solution, there was no trouble by air bubble formation (generated by accidental insertion or by presence of surfactants) throughout the measurements. This device was characterized regarding its analytical performance by evaluating the amperometric detection of acetaminophen, enabling determination from 6.60 to 66.0 μmol L^-1. This issue is important since at high concentration (e.g., as assessed in clinical analysis) the acetaminophen is known to passivate the working electrode surfaces by electrogenerated products, impairing the accuracy of the electrochemical measurements.