Electrochemical Detection of Droplets in Microfluidic Devices: Simultaneous Determination of Velocity, Size and Content

Recent advances in droplet sequential monitoring methods for droplet sorting

Droplet microfluidics is an attractive technology to run parallel experiments with high throughput and scalability while maintaining the heterogeneous features of individual samples or reactions. Droplet sorting is utilized to collect the desired droplets based on droplet characterization and in-droplet content evaluation. A proper monitoring method is critical in this process, which governs the accuracy and maximum frequency of droplet handling. Until now, numerous monitoring methods have been integrated in the microfluidic devices for identifying droplets, such as optical spectroscopy, mass spectroscopy, electrochemical monitoring, and nuclear magnetic resonance spectroscopy. In this review, we summarize the features of various monitoring methods integrated into droplet sorting workflow and discuss their suitable condition and potential obstacles in use. We aim to provide a systematic introduction and an application guide for choosing and building a droplet monitoring platform.

Micro-Droplets Parameters Monitoring in a Microfluidic Chip via Liquid-Solid Triboelectric Nanogenerator

The monitoring of micro-droplets parameters is significant to the development of droplet microfluidics. However, existing monitoring methods have drawbacks such as high cost, interference with droplet movement, and even the potential for cross-contamination. Herein, a micro-droplets monitoring method (MDMM) based on liquid-solid triboelectric nanogenerator (LS-TENG) is proposed, which can realize non-invasive and self-powered monitoring of micro-droplets in a microfluidic chip. The droplet frequency is monitored by voltage pulse frequency and a mathematical model is established to monitor the droplet length and velocity. Furthermore, this work constructs micro-droplets sensor (MDS) based on the MDMM to carry out the experiment. The coefficients of determination (R2 ) of the fitting curves of the micro-droplets frequency, length, and velocity monitoring are 0.998, 0.997, and 0.995, respectively. To prove the universal applicability of the MDMM, the micro-droplets generated by different liquid media and channel structures are monitored. Eventually, a micro-droplet monitoring system is built, which can realize the counting of micro-droplets and the monitoring of droplet frequency and length. This work provides a novel approach for monitoring micro-droplets parameters, which holds the potential to advance developments in the field of microfluidics.

Real-time precise microfluidic droplets label-sequencing combined in a velocity detection sensor

Droplets microfluidics is broadening the range of Lab on a Chip solutions that, however, still suffer from the lack of an adequate level of integration of optical detection and sensors. In fact, droplets are currently monitored by imaging techniques, mostly limited by a time-consuming data post-processing and big data storage. This work aims to overcome this weakness, presenting a fully integrated opto-microfluidic platform able to detect, label and characterize droplets without the need for imaging techniques. It consists of optical waveguides arranged in a Mach Zehnder's configuration and a microfluidic circuit both coupled in the same substrate. As a proof of concept, the work demonstrates the performances of this opto-microfluidic platform in performing a complete and simultaneous sequence labelling and identification of each single droplet, in terms of its optical properties, as well as velocity and lengths. Since the sensor is realized in lithium niobate crystals, which is also highly resistant to chemical attack and biocompatible, the future addition of multifunctional stages into the same substrate can be easily envisioned, extending the range of applicability of the final device.

Electrochemical assessments of droplet contents in microfluidic channels. Application to the titration of heterogeneous droplets

Series of aqueous droplets containing redox species were generated on-demand in a microfluidic channel and detected downstream by an electrochemical cell. Depending on the cell geometry, amperometric detections were performed to simultaneously determine the velocity, volume and content of circulating droplets in oil. Volumes and velocities were estimated from specific transition times on the chronoamperometric responses, while charge were evaluated from current integration. The results showed that the total charge within droplets was controlled by the geometry of the electrochemical cell and droplet velocity, leading to accurate determinations of droplet content under specific operating conditions. An active merging of droplets with titrating solutions was tested for analytical purposes. The results demonstrated that even if the mixing was not complete during detection, the assessment of droplet content was still valid. The performance of electrochemical detection was thus evidenced to determine the content of heterogeneous droplets. This property is pertinent since the design of sophisticated circuits is no longer required to fully homogenize the droplet content before characterization, opening broader perspectives in droplet-based microfluidics.

A micrometer head integrated microfluidic device for facile droplet size control and automatic measurement of a droplet size

A novel microfluidic droplet generator is proposed, which can control the droplet size through turning an integrated micrometer head with ease, and the size of the produced micro-droplet can be automatically and real-time monitored by an open-sourced software and off-the-shelf hardware.