Asymmetric breakup of a single droplet through a Y-junction microchannel with non-uniform flow rate

Two-phase interface fluid, bubble or droplet, has shown broad application potential in oil and gas field development, contaminated soil remediation, and medical treatment. These applications are particularly concerned about the flow characteristics of the two-phase fluid in different channels. Herein, we summarize and analyze the research progress in the flow of bubbles (or droplets) in different channels, mainly including simple, Y-junction/T-junction, and obstructed microchannels. At present, there is no systematic theory about the structure and mechanical evolution of the two-phase interface fluid, and therefore, the comprehensive study is still insufficient. Especially, current studies on the breakup of the two-phase interface in bifurcated channels mainly focus on a few of specific perspectives and a general conclusion is not achieved. In addition, to systematically verify the mechanism of bubble (or droplet) breakup, extensive studies on the three-dimensional physical model of bubbles (or droplets) are needed. Furthermore, we have also sorted out the involved influencing factors, as well as the prediction models for bubble (or droplet) breakup and retention in different channels, and in the end, we provide suggestions for the potential research and development of the two-phase interface fluid.

Two-phase flow and morphology of the gas–liquid interface for bubbles or droplets in different microchannels

Chen,

Jing,

Feng

et al. 2023

Deep reinforcement learning-based digital twin for droplet microfluidics control

Gyimah,

Scheler,

Rang

et al. 2023

This study applied deep reinforcement learning (DRL) with the Proximal Policy Optimization (PPO) algorithm within a two-dimensional computational fluid dynamics (CFD) model to achieve closed-loop control in microfluidics. The objective was to achieve the desired droplet size with minimal variability in a microfluidic capillary flow-focusing device. An artificial neural network was utilized to map sensing signals (flow pressure and droplet size) to control actions (continuous phase inlet pressure). To validate the numerical model, simulation results were compared with experimental data, which demonstrated a good agreement with errors below 11%. The PPO algorithm effectively controlled droplet size across various targets (50, 60, 70, and 80 μm) with different levels of precision. The optimized DRL + CFD framework successfully achieved droplet size control within a coefficient of variation (CV%) below 5% for all targets, outperforming the case without control. Furthermore, the adaptability of the PPO agent to external disturbances was extensively evaluated. By subjecting the system to sinusoidal mechanical vibrations with frequencies ranging from 10 Hz to 10 KHz and amplitudes between 50 and 500 Pa, the PPO algorithm demonstrated efficacy in handling disturbances within limits, highlighting its robustness. Overall, this study showcased the implementation of the DRL+CFD framework for designing and investigating novel control algorithms, advancing the field of droplet microfluidics control research.

Experimental study on the flow mixing in T-junctions with an impeller

Yin,

Huang,

et al. 2024

A good understanding of the mixing mechanism of hot and cold fluids in T-junctions is of great importance in ensuring the safe operation of T-junction piping systems. An impeller is added to T-junction ducts, and experiments are conducted using particle image velocimetry without considering the temperature difference between two fluids. The velocity field, vorticity field, and impeller speed are obtained for blade numbers Np = 3 and 4 at different momentum ratios (MR). When the impeller rotates passively in T-junctions under the impact of a branch jet, the values of MR required to initiate impeller rotation are MR = 0–0.5 for Np = 3 and 0–0.125 for Np = 4. However, an impeller with two blades cannot rotate at any momentum ratio. The relationship between the rotation speed and the flow rates of the main and branch fluids is obtained. An impeller with three blades rotates at a non-uniform speed, while that with four blades rotates uniformly. The jet flow pattern in T-junctions is converted from the impinging jet to the deflecting jet at MR ≥ 0.1. The average and root-mean-square velocity are essentially the same for a given momentum ratio, whereas the impeller speed increases with increasing flow rate for the main and branch ducts. As the momentum ratio decreases, the impeller speed also increases. For an impinging jet, the addition of an impeller effectively reduces the velocity fluctuation area and transfers the zone of the highest velocity fluctuation from the bottom wall to the middle of the main duct.