Microscopic coalescence mechanism of nanoparticle-laden droplet pairs in the coupling of electric field and flow field: A molecular dynamics study

Chemical absorption is one of the main methods for capturing and absorbing CO2 in post-combustion flue gases. The impact behaviors of the absorbent droplets on the filler have significant effects on CO2 absorption as the absorbent is sprayed down. However, the driving mechanisms behind these behaviors are not fully understood due to the increased difficulty in the absorption tower. In the present study, we numerically investigate the impact behavior of the CO2 absorbent droplet on the curved surface filler. The developed three-dimensional model is validated by our experimental results and previous studies. The driving mechanisms are revealed by focusing on the velocity and pressure field in different stages. The influence of the surface curvature and Weber number is analyzed, with particular attention to the evolution velocity. The liquid film oscillation is characterized by its amplitude and time for different surface hydrophobicity. The results show that the spreading of the liquid film is primarily influenced by the initial inertia, while its retraction and oscillation are mainly controlled by the surface tension and viscosity, leading to a longer retraction and oscillation time. Both pressure and velocity, as well as their peaks, exhibit different distributions depending on the behaviors in different stages. This is similar to the formation of the surrounding air vortex with its center above the gas–liquid interface in different stages. Both the Weber number and the curvature have little effect on the average spreading velocity, whereas the dimensionless maximum spreading diameter vs the Weber number follows a power law dependence. The results are helpful for understanding the physical mechanisms behind the impact behaviors of the CO2 absorbent droplets on the curved surface filler.

Impact behaviors of a single aqueous solution droplet on curved surface filler

Cui,

Wang,

et al. 2023

Synchronous magnetization to weaken the hindrance of surfactants to droplet coalescence during electric dehydration

Guo,

Du,

Lü

et al. 2024

Electric dehydration is the most widely used physical technology for separating water from crude oil. However, natural surfactants stabilize the oil–water interface, resulting in low dehydration efficiency and failure of the electric dehydrator due to electric field collapse. To explore the physical methods of weakening the influence of surfactants on electric dehydration, this study synchronously increased magnetization during the electric dehydration. Based on high-speed microscopy experiments, it has been demonstrated that synchronous magnetization weakens the hindrance of surfactants to droplet coalescence during electric dehydration. Within the experimental conditions, the influence of magnetization on the growth coefficient C1 ranges from 2.9% to 26.6%. In addition, based on molecular dynamics simulation, the mechanism of magnetization weakening the influence of surfactants was studied at the molecular level. It was found that water molecules and surfactant molecules undergo significant molecular clusters after magnetization, reducing the influence degree of surfactants on unit area. When the surfactant concentration increases, the decrease in the influence degree is balanced by the increase in the number of molecules, which also explains the law that the improvement rate ΔC1 decreases with the increase in surfactant concentration. The results of this work will be potentially valuable for weakening surfactant barriers to demulsification and oil–water separation.

Quantitative study of droplet generation by pressure-driven microfluidic flows in a flow-focusing microdroplet generator

Zeng,

Wang,

Chang

et al. 2024

To precisely control the size of droplets is of great importance for the applications of the droplet microfluidics. In a flow-focusing microdroplet generator, the pressure-driven microfluidic device is designed to control the flow rates of the fluids. For a specific geometry of the flow-focusing microchannel, a mathematical model of droplet formation is established, and the nonlinear relation between the droplet length and the driven-pressure ratio can be described by our model. For pressure-driven microfluidic flows, the nonlinear relation between the droplet length and the driving-pressure ratio is measured experimentally in the flow-focusing microchannel. Particularly, by using the closed-loop control method of droplet generation, good agreements are shown between the measured size of droplets and the predicted size of the droplets. As a result, the control precision of the droplet size can be increased drastically by the closed-loop control method of droplet generation. Consequently, monodisperse droplets of extremely small size can be produced in the flow-focusing microdroplet generator.