Study On The Generation Of Bubbles And Droplets In Microfluidic Chips

Microfluidic chip technology is widely used in water quality analysis,environmental monitoring,biochemical reactions,clinical testing and other fields due to its low pollution,low sample demand and high throughput.Bubbles and droplets are the new two-phase fluid motion form that has appeared in microfluidic chips in recent years.The generation mechanisms and dynamic characteristics are of great importance,they not only can be used to guide the development of related microfluidic detection instruments,but also provide theoretical support for the heat transfer in two-phase-flow chips.In this paper,the dynamic characteristics of bubble formation in a highly viscous liquid in a co-flowing microfluidic device are experimentally investigated.The evolution of gaseous thread in the co-flowing device is recorded using a high-speed camera.The bubble formation process can be divided into three stages: retraction stage,expansion stage,and collapse stage.According to an analysis of the forces acting on the gaseous thread,the bubble formation in the co-flowing device is a competitive result of the surface tension,gas pressure,squeezing and shearing effects.The surface tension effect plays an important role in the retraction stage.In the expansion stage,the gas pressure dominates the bubble's growth.While in the collapse stage,the shearing effect and squeezing effect leads to the interface breakup.Three empirical correlations are proposed according to the experimental data and can be used to predict bubble formation frequency,the diameter and length of the generated bubbles.In addition,the influence of external disturbance on the droplet formation in the flow-focusing chip is also studied experimentally,and the continuous sinusoidal perturbation applied to the dispersed phase pipeline is used to control the generation of droplets.The results show that by changing the disturbance frequency and amplitude,the generation frequency and size of the droplet can be controlled.When the applied disturbance amplitude reaches a certain value,the generation frequency of the droplet can be synchronized with the disturbance frequency,and the disturbance is controlled.The resulting droplets are highly monodisperse.Therefore,in this way,droplet generation in the microchannel can be controllable.