Study On Formation Of Droplet And Bubble In A T-Shaped Microchannel Junction

Micro-droplets and micro-bubbles find many applications in various fields, such as biological, medical, chemical and petroleum industry. Fluid flow at micro scale is with some new phenomena. This study aims to investigate the formation of droplets and bubbles in T-shaped microchannels through numerical, analytical and experimental methods.Firstly, a three-dimensional (3D) two-phase (fluid/fluid) flow model is developed using the Level Set surface-tracking method, based on the continuum model and taking the surface wettability into consideration. This model is then solved by a CFD software for droplet formation in T-shaped microchannels. It is found that:(1) droplet size decreases with the flow rate increasing of continuous phase for fixed flow rate of the dispersed phase; (2) the effective diameter of droplets increases as the surface tension force increases; (3) the effective diameter of droplets decreases as viscosity of the continuous phase increases; (4) the droplets size increases as the contact angle between the channel wall and the dispersed phase increases. Therefore, wall material with bigger contact angle can be used to obtain bigger droplets under the same microchannel geometry; (5) the effective diameter of droplet decreases with increasing of capillary number.For the experimental part, high-speed camera is used to study the formation process and mechanism of droplets in T-junction microchannels. A scaling law correlating droplet volume and microchannel wide and depth is obtained. Flow rate ratio between the dispersed phase and continuous phase has an important effect on the droplet formation process. Under fixed physical properties of the two phase fluids, both the effective diameter and generation frequency of droplets increase as the flow rate ratio increases. When the Capillary number is relatively small, a good linear relationship governs the droplet aspect ratio and the flow rate ratio. The specific correlation equation is also obtained. When the flow rate ratio remains constant, the gradual increasing of continuous phase viscosity decreases droplet size significantly, and increases the generation frequency. The main reason is that with the gradual increasing of continuous phase viscosity, the viscous shear stress becomes the dominant force during drop formation. A good agreement between numerical and analytical results is obtained. Surface tension can not be ignored in microchannels. Increasing of surface tension will increase the droplet effective diameter, due to the increasing of the Laplace force and Marangoni effect.Finally bubble formation mechanism is experimentally studied in a confined T-shaped microchannel. With fixed continuous-phase flow rate and fluid properties, the effective diameter of bubbles becomes larger as the dispersed-phase pressure increases, while the effective diameter of bubbles decreases as the continuous-phase flow rate increases under the same dispersed-phase pressure. The reason is that when continuous-phase flow rate increases, the shear force and inertia force increase, which accelerates the pinch-off speed of the bubble neck. When the continuous-phase viscosity increases under fixed dispersed-phase pressure, effective diameter of bubbles gradually decreases. With the gradual increasing of surface tension, the effective diameter of bubbles increases. Laplace and Marangoni effect cased by surface tension gradient plays an important role during bubble formation, similar to that for the droplet formation.In this work droplet/bubble formation in T-shaped mircochannels is studied. The effect of fluid properties and surface wettability of microchannel wall is investigated. The results obtained will benefit the designing of micro-droplet and micro-bubble generator and provide analytical basis for further numerical simulation and experimental study.