Investigations Of Droplets/Bubbles Generation In Microchannels With The Elastic Wall

The droplet based microfluidic, which is an indispensable method in application including the micro-detection and the micro-reaction with its advantage that operations be performed quickly using femtoliter to nanoliter in volume of reagents and samples without contamination, is a significant subcategory of the microfluidic growing with the development of the micro fabrication technology. As a crux input parameter of the microfluidic system, the droplet size is necessary to be controlled by efficient methods which are capable of producing different droplets with precisely known sizes to many applications. To provide the kind carrier to droplet microfluidic applications, this dissertation investigates the dynamic process of the interface for two immiscible fluids merging in the T-junction channel, which has the elastic thin bottom wall, documents the droplet/bubble size and then measures the deformation of the thin wall. The pressure and the velocity are monitored with the experimental and numerical method during the droplet/bubble generation. Besides, when the external vibration is forced on the wall, effects of it on the emerged droplet size are investigated by the numerical simulation. The specific research contents include:(1) The droplet generation in the T-junction channel with the elastic thin bottom wall and the deformation of the wall under the pressure from fluids flow are experimental investigated. The cross section of the main channel is measured by the confocal microscope and it is found that the elastic thin wall is deformed by the liquid flow. The deformation is varied with time as the droplet flow through the channel, that is, the elastic wall is vibrated by the pressure. The vibration frequency of the wall is measured through the images of the particle inside the wall and found to be the same with the droplet generation frequency. The droplet size performs a nice monodispersity with the elastic thin wall and this control of the wall to the droplet size is even better when the flow rate is higher or the bending rigidity of the wall is smaller.(2) The bubble generation in the T-junction channel separately with the elastic thin wall of defined dimensions and the fixed deformation thin wall is studied by the experimental method. The three layers channel is designed and fabricated, which has the groove on the bottom layer to control the position and length of the elastic wall. It is found by experiments that the closer the groove locate to the side channel, the shorter the bubble length is, when flow conditions and the elastic wall length are uniform. While the position of the elastic wall is defined, the longer wall will perform the shorter bubble is. For the channel with fixed deformation wall, the increasing length of wall induces a lower driving pressure of gas to achieve a similar bubble size. The elastic thin wall can improve the monodispersity of the bubble size but increase the flow resistance of the continuous phase, while the fixed deformation wall decreases the resistance.(3) The pressure profile of liquid and effects of the elastic sidewall during the droplet generation and flow are investigated by experimental and numerical method. The pressure evolution of the downstream is monitored for single droplet generation and flow, which proves that the interface shape and the droplet position affect the pressure on the testing section. For continuous droplets flow, the downstream pressure is affected by the break-up of the interface, the droplet quantity, the droplet position and the escaping of droplets from the outlet, where the droplet position dominate 2/3 of the whole pressure variation and the interface break-up has the limit effect in space. Besides, the uniform frequency of the thin wall vibration and the droplet generation comes from the periodic pressure variation which identifies with the droplet generation.(4) The droplet generation in the T-junction channel with the forced vibration on the wall is numerical simulated. Pressure evolutions of Pd and Pc are different with or without the external wall vibration, but their pressure difference Pd-Pc is similar with each other. The droplet generation frequency keeps 148 Hz under the flow conditions even though the vibration of walls is varied, while the droplet length occupies a wide range of about 70μm-95μm. The relationship of droplet lengths and wall vibrations is fitted and verified via amount of wall vibrating cases. When the frequency of the external wall vibration is similar with the droplet generation, the better monodispersity can also be achieved as the elastic thin wall makes.