Study On Microfluidic Droplet Technology Based On Flow Focusing

Droplet based microfluidics is a technology which can manipulate small volume liquids based on lab on a chip. It is a rapidly growing interdisciplinary field of research combining soft matter physics, biochemistry and microsystems engineering. It was broadly applied in fast analytical systems, the synthesis of advanced materials, protein crystallization and biological assays for living cells. This platform has time dimensional scaling benefits that enable controllable and rapid mixing of fluids in the droplet reactors, resulting in decreased reaction times. This benefit, coupled with the precise generation and repeatability of droplet operations, has made the droplet-based microfluidic system a potent high throughput platform for biomedical research and applications. In addition to being used as microreactors ranging from the nanoliter to femtoliter range, droplet-based systems have also been used to synthesize particles and encapsulate many biological entities for biomedicine and biotechnology applications. Due to unique advantages of droplet-based systems, this technology has the potential to provide novel solutions to today’s biomedical engineering challenges for advanced diagnostics and therapeutics. Among all the microdroplet generation techniques, flow focusing has been proven to be a very effective and broad-spectrum method in generating the droplets and bubbles. In general, the principle of flow focusing induced droplet is based on a coflow of inner phase and outer phase through an orifice, where the inner phase is pinched off by the outer phase to release droplets. A lot of work has been conducted on droplet generation and manipulation in flow focusing devices. However, among all the parameters related to this system, further study on the geometry is still needed. Besides, little work has been done about the nonlinear phenomenon happened in flow focusing device. In this dissertation, we firstly discussed all the controlling parameters. Then focused on the rich nonlinear phenomenon happened in the process of droplet generation. Finally, we gave some applications of droplet microfluidic, for example, magnetic control of droplet size, magnetic separation of non-magnetic and magnetic droplets, droplet split and particles synthesis. This work was important for further understanding the breakup mechanism of droplet. The details are as follows:1. An easy-to-realize microfluidic chip processing platform was built to facilitate the processing of microfluidic chip. In our work, a cheap collimated UV lamp instead of an expensive lithography machine was used. This technique greatly reduces the cost of production, in the context of the large investment in the early stage of microfluidic processing and the high threshold of the project. By using of the simple platform, a microfluidic chip was perfectly created to meet the requirements. For the process of long-term exploration, we used a series of measures to greatly shorten the chip molding cycle and saving time cost. This part of the work has an important role for the development of microfluidic.2. The effect of orifice size, two phase fluid properties, and two phase flow characteristics on droplet formation regimes were investigated by experimental and numerical methods. All the regimes of droplet breakup were concluded and the internal pressure field, velocity field and the evolution of two phase interface were analyzed. When the velocity of the continuous phase is less than a certain limit, the growth of the throat length will lead to the increase-decrease of the size of the droplet. When the velocity of the continuous phase is larger than the limit value, the droplet size decreases monotonously with the increase of throat length. For the throat width, the droplet size increases monotonously with the increase of throat width. When the two phase fluids belong to the low viscosity category, the whole flow process is dominated by the droplet break mechanism controlled by the flow rate. The droplet size increases with the increase of the viscosity of the dispersed phase. Most of the droplet fragmentation modes in FF structures, including squeezing, dripping, jetting, satellite and threading are summarized. The COMSOL level set method was used to simulate the droplet fragmentation in the FF pipe. The pressure distributions and velocity distributions in the pipe are obtained. It is proved that the 3D pipe is more unstable because of its unique curved surface shape, so the fracture of the droplet will be faster than that of 2D, resulting in the formation of liquid drops a little smaller. This part of the work for the FF chip design and control of droplet size provides a reference.3. The non-linear phenomena in the FF pipeline were introduced and the intrinsic mechanism of the phenomena was analyzed. The period 1-period 2 transition at low flow rates was described and the transition became period 1-period 2-period 4-chaos in high-speed. It is shown that the bifurcation of the droplet size is more likely to occur in the case of long throats at low flow rates. But its internal dominant force is capillary stress. It is concluded that the bifurcation is dominated by the inertial force and the capillary force. At high flow rates, inertia dominates, resulting in a series of nonlinear processes. This part of the work provides the support of microchip design.4. Some applications based on droplet microfluidics were introduced, including magnetic controllable droplet generation, magnetic separation, droplet splitting and synthesis of particles. First, we introduced the manipulation of ferrofluid droplets by using a microfluidic flow focusing device equipped with a magnetic tweezer. Besides the traditional flow rate controlling method, the magnetic field also can be applied to control the size of the droplet. Two major effects in magnetic manipulation process: magnetoviscous effect and magnetic drag effect were studied. Under a fixed flow rate (Qc=1mL/h, Qd=0.2mL/h), the average sizes of ferrofluid droplets were tunable from 135μm to 95μm by varying the magnetic field from OmT to 60mT. Moreover, square wave magnetic field can be used to periodically generate droplets with different sizes. These results are helpful to understand the generation mechanism of the ferrofluid droplet and supply a novel method for manipulating droplets with a predetermined size and distribution. The separation of the magnetic droplets from the non-magnetic droplets was then achieved using an external magnetic field. The droplet fission was realized by using the pipeline structure. At last, the particles with uniform particle size were prepared by the microfluidic technology. The results of this work show a wide range of applications of microfluidic droplets.