Droplet Formation And Internal Mixing Enhancement In Microchannel

With the rapid development of Micro-Electro-Mechanical System(MEMS), the miniaturization and integration of apparatus has already become one themain directions for modern equipments. Microfluidic chip is a microchip, in which the mixing, separation, attenuation and detection are integrated. This technology is widely used in biochemistry medicine, bioscience and some other fields. Nowadays, droplet based microfluidics is a new branch of microfluidics. In this type of microfluidics, each droplet can be treated as a independent microreactor. Compared with traditional chemical reactors, droplet based microfluidics can offer several advantages, such as low consumption of reactants, rapid response and no cross contamination and so on. Thus, it has great potential for chemical synthesis, nanomaterial preparation, drug sorting and cell analysis. It has been known that the droplet formation and internal mixing play important roles in the realization of the above applications. However, there still lack deep and systematic studies. Aiming at this target, this thesis is directed to the study of the droplet formation and internal mixing performance.In this thesis, the droplet formation and the internal mixing enhancement have been studied by visual experiments. First of all, the droplet formation in a T-shaped microchannel with the dimension of 200 μm ×170 μm is investigated using silicon oil and DI water as continuous phase and disperse phase respectively. The effects of the velocity of continuous phase, velocity of disperse phase and viscosity of continuous phase on the droplet formation are explored. Moreover, a novel method to produce ultra-small droplet in large size micochannel via the photothermal effect of IR laser is put forward. Secondly, the effect of spiral microchannel to enhance the mixing in droplet is studied. At last, we use the micro Particle Image Velocimetry(μPIV) technology to measure the flow fields in the droplet under the side shear flow and discuss its effect on the mixing performance.By doing the above works, Main conclusions are obtained in this thesis as follows.①It is found that in T-shaped microchannel, the droplet size decreases with the increase of continuous flow rate, increases with increasing the disperse phase flow rate. It is also found that increasing the viscosity of continuous phase can decrease the droplet size. In addition, a new method to generate ultra-small droplets is proposed based on the IR laser photothermal effect induced evaporation and condensation. It is found that during the working process, liquid water is evaporated due to the IR laser photothermal effect to produce water vapor, which is sheared by continuous phase flow to form water vapor bubbles. When the water vapor bubbles flow with the main stream, the heat of bubbles is released to the continuous phase and then the condensation occurs. Finally, ultra-small droplets can be generated in relatively large microchannels. Because the ultra-small droplets are formed by the condensation, the droplet size highly depends on the bubble size. Experimental results show that the bubble size increase with the increase of the laser power and reduces with the increase of the velocity of continuous phase.② By studying the droplet mixing efficiency in a straight micromichannel with a T junction and in a spiral microchannel, it is found that the droplet mixing efficiency in a straight micromichannel improves with increasing the continuous phase flow rate and decreases with increasing the disperse phase flow rate. The spiral microchannel also shows similar trend. More importantly, experimental results show that regardless the time and distance, the spiral microchannel can yield the enhanced mixing efficiency than did the straight microchannel with a T junction because of the second flow induced by the spiral structure.③ When the formed droplet is sheared by side shear flow, because the droplet is under the effect of the shear flow, its internal flow field is changed with the shear flow. In the meantime, the flow velocity is increased, leading to the enhancement of the mixing performance. Larger flow velocity of the shear flow and continuous shows more significant effect.