| The past decade has witnessed the considerable development of microfluidic and micromixing technology, which have been developed as a powerful platform for many different applications, such as biomedical diagnostics, engineering of tissues, drug development and chemical synthesis or assembly. Because there are several obvious advantages of microfluidics that have been proved over conventional bulk reaction. However, there is a specific small Reynolds number of the liquid flows in such microchannels, where turbulent mixing does not take place. Consequently, it is really significant for scientists to carry out experiments, in order to know the mechanism of droplet movtion, the way to enhance mixing effect under such small Re, and how to utilize mixing technique in the area of chemical systhesis and assembly. In this thesis, researches were conducted to solve some of the above problems.In order to understand the properties and movement of liquid, especially, single droplets, the control of their movement was realized using a chemical gradient surface fabricated within limited space for plasma oxidation. In this method, the properites of the chemical gradient surface can be easily tuned by changing the height, width and length of the wedge space. The direction controlled motion of single droplet was realized on this chemical gradient surface. The droplet moved towards the surface with smaller contact angle. In this work, the maximum velocity of the droplet is 0.64 mm/s, where the contact angle is 70°. This method is robust, simple, reproducible and cheap, which has a great potential in the application in biological area.Based on the understanding of the properties of a single droplet, the generation of droplets was transferred into a micro chip, where water droplets(300~1000 μm) formed continuously with good control over their size and thickness by tuning the concentration of the cross linker and reaction time. Based on the obtained droplets, a microreactor for gas capsule was fabricated using gold nanorods and ammonium bicarbonate. Due to the property of the gold nanorods of absorbing laser and giving off heat, a decomposition reaction of ammonium bicarbonate based on the heat was realized in the microdroplets. Carbon dioxide would be generated in this reaction and try to leave the main liquid droplet, thus leading to the formation of gas capsules due to the polymer on the surface of the main droplet. Functionality of the gas capsules was also realized by introducing magnetic or fluroscence nanoparticles while preparation. Addtionlly, since the gas capsules were sensitive to pressure, a pressure sensor with the detection range from 0.1 to 0.55 KPa was fabricated for the detection of localized pressure.Different passive micromixers were designed, optimized and fabricated to enhance the mixing efficiency in microdevices, including mixers with symmetrical/ asymmetrical cylindrical grooves and the array of triangle baffles. All the experimental and simulated results proved the enhancement of the mixing effect in a wide range of Re number. For the micromixers with symmetrical/asymmetrical cylindrical grooves, the mixing performance can be improved within 19 mm. While the mixing performance can be enhanced within 6.4 mm for the mixer with the array of triangle baffles. When Re is 0.1, the mixing efficiency is as high as 91.4% in the mixer with the array of triangle baffles, which is 2.02-time better than that of the common Y-mixer. When Re is 10, the mixing performance of mixers with symmetrical/ asymmetrical cylindrical grooves and the array of triangle baffles is 2.22-time, 3.87-time and 2.67-time better than that of the common Y-mixer.Based on the well understanding of miscible solutions, chemical reaction and self-assembly of products can also occur when different reactents mix with each other. The redox reaction between Ag NO3 and OPD occured while the mixing of the two solutions, where Ag NPs-modified-Po PD with different morphologies were obtained by tuning mixing conditions, such as shearing force. COMSOL software was utilized for the analysis of the possible mechanism of the mixing process as well. Furthermore, a sensor for hydrogen dioxide, with a detection range from 20 to 180 μM, was fabricated based on the Ag NPs-modified-Po PD nanospheres.Self-assembly of different building blocks based on different noble metal nanoparticles was then conducted in the microchannel, where assemblies with different morphologies or properties can be obtained with the diffusion of different flows. The investigation of different effect was also conducted, including the aspect ratio of Au NRs, polymer weight and mixing distance. Based on that, a Janus structure(0.5~4 μm) was obtained by introducing free polymer into the system, which can be utilized for the fabrication of micromotor for the transfer an d controlled release of loaded drugs. The micromotor had a tunable velocity ranging from 5 to 120 μm/s, with the change of the concentration of hydrogen peroxide. Laser was the switch for the controlled release of loaded drug. The total release will cost about 70~80 min. |
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