Study of particle suspensions in microfluidics for the development of optical devices

摘要

The vision of this PhD research project is to create a microfluidic system for controlling the locations of suspended particles in order to form three dimensional (3D) objects on demand. To realize this, the author implemented a microfluidic system that can apply suitable and desired forces on particles on demand. Particles of various refractive indices were placed close to each other in order to form a media having reconfigurable and tuneable properties. Light was coupled into such well-controlled particles in order to form dynamically tuned objects suspended in liquid such as optical waveguides. The dielectrophoretic (DEP) force was used for manipulating the locations of particles as it is capable of focusing and scattering suspended particles from pre-determined locations. Additionally, when combined with hydrodynamic forces, the DEP force was able to form densely packed areas of such particles with non-turbulent boundaries. The research was implemented in three stages. In the first stage, the author utilized a platform consisting of a microfluidic system integrated with DEP microelectrodes, microfluidics and optical peripherals for the coupling of light. Light was directly coupled into densely packed silicon dioxide (SiO2) particles with diameters of 230 and 450 nm, respectively. Light was transmitted via the closely packed 230 nm particles and in contrast was significantly scattered by the 450 nm particles. The outcomes, which were resulted from this initial stage, were the first demonstration of a dynamically tuneable optical waveguide based on the DEP focused particles in microfluidics. In the second stage of his research, the author integrated a multi mode polymeric waveguide into the microfluidic system. Tungsten trioxide (WO3) and SiO2 particles with diameters of 80 and 450 nm were investigated. The findings demonstrated that the densely packed WO3 particles were able to couple light from the polymeric waveguide, while the SiO2 particles did not affect the transmission of the optical signals significantly. The investigations of the second stage platform resulted in the first demonstration of optical waveguide tuning based on DEP focused particles. Finally, in the third stage of this research, the author implemented a quasi single mode polymeric waveguide integrated with the microfluidics. The author used WO3, zinc oxide (ZnO) and SiO2 particles with diameters of 80, 50 and 72 nm, respectively. Under the DEP force, these particles were able to interact with the optical guided modes. The results show that the WO3 particles were capable of forming layers of packed particles with anti-resonant characteristics. In particular, the fundamental mode was strongly coupled to the packed WO3 particles. However, under certain particle focusing conditions, the first order mode was anti-resonant to the closely packed WO3 particles as it was largely isolated. These findings were the first demonstration of the coupling and manipulation of optical guided modes using DEP focused particles with resonant and anti-resonant behaviors.