Integration of liquid crystals molecules in optofluidic devices

It is in the context of miniaturization that arises the microfluidics concept. It is not uncommon in chemical, biological or medical analysis laboratories to manipulate or to mix fluids while measuring the results with filters, spectrometers or all other equipment. Although standardized processes allow reliable results during manipulation in laboratory environment, they can't fulfil all the expectations. For instance, how to measure the ratio of red blood cells in a small droplet when one only gets access to a centrifuge for the cell-serum separation? It is with the goal of working with few quantities of reactants that the first microchips have been developed. They embedded components that were able to reproduce the experiments previously done in a laboratory environment, such as micro pumps, valves, bubbles generator or integrated electrodes. It leads to a better control on the manipulated fluids. The reactants being carried through channels no thicker than a hair, the flow is laminar resulting in stable system. Even though the microfluidics answer most of the miniaturisation needs, they can't enable direct measurements on-chip which results in the use of laboratory equipment, such as microscopes, spectrometers or light sources. This acts like a brake on their use on the ground, away from the laboratories. To address this problem, it could be a wise thing to integrate the optical measurements directly on the chip. That is how the optofluidics emerged, which is the merger between the microfluidics and the photonics. This lead to the development of optical or photonic components integrated to a microfluidic environment, such as the liquid-liquid waveguides, the liquid lenses, the optofluidic dye lasers or the photonic crystal fibre. In taking account of the development of novel optofluidic components, this thesis proposes and demonstrates the feasibility of using liquid crystals in such devices. Because of their fascinating soft-condensed matter nature and unique optical properties they seem to be a good candidate. Furthermore, they may rotate by the application of external fields, thereby enabling optical modulation. This principle has had a profound impact on our daily lives through the plethora of liquid-crystal displays (LCD) in use around us. This work demonstrates that these anisotropic fluids are easily integrated and aligned in a microchannel made of polydimethylsiloxane, a widely spread polymer in the prototyping of optofluidic components. Under microflows, the liquid crystals molecules tend to reorient, hence, changing their optical properties. This concept is used to develop an optofluidic modulator based on peristaltic nematogen microflows and optofluidic tunable color filters based on liquid crystal microflows.


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