Light Driven Phase Transition In Cholestric Liquid Crystal Micro-droplets

Liquid crystal micro-droplets generally refer to liquid crystal having a small size between submicron and hundreds of micrometers.In past decades,cholesteric liquid crystal micro-droplets are one of the new development directions in the field of liquid crystals.The liquid crystal micro-droplets are subjected to limited surface and space.Such small space confinement can promote a strong competition between the surface anchoring energy and the elastic energy of the liquid crystal system which endows the liquid crystal micro-droplets some unique optical properties.For example,the conventional cholesteric liquid crystal film is one-dimensional Bragg grating while the cholesteric liquid crystal micro-droplets can form a three-dimensional Bragg grating structure.Three-dimensional Bragg grating makes cholesteric liquid crystal micro-droplets attractive in the field of soft matter photonics.In addition,the liquid crystal droplets can be dispersed in an aqueous solution,and the entire surface can interact with particular reagent outside.These properties can be used to develop a variety of active devices,including biosensors and tunable droplet lasers.Most of the devices are based on the characteristics of liquid crystals responding to external stimuli.On one hand,liquid crystals have a sensitive and rapid response to external stimuli such as electromagnetic fields,stress,heat,and illumination.On the other hand,the responsive characteristics of liquid crystal micro-droplets with different sizes may change.In spite of the various potential applications of liquid crystal microdroplets,the traditional methods to change states of liquid crystal such as electricity and temperature may not effective anymore.In this thesis,the azobenzene liquid crystal was introduced into cholesteric liquid crystal micro-droplets to achieve the light-driven phase transition between the liquid crystal phase and isotropic phase.The research focused on the necessary requirements for phase transition and the dynamics of phase transition.We found that the lightdriven phase transition in cholesteric liquid crystal micro-droplets had thresholds for both the doping concentration of azobenzene liquid crystal and the intensity of ultraviolet light.We also found that the light-driven phase transition can change the topology of “stripes” in cholesteric liquid crystal micro-droplets which corresponding to the helix superstructure.Cholesteric liquid crystal micro-droplets tend to be onionlike concentric layered structure when they have parallel anchoring for molecules near droplet surface.The onion-like structure which also named radial spherical structure results from the competition between surface anchoring energy and elastic energy.Light-driven phase transition could break the competition and reduce the symmetry of the structure in cholesteric liquid crystal micro-droplets which controllably produced some unique structures.The particular structures of cholesteric liquid crystal micro-droplets are closely related with their optical properties,which may bring new applications for the cholesteric liquid crystal micro-droplets.