多场驱动下的液晶流体自组装周期畴及光控应用

Liquid crystal (LC) is a mesophase media possessing the attributes of both crystals and liquids (e.g. order and fluidity), thus it can undergo various deformations and convection under the effect of external fields, self-assembling specific micro-structures (regular domains) on the micrometer scale. In fact, due to the tunability of morphology as well as modulation depth, these domains can be used to modulate light, which is of interest in the field of photonics..Currently, the most commonly encountered domains arise from the static reorientation of molecules forced by patterned external fields, without involving the flow of molecules. However, in view of the ordered and fluid properties of LC, more attention should be paid to the dynamic flow-induced domains. Actually, investigation of their static and dynamic behavior will help us to gain deep insights into understanding the underlying mechanisms including electric transport and fluid convection processes. .Conventionally, flow-induced domains are excited by thermal or electric field. However, due to the inflexibility of the electrode geometry and the relative insensitivity of LC electrical properties to the field, the resulting morphologies of domains are relatively simple and fixed, which severely limits their applications in photonics. In contrast, the flexible properties of an optical field, such as ease and remote control of intensity, phase and polarization in space-time, will open up new opportunities for investigation of the equilibrium and non-equilibrium behavior of domains. Specially, these advantages can be used to explore the mechanisms responsible for new kinds of tunable domains in novel LC materials, such as "non-standard" convection rolls in bent-core nematics. .Several solutions, including multi-domain and reversible surface alignments by photo-alignment technique, photoresponsive LC materials whose molecular geometries and helical structure can be changed by absorbing light, and photosensitive dye producing photoelectrons to form space-charge field, together with the spatiotemporal variation of light field, are exploited in this proposal to induce the desired electro-convections, so as to realize various dynamic micro-structures integrating optical control and photonic functionality with microfluidics..The optic method we proposed addresses the new phenomenon of multi-field driving, including the coupling effects of optical-, electric-, thermal-, hydrodynamic- and elastic-field. In order to clarify this nonlinear and complex process involving static reorientation and dynamic convection, a comprehensive driving-response model will be established to demonstrate the influence of multi-field on the self-organization of LC molecules.

液晶是同时具有晶体有序性和液体流动性的软物质，目前的研究集中在利用外场调控其取向性，仅涉及到静态形变而不考虑流动。然而液晶的流动性及其在外场诱导下的自组装畴行为更应该得到关注，因为这种流体行为能完整地体现液晶作为取向有序和各向异性流体的特征。随着畴的多样性增加特别是非标准畴现象的出现，传统的电场调控手段已不敷使用。因此我们将使用时空分布的光场，结合光响应液晶、光敏杂质以及表面光学活性层来调控液晶的对流行为进而探索动力学机制，并实现微米尺度的动态周期畴结构。这种可调控的流体效应，涉及到光场、电场、弹性场、流体场和热场等相互作用，包含静态的取向形变、电荷输运和动态的对流等物理过程，需要建立多场耦合驱动模型来描述其调控机理。传统的研究中光场仅仅被动地探测流体行为而不能对其进行干涉，本项目则提出了一种光场对流体效应进行主动调控的新方法，并能实现集光场时空调控与自组装有序结构于一体的微流控光子器件。

液晶是同时具有晶体有序性和液体流动性的软物质，其自组装效应最能体现液晶作为取向有序和各向异性流体的特征。一直以来都是使用电场来激发出液晶中的周期畴结构，而我们则通过复合场（光场、电场和热场）的激励来观察液晶由于分子自组装效应诱导的多种周期畴结构。这些微结构具有完全不同的空间形貌和时间动力学行为，并且能够在改变外场条件的情况下相互转化。研究的主要工作包括：（1）光场和电场协同操控液晶自组装光栅的周期大小；（2）光场操控Fréedericksz transition效应的阈值大小，产生定制结构的光栅；（3）光控表面取向技术的改进及其对液晶自组装效应的影响；（4）光场驱动液晶自组装光栅的周期方向改变及应用。.为描述液晶微结构形貌随外场的变化关系，我们在液晶连续介质弹性理论中引入压电效应诱导的极化能、手性效应诱导的扭曲弹性能、二能级跃迁模型，能够理解由于液晶分子结构和物理参数变化带来的新型物理效应。.由于微结构的尺寸大小在微米量级，正好适合于可见光波段，并且微结构的周期大小和方向均可以根据实际需要通过外场进行操控，因此能够实现各种可调的相位光栅器件，在光子器件领域有很好的潜在应用。