Dynamic coloration of polymerized cholesteric liquid crystal networks by infiltrating organic compounds

Structural color originates from the interaction between light and periodic submicron structures and has led to many unique functions in nature, inspiring the development of material science. In sensing changes in the external environment, animals use color changes to transmit signals and accomplish behaviors such as communication, camouflage, vigilance, and courtship. Responsive photonic crystals are considered to be one of the best artificial materials for preparing color-changing functions, i.e., when induced by external stimuli, photonic crystals change their own periodic structure to modulate the properties of light waves, producing color changes in visual effects associated with wavelength changes. Among them, the cholesteric liquid crystal (CLC) exhibits unique selective reflective properties due to its self-assembled helical structure forming a periodic arrangement of dielectric constant and refractive index, is endowed with advantages such as polarization-dependent generation of structural color and dynamic tunability, and is rapidly developing in research fields such as dynamic display, information storage, and optical security. People have been changing the working wavelength and reflectivity by using optical, electrical and thermal responsive chiral molecules, multilayer composite structures, phase transitions and many other methods. However, there are still problems such as easy destabilization and limited regulation mechanisms, so it is still a challenge to design and prepare reflective liquid crystal photonic devices with multiple responses, real-time reconfigurability, and dynamic broadband tunability.

Microfluidics is a technology for the precise control and manipulation of microscale fluids, and its greatest advantage is that it allows flexible combination and scale integration of multiple unitary technologies on an overall controllable tiny platform. The selectivity and manipulability of microfluidic components can confer novel response properties to CLCs, which is one of the effective strategies to address the above challenges.

Recently, the research group led by Prof. Lu-Jian Chen at Xiamen University (XMU) achieved real-time continuous modulation of the working wavelength in anisotropic polymerized CLC (PCLC) films with a dynamic range of up to 210 nm by using a microfluidics-based "wash-out/refill" strategy. They also demonstrated structural color patterns with reversible changes and multipitch gradients. This work was published in Chinese Optics Letters 2022, Vol. 20, No. 9 (Y. Cao, et al., Dynamic coloration of polymerized cholesteric liquid crystal networks by infiltrating organic compounds) and was selected as the cover of the issue.

Principle: The acrylate-based liquid crystal monomers are cross-linked under UV initiation to form a polymer backbone, which can avoid the collapse of the polymer network during the "wash-out/refill" process and stabilize the chiral microstructure of CLCs. After integrating such chiral PCLC films into microchannels, the reversible filling of miscible organic solutions results in a continuous change in the average refractive index n_av, and a dynamically tunable photonic band gap is obtained, corresponding to rich and vivid structural colors.

The microfluidic device integrated with PCLC film is shown in Figure 1(a). By observing the macroscopic changes in the reflectance color and measuring the reflectance spectrum, the dynamic diffusion process of the flow field within the microchannel is characterized in real time. As shown in Figure 1(b), alternate pumping of the nematic phase liquid crystal E7 (n = 1.64) and benzyl alcohol (BA, n = 1.54) caused a real-time change in the refractive index of the system, making the reflection center wavelength adjustable between 450 nm and 600 nm by fluid filling. As shown in Figure 1(c), the shift of the reflection center was obtained in the range of 410 to 620 nm by the diffusion of ethanol vapor. The diffusion forms a concentration gradient leading to the coexistence of multiple pitches in this composite system. As illustrated in Figure 1 (d) and (e), the structural color patterns with reversible changes and multipitch gradients are obtained using the above two approaches, respectively.

The XMU researchers said that, unlike common scenarios using external stimuli such as light, electricity and heat, the work is a preliminary exploration of modulating broadband reflective photonic devices based on CLCs with the help of microfluidic technology. The current focus of the group is to introduce optical concepts such as geometric phase, construct microfluidic-based liquid crystal planar optical components, and expand the scope of "Liquid Crystal on Chip" for applications such as anti-counterfeit/identification and sensing.

手性液晶：光流控结构色的大范围动态调控

结构色源于光与周期性亚微米结构之间的相互作用，是大量有序结构对不同波长的光散射、衍射或干涉后产生的各种颜色。在自然界中基于结构色也形成了许多独特的功能，如动物通过颜色变化来传递信号，完成通信、伪装、警戒、求偶等行为，这启发着人类材料科学的发展。

响应性光子晶体被认为是制备具有变色功能的最佳材料之一。即在外界刺激诱导下，光子晶体改变自身周期结构调控光波特性，在视觉效果上产生与波长变化关联的颜色变化。其中，胆甾相液晶（Cholesteric Liquid Crystal, CLC）由于自组装的螺旋结构形成介电常数及折射率周期性排布，展示出独特的选择性反射特性，并被赋予偏振依赖的结构色产生与动态可调谐等优势，在动态显示、信息存储、光学安全等研究领域发展迅速。

目前，人们已通过使用光、电、热等响应性的手性分子、多层复合结构、材料相变等诸多方式改变其工作波长与反射率。然而，仍然存在易失稳与调控机制较为单一等问题。因此，设计并制备兼具多重响应、实时可重构、动态宽带可调的反射型液晶光子器件仍是挑战。

微流控是一种精确控制和操控微尺度流体的技术，其最大优势是可以将多种单元技术灵活组合和规模集成于整体可控的微小平台。微流体成分的可选择性和可操纵性赋予胆甾相液晶新颖的响应特性，是应对上述挑战的有效策略之一。

厦门大学陈鹭剑教授团队借助基于微流控技术的“浸洗-再填充”策略，在各向异性胆甾相液晶聚合物（Polymerized CLC，PCLC）薄膜中实现工作波长的实时连续调制，动态范围高达210 nm，并展示了可逆变色和多色渐变的结构色图案化应用。相关工作发表在Chinese Optics Letters 2022年20卷第9期上（Y. Cao, et al., Dynamic coloration of polymerized cholesteric liquid crystal networks by infiltrating organic compounds），并被选为当期封面。

原理：丙烯酸酯类液晶单体在紫外光引发下发生化学交联形成特定的聚合物骨架，可以避免“浸洗-再填充”过程中聚合物网络的坍塌，对胆甾相液晶的手性微结构起稳定作用。将此类手性PCLC薄膜集成于微通道后，填充互溶的有机溶液使平均折射率n_av产生连续变化，获得动态可调谐的光子带隙，对应丰富且生动的结构色。

该研究工作采用图1（a）中集成了PCLC薄膜的微流控芯片。通过观察反射色的宏观变化和测量反射谱对微通道内的流场动态扩散过程进行实时表征，见图1（b）。研究发现交替泵入向列相液晶E7（n =1.64）和苯甲醇（BA，n =1.54）使体系的平均折射率n_av发生实时改变，通过流体填充使得反射中心波长在450 nm~ 600 nm之间连续可调。从图1（c）中可以发现，通过乙醇蒸汽的扩散获得了反射波长的移动范围为410 nm~620 nm。扩散形成浓度梯度同时还导致复合体系中多螺距共存。图1（d）和（e）分别为使用上述两种方式获得的可逆变色和多色渐变的结构色图案。

展望: 研究人员表示，有别于常见的光、电、热等外场响应场景，该工作是借助微流控技术调控手性液晶宽带反射型光子器件的初步探索。目前该团队的工作重点是引入几何相位等光学概念，构建基于微流控的液晶平面光学元件，并面向防伪识别、传感等诸多应用致力于拓展“片上液晶”的范畴。