Control of Surface Anchoring Energy of Nematic Liquid Crystals via Nano-spikes of Reactive Mesogen

Liquid crystals (LCs) possess both long-range molecular ordering and fluidity, leading to anisotropic properties (e.g. elasticity, birefringence). Because the material characteristics are controllable via reorientation of LCs in response to a variety of stimuli, LCs have been widely used to design stimuli-responsive materials, including triggered release system, sensor, and actuators [1-3]. In this context, precise control on the interfacial properties (e.g., surface anchoring energy, initial orientation of molecules) is significantly important because it determines sensitivity and response time of LCs. Herein, we propose a simple and versatile approach to manipulate the surface anchoring energy (W) of LCs by doping reactive mesogens (RMs) into a vertical alignment layer of polyimide (PI). When the sandwich cells assembled with two substrates coated with RM-PI is irradiated by UV light and then LC is injected (Fig. 1a), we observe the polar anchoring energy (Wp) to increase with irradiation time (Fig. 1b). We demonstrate the result to be associated with the formation of RM nano-spikes on RM-PI. Specifically, first, we found RM monomers grew perpendicular to the substrates and form nano-spikes. The nano structures cause the interdigitation of LCs, thus enhancing Wp [4]. Second, based on surface retardation measurements, we showed a director of RMs in the spikes is aligned parallel to the normal direction of substrates. The internal orientation of RM in our nano-spike also contributes to the increase of Wp, because previous studies demonstrated the surface orientation of RMs determine the orientation of contacting LCs [5]. Overall, our findings provide additional options for programming the properties of stimuli-responsive LC materials. Furthermore, it offers access to the wide range of LC molecules for responsive materials because the proposed method can induce the vertical orientation of LCs with nontrivial shape (e.g., bent-core molecules), which are challenging to assume vertical alignment with previous alignment techniques [6].1

Regulating Surface Anchoring Energy of Nematic Liquid Crystals using Reactive Mesogen

Liquid crystals (LCs) are an intermediate phase which possess both long-range molecular ordering and fluidity. Due to these unique properties, LCs have been widely used to design stimuli-responsive materials, including triggered release systems, sensors, and actuators. In this context, precise control of interfacial properties (e.g., surface anchoring energy, the orientation of molecules) is significantly important because it determines the sensitivity and responsivity of LCs. In this regard, we proposed a simple and versatile method to manipulate the surface anchoring energy. Doping reactive mesogen (RM) into the vertical alignment polyimide (PI) layer allows us to control their polar anchoring energy with LCs. We observe the polar anchoring energy to increase with irradiation time of UV onto the RM doped PIs. In addition, we demonstrate the result to be associated with the formation of RM nano-spikes on RM-PI that facilitate vertical anchoring. we found RM monomers grew perpendicular to the substrates and form nano-spikes which cause the interdigitation of LCs. Also, the internal orientation of RM in our nano-spike contributes to the increase of polar anchoring energy.2

Surface Encapsulation of Liquid Crystals by Polymeric Amphiphiles via Thermal Trigger

2

Design of Interactive Meta-Holographic Display Using Liquid Crystallinity

The arrays of subwavelength-scaled nanostructures on a surface, often called metasurfaces, have made advances in flat optics because of their potential to display programmable hologram and miniaturize optical components. As previous metasurface systems are passive, however, their practical applications have been impeded. Accordingly, recent efforts have focused on the realization of active metasurfaces that can autonomously switch homographic images upon arrival of triggers. Nevertheless, the full potential of active metasurface system has yet to be realized due to the limitation of previous approaches, including limited design of nanostructures, complex fabrication process, and slow response. Here, we propose a simple and versatile approach to enable dynamically tunable metahologram systems by leveraging the optical anisotropic and stimuli-responsive nature of liquid crystals (LCs). We demonstrate the new class of active metahologram, a thin-layer of LCs integrated with multiplexing2

Design of Liquid Crystal Elastomer Droplets with Versatile Actuation Properties

In nature, twisting motions of many organisms enable complex mechanical function such as swimming, crawling, climbing and energy storage. Liquid crystal elastomers (LCEs) are anisotropic polymeric materials which are promising candidates for the soft-actuator owing to their capability of stimuli-responsive properties. In this work, we report the highly reversible three-dimensional torsional actuation of LCE droplet using heat. LCE droplet was obtained by removing non-polymerizable mesogen after simply mixing non-polymerizable and polymerizable mesogen. We verify the twisted configuration and twisting mechanism of LCE with fluorescence confocal polarizing microscopy. In addition, we designed asymmetric LCE droplet by phase separation of LCE and by simply tuning droplet configuration. This versatile twisting actuation of LCE droplet give us fundamental of shape control of LCE and design self-swimming particles. This work was supported by the NRF (2021R1A2C2095010, 2022M3C1A3081312).2

Self-Regulating Surface Encapsulation of Liquid Crystals

Liquid crystals (LCs) are complex fluids with both crystal-like long-range molecular ordering and liquid-like fluidity, leading to anisotropic properties (e.g., birefringence and elasticity) [1, 2]. This combination of properties provides the ability to change molecular orientations and thus report corresponding optical signals in response to a variety of physical and chemical cues, including surface modifications, pressure, and electric fields. In this work, we report a simple and versatile approach to produce LC droplets encapsulated by polymeric amphiphiles on substrates with controllable size and density upon thermal trigger. As shown in Fig. 1(a), the simplest system to observe the new approach is to place LCs between two glass substrates coated with poly (octadecyl methacrylate) (PODMA). At a room temperature (T0 = 25oC), the cells show a dark texture between crossed-polarizers (Fig. 1a) indicating a vertical orientation of LCs at substrates induced by the long aliphatic tails of PODMA. When the substrates were heated above 30oC and cooled back to T0, however, we observe the appearance of birefringent domains at substrates corresponding to radial configuration of LCs within the domains (Fig. 1b). The results suggest PODMA can play a role in thermally triggerable local encapsulation of LCs (and thus local reorientation of LCs). Specifically, with a use of polarizing optical and confocal fluorescence microscopy, we demonstrate the underlying mechanism of the thermo-optical effects to be related to thermal-induced diffusion of PODMA into LCs and their micelle formation. We found the size and density of locally encapsulated LC droplets to be controllable via heating temperature, rate of temperature changes, type of LCs, and length of aliphatic tails in PODMA, which also support our underlying mechanism. We envisage that the new thermo-optical phenomena will find applications in a variety of field, including sensors, photonics, surface modification, and structure patterning. In addition, it will be of interest to use the described mechanism to design the system that are triggered by various triggers beyond temperature.1

Design of Interactive Metahologram via Liquid Crystallinity

The arrays of subwavelength-scaled nanostructures on a surface, often called metasurfaces, have made advances in flat optics because of their potential to display programmable hologram and miniaturize optical components. Because previous metasurface systems are passive, however, their practical applications have been impeded. Accordingly, recent efforts have focused on the realization of active metasurfaces that can switch holograms upon triggers via using, for example, phase-change materials, mechanical actuations, and chemical reactions. Nevertheless, the full potential of active metasurface system has yet to be realized due to the limitation of previous approaches, including limited design of nanostructures, complex fabrication process, and slow response. Here, we propose a simple and versatile design rule to enable dynamically tunable metahologram systems by leveraging the optical anisotropic and stimuli-responsive nature of liquid crystals (LCs). We demonstrate the new class of active metahologram, a thin layer of LC integrated with multiplexing metasurface, to autonomously sense a programmed stimulus (e.g., electric field, temperature, pressure, toxic gas) and dynamically switch the holographic images [1,2,3]. These attribute provides insight into the rational design of interactive meta-hologram display that enable their full potential of multifunctional active devices. This work was supported by the Korea National Research Foundation (NRF-2021R1A4A1030944 & 2021R1A2C2095010).1

Design of Selective Chemical Reaction in Liquid Crystals via Organic Ionics

Liquid crystals (LCs) are anisotropic fluids that have both long-range molecular ordering of crystal and fluidity of liquid. This combination of properties enables LCs to be the unique material that can sense a variety of stimuli, including nano- or molecular level events, and signal them into macroscopic optical output. The capability of LCs has provided a path to the design of promising reconfigurable materials in various field, such as display, optofluidics, and sensing. Particularly in the field of chemical sensing, however, their full potential has been hindered by their poor selectivity to a target chemical specie because only the chemical affinity between LCs and target chemicals has been considered. In this work, we propose simple and versatile design rules to control not only selectivity but also sensitivity by decorating the interface of LC films with organic ionics (OIs). We demonstrated the OI-LC sensors to selectively sense and optically report the exposure of a specific gas molecule (acetic acid) even at very low concentration (< 1 ppm). In addition, we experimentally and theoretically showed that their characteristics are precisely controllable by modulating the length of carbon chain and type of counter ion in OIs. This work was supported by the Korea National Research Foundation (NRF- 2021R1A4A1030944 & 2021R1A2C2095010).1