Tunable organization of cellulose nanocrystals for controlled thermal and optical response

The biorenewable nature of cellulose nanocrystals (CNCs) has opened up new opportunities for cost-effective, sustainable materials design. By taking advantage of their distinctive structural properties and self-assembly, promising applications have started to nurture the fields of flexible electronics, biomaterials, and nanocomposites. CNCs exhibit two fundamental characteristics: rod-like morphology (5-20 nm wide, 50-500 nm long), and lyotropic behavior (i.e., liquid crystalline mesophases formed in solvents), which offer unique opportunities for structural control and fine tuning of thermal and optical properties based on a proper understanding of their individual behavior and interactions at different length scales. In the present work, we attempt to provide an integral description of the influence of single crystals in the thermal and optical response exhibited by nanostructured films. Our approach involved the connection of experimental evidence with predictions of molecular dynamics (MD) simulations. In order to assess the effect of CNC orientation in the bulk response, we produced cellulose nanostructured films under two different mechanisms, namely, self-organization and shear orientation. Self-organized nanostructured films exhibited the typical iridescent optical reflection generated by chiral nematic organization. Shear oriented films disrupted the cholesteric organization, generating highly aligned structures with high optical transparency. The resultant CNC organization present in all nanostructured films was estimated by a second order statistical orientational distribution based on two- dimensional XRD signals. A new method to determine the coefficient of thermal expansion (CTE) in a contact-free fashion was developed to properly characterize the thermal expansion of thin soft films by excluding other thermally activated phenomena. The method can be readily extended to other soft materials to accurately measure thermal strains in a non-destructive way. By evaluating the magnitude of film CTEs relative to those of individual CNC crystals, we highlighted the significant role played by crystalline interfaces. Likewise, after measuring the thermal conductivity of a single crystal and CNC films having multiple organizations, the interfacial thermal resistance arose as a governing factor for heat transport. We will offer further insights into the intricate connection of thermal and optical properties towards a future efficient manufacture and optimal CNC based-materials desig

From nanoscopic to macroscopic photo-driven motion in azobenzene-containing materials

AbstractThe illumination of azobenzene molecules with UV/visible light efficiently converts the molecules between trans and cis isomerization states. Isomerization is accompanied by a large photo-induced molecular motion, which is able to significantly affect the physical and chemical properties of the materials in which they are incorporated. In some material systems, the nanoscopic structural movement of the isomerizing azobenzene molecules can be even propagated at macroscopic spatial scales. Reversible large-scale superficial photo-patterning and mechanical photo-actuation are efficiently achieved in azobenzene-containing glassy materials and liquid crystalline elastomers, respectively. This review covers several aspects related to the phenomenology and the applications of the light-driven macroscopic effects observed in these two classes of azomaterials, highlighting many of the possibilities they offer in different fields of science, like photonics, biology, surface engineering and robotics

Spontaneous electric-polarization topology in confined ferroelectric nematics

Topological spin and polar textures have fascinated people in different areas of physics and technologies. However, the observations are limited in magnetic and solid-state ferroelectric systems. Ferroelectric nematic is the first liquid-state ferroelectric that would carry many possibilities of spatially distributed polarization fields. Contrary to traditional magnetic or crystalline systems, anisotropic liquid crystal interactions can compete with the polarization counterparts, thereby setting a challenge in understating their interplays and the resultant topologies. Here, we discover chiral polarization meron-like structures during the emergence and growth of quasi-2D ferroelectric nematic domains, which are visualized by fluorescence confocal polarizing microscopy and second harmonic generation microscopies. Such micrometre-scale polarization textures are the modified electric variants of the magnetic merons. Unlike the conventional liquid crystal textures driven solely by the elasticity, the polarization field puts additional topological constraints, e.g., head-to-tail asymmetry, to the systems and results in a variety of previously unidentified polar topological patterns. The chirality can emerge spontaneously in polar textures and can be additionally biased by introducing chiral dopants. An extended mean-field modelling for the ferroelectric nematics reveals that the polarization strength of systems plays a dedicated role in determining polarization topology, providing a guide for exploring diverse polar textures in strongly-polarized liquid crystals

Liquid Crystal Anchoring Control and its Applications in Responsive Materials

Liquid crystals (LCs), owing to their anisotropy in molecular ordering, are of interests not only in the display industry, but also in the soft matter community, e.g., to direct colloidal assembly and phase separation of surfactants, and to actuate two-dimensional (2D) sheets into three-dimension (3D). The functionality and performance of LC materials extensively rely on the molecular ordering and alignment of LCs, which are dictated by LC anchoring at various boundaries. Therefore, this thesis focuses on the study of LC anchoring from both small molecule LCs and liquid crystal monomers (LCMs), which in turn guides my design of surface topography and surface chemistry to control formation of uniform LC defect structures over cm2 samples under complex boundary conditions. The ability to precisely embed defect structures in a LC material also allows me to exploit the responsiveness of LCs to create actuators and scaffolds to (dis)assemble nano- and micro-objects. Specifically, by exploiting the bulk disclinations formed in the nematic phase of 4-octyl-4’-cyanobiphenyl (8CB) surrounding the micropillar arrays, we demonstrate (dis)assembly of gold nano-rods (AuNRs) for dynamic tuning of surface plasmon resonance (SPR). Due to the highly temperature-sensitive elastic anisotropy of 8CB, the bulk disclinations and consequently the AuNR assemblies and SPR properties can be altered reversibly by heating and cooling the LC system. Then we design and synthesize a new type of nematic LCMs with a very large nematic window. Therefore, they can be faithfully aligned at various boundary conditions, analogous to that of small molecule LCs. After crosslinking LCMs into liquid crystal polymers (LCPs), we are able to study the LC assembly, director field, and topological defects using scanning electronic microscopy (SEM) at the 100 nm resolution. We then turn our attention to direct LCM alignment through controlling of surface chemistry and topography. We demonstrate the essential role of surface chemistry in the fabrication of liquid crystal elastomer (LCE) micropillar arrays during soft lithography. A monodomain LCM alignment is achieved in a poly(2-hydroxyethyl methacrylate) coated polydimethylsiloxane (PDMS) mold. After crosslinking, the resultant LCE micropillars display a large radial strain (~30%) when heated across the nematic-isotropic phase transition temperature (TNI). The understanding of surface alignment in LCMs is then transferred to LCEs with embedded topological defects. On micron-sized one-dimensional channels with planar surface chemistry, LCMs can be faithfully oriented along the local channel direction. After crosslinking, the 2D LCE sheets show pre-programmed shape transformation to complex 3D structures through bending and stretching of local directors when heated above TNI. Last, we control LC alignment and defect formation on a flat surface simply by using chemical patterns. Planar anchored SU8 is photopatterned on homoetropically anchored dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP) coated glass. By exploiting the pattern geometry, thus, boundary conditions, in combination with anisotropy of LC elasticity, we show that LC orientation can be precisely controlled over a large area and various types of topological defects are generated. Such defect structures can be further used to trap micro- and nanoparticles

Bioinspired Light Robots from Liquid Crystal Networks

Bioinspired material research aims at learning from the sophisticated design principles of nature, in order to develop novel artificial materials with advanced functionalities. Some of the sophisticated capabilities of biological materials, such as their ability to self-heal or adapt to environmental changes, are challenging to realize in artificial systems. Nevertheless, many efforts have been recently devoted to develop artificial materials with adaptive functions, especially materials which can generate movement in response to external stimuli. One such effort is the field of soft robots, which aims towards fabrication of autonomous adaptive systems with flexibility, beyond the current capability of conventional robotics. However, in most cases, soft robots still need to be connected to hard electronics for powering and rely on complicated algorithms to control their deformation modes. Soft robots that can be powered remotely and are capable of self-regulating function, are of great interest across the scientific community.In order to realize such responsive and adaptive systems, researches across the globe are making constant efforts to develop new, ever-more sophisticated stimuliresponsive materials. Among the different stimuli-responsive materials, liquid crystal networks (LCNs) are the most suited ones to design smart actuating systems as they can be controlled and powered remotely with light and thereby obviate the need for external control circuitry. They enable pre-programable shape changes, hence equipping a single material with multiple actuation modes. In addition to light, they can also be actuated by variety of stimuli such as heat, humidity, pH, electric and magnetic fields etc., or a combination of these. Based on these advantages of LCNs, we seek inspiration from natural actuator systems present in plants and animals to devise different light controllable soft robotic systems.In this thesis, inspired from biological systems such as octopus arm movements, iris movements in eyes, object detection and capturing ability of Venus flytraps and opening and closing of certain nocturnal flowers, we demonstrate several light robots that can be programmed to show pre-determined shape changes. By employing a proper device design, these light robots can even show the characteristics of selfregulation and object recognition, which brings new advances to the field of LCNbased light robots. For instance, octopod light robot can show bidirectional bending owing to alignment programming using a commercial laser projector; artificial iris is a fully light controllable device that can self-regulate its aperture size based on intensity of incident light; the optical flytrap can not only autonomously close on an object coming into its ‘‘mouth’’ but it can also distinguish between different kinds of objects based on optical feedback, and finally, integration of light and humidity responsiveness in a single LCN actuator enables a nocturnal flower-mimicking actuator, which provides an opportunity to understand the delicate interplay between different simultaneously occurring stimuli in a monolithic actuator.We believe that besides providing a deeper understanding on the photoactuation in liquid crystal networks, at fundamental level, our work opens new avenues by providing several pathways towards next-generation intelligent soft microrobots

Enhancing Mesh Deformation Realism: Dynamic Mesostructure Detailing and Procedural Microstructure Synthesis

Propomos uma solução para gerar dados de mapas de relevo dinâmicos para simular deformações em superfícies macias, com foco na pele humana. A solução incorpora a simulação de rugas ao nível mesoestrutural e utiliza texturas procedurais para adicionar detalhes de microestrutura estáticos. Oferece flexibilidade além da pele humana, permitindo a geração de padrões que imitam deformações em outros materiais macios, como couro, durante a animação. As soluções existentes para simular rugas e pistas de deformação frequentemente dependem de hardware especializado, que é dispendioso e de difícil acesso. Além disso, depender exclusivamente de dados capturados limita a direção artística e dificulta a adaptação a mudanças. Em contraste, a solução proposta permite a síntese dinâmica de texturas que se adaptam às deformações subjacentes da malha de forma fisicamente plausível. Vários métodos foram explorados para sintetizar rugas diretamente na geometria, mas sofrem de limitações como auto-interseções e maiores requisitos de armazenamento. A intervenção manual de artistas na criação de mapas de rugas e mapas de tensão permite controle, mas pode ser limitada em deformações complexas ou onde maior realismo seja necessário. O nosso trabalho destaca o potencial dos métodos procedimentais para aprimorar a geração de padrões de deformação dinâmica, incluindo rugas, com maior controle criativo e sem depender de dados capturados. A incorporação de padrões procedimentais estáticos melhora o realismo, e a abordagem pode ser estendida além da pele para outros materiais macios.We propose a solution for generating dynamic heightmap data to simulate deformations for soft surfaces, with a focus on human skin. The solution incorporates mesostructure-level wrinkles and utilizes procedural textures to add static microstructure details. It offers flexibility beyond human skin, enabling the generation of patterns mimicking deformations in other soft materials, such as leater, during animation. Existing solutions for simulating wrinkles and deformation cues often rely on specialized hardware, which is costly and not easily accessible. Moreover, relying solely on captured data limits artistic direction and hinders adaptability to changes. In contrast, our proposed solution provides dynamic texture synthesis that adapts to underlying mesh deformations. Various methods have been explored to synthesize wrinkles directly to the geometry, but they suffer from limitations such as self-intersections and increased storage requirements. Manual intervention by artists using wrinkle maps and tension maps provides control but may be limited to the physics-based simulations. Our research presents the potential of procedural methods to enhance the generation of dynamic deformation patterns, including wrinkles, with greater creative control and without reliance on captured data. Incorporating static procedural patterns improves realism, and the approach can be extended to other soft-materials beyond skin