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Liquid crystal elastomers (LCEs) are a class of polymer networks which involve the incorporation of liquid crystal (LC) molecules into their polymer backbone or side chain. This results in anisotropy in their mechanical, optical, and electromagnetic properties similar to those exhibited by traditional LC materials. Their mechanical properties are highly coupled to the internal state of LC order, which can result in large mechanical deformations as LC order changes. This can occur in response to a variety of external stimuli such as changes in temperature, exposure to light, and application of external fields. The interplay between LC order and mechanical properties makes LCEs a highly promising class of functional materials and subsequently, they have been the subject of much research over the past several decades. However, developing an application of LCEs remains difficult in that their mechanical response is both complex and coupled to the state of liquid crystal order prior to cross-linking. Their physics are sufficiently complicated that in most cases, the use of pen-and-paper analysis is precluded. Additionally, the LCE fabrication process is complex and expensive, making trial-and-error experimental design methods unsuitable. This motivates the development and use of simulation-based methods to augment traditional experimental design methods. The two main contributors to the complexity of the design of LCE applications are the choice and imposition of liquid crystal order, or "texture", prior to cross-linking. In this work, simulation-based methods are developed and partially validated for use in applications-focused design of temperature-responsive nematic LCEs. These methods enable the simulation of LCEs of macroscopic size and of non-trivial geometry through the use of continuum mechanics and suitable numerical methods (the finite element method). LC texture is an input parameter in the presented method, allowing many choices of texture to be explored at low cost given that the textures are physically accessible. In addition to methods development and validation results, proof-of-concept simulation-based design studies were performed for two types of LCE-based actuators that are of current interest in the field: grippers and hinge mechanisms. Finally, preliminary results are presented resulting from the integration of nematic texture dynamics simulation (pre-cross-linking) and LCE mechanical simulations (post-cross-linking) which address the two main sources of complexity in the design process of LCE functional materials.
We investigate two soft matter systems that display novel behaviors when driven out of equilibrium by internal stresses, fueled by energy derived from the environment. First, we model shape-morphing dynamics of liquid crystal elastomers. We investigate photoactuation in thin polymer films that exhibit continuous, directional, macroscopic mechanical waves under constant light illumination. These polymer materials deform mechanically in response to any stimulus that modifies the strength of their nematic order. Doping the polymer with an azobenzene derivative enables the material to actuate in response to light. The trajectory of mechanical response can be controlled by patterning the orientation of the nematic director field during cross-linking, a process known as "blueprinting", which defines the local axis of induced contraction. We model the mechanics of photo-actuation via a Hamiltonian-based nonlinear finite element elastodynamics simulation. We find that the underlying mechanism enabling continuous wave generation is a feedback loop driven by self-shadowing, along with coupling between the nematic order and illumination. The model further demonstrates the mechanism by which wave direction and propagation speed depend on the blueprinted director pattern. These results explain experimental observations by our collaborators, who exploited this mechanical wave generation effect to produce robotic devices that undergo autonomous light-powered locomotion. Second, we carry out computer simulation studies of pattern formation in an active nematic fluid composed of flexible filaments, modeled as a thin layer of bead-spring polymer chains with active driving forces and thermal noise. We consider filaments that self-propel, representing e.g. gliding motility of filamentous bacteria on a smooth or bumpy surface. We investigate phase behavior and diffusive transport as a function of filament density and bending modulus, and demonstrate that Gaussian curvature of the substrate controls filament density. Next, we consider an active nematic fluid with extensile interactions, representing e.g. microtubules that slide against their neighbors driven by kinesin molecular motors. We analyze emergent non-equilibrium dynamics and microstructural evolution as a function of filament bending modulus and the magnitude of active forces. Analysis of the resulting flow includes tracking nucleation, motion, and annihilation of topological defects. Results from this filament-based model are compared to related experiments and continuum models in the literature. The projects both examine soft matter systems exhibiting emergent phenomena when driven from equilibrium via couplings to their environment. In building simulation codes for both projects, we utilize highly parallelized GPU (Graphics Processing Unit) implementations that decrease the runtime and increase the achievable length scale and duration of simulation studies. Computational models are important tools in understanding and ex- plaining the fundamental mechanisms of complex systems, and have potential for future use in design and optimization of smart material applications.
This dissertation describes stimuli-responsive liquid crystals and elastomers including thermal/electro-active ionic liquid crystal elastomers, UV responsive twist bend nematic liquid crystal dimmers and fast-switching chiral ferroelectric nematic liquid crystals with detailed studies on its nanoscale structures, electrical and optical properties for possible electric, optical and electro-optical applications. In this dissertation, the first preparation, physical properties, and electric bending actuation of a new class of active materials - ionic liquid crystal elastomers (iLCEs) are described. iLCEs can be actuated by low frequency AC or DC voltages of less than 1 V. The bending strains of the not optimized first iLCEs are already comparable to the well-developed ionic electroactive polymers (iEAPs). Additionally, iLCEs exhibit several novel and superior features. For example, pre-programmed actuation can be achived by patterning the substrates with different alignment domains at the level of cross-linking process. Since liquid crystal elastomers are also sensitive to magnetic fields, and can also be light sensitive, in addition to dual (thermal and electric) actuations in hybrid samples, iLCEs have far-reaching potentials toward multi-responsive actuations that may have so far unmatched properties in soft robotics, sensing and biomedical applications. The following two works are the understanding of the structure of the twist-bend nematic (NTB) phase. The first work presents hard and tender resonant X-ray scattering studies of two novel sulfur containing dimer materials for which we simultaneously measure the temperature dependences of the helical pitch and the correlation length of both the helical and positional order. In addition to an unexpected strong variation of the pitch with the length of the spacer connecting the monomer units, we find that at the transition to the NTB phase the positional correlation length drops. In the second work we use tender x-ray scattering to decipher the variation of the pitch and heliconical bond order of a NTB dimer containing azo groups upon polarized light illumination. It shows the first evidence of manipulation of the nanoscale heliconical structure of a twist bend nematic liquid crystal dimer containing an azo linkage by polarized light. The tender X-ray pattern reveals two different heliconical pitch values of aligned and unaligned domains under polarized light. In addition to the bulk alignment, the value of the heliconical pitch can be also tuned in two timescales by UV-violet light and recovered in a temperature dependent time. Then we studied the electrical, optical, and electro-optical properties of a ferroelectric nematic (NF) LC material doped with commercially available chiral dopants. While the NF phase of the undoped LC is only monotropic, the chiral NF phase is enantiotropic, indicating a chirality induced stabilization of the polar nematic order. Compared to undoped NF material, a remarkable improvement of the electro-optical switching time is demonstrated. The color of the chiral mixtures that exhibit a selective reflection of visible light in the chiral NF phase, can be reversibly tuned by 0.02-0.1V/[mu]m in-plane electric fields, which are much smaller than typically required in full-color cholesteric LC displays. The fast switchable reflection color at low fields has potential applications for LC displays without backlight, smart windows, shutters and e-papers. In the end, there are summaries and outlooks of all the results and the future applications.
Modeling of Mass Transport Processes in Biological Media focuses on applications of mass transfer relevant to biomedical processes and technology—fields that require quantitative mechanistic descriptions of the delivery of molecules and drugs. This book features recent advances and developments in biomedical therapies with a focus on the associated theoretical and mathematical techniques necessary to predict mass transfer in biological systems. The book is authored by over 50 established researchers who are internationally recognized as leaders in their fields. Each chapter contains a comprehensive introductory section for those new to the field, followed by recent modeling developments motivated by empirical experimental observation. Offering a unique opportunity for the reader to access recent developments from technical, theoretical, and engineering perspectives, this book is ideal for graduate and postdoctoral researchers in academia as well as experienced researchers in biomedical industries. Offers updated information related to advanced techniques and fundamental knowledge, particularly advances in computer-based diagnostics and treatment and numerical simulations Provides a bridge between well-established theories and the latest developments in the field Coverage includes dialysis, inert solute transport (insulin), electrokinetic transport, cellular molecular uptake, transdermal drug delivery and respiratory therapies
Liquid crystal elastomers are cross-linked polymer networks covalently bonded with liquid crystal mesogens. In the nematic phase, due to strong coupling between mechanical strain and orientational order, these materials display strain-induced instabilities associated with formation and evolution of orientational domains. In building a simulation model of these materials, we consider the limit in which the orientational order equilibrates rapidly compared to the strain, so that the local order tensor remains in continuously evolving quasi- static equilibrium as the strain relaxes. Our method allows us to study the onset of stripe formation in a monodomain film stretched along an axis perpendicular to the nematic director, the transition from polydomain to monodomain states, and the interaction of nematic liquid crystal elastomers with external stimuli such as an electric field. We intend through this model to further our understanding of the basic physics governing the dynamic mechanical response of nematic elastomers and also provide a useful computational tool for design and testing of potential engineering device applications.
This book introduces various advanced, smart materials and the strategies for the design and preparation for novel uses from macro to micro or from biological, inorganic, organic to composite materials. Selecting the best material is a challenging task, requiring tradeoffs between material properties and designing functional smart materials. The de
Computational Modelling of Intelligent Soft Matter: Shape Memory Polymers and Hydrogels covers the multiphysics response of various smart polymer materials, such as temperature-sensitive shape memory polymers and temperature/ chemosensitive hydrogels. Several thermo–chemo-mechanical constitutive models for these smart polymers are outlined, and their real-world applications are highlighted. The numerical counterpart of each introduced constitutive model is also presented, empowering readers to solve practical problems requiring thermomechanical responses of these materials as well as design and analyze real-world structures made of them. Introduces constitutive models based on continuum thermodynamics for intelligent soft materials Presents calibration methods for identifying material model parameters as well as finite element implementation of the featured models Allows readers to solve practical problems requiring thermomechanical responses from these materials as well as the design and analysis of real-world structures made of them
This text is a primer for liquid crystals, polymers, rubber and elasticity. It is directed at physicists, chemists, material scientists, engineers and applied mathematicians at the graduate student level and beyond.
Liquid crystal elastomers (LCEs), as an intriguing class of soft active materials, exhibit excellent actuation performances and biocompatible properties, as well as a high degree of design flexibility, which have been of increasing interest in many disciplines. This review summarizes recent developments in this inspiring area, providing an overview of fabrication methods, design schemes, actuation mechanisms, and diverse applications of LCEs. Firstly, two-stage and one-pot synthesis methods, as well as emerging fabrication techniques (e.g., 3D/4D printing and top-down microfabrication techniques) are introduced. Secondly, the design and actuation mechanisms are discussed according to the different types of stimuli (e.g., heat, light, and electric/magnetic fields, among others). Thirdly, the representative applications are summarized, including soft robotics, temperature/strain sensors, biomedical devices, stretchable displays, and smart textiles. Finally, outlooks on the scientific challenges and open opportunities are provided.