Session X53: Liquid Crystals and Self-Assembly

A Twist on Self-Assembly: Hierarchical Architectures Formed by Amphiphilic Chromonics

Amphiphilic molecules have been harnessed by biology and engineering alike for their propensity to self-assemble into complex structures. In this, steric molecular-scale interactions dominate small aggregates, whilst emergent elasticity governs as objects grow to larger length-scales. Here, we use molecular dynamics simulation of a series of coarse-grained mesogenic systems to examine the self-assembly of such supramolecular structures. The simulated systems, which are bipartite mixtures of disc-shaped and spherical particles, combine the thread-like aggregation of chromonics with the frustrations and incommensurabilities of amphiphilicity.

A veritable zoo of structures, many of which possess emergent supramolecular chirality, are reproducibly obtained. These including double helices, twisted bilayers, multi-strand ropes and tubules. By assessing the sensitivity of these final structures to the underpinning particle-scale interactions, insight is gained into how emergent length-scales can develop in grown structures. Further, from time-lines of the associated hierarchical self-assembly processes, the importance of mesogenic intermediates and size-dependent morphological changes in the development of complex aggregates is evidenced.   

An Insight of Chiral I432 Cubic Phase Formed by Polycatenar Molecules

Continuous cubic phases in liquid crystal has long been investigated for their complexity. A giant continuous cubic phase, regarded as Im3m space group then, formed by over 2500 molecules in each unit cell was discovered in 2005. That is astonishing for small molecules self-assembled into such a complex structure precisely and stably. Since then, though various hypotheses raised, the detailed structure remains unsolved until now. Recently, the chirality of this cubic phase formed by a series of achiral polycatenar molecules was reported. This chirality inspired us on the molecular arrangement in the structure. Combined the mathematical model and direct Fourier reconstruction of electron density slice, a theoretical model with helical molecular arrangement is raised by our team. Calculation concentrating on packing efficiency and simulated scattering intensities of the model has been conducted to verify the model. And more experiments based on soft X-ray scattering and electron transmission tomography are in progress to further reveal the structure.   

Insights on the smectic 'spacetime' microstructure

We numerically decompose the full four-dimensional microstructure of simulated smectics into domains, using an analogy to the theory of martensites. Smectic liquid crystals are remarkable, beautiful examples of materials microstructure, with ordered patterns of geometrically perfect ellipses and hyperbolas. The solution of the complex problem of filling three-dimensional space with domains of focal conics under constraining boundary conditions yields a set of strict rules, which are like the compatibility conditions in a martensitic crystal. In previous work, we presented the rules giving compatible conditions for the concentric circle domains found at two-dimensional smectic interfaces with planar boundary conditions. Here we discuss generalizations of our approach to describe the full four-dimensional smectic domains, where the variant symmetry group is the Weyl-Poincaré group of Lorentz boosts, translations, rotations, and dilatations. We will explore implications for the coarsening mechanism of focal conics, and possible effects of incorporating topological defects such as dislocations.   

Nematic Chainmail and the Abelian Sandpile Model

We show that topological classes of nonsingular solitons entangled with a weaved array of fixed nematic defects are in correspondence with stable recurrent states of the Abelian sandpile model. The physical correspondence is made by identifying a local Skyrmion flux with sandpile height. Using a toy model of these topological states, we discuss properties of large systems.   

Drag on a Moving Defect in a 2D Nematic Liquid Crystal

When a topological defect moves through a 2D nematic liquid crystal, its velocity is determined by a balance between elastic forces and viscous drag forces. Here, we develop an analytic theory to calculate the viscous drag in the presence of backflow, i.e. fluid motion in the liquid crystal. In this calculation, we determine the director field and the fluid velocity field as perturbation series in the defect velocity, assuming that the defect velocity is small. We then find the rate of energy dissipation, using the full Rayleigh dissipation function with all independent nematic viscosities. This calculation shows that positive topological charges experience less drag than negative topological charges, and hence move more rapidly. It also shows that certain nematic viscosity terms in the drag depend on defect orientation.   

A Unified Model Reveals the Interfacial Structure and Dynamics of Lyotropic and Multiphase, Thermotropic Liquid Crystals

In recent years, lyotropic Liquid Crystals (LCs) have attracted considerable interest due to their biological relevance and emerging applications. However, their material behaviors are not fully understood. Here, we propose a unified model to understand lyotropic and other multiphase LCs. The model is based on the coupling of a phase field framework and a Landau-de Gennes free energy. It couples composition, nematic order, and hydrodynamics. We first validate it by successfully reproducing the spindle shape of nematic tactoids and the asymmetric shape of topological defects in the nematic-isotropic (NI) coexistence phase. We argue that the irregular defect shape is due to planar anchoring at NI interface. Our model can also be applied to elucidate the dynamics of multiphase, thermotropic LCs. Specifically, we study the coalescence of two nematic droplets. We find that the associated hydrodynamic flows always accelerate the coalescence process. We analyze in detail the fusion process of two radial droplets containing +1 defects. We predict that the system crosses a free energy barrier by generating a -1 defect at the contact point. Moving forward, we anticipate that the proposed model will provide a framework within which lyotropic and multiphase LCs can be understood.   

Defect Pattern Formation, Stripe Instabilities, and Geometric Memory at the Nematic-Smectic A Liquid Crystal Phase Transition

We examine a surprising morphological sequence of defects and distortions in a hybrid-aligned liquid crystal film on a water substrate, connecting boojum defect networks in the nematic phase to self-organized focal conic domain arrays in the smectic phase upon cooling through the phase transition. Just prior to the phase transition, director field stripe distortions arise in the nematic phase with stripe orientations dictated by the pre-existing boojums. Upon the transition into the smectic A phase, the stripes break up into rows of elliptic-hyperbolic focal conic domains aligned along the stripe direction. We present a theoretical and numerical investigation of the appearance of nematic stripe distortions as driven by the increasing cost of bend distortions upon approaching the phase transition. We also demonstrate the geometrical connection between these stripe distortions and the focal conic domain rows that follow during cooling. This connection shows how memory of the original boojum network persists in the self-organized patterning of smectic focal conic domains.   

Surface Reconstruction of Glass Forming Liquid Crystals s a Tool for Sensing Volatile Organic Compounds

Glass forming cholesteric liquid crystalline oligomers (CLCO) based on siloxanes are known for their ability to form focal conic domains at temperatures higher than their glass transition temperature. These domains form a peculiar pattern on a surface of CLCOs that looks like double spirals and can be observed by Atomic Force Microscopy (AFM). This research demonstrated that the addition of low molar mass compounds of different polarity blended with oligomers changes the structure of the domains and makes the surface pattern sensitive to volatile organic compounds (VOCs) in the air. It was also demonstrated that this response is selective with respect to the nature of the particular VOC. We also discovered that the double spiral pattern is able to untwist and form less tight spirals. This new and unique behavior demonstrates the importance of mechanical tension in a glassy state and the plasticizing effect of VOCs. Interestingly, this surface reconstruction is reversible and spiral pattern appears after VOC removal. The response of these blends to many VOCs was studied by the AFM and UV-VIS spectroscopy. Each solvent uniquely reconstructs the surface and this provides a way to identify the solvent used.   

Atomistic Simulations as Bulk Elastic Probes in Liquid Crystalline Systems

Recent experimental measurements on confined liquid crystals have questioned long-established rules which place bounds upon the values of curvature elastic coefficients. We will describe how new advances in molecular simulation have enabled the direct extraction of elastic moduli from atomistic models of liquid crystals, and present results obtained from transferrable models of molecular liquid crystals, including homologues and derivatives of the common nematogen 5CB. In particular, this work explores how the surface-like elastic coefficient k₂₄ is affected by the molecular identity of the nematogens.   

Computational study of a nanoparticle-rich domain formation in a nematic liquid crystal

Past work has shown that when functionalized quantum dots are introduced into a nematic liquid crystal host and then cooled through the isotropic-nematic phase transition, structures of numerous sizes and morphologies are formed via self-assembly. The sizes of these structures are controlled by tuning the cooling rate of the liquid crystal. During the transition, the segregation of isotropic and nematic domains behaves as a classic phase separation phenomenon. We study this phenomenon by using the Cahn-Hilliard equation in conjunction with the nematic liquid crystal order parameter. By combining these two systems, we successfully model phase domain separation that follows the lower order parameter of the liquid crystal, as seen in experiments. We calculate isotropic domain size as a function of cooling rate, and find a power law relation that is differs from experimental observations, yet shows similar behavior.   

Designing Liquid Crystalline Ligands for Increased QD Photovoltaic Efficiency

High-efficiency quantum dot (QD) solar cells use nanoparticles to absorb incident light. QD photovoltaics are potential candidates for use in space missions due to their long lifetimes and stable photonic properties under high photon flux, however, one limitation of this technology is the loss of efficiency due to inter-dot energy transfer.In this project, we tried to tune the spacing between QDs by using mesogenic ligands (rod-like molecules attached to the particle by a flexible alkyl chain) to decrease the loss of energy. This strategy will provide an effective route towards improving the functional and structural characteristics of QD hybrid devices.
In the specific case of semiconducting QD nanoparticles, the photophysical properties are strongly tied to surface conditions. In a recent publication, we found that mesogenic ligand surface attachment can promote long-term QD photo-stability. The mesogenic ligands reduced inter-dot energy transfer, produced stable recombination rates and steady emission color over more than an hour of continuous photo-excitation. These effects were more prominent with a longer ligand attachment chain. After this, we present a new ligand design toward the goal of high efficiency.
  

Capping Ligands as “Atomic Orbitals” in Superlattice Self-Assembly

Materials consisting of periodic arrangements of nanocrystals, or nanoparticle superlattices, have unique properties not found in materials whose base units are atoms or molecules. In this talk, we present computational analysis of the interaction of two hydrocarbon-capped nanoparticles including cases of different core sizes, shapes, lengths and the asymmetric case, where the two particles in the pair are not identical. Binding free energies are computed using the Weighted Histogram Analysis Method and we show that these energies display a preference for symmetric pairs. A detailed analysis of ligand shell deformation shows planar interactions between the shells, which is used to explain the preference for symmetric pairs. This analysis also reveals the existence of vortex textures as predicted by the recently proposed Orbifold Topological Model.   

Self-Reporting and Self-Regulating Liquid Crystals Triggered by Motile Bacteria

Nematic liquid crystals (NLC) are structured liquids within which molecules are organized with long-range orientational order. This order leads to anisotropic elastic and dielectric properties. The orientational order of NLCs is readily perturbed by external forces, including interfacial shear stresses. In this presentation, we will show that the motion of bacterial cells near the interface between a bulk aqueous phase and an immiscible NLC can generate interfacial shear stresses that lead to reorientation of the NLC and an optical response. The reorientation of the NLC will be shown to also lead to changes in elastic and electrical double layer forces acting between colloidal inclusions (microcargo) within the NLC phase, thus triggering their release into the aqueous phase. This network of interactions within the LC leads to self-regulated release of microcargo containing biocidal agents in response to the arrival of motile bacterial cells.   

Computer Simulation of a Possible Magic Number Effect in the Phase Transition of the Eukaryotic Photosynthetic Organelle, the Pyrenoid

In most eukaryotic algae, an organelle called the pyrenoid helps concentrate CO₂ to enhance carbon fixation. We recently found that in Chlamydomonas reinhardtii, pyrenoid has liquid-like behavior including rapid condensation and dissolution during cell division. Our data suggests that the matrix is primarily composed of Rubisco and a linker protein, EPYC1. Rubisco and EPYC1 each have multiple binding sites for the other, allowing the two proteins to form a highly interconnected matrix. We now seek to understand the biophysical principles that allow the system to undergo phase transition to a state where the Rubisco and EPYC1 dissolve into the surrounding chloroplast. Here, we apply the Monte Carlo method to simulate the binding of EPYC1 and Rubisco on a 2D lattice. We find that specific numbers of Rubisco binding sites on EPYC1 promote dissolution, which we call a “magic number” effect. We explore different parameter regimes and find that the magic number effect is robust to Rubisco shape and valency. Our theoretical results provide guidance for the reconstitution of phase-separated liquid droplets from purified EPYC1 and Rubisco proteins in vitro. More broadly, our work reveals fundamental principles that may have widespread relevance to multivalent polymer systems in biology.   

Complex gels containing a hydrophobically modified polymer and phospholipid: component interactions, rheology and tribology

Polymers with phospholipids are widely used in personal care products due to their gelling and emulsifying properties. In this study we examine the underlying mechanisms through which these components interact using isothermal titration calorimetry (ITC) and their effects on rheological and tribological characteristics. Using a system containing a crosslinked hydrophobically modified polymer and phospholipids we find that the bulk rheology exhibits gel-like behavior with the elastic modulus increasing substantially upon phospholipid addition. We attribute this behavior to interactions between the hydrophobic moiety of the hydrophobically modified polymer and the phospholipids tails, resulting in a more interconnected network system, as evidenced from ITC measurements. In contrast, tribological behavior measured using a soft model contact consisting of polydimethylsiloxane (PDMS) show addition of phospholipids leads to lower friction coefficients at low entrainment speeds. Phospholipids are being adsorbed onto the PDMS surface, with the hydrated heads causing a decrease in the friction coefficients due to a hydration-lubrication mechanism. Results obtained for a system having an analogous polymer but without hydrophobes provide further credence to our hypotheses.