Linda S. Hirst

Morphology transition in lipid vesicles due to in-plane order and topological defects

Complex morphologies in lipid membranes typically arise due to chemical heterogeneity, but in the tilted gel phase, complex shapes can form spontaneously even in a membrane containing only a single lipid component. We explore this phenomenon via experiments and coarse-grained simulations on giant unilamellar vesicles of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. When cooled from the untilted Lα liquid-crystalline phase into the tilted gel phase, vesicles deform from smooth spheres to disordered, highly crumpled shapes. We propose that this shape evolution is driven by nucleation of complex membrane microstructure with topological defects in the tilt orientation that induce nonuniform membrane curvature. Coarse-grained simulations demonstrate this mechanism and show that kinetic competition between curvature change and defect motion can trap vesicles in deeply metastable, defect-rich structures.

Quantum dot/liquid crystal composite materials: self-assembly driven by liquid crystal phase transition templating

The isotropic to nematic liquid crystal (LC) phase transition is used to create organized assemblies of CdSe/ZnS core/shell quantum dots (QDs). Under controlled conditions, well dispersed QDs are expelled from the ordered domains of nematic LC into the remaining isotropic domains. The final LC phase produces three dimensional QD assemblies that are situated at the defect points in the LC volume. Through the luminescence of the QDs we are able to track the movement of the nanoparticles as the phase is formed as well as spectrally probe the resulting QD assemblies. Forster resonance energy transfer (FRET) measurements, combined with small angle X-ray scattering (SAXS) data reveal that the QD assemblies have a consistent inter-particle spacing of approximately 7.6 nm. Additionally, the location of the assemblies is shown to be controllable by utilizing beads as defect nucleation points.

Synthesis and self-assembly of coil–rod–coil molecules with lateral methyl and ethyl groups in the center of the rod segment

Rod–coil molecules, consisting of a flexible and a rigid block, have a strong capacity to self-assemble into a variety of ordered nanostructures in the bulk state. In this paper, we report the synthesis and characterization of the self-assembly behavior of coil–rod–coil oligomers. These new materials consist of five biphenyls linked together with ether bonds as a rod segment and incorporate lateral methyl or ethyl groups in the center of the rod building block and poly(ethylene oxide) (PEO) with a degree of polymerization (DP) of 7, 12 and 17 coil segments. Structural investigation of these molecules by means of differential scanning calorimetry (DSC) and X-ray scattering (XRD) in the bulk state reveals that side chains, methyl and ethyl groups in the middle of the rod segment, dramatically influence the self assembly behavior in the liquid-crystalline phase. Molecules containing a lateral methyl group based on PEO coil chain (DP of 12 and 17) self-organize into a hexagonally perforated layer (HPL) structure in the liquid- crystalline phase, while the molecule with a lateral ethyl group based on PEO coil chain (DP of 7) self-assembles into a 2-D rectangular columnar liquid-crystal structure. On further increasing of the coil length of the molecule containing ethyl group, the molecules self-organize into a 1-D lamellar structure in the crystalline phase.

Quantum dot self-assembly in liquid crystal media

In recent years the dispersion and directed assembly of nano-particles in liquid crystal media has proved an interesting field for investigation and one that may yield new hybrid materials for optical applications and fundamental research. In this paper, we investigate the dispersion of quantum dots in different liquid crystal phases, looking at aggregation and pattern formation. Quantum dot self-assembly in liquid crystals is dependent on particle surface properties and concentration in the liquid crystal medium. By varying these parameters we observe some fascinating structures and phase behavior using polarized optical microscopy and fluorescence microscopy.

Tuning Quantum-Dot Organization in Liquid Crystals for Robust Photonic Applications

Mesogenic ligands have the potential to provide control over the dispersion and stabilization of nanoparticles in liquid crystal (LC) phases. The creation of such hybrid materials is an important goal for the creation of soft tunable photonic devices, such as the LC laser. Herein, we present a comparison of isotropic and mesogenic ligands attached to the surface of CdSe (core-only) and CdSe/ZnS (core/shell) quantum dots (QDs). The mesogenic ligands flexible arm structure enhances ligand alignment, with the local LC director promoting QD dispersion in the isotropic and nematic phases. To characterize QD dispersion on different length scales, we apply fluorescence microscopy, X-ray scattering, and scanning confocal photoluminescent imaging. These combined techniques demonstrate that the LC-modified QDs do not aggregate into the dense clusters observed for dots with simple isotropic ligands when dispersed in liquid crystal, but loosely associate in a fluid-like droplet with an average interparticle spacing >10 nm. Embedding the QDs in a cholesteric cavity, we observe comparable coupling effects to those reported for more closely packed isotropic ligands.

Phase separation and critical phenomena in biomimetic ternary lipid mixtures

Lipid phase behaviour in biomimetic ternary mixtures has become a rich area of research and this review will provide a background to the many studies recently carried out, describing important advancements and future prospects for the field. In the liquid crystal community lipids are traditionally interesting as lyotropic materials and in this sense a large amount of work has been carried out on different systems. This paper will not focus on lyotropic phases but instead will concentrate on the thermotropic phase behaviour of ternary lipid mixtures. We will discuss thermotropic phase diagrams and critical phenomena in these systems, including their relationships to biological membranes. Interest in the links between lipid thermodynamics and cell function has grown steadily over the past three decades and most of the recent work in these areas has been motivated by a desire to link the phase behaviour of simple lipid mixtures to lateral organisation in the cell membrane. The literature in this field is extensive and can be intimidating; however, there are still unanswered fundamental questions. As important biological molecules, lipids have the potential to link many interesting physical ideas to how organisms function on a basic level and are certainly worthy of significant attention.

Dynamics of spontaneous emission of quantum dots in a one-dimensional cholesteric liquid crystal photonic cavity

We investigate the modulation of recombination lifetimes of CdSe/ZnS quantum dots (QDs) dispersed in a cholesteric liquid crystal (CLC) photonic cavity. Using ultrafast spectroscopic techniques we focus on the time-resolved emission from QD ensembles in CLC matrices with either planar or homeotropic alignment. In the case of planar alignment and a well-defined spectral stop-band (reflection band) we observe the emergence of a second, faster decay time of less than 2 ns. This short recombination pathway is observed only in samples where the QD emission spectrum partially overlaps the CLC stop-band by 50% or more. Samples prepared with homeotropic alignment do not have a stop-band and, consequently, do not lead to spectral or dynamical modulation of the QD emission. Our observations indicate that coupling between the excitonic and the photonic cavity modes results in an enhancement and modulation of spontaneous emission in the liquid crystal medium.

Hierarchical self-assembly of actin bundle networks: Gels with surface protein skin layers

The networklike structure of actin bundles formed with the cross-linking protein alpha-actinin has been investigated via x-ray scattering and confocal fluorescence microscopy over a wide range of alpha-actinin/F-actin ratios. We describe the hierarchical structure of bundle gels formed at high ratios. Isotropic actin bundle gels form via cluster-cluster aggregation in the diffusion-limited aggregation regime at high alpha-actinin/actin ratios. This process is clearly observed by confocal fluorescence microscopy. Polylysine is investigated as an alternative bundling agent in the high-ratio regime and the effects of F-actin length are also discussed. One particularly fascinating aspect of this system is the presence of a structured skin layer at the gel/water interface. Confocal microscopy has elucidated the full three-dimensional structure of this layer and revealed several interesting morphologies. The protein skin layer is a micron-scale structure composed of a directed network of bundles and exhibits flat, crumpled, and tubelike shapes. We show that crumpling of the skin layer results from stresses due to the underlying gel. These biologically based geometric structures may detach from the gel, demonstrating potential for the generation of biological scaffolds with defined shapes for applications in cell encapsulation and tissue engineering. We demonstrate manipulation of the skin layer, producing hemispherical structures in solution.

Dye-integrated cholesteric photonic luminescent solar concentrator

We have developed organic dye-integrated thin-film liquid crystalline photonic luminescent solar concentrators (LSCs), where the chirality of the liquid crystal (LC) results in the formation of a one-dimensional photonic cavity. By varying the different LSC parameters, including dye concentration, spectral position of the photonic band-gap and the LC phase, and by using spectroscopic and electrical characterisation, we have systematically studied the effects of self-absorption, incident absorption and confinement of down-converted emission on optical efficiency. Our results demonstrate that the efficiency of our LSCs is significantly enhanced in the LC phase when the photonic band-gap is at long wavelengths (>600 nm), overcoming associated low incident absorption and higher self-absorption. We reach the significant conclusion that focusing on improving the confinement of dye-emitted photons, rather than on increasing incident absorption, is a more promising route to enhancing thin-film LC-based LSC performance.

Fundamentals of soft matter science

Introduction Learning Objectives What Is Soft Matter? Basic Thermal Physics Intermolecular Forces Diffusion and Random Walks Self-Assembly The Phase Diagram Aggregation and Assembly Mechanical Properties of Soft Matter Questions: The Concept of Soft Materials and Their Characteristics Review of Thermal Physics Review the Mechanical Properties of Materials References Further Reading Liquid Crystals Learning Objectives Introduction to Liquid Crystals Anisotropy in Liquid Crystals The Order Parameter Thermotropic and Lyotropic Liquid Crystals Birefringence in Liquid Crystals Defect Textures Thermotropic Liquid Crystal Phases Experimental Techniques Applications of Liquid Crystals Questions The Characteristics of Liquid Crystal Materials Anisotropy and Birefringence The Structure of Liquid Crystal Phases Experimental Techniques and Liquid Crystal Technologies References Further Reading Surfactants Learning Objectives Introduction Types of Surfactants Surface Tension and Surfactants Self-Assembly and Phase Behavior Membrane Elasticity and Curvature Applications of Surfactants Experimental Methods Questions Physical and Chemical Properties of Surfactants The Hydrophobic Effect The Importance of Molecular Shape on Phase Structure and Membrane Curvature Applications and Experiments References Further Reading Polymers Learning Objectives Introduction Early Polymers Polymer Structure Liquid Crystal Polymers Polymer Solutions The Glassy and Polymer Melt Phases The Mechanical Properties of Polymers Experimental Techniques Questions Polymer Architecture Polymers in Solution Experimental Methods References Further Reading Colloidal Materials Learning Objectives Introduction Characteristics of Colloidal Systems Colloids in Suspension Competing Forces in Colloidal Dispersions Interparticle Interactions Colloidal Aggregation Colloidal Crystals Granular Materials Foams Experimental Techniques Questions Characteristics of Colloidal Systems Colloidal Aggregation and Dispersion Experimental Techniques References Further Reading Soft Biological Materials Learning Objectives Introduction The Composition of the Cell The Cell Membrane Protein Structures and Assemblies Experimental Techniques Questions Biomaterials as Soft Matter Experimental Techniques References Further Reading Glossary Appendix A: The Fourier Transform Appendix B: Physical Constants and Conversions Appendix C: Laue Scattering Theory Appendix D: Entropy and Thermodynamic Equilibrium Appendix E: The Amino Acids Index