Energetics of lipid bilayers with applications to deformations induced by inclusions

A new energy for the description of large deformations of lipid bilayers is formulated with mathematical rigor. This energy is derived by considering the smectic A liquid crystalline nature of lipid bilayers and the coupling between the deformations of the layers and their constituent lipid molecules. Analogies between smectic A liquid crystals, with an infinite number of layers, and lipid bilayers, with a finite number of layers, are further discussed. The novelty of the energy density is demonstrated by studying the large deformations of planar lipid bilayers induced by cylindrical inclusions. The results of this study are directly compared with the results obtained using May's theoretical framework [May, Eur. Biophys. J., 2000, 29, 17–28] in which small deformations are assumed. As expected, the proposed energy density predicts larger distortions of the lipid molecules and deformations of the lipid bilayers close to an inclusion

Fabrication and Actuation of Hierarchically-Patterned Polymer Substrates for Dynamic Surface and Optical Properties

Switchable optical materials, which possess reversible color and transparency change in response to external stimuli, are of wide interest for potential applications such as windows and skylights in architectural and vehicular settings or optical sensors for environmental monitoring. This thesis considers the tuning of optical properties by tailoring and actuating responsive materials. Specifically, we demonstrate the design and fabrication of tilted pillar arrays on wrinkled elastomeric polydimethylsiloxane (PDMS) as a reversibly switchable optical window. While the original PDMS film exhibits angle-dependent colorful reflection due to Bragg diffraction of light from the periodic pillar array, the tilted pillar film appears opaque due to random scattering. Upon re-stretching the film to the original pre-strain, the grating color is restored due to the straightened pillars and transmittance is recovered. Then, we develop a composite film, consisting of a thin layer of quasi-amorphous array of silica nanoparticles (NPs) embedded in bulk elastomeric PDMS, with initial high transparency and angle-independent coloring upon mechanical stretching. The color can be tuned by the silica NP size. The switch between transparency and colored states could be reversibly cycled at least 1000 times without losing the film’s structural and optical integrity. We then consider the micropatterning of nematic liquid crystal elastomers (NLCEs) as micro-actuator materials. Planar surface anchoring of liquid crystal (LC) monomers is achieved with a poly(2-hydroxyethyl methacrylate)-coated PDMS mold, leading to monodomains of vertically aligned LC monomers within the mold. After cross-linking, the resulting NLCE micropillars show a relatively large radial strain when heated above nematic to isotropic transition temperature, which can be recovered upon cooling. Finally, the understanding of liquid crystal surface anchoring under confined boundary conditions is applied to the self-assembly of gold nanorods (AuNRs) driven by LC defect structures and to dynamically tune the surface plasmon resonance (SPR) properties. By exploiting the confinement of the smectic liquid crystal, 4-octyl-4’-cyanobiphenyl (8CB), to patterned pillars treated with homeotropic surface anchoring, topological defects are formed at precise locations around each pillar and can be tuned by varying the aspect ratio of the pillars and the temperature of the system. As a result, the AuNR assemblies and SPR properties can be altered reversibly by heating and cooling between smectic, nematic and isotropic phases

Cancellation of vorticity in steady-state non-isentropic flows of complex fluids

In steady-state non-isentropic flows of perfect fluids there is always thermodynamic generation of vorticity when the difference between the product of the temperature with the gradient of the entropy and the gradient of total enthalpy is different from zero. We note that this property does not hold in general for complex fluids for which the prominent influence of the material substructure on the gross motion may cancel the thermodynamic vorticity. We indicate the explicit condition for this cancellation (topological transition from vortex sheet to shear flow) for general complex fluids described by coarse-grained order parameters and extended forms of Ginzburg-Landau energies. As a prominent sample case we treat first Korteweg's fluid, used commonly as a model of capillary motion or phase transitions characterized by diffused interfaces. Then we discuss general complex fluids. We show also that, when the entropy and the total enthalpy are constant throughout the flow, vorticity may be generated by the inhomogeneous character of the distribution of material substructures, and indicate the explicit condition for such a generation. We discuss also some aspects of unsteady motion and show that in two-dimensional flows of incompressible perfect complex fluids the vorticity is in general not conserved, due to a mechanism of transfer of energy between different levels.Comment: 12 page

Helfrich-Hurault elastic instabilities driven by geometrical frustration

The Helfrich-Hurault (HH) elastic instability is a well-known mechanism behind patterns that form as a result of strain upon liquid crystal systems with periodic ground states. In the HH model, layered structures undulate and buckle in response to local, geometric incompatibilities, in order to maintain the preferred layer spacing. Classic HH systems include cholesteric liquid crystals under electromagnetic field distortions and smectic liquid crystals under mechanical strains, where both materials are confined between rigid substrates. However, richer phenomena are observed when undulation instabilities occur in the presence of deformable interfaces and variable boundary conditions. Understanding how the HH instability is affected by deformable surfaces is imperative for applying the instability to a broader range of materials. In this review, we re-examine the HH instability and give special focus to how the boundary conditions influence the mechanical response of lamellar systems to geometrical frustration. We use lamellar liquid crystals confined within a spherical shell geometry as our model system. Made possible by the relatively recent advances in microfluidics within the past 15 years, liquid crystal shells are composed entirely of fluid interfaces and have boundary conditions that can be dynamically controlled at will. We examine past and recent work that exemplifies how topological constraints, molecular anchoring conditions, and boundary curvature can trigger the HH instability in liquid crystals with periodic ground states. We then end by identifying similar phenomena across a wide variety of materials, both biological and synthetic. With this review, we aim to highlight that the HH instability is a generic and often overlooked response of periodic materials to geometrical frustration

Hierarchical Assemblies of Soft Matters From Polymers and Liquid Crystals on Structured Surfaces

Hierarchical, multifunctional materials hold important keys to numerous advanced technologies, including electronics, optics, and medicine. This thesis encompasses generation of hierarchical structures with novel morphologies and functions through self-assembly directed by lithographically fabricated templates. Here, two soft materials, amphiphilic random copolymers of photopolymerized acryloyl chloride (ranPAC) and smectic-A liquid crystal (SmA-LC) molecule, 4\u27(5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-heptadecaflu-orododecyloxy)-biphenyl-4-carboxylic acid ethyl ester, are synthesized as model systems to investigate the governing principles at the topographic surface/interface. The ranPAC can self-organize into nanomicelles with high regularity and stability, typically not possible in random copolymer systems. The morphology can be controlled by the photopolymerization conditions and solvent; the crosslinked shell makes the micelles robust against drying and storage. Using SU-8 micropillar arrays with spatially controlled surface chemistry as templates, we construct hierarchical microporous structures with tunable pore size and symmetry (e.g. square array), and uncover a new evaporative assembly method. By functionalizing the ranPAC nanovesicles with cationic poly(ethyleneimines), we encapsulate the anticancer drug, doxorubicin hydrochloride, and mRNA at a high payload, which are delivered to HEK 293T cells in vitro at a low cytotoxicity level. SmA-LC are characterized by arrangement of molecules into thin layers with the long molecular axis parallel to the layer normal, forming a close-packed hexagonal array of topological defects known as focal conic domains (FCDs) in a thin film. Using a series of SU-8 micropillar arrays with different size, shape, height, and symmetry as topological templates, we investigate the epitaxial and hierarchical assemblies of FCDs; whether the system favors confinement or pillar edge-pinning depends on balance of the elastic energy of LCs and the surface energy imposed by the template. The conservation of toric FCD (TFCD) textures over large LC thickness manifests a remarkably unique outcome of the epitaxial growth of TFCDs. On shorter pillars, however, the system favors the pinning of FCD centers near pillar edges while avoiding the opposing effect of confinement, leading to the break of the underlying symmetry of the pillar lattice, exhibiting tunable eccentricity, and a nontrivial yet organized array of defects balancing the elastic energy of LCs and the surface energy imposed by the template

Liquid Crystal Emulsions: A Versatile Platform for Photonics, Sensing, and Active Matter

The self‐assembly of liquid crystals (LCs) is a fascinating method for controlling the organization of discrete molecules into nanostructured functional materials. Although LCs are traditionally processed in thin films, their confinement within micrometre‐sized droplets has recently revealed new properties and functions, paving the way for next‐generation soft responsive materials. These recent findings have unlocked a wealth of unprecedented applications in photonics (e.g. reflectors, lasing materials), sensing (e.g. biomolecule and pathogen detection), soft robotics (e.g. micropumps, artificial muscles), and beyond. This Minireview focuses on recent developments in LC emulsion designs and highlights a variety of novel potential applications. Perspectives on the opportunities and new directions for implementing LC emulsions in future innovative technologies are also provided