Julian S. Evans

Liquid crystals of aqueous, giant graphene oxide flakes

We report the observation of liquid crystals formed by giant graphene oxide flakes (aspect ratio above 10000) suspended in water. As their concentration increases, the flakes undergo transitions from an isotropic dispersion to a biphasic system and then to a discotic nematic liquid crystal. The gel-like liquid crystal displays an unusual defect-free uniform director alignment over hundreds of micrometres. We characterize the nematic order parameter, optical birefringence and elastic properties of this novel mesomorphic system.

Shape-dependent oriented trapping and scaffolding of plasmonic nanoparticles by topological defects for self-assembly of colloidal dimers in liquid crystals.

We demonstrate scaffolding of plasmonic nanoparticles by topological defects induced by colloidal microspheres to match their surface boundary conditions with a uniform far-field alignment in a liquid crystal host. Displacing energetically costly liquid crystal regions of reduced order, anisotropic nanoparticles with concave or convex shapes not only stably localize in defects but also self-orient with respect to the microsphere surface. Using laser tweezers, we manipulate the ensuing nanoparticle-microsphere colloidal dimers, probing the strength of elastic binding and demonstrating self-assembly of hierarchical colloidal superstructures such as chains and arrays.

Orientationally Ordered Colloidal Co‐Dispersions of Gold Nanorods and Cellulose Nanocrystals

Nematic-like and helicoidally orientational self-assemblies of gold nanorods co-dispersed with cellulose nanocrystals to form liquid crystalline phases are developed. Polarization-sensitive extinction spectra and two-photon luminescence imaging are used to characterize orientations and spatial distributions of gold nanorods. Cholesteric-isotropic phase coexistence and continuous domains of single-phase regions are observed and qualitatively discussed on the basis of entropic and electrostatic interactions in co-dispersions of rigid rods of different aspect ratios. Potential applications include biologically compatible plasmonic composite nanomaterials for solar biofuel production and polarization-sensitive plasmonic papers and fabrics.

Optical manipulation of shape-morphing elastomeric liquid crystal microparticles doped with gold nanocrystals

We demonstrate facile optical manipulation of shape of birefringent colloidal microparticles made from liquid crystal elastomers. Using soft lithography and polymerization, we fabricate elastomeric microcylinders with weakly undulating director oriented on average along their long axes. These particles are infiltrated with gold nanospheres acting as heat transducers that allow for an efficient localized transfer of heat from a focused infrared laser beam to a submicrometer region within a microparticle. Photothermal control of ordering in the liquid crystal elastomer using scanned beams allows for a robust control of colloidal particles, enabling both reversible and irreversible changes of shape. Possible applications include optomechanics, microfluidics, and reconfigurable colloidal composites with shape-dependent self-assembly.

Towards Reconfigurable Optical Metamaterials: Colloidal Nanoparticle Self-Assembly and Self-Alignment in Liquid Crystals

We explore the nanoscale colloidal self-assembly and self-alignment in liquid crystals. We use model particles with controlled shapes and sizes, including quantum dots and rods and metal nanoparticles in the form of spheres, rods, and polygonal platelets. To study these composites on the scales ranging from nanometers to millimeters and to motivate their use in metamaterial fabrication, we utilize optical microscopies, freeze-fracture transmission electron microscopy, and cryogenic transmission electron microscopy. We discuss the long-range alignment and assembly of anisotropic nanoparticles imposed by the orientational elasticity of liquid crystals, showing that these composites provide a powerful platform for self-assembly of metamaterials.

Alignment of high-aspect ratio colloidal gold nanoplatelets in nematic liquid crystals

We study elasticity-mediated alignment of anisotropic goldcolloids in liquid crystals.Colloidalgold particles of controlled shapes (spheres, rods, and polygonal platelets) and sizes are prepared using well-established biosynthesis techniques with varying solvent conditions. When introduced into liquid crystalline structured solvents, these gold particles impose tangential or vertical surface boundary conditions for the liquid crystal molecules or building blocks such as chromonic molecular aggregates. This allows for multiple types of their controlled alignment in both lyotropic and thermotropic liquid crystals and is of interest for self-assembly-based fabrication of tunable nanostructured composite materials.

Optical manipulation of self-aligned graphene flakes in liquid crystals

Graphene recently emerged as a new two-dimensional material platform with unique optical, thermal and electronic properties. Single- or few-atom-thick graphene flakes can potentially be utilized to form structured bulk composites that further enrich these properties and enable a broad range of new applications. Here we describe optical manipulation of self-aligned colloidal graphene flakes in thermotropic liquid crystals of nematic and cholesteric types. Three-dimensional rotational and translational manipulation of graphene flakes by means of holographic optical tweezers allows for non-contact spatial patterning of graphene, control of liquid crystal defects, and low-power optical realignment of the liquid crystal director using these flakes. Potential applications include optically- and electrically-controlled reconfigurable liquid crystalline dispersions of spontaneously aligning colloidal graphene flakes and new electro-optic devices with graphene-based interconnected transparent electrodes at surfaces and in the bulk of liquid crystals.

Active shape-morphing elastomeric colloids in short-pitch cholesteric liquid crystals.

Active elastomeric liquid crystal particles with initial cylindrical shapes are obtained by means of soft lithography and polymerization in a strong magnetic field. Gold nanocrystals infiltrated into these particles mediate energy transfer from laser light to heat, so that the inherent coupling between the temperature-dependent order and shape allows for dynamic morphing of these particles and well-controlled stable shapes. Continuous changes of particle shapes are followed by their spontaneous realignment and transformations of director structures in the surrounding cholesteric host, as well as locomotion in the case of a nonreciprocal shape morphing. These findings bridge the fields of liquid crystal solids and active colloids, may enable shape-controlled self-assembly of adaptive composites and light-driven micromachines, and can be understood by employing simple symmetry considerations along with electrostatic analogies.

Preparation of Nanocomposite Plasmonic Films Made from Cellulose Nanocrystals or Mesoporous Silica Decorated with Unidirectionally Aligned Gold Nanorods

Using liquid crystalline self-assembly of cellulose nanocrystals, we achieve long-range alignment of anisotropic metal nanoparticles in colloidal nanocrystal dispersions that are then used to deposit thin structured films with ordering features highly dependent on the deposition method. These hybrid films are comprised of gold nanorods unidirectionally aligned in a matrix that can be made of ordered cellulose nanocrystals or silica nanostructures obtained by using cellulose-based nanostructures as a replica. The ensuing long-range alignment of gold nanorods in both cellulose-based and nanoporous silica films results in a polarization-sensitive surface plasmon resonance. The demonstrated device-scale bulk nanoparticle alignment may enable engineering of new material properties arising from combining the orientational ordering of host nanostructures and properties of the anisotropic plasmonic metal nanoparticles. Our approach may also allow for scalable fabrication of plasmonic polarizers and nanoporous silica structures with orientationally ordered anisotropic plasmonic nanoinclusions.

Self-alignment of dye molecules in micelles and lamellae for three-dimensional imaging of lyotropic liquid crystals.

We report alignment of anisotropic amphiphilic dye molecules within oblate and prolate anisotropic micelles and lamellae, the basic building blocks of surfactant-based lyotropic liquid crystals. Absorption and fluorescence transition dipole moments of these dye molecules orient either parallel or orthogonal to the liquid crystal director. This alignment enables three-dimensional visualization of director structures and defects in different lyotropic mesophases by means of fluorescence confocal polarizing microscopy and two-photon excitation fluorescence polarizing microscopy. The studied structures include nematic tactoids, Schlieren texture with disclinations in the calamitic nematic phase, oily streaks in the lamellar phase, developable domains in the columnar hexagonal phase, and various types of line defects in the discotic cholesteric phase. Orientational three-dimensional imaging of structures in the lyotropic cholesterics reveals large Burgers vector dislocations in cholesteric layering with singular disclinations in the dislocation cores that are not common for their thermotropic counterparts.