Active nematics contain topological defects which under sufficient activity move, create and annihilate in a chaotic quasi-steady state, called active turbulence. However, understanding active defects under confinement is an open challenge, especially in three-dimensions. Here, we demonstrate the topology of three-dimensional active nematic turbulence under the spherical confinement, using numerical modelling. In such spherical droplets, we show the three-dimensional structure of the topological defects, which due to closed confinement emerge in the form of closed loops or surface-to-surface spanning line segments. In the turbulent regime, the defects are shown to be strongly spatially and time varying, with ongoing transformations between positive winding, negative winding and twisted profiles, and with defect loops of zero and non-zero topological charge.The timeline of the active turbulence is characterised by four types of bulk topology-linked events -breakup, annihilation, coalescence and cross-over of the defects -which we discuss could be used for the analysis of the active turbulence in different three-dimensional geometries. The turbulent regime is separated by a first order structural transition from a low activity regime of a steady-state vortex structure and an offset single point defect. We also demonstrate coupling of surface and bulk topological defect dynamics by changing from strong perpendicular to inplane surface alignment. More generally, this work is aimed to provide insight into three-dimensional active turbulence, distinctly from the perspective of the topology of the emergent three-dimensional topological defects. * These authors contributed equally to this work.
Achieving and exceeding diversity of colloidal analogs of chemical elements and molecules as building blocks of matter has been the central goal and challenge of colloidal science ever since Einstein introduced the colloidal atom paradigm. Recent advances in colloids assembly have been achieved by exploiting the machinery of DNA hybridization but robust physical means of defining colloidal elements remain limited. Here we introduce physical design principles allowing us to define high-order elastic multipoles emerging when colloids with controlled shapes and surface alignment are introduced into a nematic host fluid. Combination of experiments and numerical modeling of equilibrium field configurations using a spherical harmonic expansion allow us to probe elastic multipole moments, bringing analogies with electromagnetism and a structure of atomic orbitals. We show that, at least in view of the symmetry of the “director wiggle wave functions,” diversity of elastic colloidal atoms can far exceed that of known chemical elements.
Two‐photon laser writing is a powerful technique for creating intricate, high resolution features in polymerizable materials. Here, using a single‐step process to microfabricate polymer inclusions, the ability to generate read‐on‐demand images and identification codes in a liquid crystal (LC) device is demonstrated. These micrometer‐sized polymer features are encoded directly into LC devices using direct laser writing, which locks‐in the local molecular orientation at the moment of fabrication. By reading the devices with the same voltage amplitude that is used to write the polymer structures, features can be made to disappear as the director profile becomes homogeneous with the surrounding regions, effectively cloaking the structure for both polarized and unpolarized light. It is shown how this process can be used to create micrometer‐scale reconfigurable emoticons and quick‐response codes within a fully assembled LC device, with potential use in authenticity and identification applications.
The fabrication of 3D bulk metamaterials, optical materials with sub-wavelength building blocks, is an open challenge, along with the tuning of their optical properties, such as transmissivity or exit polarization where a possible approach is to embed liquid crystalline materials into metamaterials and use their tunable birefringence. In this work, we explore using numerical modelling the photonic properties of a composite of split ring resonator colloidal particles, dispersed in nematic liquid crystal, which was optimised to enable self-assembly fully. Specifically, using generalised FDTD simulations for light propagation in birefringent profiles, we demonstrate the photonic response of single particles, 2D and 3D colloidal crystals. The material transmittance is shown to exhibit clear resonant behaviour with the resonances tunable with the birefringence in the order of ~5%. Electric and magnetic field modes emergent on the particles are shown, as affected by the surrounding nematic birefringence, both the in the slit region of the split ring resonator (SRR) particles as well as around the particles. Observed photonic response is further explained by introducing basic equivalent LC circuits. Finally, this work is aimed at developing soft and fluid metamaterials, which exhibit optical anisotropy in the photonic response as a potent mechanism for controlling the flow of light at wavelength and even sub-wavelength scales.
Colloidal particles in nematic liquid crystals create elastic distortion and experience long-range forces. The symmetry of elastic distortion and consequently the complexity of interaction strongly depends largely on the liquid crystal anchoring, topology and shape of the particles. Here, we introduce a new nematic colloidal system made of peanut-shaped hematite particles. We report experimental studies on spontaneous orientation, mutual interaction, laser assisted self-assembly and the effect of external magnetic fields on the colloids. Majority of the colloids spontaneously orient either parallel or perpendicular to the nematic director. The colloids that are oriented perpendicularly exhibit two types of textures due to the out of plane tilting, which is corroborated by the Landau-de Gennes Q-tensor modelling. The transverse magnetic moment of the peanut-shaped colloids is estimated by using a simple analysis based on the competing effects of magnetic and elastic torques.
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