Experimental Insights into the Nanostructure of the Cores of Topological Defects in Liquid Crystals

Wang, Xiaoguang; Kim, Young-Ki; Büküşoğlu, Emre; Zhang, Bo; Miller, Daniel S.; Abbott, Nicholas L.

doi:10.1103/physrevlett.116.147801

Cited by 77 publications

(86 citation statements)

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“…Analytical solutions are difficult, but the numerical simulations show that the isotropic core of the point defect can have a radius between 2 and 50 nm in pentylcyanobiphenyl (5CB), a typical thermotropic nematic liquid crystal44. Recent observations of defect loops with a 10 nm radius formed at the core of point defects in thermotropic liquid crystals support these predictions45. In our experiments, we attribute the appearance of a significantly larger core with a radius r i >1 μm, to the lyotropic nature of the liquid crystal system with the low concentration of the solute.…”

Section: Discussionmentioning

confidence: 99%

Colloidal cholesteric liquid crystal in spherical confinement

et al. 2016

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The organization of nanoparticles in constrained geometries is an area of fundamental and practical importance. Spherical confinement of nanocolloids leads to new modes of packing, self-assembly, phase separation and relaxation of colloidal liquids; however, it remains an unexplored area of research for colloidal liquid crystals. Here we report the organization of cholesteric liquid crystal formed by nanorods in spherical droplets. For cholesteric suspensions of cellulose nanocrystals, with progressive confinement, we observe phase separation into a micrometer-size isotropic droplet core and a cholesteric shell formed by concentric nanocrystal layers. Further confinement results in a transition to a bipolar planar cholesteric morphology. The distribution of polymer, metal, carbon or metal oxide nanoparticles in the droplets is governed by the nanoparticle size and yields cholesteric droplets exhibiting fluorescence, plasmonic properties and magnetic actuation. This work advances our understanding of how the interplay of order, confinement and topological defects affects the morphology of soft matter.

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Section: Discussionmentioning

confidence: 99%

Colloidal cholesteric liquid crystal in spherical confinement

et al. 2016

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“…The accumulation of inclusions at the cores of disclinations is a general phenomenon reported for thermotropic liquid crystals (3,4,(8)(9)(10)(11)(12)15). Inclusions replace the strongly distorted director field and thus reduce the associated elastic energy of the system (4).…”

Section: Discussionmentioning

confidence: 99%

“…When an additive is attracted to the disclination core, two possible morphologies are expected: (i) continuous thread-like assembly, or (ii) a linear array of discrete beads. The disclination-templated assemblies reported so far had a threadlike shape, as observed for polymers in the so-called blue phases (15) and for molecular amphiphiles at the cores of nematic disclinations (9,10).…”

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“…Here, we focus on the lyotropic droplets formed by an aqueous Ch suspension of cellulose nanocrystals (CNCs) and refer to this Ch phase as the Ch-CNC phase. Lyotropic Ch liquid crystals offer a different dimension of applications because (i) they are hydrophilic, in comparison with their hydrophobic thermotropic counterparts, and (ii) the micrometer size of the core of topological defects is significantly larger (24,25) than ∼10-nm core size in droplets of thermotropic liquid crystals (10). The implications of these features are in a potentially broader range of hydrophobicity/ hydrophilicity and sizes of additives templated by disclinations.…”

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confidence: 99%

“…As one approaches the "core" of the defect, director deformation becomes so strong that the degree of orientational order varies in space. Disclinations can attract additives, e.g., colloidal particles (3)(4)(5)(6)(7)(8) and even small molecules (9,10). Such an attraction is energetically favored, as the strongly distorted disclination core region is replaced with the additive (4).…”

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Periodic assembly of nanoparticle arrays in disclinations of cholesteric liquid crystals

Prince

Cho

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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An important goal of the modern soft matter science is to discover new self-assembly modalities to precisely control the placement of small particles in space. Spatial inhomogeneity of liquid crystals offers the capability to organize colloids in certain regions such as the cores of the topological defects. Here we report two selfassembly modes of nanoparticles in linear defects-disclinations in a lyotropic colloidal cholesteric liquid crystal: a continuous helicoidal thread and a periodic array of discrete beads. The beads form one-dimensional arrays with a periodicity that matches half a pitch of the cholesteric phase. The periodic assembly is governed by the anisotropic surface tension and elasticity at the interface of beads with the liquid crystal. This mode of self-assembly of nanoparticles in disclinations expands our ability to use topological defects in liquid crystals as templates for the organization of nanocolloids. colloidal liquid crystals | nanoparticle self-assembly | liquid crystal droplets | topological defects | anisotropic surface tension T he most extensively studied liquid crystalline phase, the socalled nematic, has been so named after the linear defectdisclinations that appear as flexible threads in optical microscopy textures; "thread" is "ν«μα" in Greek (1, 2). The disclinations represent singularities of the director that describes the local orientation of molecules. As one approaches the "core" of the defect, director deformation becomes so strong that the degree of orientational order varies in space. Disclinations can attract additives, e.g., colloidal particles (3-8) and even small molecules (9, 10). Such an attraction is energetically favored, as the strongly distorted disclination core region is replaced with the additive (4). The unique templating ability of disclinations has stimulated the exploration of their applications, such as the fabrication of optical materials (11), conductive microwires (12), soft magnets (13), and electrooptical devices (5,14,15).Disclinations in nematic liquid crystals are generally onedimensional structures, as the director pattern repeats itself along the line. When an additive is attracted to the disclination core, two possible morphologies are expected: (i) continuous thread-like assembly, or (ii) a linear array of discrete beads. The disclination-templated assemblies reported so far had a threadlike shape, as observed for polymers in the so-called blue phases (15) and for molecular amphiphiles at the cores of nematic disclinations (9, 10).Here, we report the templating behavior of disclinations in the chiral version of the nematic liquid crystal, the so-called cholesteric (Ch) phase. The local directorn in this phase undergoes helicoid twisting around a helical axisv while being perpendicular to this axis (1, 2). Continuous twist leads to a pseudolayered structure, with a well-defined pitch but no modulation of density. Experiments were performed for spherical Ch droplets, in which disclinations correspond to the equilibrium state, thus ensuring...

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Taming Liquid Crystal Self‐Assembly: The Multifaceted Response of Nematic and Smectic Shells to Polymerization

2016

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Liquid crystal-mediated self-assembly has emerged as a potent tool for advanced materials research and development, with particularly compelling opportunities opened by operating with topological defects. Demonstrated concepts range from nanoscopic assembly in defect lines [1][2][3][4] to actuators with specific folding or pumping actions. [5][6][7][8] Defects can be introduced in flat samples by advanced photopatterning, [9] but they in fact occur spontaneously in samples with spherical topology, like droplets or shells. If the director (the average molecule orientation) has a component in the spherical interface, the director field n(r) must contain topological defects, summing up to a total topological charge of +2.[10] These curvature-induced defects are attractive e.g. in sensing: when threading nematic (orientational but no positional long-range order) droplets on biofibers, defect rings develop that reveal information on the fiber morphology, [11] and topological defects in nematic droplets enable detection of endotoxin with extraordinary sensitivity.[12] Another application avenue was proposed by Nelson: [13] because a tetrahedral defect arrangement minimizes the free energy of spherically symmetric nematic shells, they could generate colloidal particles with interactions directed for diamond-like colloid crystallization, of large interest for the photonics industry. However, when the first nematic shells were realized experimentally, [14] it turned out that the free energy is further reduced by breaking spherical symmetry, collecting all defects in a localized region with minimum shell thickness. Although the tetrahedral arrangement can be recovered by making the shell very thin, [15] this illustrates the challenge in harnessing liquid crystal-mediated self-assembly. The large parameter space is a blessing through the many possible outcomes, but a curse through the di culty in accounting for all of them. There is thus a clear need to identify a method to tame the self-assembly in liquid crystal shells, without restricting the opportunities.Another serious problem is the limited shell lifetime, on the order of days or less. Adjacent shells coalesce into a droplet and isolated shells collapse due to di↵usion of the 1 This is the pre-peer reviewed version

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Experimental Insights into the Nanostructure of the Cores of Topological Defects in Liquid Crystals

Cited by 77 publications

References 40 publications

Colloidal cholesteric liquid crystal in spherical confinement

Colloidal cholesteric liquid crystal in spherical confinement

Periodic assembly of nanoparticle arrays in disclinations of cholesteric liquid crystals

Taming Liquid Crystal Self‐Assembly: The Multifaceted Response of Nematic and Smectic Shells to Polymerization

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