Topological defect transformation and structural transition of two-dimensional colloidal crystals across the nematic to smectic-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>A</mml:mi></mml:math>phase transition

Zuhail, K. P.; Paladugu, Sathyanarayana; Seč, David; Čopar, Simon; Škarabot, Miha; Muševič, Igor; Dhara, Surajit

doi:10.1103/physreve.91.030501

Cited by 21 publications

(26 citation statements)

References 35 publications

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“…The reshaping of the boojum defect is somewhat similar to that observed in case of hyperbolic hedgehog defect in dipolar colloids across the N-SmA transition. 29 In the latter case, using Landau-de Gennes Q-tensor modeling (in the N phase), we showed that the increase of bend elastic constant (K 33 ) replaces the energetically costly bend distortion by more favorable splay distortion, concentrated in an elongated tail. However, one important difference is that, in case of dipolar colloid, the point defect moves toward the surface of the particle just before the transition, whereas in case of boojum defect similar movement is not observed.…”

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confidence: 81%

“…The phase transition of liquid crystal has a strong influence on the pairwise colloidal interaction. 29 In this letter, we report on the temperature dependence of equilibrium separation of a pair of isolated nematic boojum colloids. We show that the interparticle separation is linearly proportional to the orientation angle with respect to the rubbing direction and is the direct verification of the recent prediction of computer simulation.…”

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confidence: 99%

“…In case of dipolar colloids, the interparticle separation shows opposite behavior in the sense that it increases with decreasing temperature. 29 Recently, James and Fukuda 32 theoretically studied the effect of anchoring strength and elastic anisotropy on the separation of a pair of dipolar colloids in the N phase. They showed that in the strong anchoring regime (K 33 / WR 0 0.04, where K 33 being the bend elastic constant and W, the anchoring energy), the equilibrium separation increases linearly with the elastic anisotropy.…”

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confidence: 99%

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Temperature dependence of equilibrium separation and lattice parameters of nematic boojum-colloids

Zuhail

Dhara

2015

Applied Physics Letters

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We report experimental studies on the temperature dependence of equilibrium separation of a pair of nematic boojum colloids. The interparticle separation is linearly proportional to the elastic anisotropy and the orientation angle with respect to the rubbing direction. The latter result is in agreement with the recent prediction of computer simulations. We prepared two-dimensional colloidal crystals assisted by a laser tweezer and measured the temperature dependence of the lattice parameters. We observe a structural transition in boojum colloidal crystal across the N-SmA transition. The structural transition is mediated by the transformation of boojum defects into focal conic lines across the phase transition. The hexagonal close pack structure of the colloidal crystal in the SmA phase could be useful for designing photonic crystal.

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Temperature dependence of equilibrium separation and lattice parameters of nematic boojum-colloids

Zuhail

Dhara

2015

Applied Physics Letters

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“…In aqueous suspensions, the colloidal interaction is mostly driven by entropic or electrostatic forces, which are short‐range and isotropic, thus limiting the diversity in the resulting colloidal structures. Colloidal dispersions in nematic liquid crystals (NLCs) are very promising as they induce variety of topological defects and interact via long‐range elastic forces of the medium . They are especially interesting because the interaction is anisotropic and shape dependent, thereby providing a route to targeted colloidal assemblies .…”

Section: Introductionmentioning

confidence: 99%

Orientation Dependent Interaction and Self‐Assembly of Cubic Magnetic Colloids in a Nematic Liquid Crystal

Sudhakaran

Pujala

Dhara

2020

Advanced Optical Materials

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with their electric counterparts. [5,19] The defect mediated elastic interaction gives rise to 2D and 3D colloidal structures and superstructures, bearing strong potential for photonic applications such as photonic bandgap crystals. [10,[20][21][22] Research on nematic colloids is mostly concentrated so far on spherical particles. [19] Due to the advancement of synthesis and fabrication techniques, recently a surge has been observed in the number of investigations on nonspherical colloids, such as micro-and nanorods, [14,15] ellipsoids, [23] peanuts, [24] microbullets, [25] circular and polygonal platelets, [16,17,26] handle-bodies of varying genus, [7] and Mobius strips. [27] These investigations have provided ample of new results on the topological defects and ensuing self-assembled structures. However, experimental studies on faceted colloids such as cubes with flat faces are largely unexplored. There are of course a few simulation studies on microcubes in NLC showing the sharp edges act as pinning sites of Saturn ring defect. [28,29] The cube is modeled using a special case of the superellipsoid equation: (x) 2m + (y) 2m + (z) 2m = a 2m , where m is a shape parameter, defining the roundedness of the corners. It represents a sphere of radius a for m = 1, and a cube of side length L = 2a for m → ∞ as shown in Figure 1. As m interpolates the shape between a sphere and a cube, the defect loop evolves from a circular to distorted ring that wraps around the colloid while following only the edges. [28] In this paper, we for the first time report experimental studies on magnetic microcubes dispersed in a thin film of nematic liquid crystal. We study spontaneous orientation, elastic interaction and laser tweezer assisted colloidal assembly. We show that the cubic colloids stabilize diverse assemblies, which are not viable in spherical colloids. The cubic particles are made of hematite and hence respond to external magnetic fields, thereby enabling us to determine the magnetic moment from the competing effects of magnetic and elastic torques. The magnetic response provides an additional degree of freedom for manipulation and controlled assembly of colloids in liquid crystals. Results and DiscussionThe microcubes are nearly monodispersed with slightly round edges as seen in the scanning electron microscope (SEM) image presented in Figure 2a. The shape of our microcubes closely resembles the simulated cube with sharpness Spherical microparticles dispersed in nematic liquid crystals have been extensively investigated in the past years. Here, experimental studies are reported on the elastic deformation, colloidal interaction, and self-assembly of hematite microcubes with homeotropic surface anchoring in a nematic liquid crystal. It is demonstrated that the colloidal interaction and self-assembly of cubic colloids are orientation dependent. In a notable departure from the conventional microspheres, the microcubes stabilize diverse structures, such as bent chains, branches, kinks, and closed-loops. The microcubes reorient un...

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“…Thus defects in liquid crystals are undesirable in any practical application of liquid crystals although they are very important and interesting from the standpoint of fundamental physics. Apart from their spontaneous creation during a phase transition, these defects can also be harvested and manipulated in a controlled manner by dispersing foreign micro particles in the liquid crystals [2][3][4][5][6][7].…”

Section: Introductionmentioning

confidence: 99%

Magnetic-field tuning of whispering gallery mode lasing from ferromagnetic nematic liquid crystal microdroplets

et al. 2017

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We report magnetic field tuning of the structure and Whispering Gallery Mode lasing from ferromagnetic nematic liquid crystal micro-droplets. Microlasers were prepared by dispersing a nematic liquid crystal, containing magnetic nanoparticles and fluorescent dye, in a glycerol-lecithin matrix. The droplets exhibit radial director structure, which shows elastic distortion at a very low external magnetic field. The fluorescent dye doped ferromagnetic nematic droplets show Whispering Gallery Mode lasing, which is tunable by the external magnetic field. The tuning of the WGM lasing modes is linear in magnetic field with a wavelength-shift of the order of 1 nm/100 mT. Depending on the lasing geometry, the WGMs are red- or blue-shifted.

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Topological defect transformation and structural transition of two-dimensional colloidal crystals across the nematic to smectic-Aphase transition

Cited by 21 publications

References 35 publications

Temperature dependence of equilibrium separation and lattice parameters of nematic boojum-colloids

Temperature dependence of equilibrium separation and lattice parameters of nematic boojum-colloids

Orientation Dependent Interaction and Self‐Assembly of Cubic Magnetic Colloids in a Nematic Liquid Crystal

Magnetic-field tuning of whispering gallery mode lasing from ferromagnetic nematic liquid crystal microdroplets

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