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

Zuhail, K. P.; Dhara, Surajit

doi:10.1063/1.4921634

Cited by 21 publications

(28 citation statements)

References 33 publications

(41 reference statements)

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“…Another example concerns the development of defect topologies that form when a colloidal particle is immersed into a nematic or smectic A carrier fluid [62]. Our theoretical results [63] are in good qualitative agreement with experimental findings [64,65]. Our last example concerns biaxial binary mixtures of two uniaxially symmetric compounds that pertain also to the present class of model systems [25].…”

Section: Discussionsupporting

confidence: 72%

Emergent biaxiality in nematic microflows illuminated by a laser beam

et al. 2019

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Anisotropic fluids (e.g. liquid crystals) offer a remarkable promise as optofluidic materials owing to the directional, tunable, and coupled interactions between the material, flow, and the optical fields. Here we present a comprehensive in silico treatment of this anisotropic interaction by performing nonequilibrium molecular dynamics simulations. We quantify the response of a nematic liquid crystal (NLC) undergoing a Poiseuille flow in the Stokes regime, while being illuminated by a laser beam incident perpendicular to the flow direction. We adopt a minimalistic model to capture the interactions, accounting for two features: first, the laser heats up the NLC locally; and second, the laser polarises the NLC and exerts an optical torque that tends to reorient molecules of the nematic phase. Because of this reorientation the liquid crystal exhibits small regions of biaxiality, where the nematic director is one symmetry axis and the axis of rotation for the reorientation of the molecules is the other one. We find that the relative strength of the viscous and the optical torques mediates the flow-induced response of the biaxial regions, thereby tuning the emergence, shape and location of the regions of enhanced biaxiality. The mechanistic framework presented here promises experimentally tractable routes toward novel optofluidic applications based on material-flow-light interactions.

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Section: Discussionsupporting

confidence: 72%

Emergent biaxiality in nematic microflows illuminated by a laser beam

et al. 2019

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“…This can be contrasted with the past experiments with different defect states accompanying the particles: for dipolar colloids [22], it was found that the interparticle separation increases linearly with the elastic anisotropy. In the case of boojums [23], D decreases linearly with the elastic anisotropy. Thus the separation between a pair of microspheres is governed by the type of the associated defect and the elastic anisotropy.…”

Section: Resultsmentioning

confidence: 95%

“…We recently studied colloidal particles with hyperbolic hedgehog and boojum defects across the N -SmA phase transition. We showed that both types of nematic defects are transformed into focal conic line defects in the SmA phase [22,23]. The phase transition has a strong influence on the pairwise colloidal interaction and is responsible for structural transitions in two-dimensional colloidal crystals.…”

Section: Introductionmentioning

confidence: 91%

Spherical microparticles with Saturn ring defects and their self-assembly across the nematic to smectic-Aphase transition

et al. 2015

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We report experimental studies on the Saturn ring defect associated with a spherical microparticle across the nematic (N) to smectic-A (SmA) phase transition. We observe that the director distortion around the microparticle changes rapidly with temperature. The equilibrium interparticle separation and the angle between two quadrupolar particles in the N phase are larger than those of the SmA phase. They are almost independent of the temperature in both phases, except for a discontinuous jump at the transition. We assembled a few particles using a laser tweezer to form a two-dimensional colloidal crystal in the N phase. The lattice structure of the crystal dissolves irreversibly across the N-SmA phase transition. The results on the pretransitional behavior of the defect are supported by the Landau-de Gennes Q-tensor modeling.

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

Cited by 21 publications

References 33 publications

Emergent biaxiality in nematic microflows illuminated by a laser beam

Emergent biaxiality in nematic microflows illuminated by a laser beam

Spherical microparticles with Saturn ring defects and their self-assembly across the nematic to smectic-Aphase transition

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

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