Topological defects and interactions in nematic emulsions

Lubensky, T. C.; Pettey, David; Currier, Nathan; Stark, Holger

doi:10.1103/physreve.57.610

Cited by 585 publications

(686 citation statements)

References 46 publications

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“…Experimental images are consistent with director structures (Fig. 1 E-L) that we computer-simulate by using the Ansatzen minimizing the Frank elastic free energy (12,13,25,26) for respective boundary conditions at the particle's surface. We also calculate the patterns of n eff for laser light linearly polarized perpendicular to the far-field director N 0 ( Fig.…”

Section: Resultsmentioning

confidence: 66%

Laser trapping in anisotropic fluids and polarization-controlled particle dynamics

Smalyukh

Kachynski

Kuzmin

et al. 2006

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

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Anisotropic fluids are widespread, ranging from liquid crystals used in displays to ordered states of a biological cell interior. Optical trapping is potentially a powerful technique in the fundamental studies and applications of anisotropic fluids. We demonstrate that laser beams in these fluids can generate anisotropic optical trapping forces, even for particles larger than the trapping beam wavelength. Immersed colloidal particles modify the fluid's ordered molecular structures and locally distort its optic axis. This distortion produces a refractive index ''corona'' around the particles that depends on their surface characteristics. The laser beam can trap such particles not only at their center but also at the high-index corona. Trapping forces in the beam's lateral plane mimic the corona and are polarizationcontrolled. This control allows the optical forces to be reversed and cause the particle to follow a prescribed trajectory. Anisotropic particle dynamics in the trap varies with laser power because of the anisotropy of both viscous drag and trapping forces. Using thermotropic liquid crystals and biological materials, we show that these phenomena are quite general for all anisotropic fluids and impinge broadly on their quantitative studies using laser tweezers. Potential applications include modeling thermodynamic systems with anisotropic polarization-controlled potential wells, producing optically tunable photonic crystals, and fabricating light-controlled nano-and micropumps.colloids ͉ laser tweezers ͉ liquid crystals ͉ optical trapping ͉ defects

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Section: Resultsmentioning

confidence: 66%

Laser trapping in anisotropic fluids and polarization-controlled particle dynamics

Smalyukh

Kachynski

Kuzmin

et al. 2006

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

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“…2I) would lead to reorganization of the microparticle chains. Here we note that the energy of a pair of microparticles interacting through LC-mediated forces with dipolar symmetry can be estimated as U ¼ 4πKCa 4 ð1 − 3 cos 2 ðθÞÞ∕R 3 , where K is the elastic constant of the LC (10 −11 N for 5CB), C is a constant [estimated to be ∼25 from theory (14) and ∼6 from experimental measurements (15)], a is the radius of the microparticle, θ is the angle of the dipole of one microparticle with respect to a second microparticle, and R is the separation between the microparticles. Because θ is 0°for planar ordering of LC at the 5CB-aqueous interface (Fig.…”

Section: Resultsmentioning

confidence: 99%

Chemoresponsive assemblies of microparticles at liquid crystalline interfaces

Koenig

Lin

Abbott

2010

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

108

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Assemblies formed by solid particles at interfaces have been widely studied because they serve as models of molecular phenomena, including molecular self-assembly. Solid particles adsorbed at interfaces also provide a means of stabilizing liquid-liquid emulsions and synthesizing materials with tunable mechanical, optical, or electronic properties. Whereas many past studies have investigated colloids at interfaces of isotropic liquids, recently, new types of intercolloidal interactions have been unmasked at interfaces of liquid crystals (LCs): The long-range ordering of the LCs, as well as defects within the LCs, mediates intercolloidal interactions with symmetries that differ from those observed with isotropic liquids. Herein, we report the decoration of interfaces formed between aqueous phases and nematic LCs with prescribed densities of solid, micrometer-sized particles. The microparticles assemble into chains with controlled interparticle spacing, consistent with the dipolar symmetry of the defects observed to form about each microparticle. Addition of a molecular surfactant to the aqueous phase results in a continuous ordering transition in the LC, which triggers reorganization of the microparticles, first by increasing the spacing between microparticles within chains and ultimately by forming two-dimensional arrays with local hexagonal symmetry. The ordering transition of the microparticles is reversible and is driven by surfactant-induced changes in the symmetry of the topological defects induced by the microparticles. These results demonstrate that the orderings of solid microparticles and molecular adsorbates are strongly coupled at the interfaces of LCs and that LCs offer the basis of methods for reversible, chemosensitive control of the interfacial organization of solid microparticles. colloidal interactions | interfacial assemblies | liquid crystals | ordering transitionsA liquid crystal (LC) is a phase of matter that blends properties that are typically associated with either crystalline solids or isotropic liquids (1). The molecules that comprise nematic LC phases exhibit long-range orientational order, yet they also possess mobilities characteristic of an isotropic liquid. LCs are most widely known for their use in electrooptic displays; however, they are increasingly being studied in the context of biology, materials science, and analytical chemistry (1). Recently, for example, microdroplets and microparticles dispersed in bulk LCs have been shown to form ordered assemblies, demonstrating that LCs can provide routes to stabilizing emulsions and assembling solid particles into colloidal crystals (2, 3). Although the origins of the interparticle forces that direct the formation of these assemblies in bulk LCs are not yet completely understood, it is clear that the long-range orientational ordering of molecules within the LC phase gives rise to interparticle forces that can be described in terms of the elasticity of the LCs and formation of topological defects within the LC. The LC-mediated interpart...

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“…A rough estimate of the order of magnitude of the elastic energy associated with the director distortions around a strongly anchored micron-size particle placed in an otherwise uniform nematic cell, is 56 . Forces of the same nature are also responsible for attraction of colloidal particles to (particle-free) distortions and defects in the director field in nematics 47,57,58 , smectics [59][60][61] and blue phases 62 , for trapping and ordering of particles at the LC surfaces [63][64][65][66] , and even for symmetry-breaking that enables transport phenomena such as nonlinear electrophoresis in LCs 45,67 . As discussed in the next…”

Section: Surface Anchoring and Two Types Of Liquid Crystal Colloidsmentioning

confidence: 99%