Colloids in Cholesterics: Size-Dependent Defects and Non-Stokesian Microrheology

Lintuvuori, Juho S.; Stratford, Kevin; Cates, Michael E.; Marenduzzo, Davide

doi:10.1103/physrevlett.105.178302

Cited by 81 publications

(75 citation statements)

References 29 publications

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“…To confront these difficulties, in the past decade, several hybrid simulation techniques [28][29][30][31][32] as well as other mesoscopic methods [33,34] for the dynamics of complex colloidal suspensions have been developed. Of these methods, we here adopt the fluid-particle-dynamics (FPD) method [30,31] to incorporate HIs into the study of a model active suspension.…”

Section: Methodsmentioning

confidence: 99%

Activity-induced clustering in model dumbbell swimmers: The role of hydrodynamic interactions

2014

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Using a fluid-particle dynamics approach, we numerically study the effects of hydrodynamic interactions on the collective dynamics of active suspensions within a simple model for bacterial motility: each microorganism is modeled as a stroke-averaged dumbbell swimmer with prescribed dipolar force pairs. Using both simulations and qualitative arguments, we show that, when the separation between swimmers is comparable to their size, the swimmers' motions are strongly affected by activity-induced hydrodynamic forces. To further understand these effects, we investigate semi-dilute suspensions of swimmers in the presence of thermal fluctuations. A direct comparison between simulations with and without hydrodynamic interactions shows these to enhance the dynamic clustering at a relatively small volume fraction; with our chosen model the key ingredient for this clustering behavior is hydrodynamic trapping of one swimmer by another, induced by the active forces. Furthermore, the density dependence of the motility (of both the translational and rotational motions) exhibits distinctly different behaviors with and without hydrodynamic interactions; we argue that this is linked to the clustering tendency. Our study illustrates the fact that hydrodynamic interactions not only affect kinetic pathways in active suspensions, but also cause major changes in their steady state properties.

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Section: Methodsmentioning

confidence: 99%

Activity-induced clustering in model dumbbell swimmers: The role of hydrodynamic interactions

2014

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“…Recent studies also showed the importance of size [18][19][20][21][22][23][24] and shape [24][25][26][27][28][29][30][31] in elastic interactions and ensuing assemblies of colloidal particles in nematics. On the other hand, colloidal dispersions in twisted nematic and cholesteric LCs (CLCs) have received less attention [32][33][34][35][36][37][38][39][40][41][42][43][44] because of the complexity of the problem caused by, for example, the screening of elastic interactions by periodic structure of cholesterics. 44 This complexity increases as the ratio between the particle's size and cholesteric periodicity increases.…”

Section: Introductionmentioning

confidence: 99%

Magnetically responsive gourd-shaped colloidal particles in cholesteric liquid crystals

et al. 2014

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Particle shape and medium chirality are two key features recently used to control anisotropic colloidal self-assembly and dynamics in liquid crystals. Here, we study magnetically responsive gourd-shaped colloidal particles dispersed in cholesteric liquid crystals with periodicity comparable or smaller than the particle's dimensions. Using magnetic manipulation and optical tweezers, which allow one to position colloids near the confining walls, we measured the elastic repulsive interactions of these particles with confining surfaces and found that separation-dependent particlewall interaction force is a non-monotonic function of separation and shows oscillatory behavior. We show that gourd-shaped particles in cholesterics reside not on a single sedimentation level, but on multiple long-lived metastable levels separated by a distance comparable to cholesteric periodicity.Finally, we demonstrate three-dimensional laser tweezers assisted assembly of gourd-shaped particles taking advantage of both orientational order and twist periodicity of cholesterics, potentially allowing new forms of orientationally and positionally ordered colloidal organization in these media.

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“…4), is also the ground state in cholesterics--the knots are metastable--although the difference in energies is small (of order 1-2%) and decreases both with increasing chirality and knot complexity p. The behavior with increasing p can be understood in terms of the total length of disclination line, which scales as p for the isolated loops and as ffiffiffiffiffiffiffiffiffiffiffiffi 4 + p 2 p for the knots. If the chirality is increased such that the pitch becomes smaller than the width of the colloid, then the disclinations develop twists analogous to those around spherical colloids (37,48).…”

Section: Möbius Strips and Knotted Defectsmentioning

confidence: 99%

Knots and nonorientable surfaces in chiral nematics

Machon

Alexander

2013

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

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Knots and knotted fields enrich physical phenomena ranging from DNA and molecular chemistry to the vortices of fluid flows and textures of ordered media. Liquid crystals provide an ideal setting for exploring such topological phenomena through control of their characteristic defects. The use of colloids in generating defects and knotted configurations in liquid crystals has been demonstrated for spherical and toroidal particles and shows promise for the development of novel photonic devices. Extending this existing work, we describe the full topological implications of colloids representing nonorientable surfaces and use it to construct torus knots and links of type (p,2) around multiply twisted Möbius strips.topological defects | homotopy theory | metamaterials C ontrolling and designing complex 3D textures in ordered media is central to the development of advanced materials, photonic crystals, tunable devices or sensors, and metamaterials (1-10), as well as to furthering our basic understanding of mesophases (11-13). Topological concepts, in particular, have come to play an increasingly significant role in characterizing materials across a diverse range of topics from helicity in fluid flows (14, 15) and transitions in soap films (16) to molecular chemistry (17), knots in DNA (18), defects in ordered media (19, 20), quantum computation (21, 22), and topological insulators (23). Topological properties are robust, because they are protected against all continuous deformations, and yet flexible for the same reason, allowing for tunability without loss of functionality.Some of the most intricate and interesting textures in ordered media involve knots. Originating with Lord Kelvin's celebrated "vortex atom" theory (24), the idea of encoding knotted structures in continuous fields has continued in magnetohydrodynamics (25), fluid dynamics (15), high-energy physics (26-28), and electromagnetic fields (29, 30), and has seen recent experimental realizations in optics (31), liquid crystals (32), and fluid vortices (33). Tying knots in a continuous field involves a much greater level of complexity than in a necktie, or rope, or even a polymer or strand of DNA. In a field, the knot is surrounded by material that has to be precisely configured so as to be compatible with the knotted curve. However, this complexity brings its own benefits, for the full richness of the mathematical theory of knots is naturally expressed in terms of the properties of the knot complement: everything that is not the knot. In this sense, knotted fields are ideally suited to directly incorporate and experimentally realize the full scope of modern knot theory.Liquid crystals are orientationally ordered mesophases, whose unique blend of soft elasticity, optical activity, and fluid nature offers a fertile setting for the development of novel metamaterials and the study of low-dimensional topology in ordered media. Much of the current focus centers on colloidal systems--colloidal particles dispersed in a liquid crystal host--which have a dual charac...

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Colloids in Cholesterics: Size-Dependent Defects and Non-Stokesian Microrheology

Cited by 81 publications

References 29 publications

Activity-induced clustering in model dumbbell swimmers: The role of hydrodynamic interactions

Activity-induced clustering in model dumbbell swimmers: The role of hydrodynamic interactions

Magnetically responsive gourd-shaped colloidal particles in cholesteric liquid crystals

Knots and nonorientable surfaces in chiral nematics

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