Particle-Stabilized Defect Gel in Cholesteric Liquid Crystals

Zapotocky, Martin; Ramos, Laurence; Poulin, Philippe; Lubensky, T. C.; Weitz, David A.

doi:10.1126/science.283.5399.209

Cited by 232 publications

(190 citation statements)

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“…As the disclination network starts to flow the periodicity of the blue phase gives rise to an oscillating state which would leave a measurable signature in the timedependent stress curve. Elastic networks are of interest in many contexts and we compare our results to very recent studies of the rheological properties of disclinations in colloidal suspensions in liquid crystals [9].The equilibrium properties of BPs are described by a Landau-de Gennes free energy density, F , written in terms of a tensor order parameter Q αβ [1,3]. This comprises a bulk term…”

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confidence: 82%

Rheology of Cholesteric Blue Phases

Dupuis¹,

Marenduzzo

Orlandini

et al. 2005

Phys. Rev. Lett.

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Blue phases of cholesteric liquid crystals offer a spectacular example of naturally occurring disclination line networks. Here we numerically solve the hydrodynamic equations of motion to investigate the response of three types of blue phases to an imposed Poiseuille flow. We show that shear forces bend and twist and can unzip the disclination lines. Under gentle forcing the network opposes the flow and the apparent viscosity is significantly higher than that of an isotropic liquid. With increased forcing we find strong shear thinning corresponding to the disruption of the defect network. As the viscosity starts to drop, the imposed flow sets the network into motion. Disclinations break-up and re-form with their neighbours in the flow direction. This gives rise to oscillations in the time-dependent measurement of the average stress.61.30. Mp,83.80.Xz In many cholesteric liquid crystals the transition between the regular cholesteric phase and the isotropic phase occurs through one or more intermediate phases known as "blue phases" (BPs) [1][2][3]. Understanding the structure of the blue phases was a theoretical and experimental tour de force clearly summarised in [3].Blue phases occur because the locally favoured state is a double twist (i.e. a rotation of the director field in two directions, as opposed to the single twist of the cholesteric) which is globally incompatible with the requirement of continuity [3]. This results in the formation of networks of defects that separate local regions of double twist with axes aligned in different directions.The networks can be intricate and lead to structures that are periodic at large length scales. For example the BPI and BPII phases are 3-dimensional cubic phases where the orientational order can be periodic on the scale of the wavelength of visible light. Due to this, they have potential applications in fast light modulators [4], tunable photonic crystals [5] and lasers [6]. The main factor limiting the technological exploitation of BPs is that they are generally only stable over a temperature range ∼ 1 K. However recently it has been shown that the addition of polymers can stabilise BPI over more than 60 K opening perspectives in BP technology.Though the static structure of BPs is well accepted essentially nothing is known of their rheology. Given the intricacy of the director configurations one might expect that elastic and viscous responses to an external stress combine to give highly non-Newtonian behaviour under flow. Cholesterics show a strong dependence of viscosity on stress and, in particular, a striking permeation mode where the backflow acts to minimise director distortions [8]. In blue phases there is also a fully threedimensional network of defects whose elastic properties will increase the richness of their rheology.Therefore this paper aims to investigate the rheology of cholesteric BPs by numerically solving the hydrodynamic equations of motion for BPI, BPII and, for comparison, a simplified metastable network which we will term double twist (DT). ...

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mentioning

confidence: 82%

Rheology of Cholesteric Blue Phases

Dupuis¹,

Marenduzzo

Orlandini

et al. 2005

Phys. Rev. Lett.

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“…The repulsion arises from the competition between the surface aligning property, which favors, e.g., a radial orientation of the nematic molecules, and the bulk elasticity, which favors a uniform nematic orientation [3]. This effect was shown to arise also in other anisotropic fluid hosts, e.g., lyotropic solutions of anisotropic micelles [4], and in different liquid crystal phases, e.g., cholesterics [5]. Lately, such systems were shown to form liquid or solid composite materials with unusual properties [6,7].…”

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confidence: 99%

Nematic-Wetted Colloids in the Isotropic Phase: Pairwise Interaction, Biaxiality, and Defects

Galatola

Fournier

2001

Phys. Rev. Lett.

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We calculate the interaction between two spherical colloidal particles embedded in the isotropic phase of a nematogenic liquid. The surface of the particles induces wetting nematic coronas that mediate an elastic interaction. In the weak wetting regime, we obtain exact results for the interaction energy and the texture, showing that defects and biaxiality arise, although they are not topologically required. We evidence rich behaviors, including the possibility of reversible colloidal aggregation and dispersion. Complex anisotropic self-assembled phases might be formed in dense suspensions.Pacs numbers: 68.08.Bc, 82.70.Dd Dispersions of small particles or liquid droplets in a host fluid, namely colloidal suspensions, are a widespread and important state of matter [1]. They are long-lived metastable states, of fundamental interest from the point of view of collective interactions in complex matter. They also have considerable technological importance, e.g., in paints, coatings, foods and drugs. To prevent coagulation due to attractive van der Waals forces, usually the particles are treated in order to produce Coulombic or steric repulsive interactions. Recently, a novel source of repulsion has been reported: the elastic distortion of a liquid crystal host [2], i.e., a fluid phase with long-range orientational order of the molecules. The repulsion arises from the competition between the surface aligning property, which favors, e.g., a radial orientation of the nematic molecules, and the bulk elasticity, which favors a uniform nematic orientation [3]. This effect was shown to arise also in other anisotropic fluid hosts, e.g., lyotropic solutions of anisotropic micelles [4], and in different liquid crystal phases, e.g., cholesterics [5]. Lately, such systems were shown to form liquid or solid composite materials with unusual properties [6,7].It is well known that solid surfaces influence not only the orientation of liquid crystals, but also, in general, the degree of order of their constituent molecules. In particular, a surface can induce a wetting nematic layer even above the transition temperature at which the nematic becomes an ordinary isotropic liquid [8][9][10]. Therefore, an elastic colloidal stabilization could be achieved also above the nematic-isotropic transition. In addition, the vicinity of a phase transition may give a critical character to the stabilization mechanism and yield rich phaseseparation behaviors, as predicted in [11] for a simpler system with a scalar order-parameter.To understand these complex collective effects, it is essential to calculate the interaction between the particles precisely. An attempt, based on a quasi-planar approximation and assuming uniaxiality of the nematic tensorial order, has been proposed in Ref. [12]. Here, we give the first exact solution to the problem, including biaxiality. Our calculation, based on a multipolar expansion, is valid for weak surface ordering. We obtain numerically the texture between two spherical particles imposing normal boundary condition...

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“…Importantly, since the elastic energy of defects, containing lines, loops, and knots, is positively correlated with the pitch length,69, 70, 71 an increasing energy barrier occurs between the free energy of the FC and planar reconfiguration in the photochemical reaction 72, 73. However, the light scattering performance spontaneously degenerated at the low excitation power density (<20.0 mW cm −2 ), as a result of the advantage of the surface anchoring energy of CLCs on the alignment layer over the low energy barrier with a large pitch length 74, 75. When the helical pitch was short enough at the high power density (>40.0 mW cm −2 ), the high energy barrier allowed the disordered helixes to be stabilized in FC domains at the PSS (Figure 1b(ii)).…”

Section: Resultsmentioning

confidence: 99%

Stimuli‐Directed Dynamic Reconfiguration in Self‐Organized Helical Superstructures Enabled by Chemical Kinetics of Chiral Molecular Motors

et al. 2017

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Dynamic controllability of self‐organized helical superstructures in spatial dimensions is a key step to promote bottom‐up artificial nanoarchitectures and functional devices for diverse applications in a variety of areas. Here, a light‐driven chiral overcrowded alkene molecular motor with rod‐like substituent is designed and synthesized, and its thermal isomerization reaction exhibits an increasing structural entropy effect on chemical kinetic analysis in anisotropic achiral liquid crystal host than that in isotropic organic liquid. Interestingly, the stimuli‐directed angular orientation motion of helical axes in the self‐organized helical superstructures doped with the chiral motors enables the dynamic reconfiguration between the planar (thermostationary) and focal conic (photostationary) states. The reversible micromorphology deformation processes are compatible with the free energy fluctuation of self‐organized helical superstructures and the chemical kinetics of chiral motors under different conditions. Furthermore, stimuli‐directed reversible nonmechanical beam steering is achieved in dynamic hidden periodic photopatterns with reconfigurable attributes prerecorded with a corresponding photomask and photoinduced polymerization.

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Particle-Stabilized Defect Gel in Cholesteric Liquid Crystals

Cited by 232 publications

References 10 publications

Rheology of Cholesteric Blue Phases

Rheology of Cholesteric Blue Phases

Nematic-Wetted Colloids in the Isotropic Phase: Pairwise Interaction, Biaxiality, and Defects

Stimuli‐Directed Dynamic Reconfiguration in Self‐Organized Helical Superstructures Enabled by Chemical Kinetics of Chiral Molecular Motors

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