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Napoli G, Pylypovskyi OV, Sheka DD, Vergori L. Nematic shells: new insights in topology- and curvature-induced effects. Soft Matter 2021; 17:10322-10333. [PMID: 34734955 DOI: 10.1039/d1sm00719j] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
Within the framework of continuum theory, we draw a parallel between ferromagnetic materials and nematic liquid crystals confined on curved surfaces, which are both characterized by local interaction and anchoring potentials. We show that the extrinsic curvature of the shell combined with the out-of-plane component of the director field gives rise to chirality effects. This interplay produces an effective energy term reminiscent of the chiral term in cholesteric liquid crystals, with the curvature tensor acting as a sort of anisotropic helicity. We discuss also how the different nature of the order parameter, a vector in ferromagnets and a tensor in nematics, yields different textures on surfaces with the same topology as the sphere. In particular, we show that the extrinsic curvature governs the ground state configuration on a nematic spherical shell, favouring two antipodal disclinations of charge +1 on small particles and four +1/2 disclinations of charge located at the vertices of a square inscribed in a great circle on larger particles.
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Affiliation(s)
- Gaetano Napoli
- Dipartimento di Matematica e Fisica "E. De Giorgi", Università del Salento, Lecce, Italy.
| | - Oleksandr V Pylypovskyi
- Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Ion Beam Physics and Materials Research, 01328 Dresden, Germany
| | - Denis D Sheka
- Taras Shevchenko National University of Kyiv, 01601 Kyiv, Ukraine
| | - Luigi Vergori
- Dipartimento di Ingegneria, Università di Perugia, Perugia, Italy
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2
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Abstract
Intense electric fields applied to an asymmetric π-cell containing a nematic liquid crystal subjected to strong mechanical stresses induce distortions that are relaxed through a fast-switching mechanism: the order reconstruction transition. Topologically different nematic textures are connected by such a mechanism that is spatially driven by the intensity of the applied electric fields and by the anchoring angles of the nematic molecules on the confining plates of the cell. Using the finite element method, we implemented the moving mesh partial differential equation numerical technique, and we simulated the nematic evolution inside the cell in the context of the Landau–de Gennes order tensor theory. The order dynamics have been well captured, putting in evidence the possible existence of a metastable biaxial state, and a phase diagram of the nematic texture has been built, therefore confirming the appropriateness of the used technique for the study of this type of problem.
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3
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Ferris AJ, Rosenblatt C, Atherton TJ. Spontaneous Anchoring-Mediated Topography of an Orientable Fluid. Phys Rev Lett 2021; 126:057803. [PMID: 33605760 DOI: 10.1103/physrevlett.126.057803] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/14/2020] [Accepted: 01/13/2021] [Indexed: 06/12/2023]
Abstract
A topography in a Newtonian fluid occurs if there is a disturbance near the surface. But what if there is no such disturbance? We show by optical profilometry that a thin nematic film resting on a topological-defect-patterned substrate can exhibit a hill or divot at the opposing free (air) interface in the absence of a topological disturbance at that interface. We propose a model that incorporates several material properties and that predicts the major experimental features. This work demonstrates the importance of, in particular, anisotropic surface interactions in the creation of a free-surface topography.
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Affiliation(s)
- Andrew J Ferris
- Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, USA
| | - Charles Rosenblatt
- Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, USA
| | - Timothy J Atherton
- Department of Physics, Tufts University, Medford, Massachusetts 02155, USA
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4
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Salgado-Blanco D, Mendoza CI, Díaz-Herrera E. Orientational dependence of the potential of mean force for a discotic liquid crystal near a substrate. Mol Phys 2020. [DOI: 10.1080/00268976.2020.1748241] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
Affiliation(s)
- Daniel Salgado-Blanco
- Cátedras CONACyT – Centro Nacional de Supercómputo, Instituto Potosino de Investigación Científica y Tecnológica, San Luis Potosí, México
| | - Carlos I. Mendoza
- Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Ciudad de México, Mexico
| | - Enrique Díaz-Herrera
- Departamento de Física, Universidad Autónoma Metropolitana-Iztapalapa, Ciudad de México, Mexico
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5
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Abstract
An escaped radial director profile in a nematic liquid crystal cell can be transformed into a pair of strength m = +1/2 surface defects (and their associated disclination lines) at a threshold electric field. Analogously, a half-integer defect pair can be transformed at a threshold electric field into a director profile that escapes into the third dimension. These transitions were demonstrated experimentally and numerically, and are discussed in terms of topologically discontinuous and continuous pathways that connect the two states. Additionally, we note that the pair of disclination lines associated with the m = +1/2 surface defects were observed to co-rotate around a common point for a sufficiently large electric field at a sufficiently low frequency.
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Affiliation(s)
- Adam L Susser
- Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, USA.
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6
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Liu Z, Luo D, Yang KL. Flow-driven disclination lines of nematic liquid crystals inside a rectangular microchannel. Soft Matter 2019; 15:5638-5643. [PMID: 31243417 DOI: 10.1039/c9sm00900k] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
For nematic liquid crystals (LCs), a disclination line is formed when the director of the LCs changes abruptly. In this study, we demonstrate an approach to form dynamic disclination lines by flowing the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) in rectangular microchannels with a large aspect ratio. The dynamic disclination line moves gradually from the side toward the centre of the microchannel when the Ericksen number reaches 8.5. At the critical Ericksen number, influence of the anchoring energy on the side wall extends to the centre of the microchannel and determines the final position of the dynamic disclination line. As a result, the orientation of the LC is influenced by surface defects of the side wall. This phenomenon can be used to detect minute surface defects on the side wall and is potentially useful for visual sensing applications that require high sensitivity.
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Affiliation(s)
- Zongdai Liu
- Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore. and Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Xueyuan Road 1088, Shenzhen, Guangdong 518055, China.
| | - Dan Luo
- Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Xueyuan Road 1088, Shenzhen, Guangdong 518055, China.
| | - Kun-Lin Yang
- Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore.
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7
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Salgado-Blanco D, Díaz-Herrera E, Mendoza CI. Effect of the anchoring strength on the phase behaviour of discotic liquid crystals under face-on confinement. J Phys Condens Matter 2019; 31:105101. [PMID: 30540979 DOI: 10.1088/1361-648x/aaf843] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
In this study we have performed molecular dynamics simulations to study a Gay-Berne discotic fluid confined in a slab geometry for a fixed confinement length. Four different anchoring strengths with a homeotropic (face-on) configuration were studied. We found that changing the anchoring strength changes the normal component of the stress tensor, which in turn changes the density of the system's bulk. This phenomenon leads to a shift in the isotropic-nematic transition temperature. We observe that the temperature regions where the nematic phase is present diminishes as the anchoring strength increases. The anchoring strength also affects the nematic-columnar coexistence temperature-region: it spans over more temperatures at higher anchoring strengths.
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Affiliation(s)
- Daniel Salgado-Blanco
- Cátedras CONACyT-Centro Nacional de Supercómputo, Instituto Potosino de Investigación Científica y Tecnológica, Camino a la Presa San José 2055, 78216, San Luis Potosí, Mexico
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8
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Rahimi M, Ramezani-Dakhel H, Zhang R, Ramirez-Hernandez A, Abbott NL, de Pablo JJ. Segregation of liquid crystal mixtures in topological defects. Nat Commun 2017; 8:15064. [PMID: 28452347 PMCID: PMC5414351 DOI: 10.1038/ncomms15064] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2016] [Accepted: 02/24/2017] [Indexed: 01/29/2023] Open
Abstract
The structure and physical properties of liquid crystal (LC) mixtures are a function of composition, and small changes can have pronounced effects on observables, such as phase-transition temperatures. Traditionally, LC mixtures have been assumed to be compositionally homogenous. The results of chemically detailed simulations presented here show that this is not the case; pronounced deviations of the local order from that observed in the bulk at defects and interfaces lead to significant compositional segregation effects. More specifically, two disclination lines are stabilized in this work by introducing into a nematic liquid crystal mixture a cylindrical body that exhibits perpendicular anchoring. It is found that the local composition deviates considerably from that of the bulk at the interface with the cylinder and in the defects, thereby suggesting new assembly and synthetic strategies that may capitalize on the unusual molecular environment provided by liquid crystal mixtures. Liquid crystal mixtures are used in commercial applications and their composition affects their properties. Here Rahimi et al. use atomistic simulations to show that defects influence the molecular arrangement of the mixture components leading to a deviation of the local order from that of the bulk.
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Affiliation(s)
- Mohammad Rahimi
- Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA
| | - Hadi Ramezani-Dakhel
- Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA
| | - Rui Zhang
- Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA
| | - Abelardo Ramirez-Hernandez
- Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA.,Argonne National Laboratory, Argonne, Illinois 60439, USA
| | - Nicholas L Abbott
- Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
| | - Juan J de Pablo
- Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA.,Argonne National Laboratory, Argonne, Illinois 60439, USA
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Salili SM, Tamba MG, Sprunt SN, Welch C, Mehl GH, Jákli A, Gleeson JT. Anomalous Increase in Nematic-Isotropic Transition Temperature in Dimer Molecules Induced by a Magnetic Field. Phys Rev Lett 2016; 116:217801. [PMID: 27284674 DOI: 10.1103/physrevlett.116.217801] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/29/2016] [Indexed: 06/06/2023]
Abstract
We have determined the nematic-isotropic transition temperature as a function of an applied magnetic field in three different thermotropic liquid crystalline dimers. These molecules are comprised of two rigid calamitic moieties joined end to end by flexible spacers with odd numbers of methylene groups. They show an unprecedented magnetic field enhancement of nematic order in that the transition temperature is increased by up to 15 K when subjected to a 22 T magnetic field. The increase is conjectured to be caused by a magnetic-field-induced decrease of the average bend angle in the aliphatic spacers connecting the rigid mesogenic units of the dimers.
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Affiliation(s)
- S M Salili
- Chemical Physics Interdisciplinary Program & Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
| | - M G Tamba
- Department of Nonlinear Phenomena, Institute for Experimental Physics, Otto von Guericke University Magdeburg, Magdeburg 39106, Germany
| | - S N Sprunt
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
| | - C Welch
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - G H Mehl
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - A Jákli
- Chemical Physics Interdisciplinary Program & Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
| | - J T Gleeson
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
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10
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Vanzo D, Ricci M, Berardi R, Zannoni C. Wetting behaviour and contact angles anisotropy of nematic nanodroplets on flat surfaces. Soft Matter 2016; 12:1610-1620. [PMID: 26670582 DOI: 10.1039/c5sm02179k] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
We have studied the wetting behaviour of liquid crystal nanodroplets deposited on a planar surface, modelling the mesogens with Gay-Berne ellipsoids and the support surface with a slab of Lennard-Jones (LJ) spherical particles whose mesogen-surface affinity can be tuned. A crystalline and an amorphous planar surface, both showing planar anchoring, have been investigated: the first is the (001) facet of a LJ fcc crystal, the second is obtained from a disordered LJ glass. In both cases we find that the deposited nanodroplet is, in general, elongated and that the contact angle changes around its contour. Simulations for the crystalline substrate show that the angle of contact turns reversibly from anisotropic to isotropic when crossing the clearing transition. As far as we know this is a novel, not yet explored effect for thermotropic liquid crystals, that we hope will stimulate experimental investigations.
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Affiliation(s)
- Davide Vanzo
- Department of Industrial Chemistry "Toso Montanari", University of Bologna, viale Risorgimento 4, 40136 Bologna, Italy.
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11
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Abstract
We numerically study structural transitions inside shallow sub-micrometre scale wells with square cross section, filled with nematic liquid crystal material. We model the wells within the Landau–de Gennes theory. We obtain two qualitatively different states: (i) the diagonal state with defects for relatively large wells with lateral dimension greater than a critical threshold and (ii) a new, two-dimensional star-like biaxial order reconstruction pattern called the well order-reconstruction structure (WORS), for wells smaller than the critical threshold. The WORS is defined by an uniaxial cross connecting the four vertices of the square cross section. We numerically compute the critical threshold in terms of the bare biaxial correlation length and study its dependence on the temperature and on the anchoring strength on the lateral well surfaces.
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Affiliation(s)
- Samo Kralj
- Faculty of Natural Sciences and Mathematics, Department of Physics, University of Maribor, Koroška cesta 160, Maribor 2000, Slovenia
- Jožef Stefan Institute, PO Box 3000, Ljubljana 1000, Slovenia
| | - Apala Majumdar
- Department of Mathematical Sciences, University of Bath, Bath BA2 7AY, UK
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12
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Abstract
Chiral discrimination of vapours plays an important role in olfactory perception of biological systems and its realization by artificial sensors has been an intriguing challenge. Here, we report a simple method that tangibly visualizes the chirality of a diverse variety of molecules dissolved from vapours with high sensitivity, by making use of a structural change in a periodic microstructure of a nematic liquid crystal confined in open microchannels. This microstructure is accompanied by a topological line defect of a zigzag form with equal lengths of ‘zig’ and ‘zag.’ We find that a tiny amount of vapour of chiral molecules injected onto the liquid crystal induces the imbalance of ‘zig’ and ‘zag’ depending on its enantiomeric excess within a few seconds. Our liquid-crystal-based ‘chirality balance’ offers a simple, quick and versatile chirality-sensing/-screening method for gas-phase analysis (for example, for odours, environmental chemicals or drugs). Chiral determination of vapours is possible in biological systems as an important part of the olfactory system. Here, the authors describe a system that is capable of visually detecting and distinguishing the chirality of vapour-phase molecules by structural changes in a liquid crystal confined in open microchannels.
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13
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Gharbi MA, Nobili M, Blanc C. Use of topological defects as templates to direct assembly of colloidal particles at nematic interfaces. J Colloid Interface Sci 2014; 417:250-5. [DOI: 10.1016/j.jcis.2013.11.051] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/04/2013] [Revised: 11/18/2013] [Accepted: 11/19/2013] [Indexed: 10/25/2022]
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14
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Zhou X, Zhang Z. Dynamics of order reconstruction in a nanoconfined nematic liquid crystal with a topological defect. Int J Mol Sci 2013; 14:24135-53. [PMID: 24351807 PMCID: PMC3876101 DOI: 10.3390/ijms141224135] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2013] [Revised: 11/26/2013] [Accepted: 11/27/2013] [Indexed: 11/16/2022] Open
Abstract
At the wall in a hybrid nematic cell with strong anchoring, the nematic director is parallel to one wall and perpendicular to the other. Within the Landau-de Gennes theory, we have investigated the dynamics of s = ±1/2 wedge disclinations in such a cell, using the two-dimensional finite-difference iterative method. Our results show that with the cell gap decreasing, the core of the defect explodes, and the biaxiality propagates inside the cell. At a critical value of dc* ≈ 9ξ (where ξ is the characteristic length for order-parameter changes), the exchange solution is stable, while the defect core solution becomes metastable. Comparing to the case with no initial disclination, the value at which the exchange solution becomes stable increases relatively. At a critical separation of dc ≈ 6ξ, the system undergoes a structural transition, and the defect core merges into a biaxial layer with large biaxiality. For weak anchoring boundary conditions, a similar structural transition takes place at a relative lower critical value. Because of the weakened frustration, the asymmetric boundary conditions repel the defect to the weak anchoring boundary and have a relatively lower critical value of da, where the shape of the defect deforms. Further, the response time between two very close cell gaps is about tens of microseconds, and the response becomes slower as the defect explodes.
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Affiliation(s)
- Xuan Zhou
- Department of physics, Hebei University of Technology, Tianjin 300401, China; E-Mail:
| | - Zhidong Zhang
- Department of physics, Hebei University of Technology, Tianjin 300401, China; E-Mail:
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15
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Hamdi R, Lombardo G, de Santo MP, Barberi R. Biaxial coherence length in a nematic π-cell. Eur Phys J E Soft Matter 2013; 36:115. [PMID: 24122278 DOI: 10.1140/epje/i2013-13115-y] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/08/2013] [Revised: 08/01/2013] [Accepted: 09/11/2013] [Indexed: 06/02/2023]
Abstract
In a highly frustrated calamitic nematic phase, the strain can be relaxed by lowering the nematic order: the starting uniaxial symmetry can be broken and it can be replaced locally with transient biaxial domains. Using simple optical retardation measurements, we estimate the length scale over which the biaxial disturbance decays in space within a π-cell submitted to a weak electric field. We also characterise the transition cascade from the uniaxial splay texture to a bend texture through slow defect motion.
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Affiliation(s)
- R Hamdi
- Physics Department, University of Calabria, 87036, Rende, Italy,
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Roscioni OM, Muccioli L, Della Valle RG, Pizzirusso A, Ricci M, Zannoni C. Predicting the anchoring of liquid crystals at a solid surface: 5-cyanobiphenyl on cristobalite and glassy silica surfaces of increasing roughness. Langmuir 2013; 29:8950-8. [PMID: 23597166 DOI: 10.1021/la400857s] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
We employ atomistic molecular dynamics simulations to predict the alignment and anchoring strength of a typical nematic liquid crystal, 4-n-pentyl-4'-cyano biphenyl (5CB), on different forms of silica. In particular, we study a thin (~20 nm) film of 5CB supported on surfaces of crystalline (cristobalite) and amorphous silica of different roughness. We find that the orientational order at the surface and the anchoring strength depend on the morphology of the silica surface and its roughness. Cristobalite yields a uniform planar orientation and increases the order at the surface with respect to the bulk whereas amorphous glass has a disordering effect. Despite the low order at the amorphous surfaces, a planar orientation is established with a persistence length into the film higher than the one obtained for cristobalite.
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Affiliation(s)
- Otello Maria Roscioni
- Diartimento di Chimica Industriale Toso Montanari and INSTM, Università di Bologna, Bologna, Italy
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Ohzono T, Fukuda JI, Suzuki K, Yamaguchi T. ±1/2 wedge disclinations stabilized by a sinusoidal boundary in a thin hybrid nematic liquid-crystal film. Phys Rev E Stat Nonlin Soft Matter Phys 2012; 86:030701. [PMID: 23030857 DOI: 10.1103/physreve.86.030701] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/13/2012] [Indexed: 06/01/2023]
Abstract
As an interesting example of how geometry affects the formation of defects, we study the defect structures of a hybrid nematic liquid-crystal film in a wedge-shaped cell made up of sinusoidal microwrinkles and an elastomer sheet. When the cell thickness is larger than a threshold value h(c), +1/2 and -1/2 disclinations are simultaneously stabilized along concave grooves and convex crests, respectively. A simple theoretical analysis gives a good estimate of h(c). The disclinations also show alternating optical rotations resulting from the curved boundary and liquid-crystal elastic anisotropy.
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Affiliation(s)
- Takuya Ohzono
- Nanosystem Research Institute (NRI), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8565, Japan.
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Amoddeo A, Barberi R, Lombardo G. Surface and bulk contributions to nematic order reconstruction. Phys Rev E Stat Nonlin Soft Matter Phys 2012; 85:061705. [PMID: 23005112 DOI: 10.1103/physreve.85.061705] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/20/2011] [Indexed: 06/01/2023]
Abstract
Nematic molecules confined in an asymmetric π cell and subjected to strong electric fields exhibit textural distortions involving nematic order variations, described by the Landau-de Gennes Q-tensor theory. We investigated the evolution of order variations as function of the applied electric pulse amplitude and of the nematic surface pretilt anchoring angles by implementing a Q-tensor model with a moving mesh finite element method. The proposed technique is able to clearly distinguish the bulk and the surface order reconstruction which occur in the cell.
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Affiliation(s)
- A Amoddeo
- Mechanics and Materials Department, Università "Mediterranea" di Reggio Calabria, I-89122 Reggio Calabria, Italy.
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19
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Lombardo G, Amoddeo A, Hamdi R, Ayeb H, Barberi R. Biaxial surface order dynamics in calamitic nematics. Eur Phys J E Soft Matter 2012; 35:32. [PMID: 22592817 DOI: 10.1140/epje/i2012-12032-y] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/19/2012] [Revised: 03/19/2012] [Accepted: 04/23/2012] [Indexed: 05/31/2023]
Abstract
Thermotropic nematic materials relax strong distortions by lowering the nematic order: the uniaxial symmetry is broken and is locally replaced by biaxial domains. We investigated the dynamics of the nematic order near a boundary surface of an asymmetric π-cell submitted to an external electric field, close to the electric order reconstruction threshold. An unexpected phenomenon is observed close, but below the threshold: the biaxial order spreads on the surface inducing a consequent bulk topological behaviour equivalent to the splay-bend fast transition allowed by order reconstruction at higher voltage.
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Affiliation(s)
- G Lombardo
- CNR-IPCF UOS di Cosenza, c/o University of Calabria, 87036 Rende, Italy
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20
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Ohzono T, Fukuda JI. Zigzag line defects and manipulation of colloids in a nematic liquid crystal in microwrinkle grooves. Nat Commun 2012; 3:701. [PMID: 22426222 PMCID: PMC3293426 DOI: 10.1038/ncomms1709] [Citation(s) in RCA: 79] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2011] [Accepted: 01/25/2012] [Indexed: 11/21/2022] Open
Abstract
Spatially confined liquid crystals exhibit non-uniform alignment, often accompanied by self-organised topological defects of non-trivial shape in response to imposed boundary conditions and geometry. Here we show that a nematic liquid crystal, when confined in a sinusoidal microwrinkle groove, exhibits a new periodic arrangement of twist deformations and a zigzag line defect. This periodic ordering results from the inherent liquid crystal elastic anisotropy and the antagonistic boundary conditions at the flat liquid crystal-air and the curved liquid crystal-groove interfaces. The periodic structure can be tuned by controlling the groove geometry and the molecular chirality, which demonstrates the importance of boundary conditions and introduced asymmetry for the engineering of topological defects. Moreover, the kinks in the zigzag defects can trap small particles, which may afford a new method for manipulation of colloids. Our system, which uses easily fabricated microwrinkle grooves, provides a new microfabrication method based on the arrangement of controllable defects.
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Affiliation(s)
- Takuya Ohzono
- Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Japan.
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21
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Pizzirusso A, Berardi R, Muccioli L, Ricci M, Zannoni C. Predicting surface anchoring: molecular organization across a thin film of 5CB liquid crystal on silicon. Chem Sci 2012. [DOI: 10.1039/c1sc00696g] [Citation(s) in RCA: 63] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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Abstract
We show numerically that the interaction potential or force mediated by a liquid crystalline blue phase (BP) between two parallel plates exhibits oscillatory behavior with variation of the interplate distance, when the parallel plates impose strong normal anchoring. Its periodicity is approximately half of the unit-cell dimension of the bulk BP. The interaction arises from the deformation of the confined BP structure around the midplane of the system. The oscillatory interaction can be regarded as a clear manifestation of the BP ordering, because the cholesteric helical alignment adopted by a chiral liquid crystal cannot yield an oscillatory interaction.
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Affiliation(s)
- Jun-ichi Fukuda
- Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305-8568, Japan.
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Carbone G, Zappone B, Barberi R, Bartolino R, Musevic I. Direct nanomechanical measurement of layer thickness and compressibility of smectic liquid crystals. Phys Rev E Stat Nonlin Soft Matter Phys 2011; 83:051707. [PMID: 21728556 DOI: 10.1103/physreve.83.051707] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/28/2010] [Revised: 04/12/2011] [Indexed: 05/31/2023]
Abstract
Using an atomic force microscope (AFM) we confined a smectic-A liquid crystal (LC) between a flat glass plate and a 10-μm glass sphere attached to the free end of the AFM cantilever. Both surfaces were treated with a surfactant that induces normal alignment of the LC molecules. We measured the force F acting on the cantilever while varying the plate-sphere distance D with subnanometer precision. For D < 50 nm, the force was periodically oscillating and decayed as D was increased. Analyzing the force in the framework of a simple model of elastic deformation of the smectic layers, we have evaluated the undeformed layer thickness a(0) and compressibility modulus B. Compared to other techniques used to determine a(0) and B, AFM measurements are faster and require a much smaller amount (microliters) of LC. Moreover, they are based on purely mechanical deformations of the LC structure and do not require any static or radiative electromagnetic field.
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Davidson AJ, Brown CV, Mottram NJ, Ladak S, Evans CR. Defect trajectories and domain-wall loop dynamics during two-frequency switching in a bistable azimuthal nematic device. Phys Rev E Stat Nonlin Soft Matter Phys 2010; 81:051712. [PMID: 20866252 DOI: 10.1103/physreve.81.051712] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/09/2010] [Indexed: 05/29/2023]
Abstract
Bistable azimuthal nematic alignment textures have been created in micrometer-scale channels for which one sidewall is smooth and straight and the other possesses a symmetric sawtooth morphology. The optical textures have been observed during dynamic switching between the two stable states in response to dual frequency ac waveform driving of a highly dispersive nematic liquid crystal. The switching processes involves collapsing of filamentlike director reorientation (tilt-wall) loops and the associated motion and annihilation of surface defects along and close to the edge at the sawtooth sidewall. The predictions from both the n-director-based Ericksen-Leslie theory and the Q-tensor theory are in good agreement with the experimental observations.
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Affiliation(s)
- A J Davidson
- Department of Mathematics and Statistics, University of Strathclyde, 26 Richmond Street, Glasgow G1 1XH, United Kingdom
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Varga S, Martinez-Ratón Y, Velasco E. Competition between capillarity, layering and biaxiality in a confined liquid crystal. Eur Phys J E Soft Matter 2010; 32:89-101. [PMID: 20521078 DOI: 10.1140/epje/i2010-10601-8] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/21/2010] [Accepted: 05/03/2010] [Indexed: 05/29/2023]
Abstract
The effect of confinement on the phase behaviour and structure of fluids made of biaxial hard particles (cuboids) is examined theoretically by means of Onsager second-order virial theory in the limit where the long particle axes are frozen in a mutually parallel configuration. Confinement is induced by two parallel planar hard walls (slit-pore geometry), with particle long axes perpendicular to the walls (perfect homeotropic anchoring). In bulk, a continuous nematic-to-smectic transition takes place, while shape anisotropy in the (rectangular) particle cross-section induces biaxial ordering. As a consequence, four bulk phases, uniaxial and biaxial nematic and smectic phases, can be stabilised as the cross-sectional aspect ratio is varied. On confining the fluid, the nematic-to-smectic transition is suppressed, and either uniaxial or biaxial phases, separated by a continuous transition, can be present. Smectic ordering develops continuously from the walls for increasing particle concentration (in agreement with the supression of nematic-smectic second-order transition at confinement), but first-order layering transitions, involving structures with n and n + 1 layers, arise in the confined fluid at high concentration. Competition between layering and uniaxial-biaxial ordering leads to three different types of layering transitions, at which the two coexisting structures can be both uniaxial, one uniaxial and another biaxial, or both biaxial. Also, the interplay between molecular biaxiality and wall interactions is very subtle: while the hard wall disfavours the formation of the biaxial phase, biaxiality is against the layering transitions, as we have shown by comparing the confined phase behaviour of cylinders and cuboids. The predictive power of Onsager theory is checked and confirmed by performing some calculations based on fundamental-measure theory.
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Affiliation(s)
- S Varga
- Departamento de Física Téorica de la Materia Condensada, Universidad Autónoma de Madrid, E-28049 Madrid, Spain
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