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Patel M, Shimizu S, Bates MA, Fernandez-Nieves A, Guldin S. Long term phase separation dynamics in liquid crystal-enriched microdroplets obtained from binary fluid mixtures. SOFT MATTER 2023; 19:1017-1024. [PMID: 36647716 DOI: 10.1039/d2sm01348g] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/17/2023]
Abstract
The dynamics of long term phase separation in binary liquid mixtures remains a subject of fundamental interest. Here, we study a binary liquid mixture, where the minority phase is confined to a liquid crystal (LC)-rich droplet, by investigating the evolution of size, defect and mesogen alignment over time. We track the binary liquid mixture evolving towards equilibrium by visualising the configuration of the liquid crystal droplet through polarisation microscopy. We compare our experimental findings with computational simulations and elucidate differences between bulk phases and confined droplets based on the respective thermodynamics of phase separation. Our work provides insights on how phase transitions on the microscale can deviate from bulk phase diagrams with relevance to other material systems, such as the liquid-liquid phase separation of polymer and protein solutions.
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Affiliation(s)
- Mehzabin Patel
- Department of Chemical Engineering, University College London, Torrington Place, London, WC1E 7JE, UK.
| | - Seishi Shimizu
- Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
| | - Martin A Bates
- Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
| | - Alberto Fernandez-Nieves
- Department of Condensed Matter Physics, University of Barcelona, 08028 Barcelona, Spain
- ICREA-Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain
- Institute for Complex Systems (UBICS), University of Barcelona, 08028, Barcelona, Spain
| | - Stefan Guldin
- Department of Chemical Engineering, University College London, Torrington Place, London, WC1E 7JE, UK.
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Polanšek J, Holbl A, Starzonek S, Drozd-Rzoska A, Rzoska SJ, Kralj S. History-dependent phase transition character. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2022; 45:70. [PMID: 35997865 PMCID: PMC9399213 DOI: 10.1140/epje/s10189-022-00221-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 05/23/2022] [Accepted: 07/26/2022] [Indexed: 06/15/2023]
Abstract
We consider history-dependent behavior in domain-type configurations in orientational order that are formed in configurations reached via continuous symmetry-breaking phase transitions. In equilibrium, these systems exhibit in absence of impurities a spatially homogeneous order. We focus on cases where domains are formed via (i) Kibble-Zurek mechanism in fast enough quenches or by (ii) Kibble mechanism in strongly supercooled phases. In both cases, domains could be arrested due to pinned topological defects that are formed at domain walls. In systems exhibiting polar or quadrupolar order, point and line defects (disclinations) dominate, respectively. In particular, the disclinations could form complex entangled structures and are more efficient in stabilizing domains. Domain patterns formed by fast quenches could be arrested by impurities imposing a strong enough random-field type disorder, as suggested by the Imry-Ma theorem. On the other hand, domains formed in supercooled systems could be also formed if large enough energy barriers arresting domains are established due to large enough systems' stiffness. The resulting effective interactions in established domain-type patterns could be described by random matrices. The resulting eigenvectors reveal expected structural excitations formed in such structures. The most important role is commonly played by the random matrix largest eigenvector. Qualitatively different behavior is expected if this eigenvector exhibits a localized or extended character. In the former case, one expects a gradual, non-critical-type transition into a glass-type structure. However, in the latter case, a critical-like phase behavior could be observed.
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Affiliation(s)
- Juš Polanšek
- Faculty of Natural Sciences and Mathematics, University of Maribor, Koroska 160, 2000, Maribor, Slovenia
| | - Arbresha Holbl
- Faculty of Natural Sciences and Mathematics, University of Maribor, Koroska 160, 2000, Maribor, Slovenia
| | - Szymon Starzonek
- Institute of High Pressure Physics Polish Academy of Sciences, ul. Sokołowska 29/37, 01-142, Warsaw, Poland.
| | - Aleksandra Drozd-Rzoska
- Institute of High Pressure Physics Polish Academy of Sciences, ul. Sokołowska 29/37, 01-142, Warsaw, Poland
| | - Sylwester J Rzoska
- Institute of High Pressure Physics Polish Academy of Sciences, ul. Sokołowska 29/37, 01-142, Warsaw, Poland
| | - Samo Kralj
- Faculty of Natural Sciences and Mathematics, University of Maribor, Koroska 160, 2000, Maribor, Slovenia
- Condensed Matter Physics Department, Jožef Stefan Institute, Jamova 39, 1000, Ljubljana, Slovenia
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Salgado-Blanco D, Llanas-García AH, Díaz-Herrera E, Martínez-González JA, Mendoza CI. Structural properties and ring defect formation in discotic liquid crystal nanodroplets. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2022; 34:254001. [PMID: 35358952 DOI: 10.1088/1361-648x/ac630b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/15/2021] [Accepted: 03/31/2022] [Indexed: 06/14/2023]
Abstract
In this work, we performedNpTMonte Carlo simulations of a Gay-Berne discotic liquid crystal confined in a spherical droplet under face-on anchoring and fixed pressure. We find that, in contrast to the unbounded system, a plot of the order parameter as function of temperature does not show a clear evidence of a first-order isotropic-nematic transition. We also find that the impossibility of simultaneously satisfy the uniform director field requirement of a nematic phase with the radial boundary conditions, results in the appearance of a ring disclination line as a stress release mechanism in the interior of the droplet. Under further cooling, a columnar phase appears at the center of the droplet.
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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, México
- División de Materiales Avanzados, IPICYT, Camino a la Presa San José 2055, Col. Lomas 4a Sección, San Luis Potosí, S. L. P., 78216, México
| | - Andrea H Llanas-García
- División de Materiales Avanzados, IPICYT, Camino a la Presa San José 2055, Col. Lomas 4a Sección, San Luis Potosí, S. L. P., 78216, México
| | - Enrique Díaz-Herrera
- Departamento de Física, Universidad Autónoma Metropolitana-Iztapalapa, Ave. San Rafael Atlixco 186, Col. Vicentina, 09340 México, Ciudad de México, Mexico
| | - José A Martínez-González
- Facultad de Ciencias, Universidad Autónoma de San Luis Potosí, Av. Parque Chapultepec 1570, San Luis Potosí 78210 SLP, Mexico
| | - Carlos I Mendoza
- Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Apdo. Postal 70-360, 04510 México, Ciudad de México, Mexico
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Bhattacharjee AK. Stochastic kinetics reveal imperative role of anisotropic interfacial tension to determine morphology and evolution of nucleated droplets in nematogenic films. Sci Rep 2017; 7:40059. [PMID: 28054600 PMCID: PMC5213422 DOI: 10.1038/srep40059] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2016] [Accepted: 11/30/2016] [Indexed: 11/19/2022] Open
Abstract
For isotropic fluids, classical nucleation theory predicts the nucleation rate, barrier height and critical droplet size by ac- counting for the competition between bulk energy and interfacial tension. The nucleation process in liquid crystals is less understood. We numerically investigate nucleation in monolayered nematogenic films using a mesoscopic framework, in par- ticular, we study the morphology and kinetic pathway in spontaneous formation and growth of droplets of the stable phase in the metastable background. The parameter κ that quantifies the anisotropic elastic energy plays a central role in determining the geometric structure of the droplets. Noncircular nematic droplets with homogeneous director orientation are nucleated in a background of supercooled isotropic phase for small κ. For large κ, noncircular droplets with integer topological charge, accompanied by a biaxial ring at the outer surface, are nucleated. The isotropic droplet shape in a superheated nematic background is found to depend on κ in a similar way. Identical growth laws are found in the two cases, although an unusual two-stage mechanism is observed in the nucleation of isotropic droplets. Temporal distributions of successive events indi- cate the relevance of long-ranged elasticity-mediated interactions within the isotropic domains. Implications for a theoretical description of nucleation in anisotropic fluids are discussed.
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Affiliation(s)
- Amit Kumar Bhattacharjee
- Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560064, India
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Kang K, Dhont JKG. An electric-field induced dynamical state in dispersions of highly charged colloidal rods: comparison of experiment and theory. Colloid Polym Sci 2015; 293:3325-3336. [PMID: 26617428 PMCID: PMC4654752 DOI: 10.1007/s00396-015-3707-4] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2015] [Revised: 07/08/2015] [Accepted: 07/14/2015] [Indexed: 11/30/2022]
Abstract
Concentrated dispersions of highly charged rod-like colloids (fd-virus particles) in isotropic-nematic coexistence exhibit a dynamical state when subjected to low-frequency electric fields [Soft Matter, 2010, 6, 273]. This dynamical state consists of nematic domains which persistently melt and form on time scales typically of the order of seconds. The origin of the dynamical state has been attributed to a field-induced, cyclic dissociation and association of condensed ions [Soft Matter, 2014, 10, 1987, Soft Matter, 2015, 11, 2893]. The ionic strength increases on dissociation of condensed ions, rendering the nematic domains unstable, while the subsequent decrease of the ionic strength due to association of condensed ions leads to a recurrent stabilization of the nematic state. The role of dissociation/association of condensed ions in the phase/state behaviour of charged colloids in electric fields has not been addressed before. The electric field strength that is necessary to dissociate sufficient condensed ions to render a nematic domain unstable, depends critically on the ambient ionic strength of the dispersion without the external field, as well as the rod-concentration. The aim of this paper is to compare experimental results for the location of transition lines and the dynamics of melting and forming of nematic domains at various ionic strengths and rod-concentrations with the ion-dissociation/association model. Phase/state diagrams in the field-amplitude versus frequency plane at two different ambient ionic strengths and various rod-concentrations are presented, and compared to the theory. The time scale on which melting and forming of the nematic domains occurs diverges on approach of the transition line where the dynamical state appears. The corresponding critical exponents have been measured by means of image time-correlation spectroscopy [Eur. Phys. J. E, 2009, 30, 333], and are compared to the theoretical values predicted by the ion-dissociation/association model.
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Affiliation(s)
- K. Kang
- />Soft Condensed Matter, Forschungszentrum Jülich Institute of Complex Systems (ICS-3), D-52425 Jülich, Germany
| | - J. K. G. Dhont
- />Soft Condensed Matter, Forschungszentrum Jülich Institute of Complex Systems (ICS-3), D-52425 Jülich, Germany
- />Department of Physics, Heinrich-Heine Universität Düsseldorf, D-40225 Düsseldorf, Germany
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Dhont JKG, Kang K. An electric-field induced dynamical state in dispersions of charged colloidal rods. SOFT MATTER 2014; 10:1987-2007. [PMID: 24652225 DOI: 10.1039/c3sm52277f] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
The response of concentrated dispersions of charged colloids to low-frequency electric fields is governed by field-induced inter-colloidal interactions resulting from the polarization of electric double layers and the layer of condensed ions, association and dissociation of condensed ions, as well as hydrodynamic interactions through field-induced electro-osmotic flow. The phases and states that can be formed by such field-induced interactions are an essentially unexplored field of research. Experiments on concentrated suspensions of rod-like colloids (fd-virus particles), within the isotropic-nematic phase coexistence region, showed that a number of phases/states are induced, depending on the field amplitude and frequency [Soft Matter, 2010, 6, 273]. In particular, a dynamical state is found where nematic domains form and melt on a time scale of the order of seconds. We discuss the microscopic origin of this dynamical state, which is attributed to the cyclic, electric-field induced dissociation and association of condensed ions. A semi-quantitative theory is presented for the dynamics of melting and formation of nematic domains, including a model for the field-induced dissociation/association of condensed ions. The resulting equation of motion for the orientational order parameter is solved numerically for parameters complying with the fd-virus system. A limit-cycle is found, with a cycling-time that diverges at the transition line in the field-amplitude versus frequency plane where the dynamical state first appears, in accord with experimental findings.
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Affiliation(s)
- Jan K G Dhont
- Forschungszentrum Jülich, Institute of Complex Systems (ICS), Soft Condensed Matter, D-52425 Jülich, Germany.
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Repnik R, Ranjkesh A, Simonka V, Ambrozic M, Bradac Z, Kralj S. Symmetry breaking in nematic liquid crystals: analogy with cosmology and magnetism. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2013; 25:404201. [PMID: 24025777 DOI: 10.1088/0953-8984/25/40/404201] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
Universal behavior related to continuous symmetry breaking in nematic liquid crystals is studied using Brownian molecular dynamics. A three-dimensional lattice system of rod-like objects interacting via the Lebwohl-Lasher interaction is considered. We test the applicability of predictions originally derived in cosmology and magnetism. In the first part we focus on coarsening dynamics following the temperature driven isotropic-nematic phase transition for different quench rates. The behavior in the early coarsening regime supports predictions made originally by Kibble in cosmology. For fast enough quenches, symmetry breaking and causality give rise to a dense tangle of defects. When the degree of orientational ordering is large enough, well defined protodomains characterized by a single average domain length are formed. With time subcritical domains gradually vanish and supercritical domains grow with time, exhibiting a universal scaling law. In the second part of the paper we study the impact of random-field-type disorder on a range of ordering in the (symmetry broken) nematic phase. We demonstrate that short-range order is observed even for a minute concentration of impurities, giving rise to disorder in line with the Imry-Ma theorem prediction only for the appropriate history of systems.
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Affiliation(s)
- R Repnik
- Faculty of Natural Sciences and Mathematics, University of Maribor, Koroska 160, 2000 Maribor, Slovenia
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Dan B, Ma AWK, Hároz EH, Kono J, Pasquali M. Nematic-Like Alignment in SWNT Thin Films from Aqueous Colloidal Suspensions. Ind Eng Chem Res 2012. [DOI: 10.1021/ie3001925] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Budhadipta Dan
- Department of Physics and Astronomy, §Department of Chemical and Biomolecular Engineering, ⊥Department of Electrical and Computer Engineering, #Department of Chemistry, and ‡R.E. Smalley Institute for Nanoscale Science & Technology, Rice University, 6100 Main Street, Houston, Texas 77005, United States
| | - Anson W. K. Ma
- Department of Physics and Astronomy, §Department of Chemical and Biomolecular Engineering, ⊥Department of Electrical and Computer Engineering, #Department of Chemistry, and ‡R.E. Smalley Institute for Nanoscale Science & Technology, Rice University, 6100 Main Street, Houston, Texas 77005, United States
| | - Erik H. Hároz
- Department of Physics and Astronomy, §Department of Chemical and Biomolecular Engineering, ⊥Department of Electrical and Computer Engineering, #Department of Chemistry, and ‡R.E. Smalley Institute for Nanoscale Science & Technology, Rice University, 6100 Main Street, Houston, Texas 77005, United States
| | - Junichiro Kono
- Department of Physics and Astronomy, §Department of Chemical and Biomolecular Engineering, ⊥Department of Electrical and Computer Engineering, #Department of Chemistry, and ‡R.E. Smalley Institute for Nanoscale Science & Technology, Rice University, 6100 Main Street, Houston, Texas 77005, United States
| | - Matteo Pasquali
- Department of Physics and Astronomy, §Department of Chemical and Biomolecular Engineering, ⊥Department of Electrical and Computer Engineering, #Department of Chemistry, and ‡R.E. Smalley Institute for Nanoscale Science & Technology, Rice University, 6100 Main Street, Houston, Texas 77005, United States
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Bradač Z, Kralj S, Žumer S. Early stage domain coarsening of the isotropic-nematic phase transition. J Chem Phys 2011; 135:024506. [DOI: 10.1063/1.3609102] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Krasna M, Cvetko M, Ambrozic M. Symmetry breaking and structure of a mixture of nematic liquid crystals and anisotropic nanoparticles. Beilstein J Org Chem 2010; 6. [PMID: 20703373 PMCID: PMC2919260 DOI: 10.3762/bjoc.6.74] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2010] [Accepted: 06/11/2010] [Indexed: 11/23/2022] Open
Abstract
Orientational ordering of a homogeneous mixture of uniaxial liquid crystalline (LC) molecules and magnetic nanoparticles (NPs) is studied using the Lebwohl-Lasher lattice model. We consider cases where NPs tend to be oriented perpendicularly to LC molecules due to elastic forces. We study domain-type configurations of ensembles, which are quenched from the isotropic phase. We show that for large enough concentrations of NPs the long range uniaxial nematic ordering is replaced by short range order exhibiting strong biaxiality. This suggests that the impact of NPs on orientational ordering of LCs for appropriate concentrations of NPs is reminiscent to the influence of quenched random fields which locally enforce a biaxial ordering.
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Affiliation(s)
- Marjan Krasna
- University of Maribor, Faculty of Natural Sciences and Mathematics, Koroska cesta 160, 2000 Maribor, Slovenia.
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Nespoulous M, Blanc C, Nobili M. Orientational quenched disorder of a nematic liquid crystal. PHYSICAL REVIEW LETTERS 2010; 104:097801. [PMID: 20367011 DOI: 10.1103/physrevlett.104.097801] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/31/2009] [Indexed: 05/29/2023]
Abstract
By means of direct imaging, we map the surface heterogeneities of the nematic director orientation on a SiOx anchoring layer. The spatial correlations of surface director orientations are well fitted with a compressed exponential with exponent of 1.5 and typical correlation length of few microns. To discuss these results a formal analogy is established between the equation governing the nematic surface torques and the Langevin equation. Based on this analogy we prove that the disorder is spatially correlated orientational quenched disorder. The measured correlation length is discussed in terms of substrate morphology and molecular adsorption.
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Affiliation(s)
- Mathieu Nespoulous
- Laboratoire des Colloïdes, Verres et Nanomatériaux, Université Montpellier II, Place Eugène Bataillon, 34090 Montpellier, France
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Dhara S, Arun Kumar T, Ishikawa K, Takezoe H. Thermotropic uniaxial nematic liquid crystal with ± 1/2 strength disclinations: defect-antidefect production and correlation. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2009; 21:505103. [PMID: 21836225 DOI: 10.1088/0953-8984/21/50/505103] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
We report the observation of mostly wedge disclinations of strength ± 1/2 in a uniaxial nematic liquid crystal. The dense network of the disclination is stabilized in cells of thickness (∼6 µm) treated with perfluoropolymer. The distribution of net defect strength was found to be Gaussian. The scaling exponent obtained from the width of the distribution is ν = 0.29 ± 0.08 and is in good agreement with the prediction based on the cosmological Kibble mechanism. The exponent and the constant of the scaling relation are compared with the values reported for lyotropic liquid crystal with half-strength disclination by Mukai and co-workers.
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Affiliation(s)
- Surajit Dhara
- School of Physics, University of Hyderabad, Hyderabad 500046, India
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The influence of disorder on thermotropic nematic liquid crystals phase behavior. Int J Mol Sci 2009; 10:3971-4008. [PMID: 19865529 PMCID: PMC2769155 DOI: 10.3390/ijms10093971] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2009] [Revised: 08/07/2009] [Accepted: 08/14/2009] [Indexed: 11/23/2022] Open
Abstract
We review the theoretical research on the influence of disorder on structure and phase behavior of condensed matter system exhibiting continuous symmetry breaking focusing on liquid crystal phase transitions. We discuss the main properties of liquid crystals as adequate systems in which several open questions with respect to the impact of disorder on universal phase and structural behavior could be explored. Main advantages of liquid crystalline materials and different experimental realizations of random field-type disorder imposed on liquid crystal phases are described.
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14
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Chen X, Hamlington BD, Shen AQ. Isotropic-to-nematic phase transition in a liquid-crystal droplet. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2008; 24:541-546. [PMID: 18081328 DOI: 10.1021/la701844s] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/25/2023]
Abstract
In this paper, we focus on the isotropic-to-nematic phase transition in a liquid-crystal droplet. We present the results of an experiment to measure the growth of the nematic phase within an isotropic phase liquid-crystal droplet. Experimentally, we observe two primary phase transition regimes. At short time scales, our experimental results (R(t) approximately t0.51) show good agreement with a Stefan-type model of the evolution of the nematic phase within the isotropic phase of a liquid crystal. As time progresses, the growth of the nematic phase is restricted by increased confinement of the droplet boundary. During this stage of growth, the nematic phase grows at a slower rate of R(t) approximately t0.31. The slower growth at later stages might be due to a variety of factors such as confinement-induced latent heat reduction; a change of defect strength during its evolution; or interactions between the defect and the interface between the liquid crystal and oil or between adjacent defects. The presence of two growth regimes is also consistent with the molecular simulations of Bradac et al. (Bradac, Z.; Kralj, S.; Zumer, S. Phys. Rev. E 2002, 65, 021705) who identify an early stage domain regime and a late stage confinement regime. For the domain and confinement regimes, Bradac et al. (Bradac, Z.; Kralj, S.; Zumer, S. Phys. Rev. E 2002, 65, 021705) obtained growth exponents of 0.49 +/- 0.05 and 0.25 +/- 0.05. These are remarkably close to the values 0.51 and 0.31 observed in our experiments.
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Affiliation(s)
- Xuemei Chen
- Mechanical and Aerospace Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, USA
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Ambrozic M, Kralj S, Virga EG. Defect-enhanced nematic surface order reconstruction. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2007; 75:031708. [PMID: 17500712 DOI: 10.1103/physreve.75.031708] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/13/2006] [Indexed: 05/15/2023]
Abstract
Within the Landau-de Gennes phenomenological theory, we study the influence of an applied electric field with average strength E{a} on the position of a nematic line defect with topological charge M=+/-1/2 in a hybrid cell. We explore the biaxial structure of the defect core and we describe its expulsion from the cell upon increasing E{a}. We show that prior to the expulsion the defect core displays dramatic changes for strong enough surface anchorings. At a critical value of E{a}, the core broadens and merges into a surface layer with a large biaxiality. This transition corresponds to the reconstruction of the nematic order already observed in the bulk in response to an applied electric field. A similar order reconstruction could take place even in the absence of defects, but at a higher threshold.
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Affiliation(s)
- Milan Ambrozic
- Engineering Ceramics Department, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
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16
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Svetec M, Kralj S, Bradac Z, Zumer S. Annihilation of nematic point defects: pre-collision and post-collision evolution. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2006; 20:71-9. [PMID: 16733641 DOI: 10.1140/epje/i2005-10120-9] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/11/2005] [Accepted: 03/27/2006] [Indexed: 05/09/2023]
Abstract
The annihilation of the nematic hedgehog and anti-hedgehog within an infinite cylinder of radius R is studied. The semi-microscopic lattice-type model and Brownian molecular dynamics are used. We distinguish among the i) early pre-collision, ii) late pre-collision, iii) early post-collision, and iv) late post-collision stages. In the pre-collision stage our results agree qualitatively with the existing experimental observations and also continuum-type simulations. The core of each defect exhibits a ring-like structure, where the ring axis is set perpendicular to the cylinder symmetry axis. For xi(0)d/(2R) > 1 the interaction between defects is negligible, where xi(0)d describes the initial separation of defects. Consequently, the defects annihilate within the simulation time window for xi(0)d/(2R) < 1. For close enough defects their separation scales as xi(d) [see text] (t(c)- t)(0.4+/-0.1), where t(c) stands for the collision time. In elastically anisotropic medium the hedgehog is faster than the anti-hedgehog. In the early pre-collision stage the defects can be treated as point-like particles, possessing inherent core structure, that interact via the nematic director field. In the late pre-collision stage the cores reflect the interaction between defects. After the collision a charge-less ring structure is first formed. In the early post-collision stage the ring adopts an essentially untwisted circular structure of the radius xi(r). In the late post-collision stage we observe two qualitatively different scenarios. For mu = xi(r)/R < mu(c) approximately 0.25 the ring collapses leading to the escaped radial equilibrium structure. For mu > mu(c) the chargeless ring triggers the nucleation growth into the planar polar structure with line defects.
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Affiliation(s)
- M Svetec
- Regional Development Agency Ltd., Lendavska 5a, 9000 Murska Sobota, Slovenia.
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Popa-Nita V, Kralj S. Random anisotropy nematic model: nematic-non-nematic mixture. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2006; 73:041705. [PMID: 16711825 DOI: 10.1103/physreve.73.041705] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/07/2005] [Indexed: 05/09/2023]
Abstract
The influence of a random-anisotropy- (RA-) type disorder on the phase separation of the nematogen-non-nematogen mixture is studied. A combination of the phenomenological Landau-de Gennes and Flory-Huggins theories is used. We assume that the non-nematogen component (i.e., impurity) enforces the RA disorder to the enclosing thermotropic liquid-crystal (LC) phase. The Imry-Ma argument is used according to which the lower-temperature phase exhibits a domain-type pattern. The disorder strength is measured in terms of the dimensionless parameter Lambda. We consider the case in which the LC molecules and impurities mix in the isotropic phase for Lambda=0. The impurities enforce a finite degree of orientational ordering even in the high-temperature paranematic phase. In the low-temperature phase they give rise to a domain-type structure, resulting in the distorted nematic (speronematic) phase. We show that the onset of orientational ordering increases the phase separation tendency. The RA field, however, opposes this tendency. With increasing value of Lambda the difference between the paranematic and speronematic ordering decreases. Consequently the structure of the phase-separated pattern can be much more complex in comparison to the Lambda=0 case.
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Affiliation(s)
- V Popa-Nita
- Faculty of Physics, University of Bucharest, P.O. Box MG-11, Bucharest 077125, Romania.
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Bradac Z, Kralj S, Svetec M, Zumer S. Annihilation of nematic point defects: postcollision scenarios. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2003; 67:050702. [PMID: 12786125 DOI: 10.1103/physreve.67.050702] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/22/2002] [Indexed: 05/24/2023]
Abstract
We perform a study of the annihilation of a nematic radial and hyperbolic point defects with the main focus on the confinement induced collision and postcollision scenarios. Brownian molecular dynamics on a semimicroscopic lattice is used. Initially a pair of defects, separated for 1.4-1.7 radii, is induced at the axis of the cylindrical capillary. In such a configuration defects start to approach slowly. In the early stage, their cores are negligibly influenced by the mutual interaction. When the distance becomes comparable to the nematic correlation length, the cores significantly deform. In the collision regime, defects gradually merge. We observe two qualitatively different scenarios in the postcollision regime, depending on the degree of (meta) stability of the initially imposed escaped structure with point defects.
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Affiliation(s)
- Zlatko Bradac
- Laboratory of Physics of Complex Systems, Faculty of Education, University of Maribor, Koroska 160, 2000 Maribor, Slovenia
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