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Bartsch H, Bier M, Dietrich S. Interface structures in ionic liquid crystals. SOFT MATTER 2019; 15:4109-4126. [PMID: 31080982 DOI: 10.1039/c9sm00062c] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Ionic liquid crystals (ILCs) are anisotropic mesogenic molecules which additionally carry charges. This combination gives rise to a complex interplay of the underlying (anisotropic) contributions to the pair interactions. It promises interesting and distinctive structural and orientational properties to arise in systems of ILCs, combining properties of liquid crystals and ionic liquids. While previous theoretical studies have focused on the phase behavior of ILCs and the structure of the respective bulk phases, in the present study we provide new results, obtained within density functional theory, concerning (planar) free interfaces between an isotropic liquid L and two types of smectic-A phases (SA or SAW). We discuss the structural and orientational properties of these interfaces in terms of the packing fraction profile η(r) and the orientational order parameter profile S2(r) concerning the tilt angle α between the (bulk) smectic layer normal and the interface normal. The asymptotic decay of η(r) and of S2(r) towards their values in the isotropic bulk is discussed, too.
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Affiliation(s)
- Hendrik Bartsch
- Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, 70569, Stuttgart, Germany.
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2
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Huang CC, Baus M, Ryckaert JP. On the calculation of the absolute grand potential of confined smectic-A phases. Mol Phys 2015. [DOI: 10.1080/00268976.2015.1028500] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Affiliation(s)
- Chien-Cheng Huang
- Physique des Polymères, Université Libre de Bruxelles , Brussels, Belgium
| | - Marc Baus
- Physique des Polymères, Université Libre de Bruxelles , Brussels, Belgium
| | - Jean-Paul Ryckaert
- Physique des Polymères, Université Libre de Bruxelles , Brussels, Belgium
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3
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Praetorius S, Voigt A, Wittkowski R, Löwen H. Structure and dynamics of interfaces between two coexisting liquid-crystalline phases. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 87:052406. [PMID: 23767553 DOI: 10.1103/physreve.87.052406] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/01/2013] [Indexed: 06/02/2023]
Abstract
A phase-field-crystal model is used to access the structure and thermodynamics of interfaces between two coexisting liquid-crystalline phases in two spatial dimensions. Depending on the model parameters, there is a variety of possible coexistences between two liquid-crystalline phases, including a plastic triangular crystal (PTC). Here, we numerically calculate the profiles for the mean density and for the nematic order tensor across the interface for isotropic-PTC and columnar-PTC (or equivalently smectic-A-PTC) phase coexistence. As a general finding, the width of the interface with respect to the nematic order parameter characterizing the orientational order is larger than the width of the mean-density interface. In approaching the interface from the PTC side, at first, the mean density goes down, and then the nematic order parameter follows. The relative shift in the two profiles can be larger than a full lattice constant of the plastic crystal. Finally, we also present numerical results for the dynamic relaxation of an initial order-parameter profile towards its equilibrium interfacial profile. Our predictions for the interfacial profiles can, in principle, be verified in real-space experiments of colloidal dispersions.
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Affiliation(s)
- Simon Praetorius
- Institute of Scientific Computing, Technical University Dresden, D-01062 Dresden, Germany
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4
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Zakharov AV, Vakunenko AA. Influence of an external field on the surface tension of free-standing smectic films. Phys Rev E 2012; 86:031701. [PMID: 23030927 DOI: 10.1103/physreve.86.031701] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2012] [Revised: 08/01/2012] [Indexed: 11/07/2022]
Abstract
We have carried out a theoretical study of the effect of the electric field E on the thermodynamical properties and surface tension γ of free-standing smectic films. Calculations, based upon the extended McMillan approach with anisotropic forces, show a stepwise reduction and increase of the values of the Helmholtz free energy f and γ, respectively, per partially fluorinated 5-n-alkyl-2-[4-n-(perfluoroalkyl-metheleneoxy)phenyl] (H10F5MOPP) molecule, as the temperature is raised above that for the bulk smectic-A-isotropic transition. Calculations show that E may not only affect the layer-thinning transition sequence, but also change the first multilayer jump in the thickness and increase the value of γ per H10F5MOPP molecule. Reasonable agreement between the theoretically predicted and the experimentally obtained data on γ of the partially fluorinated H10F5MOPP film has been obtained.
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Affiliation(s)
- A V Zakharov
- Saint Petersburg Institute for Machine Sciences, The Russian Academy of Sciences, Saint Petersburg 199178, Russia.
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5
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Zakharov AV, Sullivan DE. Transition entropy, Helmholtz free energy, and heat capacity of free-standing smectic films above the bulk smectic-A-isotropic transition temperature: a mean-field treatment. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 82:041704. [PMID: 21230292 DOI: 10.1103/physreve.82.041704] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/24/2009] [Revised: 07/02/2010] [Indexed: 05/30/2023]
Abstract
We have carried out a numerical study of both the structural and thermodynamic properties of free-standing smectic films (FSSFs) for two cases of enhanced pair interactions in the bounding layers. Calculations, based upon the extended McMillan's approach with anisotropic forces, shows a stepwise reduction of the value of the heat capacity as the temperature is raised above the bulk smectic A-isotropic transition. The effects of surface "enhanced" pair interactions in the bounding layers and of film thickness on the orientational and translational order parameters, the Helmholtz free energy, and entropy of FSSFs have also been investigated. Reasonable agreement between the theoretically predicted and the experimentally obtained--by means of calorimetric techniques--data on the heat capacity of the partially fluorinated 5-n-alkyl-2-(4-n-(perfluoroalkyl-metheleneoxy)phenyl) (H10F5MOPP) films has been obtained.
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Affiliation(s)
- A V Zakharov
- Saint Petersburg Institute for Machine Sciences, The Russian Academy of Sciences, Saint Petersburg 199178, Russia.
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de las Heras D, Martínez-Ratón Y, Velasco E. Surface and smectic layering transitions in binary mixtures of parallel hard rods. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 81:021706. [PMID: 20365581 DOI: 10.1103/physreve.81.021706] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/13/2009] [Indexed: 05/29/2023]
Abstract
The surface phase behavior of binary mixtures of colloidal hard rods in contact with a solid substrate (hard wall) is studied, with special emphasis on the region of the phase diagram that includes the smectic A phase. The colloidal rods are modeled as hard cylinders of the same diameter and different lengths, in the approximation of perfect alignment. A fundamental-measure density functional is used to obtain equilibrium density profiles and thermodynamic properties such as surface tensions and adsorption coefficients. The bulk phase diagram exhibits nematic-smectic and smectic-smectic demixing, with smectic phases having different compositions; in some cases they are microfractionated. The calculated surface phase diagram of the wall-nematic interface shows a very rich phase behavior, including layering transitions and complete wetting at high pressures, whereby an infinitely thick smectic film grows at the wall via an infinite sequence of stepwise first-order layering transitions. For lower pressures complete wetting also obtains, but here the smectic film grows in a continuous fashion. Finally, at very low pressures, the wall-nematic interface exhibits critical adsorption by the smectic phase, due to the second-order character of the bulk nematic-smectic transition.
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Affiliation(s)
- Daniel de las Heras
- Departamento de Física Teórica de Materia Condensada, Universidad Autónoma de Madrid, E-28049 Madrid, Spain.
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7
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Kadivar E. Influence of the magnetic field on isotropic wetting behavior of a nematic liquid crystal. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2008; 78:031706. [PMID: 18851054 DOI: 10.1103/physreve.78.031706] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/01/2008] [Indexed: 05/26/2023]
Abstract
I present a theoretical investigation of the temperature and magnetic field dependence of isotropic (paranematic) wetting layers close to an aligning substrate within a semi-infinite nematic liquid crystal with positive magnetic anisotropy under condition of weak homeotropic anchoring. Using the Landau-de Gennes model supplement by Nobili-Durand surface free energy, the existence and stability of paranematic wetting layers close to the substrate and below the nematic-isotropic temperature are discussed. Numerical results are presented showing the phase diagram for the isotropic (paranematic), nematic, and wetting layer states. In the present work, the dependence of the transition kind to the magnetic field is discussed.
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Affiliation(s)
- Erfan Kadivar
- Department of Physics, Faculty of Sciences, Persian Gulf University, 75168 Bushehr, Iran.
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8
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Tarazona P, Cuesta J, Martínez-Ratón Y. Density Functional Theories of Hard Particle Systems. THEORY AND SIMULATION OF HARD-SPHERE FLUIDS AND RELATED SYSTEMS 2008. [DOI: 10.1007/978-3-540-78767-9_7] [Citation(s) in RCA: 75] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
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Canabarro AA, de Oliveira IN, Lyra ML. Homeotropic surface anchoring and the layer-thinning transition in free-standing films. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2008; 77:011704. [PMID: 18351863 DOI: 10.1103/physreve.77.011704] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/04/2007] [Indexed: 05/26/2023]
Abstract
In this work, we investigate the interplay between surface anchoring and finite-size effects on the smectic-isotropic transition in free-standing smectic films. Using an extended McMillan model, we study how a homeotropic anchoring stabilizes the smectic order above the bulk transition temperature. In particular, we determine how the transition temperature depends on the surface ordering and film thickness. We identify a characteristic anchoring for which the transition temperature does not depend on the film thickness. For strong surface ordering, we found that the thickness dependence of the transition temperature can be well represented by a power-law relation. The power-law exponent exhibits a weak dependence on the range of film thicknesses, as well as on the molecular alkyl tail length. Our results reproduce the main experimental findings concerning the layer-thinning transitions in free-standing smectic films.
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Affiliation(s)
- A A Canabarro
- Instituto de Física, Universidade Federal de Alagoas, 57072-970 Maceió, Alagoas, Brazil
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Bahr C. Surface triple points and multiple-layer transitions observed by tuning the surface field at smectic liquid-crystal-water interfaces. PHYSICAL REVIEW LETTERS 2007; 99:057801. [PMID: 17930793 DOI: 10.1103/physrevlett.99.057801] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/20/2007] [Indexed: 05/25/2023]
Abstract
We present an ellipsometric study of the interface between a smectic liquid crystal and water in the presence of a nonionic surfactant. The surfactant concentration serves as a handle to tune the surface field. For sufficiently large surfactant concentrations, a smectic phase is present at the interface in the temperature range above the smectic-A-isotropic bulk transition; when the bulk transition is approached, the thickness of this surface phase grows via a series of layer-by-layer transitions at which single smectic layers are formed. At lower surfactant concentrations, transitions appear at which the thickness of the surface phase jumps by multiple smectic layers, thereby implying the existence of triple points at which surface phases with different smectic layer numbers coexist. This is the first experimental demonstration of such surface triple points which are predicted by theoretical models.
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Affiliation(s)
- Ch Bahr
- Max Planck Institute for Dynamics and Self-Organization, Bunsenstrasse 10, D-37073 Göttingen, Germany.
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11
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Kadivar E, Bahr C, Stark H. Crossover in the wetting behavior at surfactant-laden liquid-crystal-water interfaces: experiment and theory. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2007; 75:061711. [PMID: 17677287 DOI: 10.1103/physreve.75.061711] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/22/2007] [Indexed: 05/16/2023]
Abstract
The behavior of a nematic liquid crystal at a surfactant-laden interface to an aqueous phase is studied under the condition of homeotropic anchoring. It is shown that with decreasing surfactant concentration the system shifts from surface-enhanced to surface-decreased order, i.e., the behavior changes from complete nematic wetting when the nematic-isotropic phase transition is approached from above to a different wetting behavior below the transition, characterized by a considerably decreased Maier-Saupe order parameter at the interface. The experimental behavior is analyzed within the framework of the Landau-de Gennes theory supplemented by a surface free energy, in which the wetting behavior is controlled by the magnitude of the anchoring strength and the preferred surface order parameter in comparison to the bulk order parameter. The theoretical modeling is able to account for all experimental observations.
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Affiliation(s)
- Erfan Kadivar
- Max Planck Institute for Dynamics and Self-Organization, Bunsenstr. 10, D-37073 Göttingen, Germany
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12
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Correia NT, Moura Ramos JJ, Adão MHCV, Saramago BJV. Temperature Dependence of the Surface Behaviour of a Side-Chain Liquid Crystalline Polymer Probed by Contact Angle Measurements. ACTA ACUST UNITED AC 2006. [DOI: 10.1080/10587259708042338] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Affiliation(s)
- Natália T. Correia
- a Centro de Química-Fisica Molecular , Complexo I, IST, Av. Rovisco Pais, 1096 , Lisboa Codex , Portugal
| | - Joaquim J. Moura Ramos
- a Centro de Química-Fisica Molecular , Complexo I, IST, Av. Rovisco Pais, 1096 , Lisboa Codex , Portugal
| | - Maria Helena C. V. Adão
- b Centro de Química Estrutural , Complexo I, IST, Av. Rovisco Pais, 1096 , Lisboa Codex , Portugal
| | - Benilde J. V. Saramago
- b Centro de Química Estrutural , Complexo I, IST, Av. Rovisco Pais, 1096 , Lisboa Codex , Portugal
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13
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Cheung DL, Schmid F. A density-functional theory study of the confined soft ellipsoid fluid. J Chem Phys 2004; 120:9185-91. [PMID: 15267855 DOI: 10.1063/1.1703522] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
A system of soft ellipsoid molecules confined between two planar walls is studied using classical density-functional theory. Both the isotropic and nematic phases are considered. The excess free energy is evaluated using two different Ansätze and the intermolecular interaction is incorporated using two different direct correlation functions (DCF's). The first is a numerical DCF obtained from simulations of bulk soft ellipsoid fluids and the second is taken from the Parsons-Lee theory. In both the isotropic and nematic phases the numerical DCF gives density and order parameter profiles in reasonable agreement with simulation. The Parsons-Lee DCF also gives reasonable agreement in the isotropic phase but poor agreement in the nematic phase.
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Affiliation(s)
- David L Cheung
- Theoretische Physik, Universitat Bielefeld, D-33615 Bielefeld, Germany
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14
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Steuer H, Hess S, Schoen M. Phase behavior of liquid crystals confined by smooth walls. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2004; 69:031708. [PMID: 15089311 DOI: 10.1103/physreve.69.031708] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/12/2003] [Indexed: 05/24/2023]
Abstract
Monte Carlo simulations for a simple model liquid crystal are presented. The influence of flat walls on the phase behavior is analyzed for two different anchoring mechanisms, one favoring homeotropic alignment and one simulating a twisted nematic cell without external fields, e.g., two walls with different homogeneous planar alignment. The simulations are performed in the constant pressure ensemble. The box volume may change in the directions perpendicular to the wall normal. The isotropic-nematic phase transition in the bulk system is first studied for different isobars. For the weak first order transition we do not observe any hysteresis down to a temperature accuracy of deltaT=0.001. The isotherm T=1 is then studied in the bulk as well as in the confined geometries. The walls stabilize the positional order in the systems due to the formation of layers. The orientational order is weakly stabilized.
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Affiliation(s)
- Haiko Steuer
- Institut für Theoretische Physik, Technische Universität Berlin, PN7-1, Hardenbergstrasse 36, D-10623 Berlin, Germany.
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Lucht R, Marczuk P, Bahr C, Findenegg GH. X-ray reflectivity study of smectic wetting and prewetting at the free surface of isotropic liquid crystals. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2001; 63:041704. [PMID: 11308864 DOI: 10.1103/physreve.63.041704] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/15/2000] [Indexed: 05/23/2023]
Abstract
We study the structures of free-surface-wetting layers above the isotropic to smectic-A transition of three liquid-crystal compounds that show different kinds of growth of the wetting film as the bulk transition is approached: layer-by-layer, continuous, and continuous with prewetting. The smectic-A surface phase of the layer-by-layer compound consists of well-defined layers and possesses a sharp boundary to the isotropic bulk phase, whereas in the two continuous compounds sinusoidal density oscillations with a continuously decaying amplitude are found. In the continuous case with prewetting, the wetting film below the prewetting transition does not show an essential difference to the continuous case without prewetting.
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Affiliation(s)
- R Lucht
- Institute of Physical Chemistry, University Marburg, D-35032 Marburg, Germany
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Lucht R, Bahr C, Heppke G. Wetting behavior above the liquid-crystal-isotropic transition in a homologous series. PHYSICAL REVIEW. E, STATISTICAL PHYSICS, PLASMAS, FLUIDS, AND RELATED INTERDISCIPLINARY TOPICS 2000; 62:2324-2333. [PMID: 11088700 DOI: 10.1103/physreve.62.2324] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/17/2000] [Indexed: 05/23/2023]
Abstract
An ellipsometric study of the wetting behavior at the free surface above the isotropic to nematic or isotropic to smectic-A transition of nine homologous compounds with even alkyl chain lengths n in the range from four to twenty carbon atoms is presented. All compounds show a pretransitional increase of the nematic or smectic surface coverage as the bulk isotropic to liquid-crystal transition is approached from above. The behavior of the nematic compounds (n=4 to 10) can be interpreted, within the framework of a Landau model, as complete wetting. In short nematic homologs the divergence of the nematic coverage is strongly reduced by a decrease of the nematic susceptibility of the isotropic phase. The elastic coefficient L of the Landau model shows a pronounced increase with increasing n, resulting in the occurrence of a discontinuous prewetting transition in the shortest smectic homolog (n=12) that is still describable by the nematic Landau model. In the longer smectic homologs (n=14 to 20), layering steps appear in the pretransitional increase of the coverage. The results indicate probable partial wetting for the longest homolog, whereas for the other smectic compounds the distinction between complete and partial wetting is difficult on the basis of ellipsometry.
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Affiliation(s)
- R Lucht
- Institute of Physical Chemistry, University Marburg, D-35032 Marburg, Germany
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Lucht R, Bahr C, Heppke G. Layering Transitions at the Free Surface of a Smectic Liquid Crystal. J Phys Chem B 1998. [DOI: 10.1021/jp981832a] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- R. Lucht
- Institute of Physical Chemistry, University Marburg, D-35032 Marburg, Germany
| | - Ch. Bahr
- Institute of Physical Chemistry, University Marburg, D-35032 Marburg, Germany
| | - G. Heppke
- I.-N.-Stranski-Institut of Physical and Theoretical Chemistry, Technical University Berlin, D-10623 Berlin, Germany
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Lucht R, Bahr C, Heppke G, Goodby JW. Variety of wetting behaviors at the free surface of isotropic liquid crystals. J Chem Phys 1998. [DOI: 10.1063/1.475765] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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21
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Hamraoui A, Privat M. Prewetting transition influenced by layeringlike transitions at a binary liquid/solid interface. J Chem Phys 1997. [DOI: 10.1063/1.474933] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
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Martinez Y, Somoza AM, Mederos L, Sullivan DE. Metastability of freely suspended liquid-crystal films. PHYSICAL REVIEW. E, STATISTICAL PHYSICS, PLASMAS, FLUIDS, AND RELATED INTERDISCIPLINARY TOPICS 1996; 53:2466-2472. [PMID: 9964532 DOI: 10.1103/physreve.53.2466] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
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23
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Martínez-Ratón Y, Somoza AM, Mederos L, Sullivan DE. Smectic-A ordering in liquid crystal films. Faraday Discuss 1996. [DOI: 10.1039/fd9960400111] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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