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Huang Y, Wang W, Whitmer JK, Zhang R. Structures, thermodynamics and dynamics of topological defects in Gay-Berne nematic liquid crystals. SOFT MATTER 2023; 19:483-496. [PMID: 36533944 DOI: 10.1039/d2sm01178f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/17/2023]
Abstract
Topological defects are a ubiquitous phenomenon across different physical systems. A better understanding of defects can be helpful in elucidating the physical behaviors of many real materials systems. In nematic liquid crystals, defects exhibit unique optical signatures and can segregate impurities, showing their promise as molecular carriers and nano-reactors. Continuum theory and simulations have been successfully applied to link static and dynamical behaviors of topological defects to the material constants of the underlying nematic. However, further evidence and molecular details are still lacking. Here we perform molecular dynamics simulations of Gay-Berne particles, a model nematic, to examine the molecular structures and dynamics of +1/2 defects in a thin-film nematic. Specifically, we measure the bend-to-splay ratio K3/K1 using two independent, indirect measurements, showing good agreement. Next, we study the annihilation event of a pair of ±1/2 defects, of which the trajectories are consistent with experiments and hydrodynamic simulations. We further examine the thermodynamics of defect annihilation in an NVE ensemble, leading us to correctly estimate the elastic modulus by using the energy conservation law. Finally, we explore effects of defect annihilation in regions of nonuniform temperature within these coarse-grained molecular models which cannot be analysed by existing continuum level simulations. We find that +1/2 defects tend to move toward hotter areas and their change of speed in a temperature gradient can be quantitatively understood through a term derived from the temperature dependence of the elastic modulus. As such, our work has provided molecular insights into structures and dynamics of topological defects, presented unique and accessible methods to measure elastic constants by inspecting defects, and proposed an alternative control parameter of defects using temperature gradient.
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Affiliation(s)
- Yulu Huang
- Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China.
| | - Weiqiang Wang
- Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China.
| | - Jonathan K Whitmer
- Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA
| | - Rui Zhang
- Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China.
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Affiliation(s)
- Michael P. Allen
- Department of Physics, University of Warwick, Coventry, UK
- H. H. Wills Physics Laboratory, Royal Fort, Bristol, UK
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Sarman S, Wang YL, Laaksonen A. Shear flow simulations of smectic liquid crystals based on the Gay–Berne fluid and the soft sphere string-fluid. Phys Chem Chem Phys 2019; 21:292-305. [DOI: 10.1039/c8cp05077e] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Smectic liquid crystal undergoing shear flow.
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Affiliation(s)
- Sten Sarman
- Department of Materials and Environmental Chemistry
- Arrhenius Laboratory
- Stockholm University
- 106 91 Stockholm
- Sweden
| | - Yong-Lei Wang
- Department of Chemistry
- Stanford University
- Stanford
- USA
| | - Aatto Laaksonen
- Department of Materials and Environmental Chemistry
- Arrhenius Laboratory
- Stockholm University
- 106 91 Stockholm
- Sweden
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Chen W, Kulju S, Foster AS, Alava MJ, Laurson L. Boundary lubrication with a liquid crystal monolayer. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 90:012404. [PMID: 25122312 DOI: 10.1103/physreve.90.012404] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/17/2014] [Indexed: 06/03/2023]
Abstract
We study boundary lubrication characteristics of a liquid crystal (LC) monolayer sheared between two crystalline surfaces by nonequilibrium molecular dynamics simulations, using a simplified rigid bead-necklace model of the LC molecules. We consider LC monolayers confined by surfaces with three different atomic structures, subject to different shearing velocities, thus approximating a wide variety of materials and driving conditions. The time dependence of the friction force is studied and correlated with that of the orientational order exhibited by the LC molecules, arising from the competition between the effect of the structure of the confining surfaces and that of the imposed sliding direction. We show that the observed stick-slip events for low shear rates involve order-disorder transitions, and that the LC monolayer no longer has enough time to reorder at high shear rates, resulting in a smooth sliding regime. An irregular stick-slip phase between the regular stick-slip and smooth sliding is observed for intermediate shear rates regardless of the surface structure.
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Affiliation(s)
- W Chen
- COMP Centre of Excellence, Department of Applied Physics, Aalto University, P.O. Box 11100, FI-00076 AALTO, Espoo, Finland
| | - S Kulju
- Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33010 Tampere, Finland
| | - A S Foster
- COMP Centre of Excellence, Department of Applied Physics, Aalto University, P.O. Box 11100, FI-00076 AALTO, Espoo, Finland
| | - M J Alava
- COMP Centre of Excellence, Department of Applied Physics, Aalto University, P.O. Box 11100, FI-00076 AALTO, Espoo, Finland
| | - L Laurson
- COMP Centre of Excellence, Department of Applied Physics, Aalto University, P.O. Box 11100, FI-00076 AALTO, Espoo, Finland
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Sarman S, Laaksonen A. Director alignment relative to the temperature gradient in nematic liquid crystals studied by molecular dynamics simulation. Phys Chem Chem Phys 2014; 16:14741-9. [DOI: 10.1039/c4cp01451k] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
A nematic liquid crystal oriented by the temperature gradient.
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Affiliation(s)
- Sten Sarman
- Department of Materials and Environmental Chemistry
- Arrhenius Laboratory
- Stockholm University
- 106 91 Stockholm, Sweden
| | - Aatto Laaksonen
- Stellenbosch Institute of Advanced Studies (STIAS)
- Wallenberg Research Centre at Stellenbosch University
- Stellenbosch 7600, South Africa
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Sarman S, Laaksonen A. Thermomechanical coupling, heat conduction and director rotation in cholesteric liquid crystals studied by molecular dynamics simulation. Phys Chem Chem Phys 2013; 15:3442-53. [PMID: 23223192 DOI: 10.1039/c2cp42582c] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Affiliation(s)
- Sten Sarman
- Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm, Sweden.
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Börzsönyi T, Szabó B, Törös G, Wegner S, Török J, Somfai E, Bien T, Stannarius R. Orientational order and alignment of elongated particles induced by shear. PHYSICAL REVIEW LETTERS 2012; 108:228302. [PMID: 23003661 DOI: 10.1103/physrevlett.108.228302] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/23/2011] [Indexed: 05/03/2023]
Abstract
Shear induced alignment of elongated particles is studied experimentally and numerically. We show that shear alignment of ensembles of macroscopic particles is comparable even on a quantitative level to simple molecular systems, despite the completely different types of particle interactions. We demonstrate that for dry elongated grains the preferred orientation forms a small angle with the streamlines, independent of shear rate across three decades. For a given particle shape, this angle decreases with increasing aspect ratio of the particles. The shear-induced alignment results in a considerable reduction of the effective friction of the granular material.
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Affiliation(s)
- Tamás Börzsönyi
- Institute for Solid State Physics and Optics, Wigner Research Center for Physics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary.
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Sarman S, Laaksonen A. Twist viscosities and flow alignment of biaxial nematic liquid crystal phases of a soft ellipsoid-string fluid studied by molecular dynamics simulation. Phys Chem Chem Phys 2012; 14:11999-2013. [DOI: 10.1039/c2cp41456b] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Affiliation(s)
- Sten Sarman
- Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, 106 91 Stockholm, Sweden.
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Zakharov AV, Vakulenko AA, Romano S. Thermal and flexoelectric effects on nematodynamics in a microvolume cylindrical cavity. J Chem Phys 2010; 132:094901. [PMID: 20210410 DOI: 10.1063/1.3340505] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We have considered a homogeneously aligned liquid crystal (HALC) microvolume, confined between two infinitely long horizontal coaxial cylinders subjected to both a temperature gradient nabla T and radially applied electric field E. We have investigated dynamic field pumping, i.e., studied the interaction between director, velocity, electric fields, as well as, a radially applied temperature gradient, where the inner cylinder is kept at a lower temperature than the outer one. Flexoelectric polarization P has been taken into account as well, and modeled via the classical Meyer treatment. In order to elucidate the role of nabla T, E, and P in producing hydrodynamic flow, we have carried out a numerical study of a system of hydrodynamic equations including director reorientation, fluid flow, and temperature redistribution across the HALC cavity. Calculations show that there exists a range of parameter values (voltage and curvature of the inner cylinder) producing a kinklike orientation process in the system, as well as a nonstandard pumping regime with maximum flow near the hot cylinder.
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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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Sarman S, Laaksonen A. The temperature dependence of the heat conductivity of a liquid crystal studied by molecular dynamics simulation. Chem Phys Lett 2010. [DOI: 10.1016/j.cplett.2009.11.073] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
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