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Safdari M, Zandi R, van der Schoot P. Dynamics of elongation of nematic tactoids in an electric field. Phys Rev E 2024; 109:054706. [PMID: 38907476 DOI: 10.1103/physreve.109.054706] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/24/2023] [Accepted: 04/22/2024] [Indexed: 06/24/2024]
Abstract
Nematic tactoids are spindle-shaped droplets of a nematic phase nucleated in the co-existing isotropic phase. According to equilibrium theory, their internal structure and shape are controlled by a balance between the elastic deformation of the director field, induced by the preferred anchoring of that director field to the interface, and the interfacial free energy. Recent experiments on tactoids of chitin nanocrystals dispersed in water show that electrical fields can very strongly elongate tactoids, at least if the tactoids are sufficiently large in volume. However, this observation contradicts the predictions of equilibrium theory as well as findings from Monte Carlo simulations that do not show this kind of extreme elongation to take place at all. To explain this, we put forward a relaxational model based on the Oseen-Frank free energy of elastic deformation of a director field coupled to an anisotropic surface free energy. In our model, we use two reaction coordinates to describe the director field and the extent of elongation of the droplets and evaluate the evolution of both as a function of time following the switching on of an electric field. Depending on the relative magnitude of the fundamental relaxation rates associated with the two reaction coordinates, we find that the aspect ratio of the drops may develop a large and very long-lived overshoot before eventually relaxing to the much smaller equilibrium value. In that case, the response of the curvature of the director field lags behind, explaining the experimental observations. Our theory describes the experimental data reasonably well.
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Kuhnhold A, van der Schoot P. Structure of nematic tactoids of hard rods. J Chem Phys 2022; 156:104501. [DOI: 10.1063/5.0078056] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We study by means of Monte Carlo simulations the internal structure of nematic droplets or tactoids formed by hard, rod-like particles in a gas of spherical ghost particles that act as depletion agents for the rods. We find that the shape and internal structure of tactoids are strongly affected by the size of the droplets. The monotonically increasing degree of nematic order with increasing particle density that characterizes the bulk nematic phase is locally violated and more so the smaller the tactoid. We also investigate the impact of an external quadrupolar alignment field on tactoids and find that this tends to make the director field more uniform, but not to very significantly increase the tactoid’s aspect ratio. This agrees with recent theoretical predictions yet is at variance with experimental observations and dynamical simulations. We explain this discrepancy in terms of competing relaxation times.
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Affiliation(s)
- Anja Kuhnhold
- Institute of Physics, University of Freiburg, 79104 Freiburg (Breisgau), Germany
| | - Paul van der Schoot
- Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
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Safdari M, Zandi R, van der Schoot P. Effect of electric fields on the director field and shape of nematic tactoids. Phys Rev E 2021; 103:062703. [PMID: 34271629 DOI: 10.1103/physreve.103.062703] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2021] [Accepted: 04/06/2021] [Indexed: 01/30/2023]
Abstract
Tactoids are spindle-shaped droplets of a uniaxial nematic phase suspended in the coexisting isotropic phase. They are found in dispersions of a wide variety of elongated colloidal particles, including actin, fd virus, carbon nanotubes, vanadium peroxide, and chitin nanocrystals. Recent experiments on tactoids of chitin nanocrystals in water show that electric fields can very strongly elongate tactoids even though the dielectric properties of the coexisting isotropic and nematic phases differ only subtly. We develop a model for partially bipolar tactoids, where the degree of bipolarness of the director field is free to adjust to optimize the sum of the elastic, surface, and Coulomb energies of the system. By means of a combination of a scaling analysis and a numerical study, we investigate the elongation and director field's behavior of the tactoids as a function of their size, the strength of the electric field, the surface tension, anchoring strength, the elastic constants, and the electric susceptibility anisotropy. We find that tactoids cannot elongate significantly due to an external electric field, unless the director field is bipolar or quasibipolar and somehow frozen in the field-free configuration. Presuming that this is the case, we find reasonable agreement with experimental data.
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Affiliation(s)
- Mohammadamin Safdari
- Department of Physics, University of California, Riverside, California 92521, USA
| | - Roya Zandi
- Department of Physics, University of California, Riverside, California 92521, USA
| | - Paul van der Schoot
- Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
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Metselaar L, Dozov I, Antonova K, Belamie E, Davidson P, Yeomans JM, Doostmohammadi A. Electric-field-induced shape transition of nematic tactoids. Phys Rev E 2017; 96:022706. [PMID: 28950460 DOI: 10.1103/physreve.96.022706] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2017] [Indexed: 06/07/2023]
Abstract
The occurrence of new textures of liquid crystals is an important factor in tuning their optical and photonics properties. Here, we show, both experimentally and by numerical computation, that under an electric field chitin tactoids (i.e., nematic droplets) can stretch to aspect ratios of more than 15, leading to a transition from a spindlelike to a cigarlike shape. We argue that the large extensions occur because the elastic contribution to the free energy is dominated by the anchoring. We demonstrate that the elongation involves hydrodynamic flow and is reversible: the tactoids return to their original shapes upon removing the field.
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Affiliation(s)
- Luuk Metselaar
- The Rudolf Peierls Centre for Theoretical Physics, 1 Keble Road, Oxford OX1 3NP, United Kingdom
| | - Ivan Dozov
- Laboratoire de Physique des Solides, Université Paris-Sud, Université Paris-Saclay, CNRS, UMR 8502, Orsay, France
| | - Krassimira Antonova
- Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia, Bulgaria
| | - Emmanuel Belamie
- Institut Charles Gerhardt Montpellier, ENSCM, Montpellier, France
| | - Patrick Davidson
- Laboratoire de Physique des Solides, Université Paris-Sud, Université Paris-Saclay, CNRS, UMR 8502, Orsay, France
| | - Julia M Yeomans
- The Rudolf Peierls Centre for Theoretical Physics, 1 Keble Road, Oxford OX1 3NP, United Kingdom
| | - Amin Doostmohammadi
- The Rudolf Peierls Centre for Theoretical Physics, 1 Keble Road, Oxford OX1 3NP, United Kingdom
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Everts JC, Punter MTJJM, Samin S, van der Schoot P, van Roij R. A Landau-de Gennes theory for hard colloidal rods: Defects and tactoids. J Chem Phys 2016; 144:194901. [DOI: 10.1063/1.4948785] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Affiliation(s)
- J. C. Everts
- Institute for Theoretical Physics, Center for Extreme Matter and Emergent Phenomena, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
| | | | - S. Samin
- Institute for Theoretical Physics, Center for Extreme Matter and Emergent Phenomena, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
| | - P. van der Schoot
- Institute for Theoretical Physics, Center for Extreme Matter and Emergent Phenomena, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
- Theory of Polymers and Soft Matter Group, Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
| | - R. van Roij
- Institute for Theoretical Physics, Center for Extreme Matter and Emergent Phenomena, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
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Rudyak VY, Emelyanenko AV, Loiko VA. Structure transitions in oblate nematic droplets. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 88:052501. [PMID: 24329282 DOI: 10.1103/physreve.88.052501] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/18/2013] [Revised: 10/01/2013] [Indexed: 06/03/2023]
Abstract
We consider the structure transitions in oblate supramicrometer nematic droplets related to reorientation of the line defect in the electric field. These transitions can be used in optical devices based on polymer dispersed liquid crystal materials with high contrast ratio. We suggest a simple method for determination of director distribution in nematic droplets of an arbitrary shape with surface interaction and in the presence of constant electric field. Point and linear defects are taken into account. This method does not require any presuppositions about symmetry of the director distribution. The elasticity continuum theory is treated with Monte Carlo annealing on a simple lattice. A special triangulation-based technique is applied for accurate representation of the droplet boundaries. The method is tested on 5CB material.
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Affiliation(s)
- V Yu Rudyak
- Department of Physics, M.V. Lomonosov Moscow State University, Moscow, 119991, Russia
| | - A V Emelyanenko
- Department of Physics, M.V. Lomonosov Moscow State University, Moscow, 119991, Russia
| | - V A Loiko
- B.I. Stepanov Institute of Physics of the National Academy of Sciences of Belarus, Nezalezhnasti Avenue 68, Minsk 220072, Belarus
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