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Pieranski P, Godinho MH. Tropisms of the Dowser Texture. MATERIALS (BASEL, SWITZERLAND) 2020; 13:ma13204681. [PMID: 33096641 PMCID: PMC7589004 DOI: 10.3390/ma13204681] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/07/2020] [Revised: 10/12/2020] [Accepted: 10/14/2020] [Indexed: 06/11/2023]
Abstract
Due to its low symmetry C2v, the dowser texture is characterised by a 2D unitary vector field or alternatively by a unitary complex field. For the same symmetry reasons, the dowser texture is sensitive, in first order, to perturbations such as thickness gradients, electric fields or flows. We will focus on corresponding properties called respectively: cuneitropism, electrotropism and rheotropism. In particular we will show that topological defects, known as dowsons or monopoles, can be manipulated by means of these tropisms.
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Affiliation(s)
- Pawel Pieranski
- Laboratoire de Physique des Solides, Université Paris-Saclay, Bât. 510, 91405 Orsay, France
| | - Maria Helena Godinho
- Departamento Ciência dos Materiais, CENIMAT, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal;
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Tang X, Selinger JV. Minimization principle for shear alignment of liquid crystals. Phys Rev E 2020; 101:032701. [PMID: 32289986 DOI: 10.1103/physreve.101.032701] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/26/2019] [Accepted: 02/19/2020] [Indexed: 11/07/2022]
Abstract
If a static perturbation is applied to a liquid crystal, then the director configuration changes to minimize the free energy. If a shear flow is applied to a liquid crystal, then one might ask: Does the director configuration change to minimize any effective potential? To address that question, we derive the Leslie-Ericksen equations for dissipative dynamics and determine whether they can be expressed as relaxation toward a minimum. The answer may be yes or no, depending on the number of degrees of freedom. Using theory and simulations, we consider two specific examples, reverse tilt domains under simple shear flow and dowser configurations under plane Poiseuille flow, and we demonstrate that each example shows relaxation toward the minimum of an effective potential.
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Affiliation(s)
- Xingzhou Tang
- Department of Physics, Advanced Materials and Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
| | - Jonathan V Selinger
- Department of Physics, Advanced Materials and Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
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Kos Ž, Ravnik M. Field generated nematic microflows via backflow mechanism. Sci Rep 2020; 10:1446. [PMID: 31996700 PMCID: PMC6989461 DOI: 10.1038/s41598-020-57944-5] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2019] [Accepted: 11/26/2019] [Indexed: 01/08/2023] Open
Abstract
Generation of flow is an important aspect in microfluidic applications and generally relies on external pumps or embedded moving mechanical parts which pose distinct limitations and protocols on the use of microfluidic systems. A possible approach to avoid moving mechanical parts is to generate flow by changing some selected property or structure of the fluid. In fluids with internal orientational order such as nematic liquid crystals, this process of flow generation is known as the backflow effect. In this article, we demonstrate the contact-free generation of microfluidic material flows in nematic fluids -including directed contact-free pumping- by external electric and optical fields based on the dynamic backflow coupling between nematic order and material flow. Using numerical modelling, we design efficient shaping and driving of the backflow-generated material flow using spatial profiles and time modulations of electric fields with oscillating amplitude, rotating electric fields and optical fields. Particularly, we demonstrate how such periodic external fields generate efficient net average nematic flows through a microfluidic channel, that avoid usual invariance under time-reversal limitations. We show that a laser beam with rotating linear polarization can create a vortex-like flow structure and can act as a local flow pump without moving mechanical parts. The work could be used for advanced microfluidic applications, possibly by creating custom microfluidic pathways without predefined channels based on the adaptivity of an optical set-up, with a far reaching unconventional idea to realize channel-less microfluidics.
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Affiliation(s)
- Žiga Kos
- University of Ljubljana, Faculty of Mathematics and Physics, Jadranska 19, 1000, Ljubljana, Slovenia
| | - Miha Ravnik
- University of Ljubljana, Faculty of Mathematics and Physics, Jadranska 19, 1000, Ljubljana, Slovenia.
- Jožef Stefan Institute, Jamova 39, 1000, Ljubljana, Slovenia.
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Čopar S, Kos Ž, Emeršič T, Tkalec U. Microfluidic control over topological states in channel-confined nematic flows. Nat Commun 2020; 11:59. [PMID: 31896755 PMCID: PMC6940393 DOI: 10.1038/s41467-019-13789-9] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2019] [Accepted: 11/28/2019] [Indexed: 12/02/2022] Open
Abstract
Compared to isotropic liquids, orientational order of nematic liquid crystals makes their rheological properties more involved, and thus requires fine control of the flow parameters to govern the orientational patterns. In microfluidic channels with perpendicular surface alignment, nematics discontinuously transition from perpendicular structure at low flow rates to flow-aligned structure at high flow rates. Here we show how precise tuning of the driving pressure can be used to stabilize and manipulate a previously unresearched topologically protected chiral intermediate state which arises before the homeotropic to flow-aligned transition. We characterize the mechanisms underlying the transition and construct a phenomenological model to describe the critical behaviour and the phase diagram of the observed chiral flow state, and evaluate the effect of a forced symmetry breaking by introduction of a chiral dopant. Finally, we induce transitions on demand through channel geometry, application of laser tweezers, and careful control of the flow rate. It is interesting phenomenon that chiral order can emerge in intrinsically achiral liquid crystals. Here Čopar et al. demonstrate achiral-to-chiral transition of the nematic liquid crystals flow in microfluidic channels and their behaviour, stability, and dependence on geometric and material parameters.
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Affiliation(s)
- Simon Čopar
- Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000, Ljubljana, Slovenia
| | - Žiga Kos
- Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000, Ljubljana, Slovenia
| | - Tadej Emeršič
- Faculty of Medicine, Institute of Biophysics, University of Ljubljana, Vrazov trg 2, 1000, Ljubljana, Slovenia.,Jožef Stefan Institute, Jamova 39, 1000, Ljubljana, Slovenia
| | - Uroš Tkalec
- Faculty of Medicine, Institute of Biophysics, University of Ljubljana, Vrazov trg 2, 1000, Ljubljana, Slovenia. .,Jožef Stefan Institute, Jamova 39, 1000, Ljubljana, Slovenia. .,Faculty of Natural Sciences and Mathematics, University of Maribor, Koroška 160, 2000, Maribor, Slovenia.
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Pieranski P, Godinho MH. Electro-osmosis and flexo-electricity in the dowser texture. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2019; 42:69. [PMID: 31147854 DOI: 10.1140/epje/i2019-11832-9] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/24/2019] [Accepted: 04/30/2019] [Indexed: 06/09/2023]
Abstract
Due to its low symmetry, the long-lived pseudo-planar texture, dubbed "the dowser texture", has a flexo-electric spontaneous polarisation [Formula: see text]. Being degenerated, the dowser texture is easily aligned by the electric torque [Formula: see text] acting on [Formula: see text]. The dowser texture can also be aligned by Poiseuille flows driven by electro-osmosis. The hydrodynamic torques due to the electro-osmotic flows are linear in [Formula: see text] like the electrical one. It is shown that in 5CB the electro-osmotic flows can alter measurements of the flexo-electric polarisation.
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Affiliation(s)
- Pawel Pieranski
- Laboratoire de Physique des Solides, Université Paris-Sud, Bât. 510, 91405, Orsay, France.
| | - Maria Helena Godinho
- CENIMAT, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829 - 516, Caparica, Portugal
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Pieranski P, Godinho MH. Flexo-electricity of the dowser texture. SOFT MATTER 2019; 15:1469-1480. [PMID: 30607421 DOI: 10.1039/c8sm02329h] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
The persistent quasi-planar nematic texture known also as the dowser texture is characterized by a 2D unitary vector field d. We show here that the dowser texture is sensitive, in first order, to electric fields. This property is due to the flexo-electric polarisation P collinear with d expected from R. B. Meyer's considerations on flexo-electricity in nematics. It is pointed out that due to the flexo-electric polarisation nematic monopoles can be manipulated by electric fields of appropriate geometry.
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Affiliation(s)
- Pawel Pieranski
- Laboratoire de Physique des Solides, Université Paris-Sud, 91405 Orsay, France.
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Emeršič T, Zhang R, Kos Ž, Čopar S, Osterman N, de Pablo JJ, Tkalec U. Sculpting stable structures in pure liquids. SCIENCE ADVANCES 2019; 5:eaav4283. [PMID: 30793033 PMCID: PMC6377271 DOI: 10.1126/sciadv.aav4283] [Citation(s) in RCA: 17] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/14/2018] [Accepted: 12/21/2018] [Indexed: 05/19/2023]
Abstract
Pure liquids in thermodynamic equilibrium are structurally homogeneous. In liquid crystals, flow and light pulses are used to create reconfigurable domains with polar order. Moreover, through careful engineering of concerted microfluidic flows and localized optothermal fields, it is possible to achieve complete control over the nucleation, growth, and shape of such domains. Experiments, theory, and simulations indicate that the resulting structures can be stabilized indefinitely, provided the liquids are maintained in a controlled nonequilibrium state. The resulting sculpted liquids could find applications in microfluidic devices for selective encapsulation of solutes and particles into optically active compartments that interact with external stimuli.
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Affiliation(s)
- Tadej Emeršič
- Institute of Biophysics, Faculty of Medicine, University of Ljubljana, Vrazov trg 2, 1000 Ljubljana, Slovenia
| | - Rui Zhang
- Institute for Molecular Engineering, University of Chicago, Chicago, IL 60637, USA
| | - Žiga Kos
- Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
| | - Simon Čopar
- Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
| | - Natan Osterman
- Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
- Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
| | - Juan J. de Pablo
- Institute for Molecular Engineering, University of Chicago, Chicago, IL 60637, USA
- Material Science Division, Argonne National Laboratory, Lemont, IL 60439, USA
- Corresponding author. (J.J.d.P.); (U.T.)
| | - Uroš Tkalec
- Institute of Biophysics, Faculty of Medicine, University of Ljubljana, Vrazov trg 2, 1000 Ljubljana, Slovenia
- Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
- Faculty of Natural Sciences and Mathematics, University of Maribor, Koroška 160, 2000 Maribor, Slovenia
- Corresponding author. (J.J.d.P.); (U.T.)
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Kitzerow H. Pawel Pieranski – crystallographer of liquids and Alfred-Saupe-prize laureate 2019. LIQUID CRYSTALS TODAY 2019. [DOI: 10.1080/1358314x.2019.1625161] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Affiliation(s)
- Heinz Kitzerow
- Center for Optoelectronics and Photonics (CeOPP) and Department of Chemistry, Paderborn University, Paderborn, Germany
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