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Zheng W. Anchoring-mediated stick-slip winding of cholesteric liquid crystals. Phys Rev E 2024; 109:044701. [PMID: 38755887 DOI: 10.1103/physreve.109.044701] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2023] [Accepted: 03/13/2024] [Indexed: 05/18/2024]
Abstract
The stick-slip phenomenon widely exists in contact mechanics, from the macroscale to the nanoscale. During cholesteric-nematic unwinding by external fields, there is controversy regarding the role of planar surface anchoring, which may induce discontinuous stick-slip behaviors despite the well-known continuous transitions observed in past experiments. Here we observe three regimes, namely, constrained, stick-slip, and sliding-slip, under mechanical winding with different anchoring conditions, and measure the corresponding forces by the surface force balance. These behaviors result from a balance of cholesteric elastic torque and surface torque, reminiscent of the slip morphology on frictional substrates [T. G. Sano et al., Phys. Rev. Lett. 118, 178001 (2017)10.1103/PhysRevLett.118.178001], and provide evidence of dynamics in static rotational friction.
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Affiliation(s)
- Weichao Zheng
- Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ, United Kingdom
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2
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Oswald P. Surface-field-induced heliconical instability in the cholesteric phase of a mixture of a flexible dimer (CB7CB) and a rodlike molecule (8CB). Phys Rev E 2022; 105:024704. [PMID: 35291181 DOI: 10.1103/physreve.105.024704] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2021] [Accepted: 12/01/2021] [Indexed: 06/14/2023]
Abstract
In 1968, de Gennes and Meyer independently predicted that a cholesteric phase can form a stable oblique helicoidal (or heliconical) structure provided that K_{3}<K_{2} where K_{3} (K_{2}) is the bend (twist) constant. This structure usually develops under electric field when the material is of positive dielectric anisotropy and was observed for the first time in 2014 by Xiang et al. in a cholesteric phase made of a liquid crystal dimer material (CB7CB) in which K_{3} is anomalously small. Following a recent theoretical prediction by Poy and Žumer, I show that confining a similar cholesteric phase between two glass plates treated for unidirectional anchoring can lead to a similar heliconical instability. In that case, the confinement induces a surface field that acts as an effective electric field E with E≡1/d where d is the sample thickness. The experiment was conducted in a mixture of CB7CB +50 wt% 8CB doped with a small amount of the chiral molecule R811. In addition, I show that this mixture presents an unexpected compensation point near the transition to the N_{TB} phase.
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Affiliation(s)
- Patrick Oswald
- Univ Lyon, ENS de Lyon, Univ Claude Bernard, CNRS, Laboratoire de Physique, F-69342 Lyon, France
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3
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Oswald P, Dequidt A. Heliconical-fluctuation-induced compensation point in the diluted cholesteric phase of mixtures containing the flexible dimer CB7CB. Phys Rev E 2022; 105:024707. [PMID: 35291088 DOI: 10.1103/physreve.105.024707] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2021] [Accepted: 02/14/2022] [Indexed: 06/14/2023]
Abstract
We show experimentally and theoretically that the heliconical fluctuations that develop in a cholesteric phase (Ch) close to a transition to a chiral twist-bend nematic phase (N_{TB}) may lead to the appearance of a compensation point. At this point, the equilibrium twist of the cholesteric phase vanishes and changes sign. Mixtures of the flexible dimer CB7CB and the rodlike molecules 8CB or 5CB, doped with a small amount of the chiral molecules R811, S2011, CC, or CB15, are used in experiments to determine the conditions for the appearance of a compensation point.
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Affiliation(s)
- Patrick Oswald
- Université Lyon, ENS de Lyon, Université Claude Bernard, CNRS, Laboratoire de Physique, F-69342 Lyon, France
| | - Alain Dequidt
- Université Clermont Auvergne, CNRS, SIGMA Clermont, ICCF, F-63000 Clermont-Ferrand, France
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Oswald P. Role of an oscillatory electric field on the Lehmann rotation of cholesteric droplets. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2020; 43:11. [PMID: 32048057 DOI: 10.1140/epje/i2020-11935-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/03/2019] [Accepted: 01/29/2020] [Indexed: 06/10/2023]
Abstract
This paper deals with the Lehmann rotation of banded cholesteric droplets subjected to a temperature gradient when they coexist with their own isotropic liquid. I show that their angular rotation velocity increases --in absolute value-- when they are subjected to an additional AC electric field in the conducting regime. This velocity increase is correlated with a prolate distortion of the droplets and the probable presence of electrohydrodynamical toroidal circulation flows inside and outside the droplets. I propose that the coupling between these flows and the director field is responsible for the increase of the angular velocity of the texture. The origin of these flows is discussed qualitatively in the framework of the leaky dielectric model by taking into account the generation of charges both in the bulk via a Carr-Helfrich mechanism (Tarasov, Krekhow and Kramer model) and at the surface of the droplet (Taylor-Melcher model).
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Affiliation(s)
- P Oswald
- Univ Lyon, ENS de Lyon, Univ Claude Bernard, CNRS, Laboratoire de Physique, F-69342, Lyon, France.
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Boniello G, Luo Y, Beller DA, Serra F, Stebe KJ. Colloids in confined liquid crystals: a plot twist in the lock-and-key mechanism. SOFT MATTER 2019; 15:5220-5226. [PMID: 31172164 DOI: 10.1039/c9sm00788a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
By confining soft materials within tailored boundaries it is possible to design energy landscapes to address and control colloidal dynamics. This provides unique opportunities to create reconfigurable, hierarchically organized structures, a leading challenge in materials science. Example soft matter systems include liquid crystals. For instance, when nematic liquid crystals (NLCs) are confined in a vessel with undulated boundaries, bend and splay distortions can be used to position particles. Here we confine this system in a twist cell. We also study cholesteric liquid crystals, which have an "intrinsic" twist distortion which adds to the ones imposed by the solid boundaries. The cholesteric pitch competes with the other length scales in the system (colloid radius, vessel thickness, wavelength of boundary undulations), enriching the possible configurations. Depending on the pitch-to-radius and pitch-to-thickness ratios the interaction can be attractive or repulsive. By tuning the pitch (i.e. changing the concentration of the chiral dopant), it is possible to selectively promote or inhibit particle trapping at the docking sites.
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Affiliation(s)
- Giuseppe Boniello
- Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA.
| | - Yimin Luo
- Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA.
| | - Daniel A Beller
- Department of Physics, University of California, Merced, CA 95343, USA
| | - Francesca Serra
- Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA.
| | - Kathleen J Stebe
- Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA.
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Tóth-Katona T, Jánossy I. Photoalignment at the nematic liquid crystal-polymer interface: Experimental evidence of three-dimensional reorientation. J Mol Liq 2019. [DOI: 10.1016/j.molliq.2019.04.074] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
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de Souza RF, Lenzi EK, de Souza RT, Evangelista LR, Li Q, Zola RS. Surface induced twist in nematic and chiral nematic liquid crystals: stick-slip-like and constrained motion. SOFT MATTER 2018; 14:2084-2093. [PMID: 29485156 DOI: 10.1039/c7sm02122d] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Surface driven pattern formation is an intriguing phenomenon in the liquid crystal field. Owing to its ability to transmit torque, one can generate different patterns by propagating distortions on the optical wavelength scale in the sample from the surface. Here, we theoretically investigate (from the elasticity point of view) twist deformations induced by a rotating easy axis at one surface, by considering the anchoring energy and surface viscosity of nematic and chiral nematic samples. The model is solved analytically in the limit of strong anchoring and numerically for a low anchoring strength situation. Such rotation could be induced, in principle, by light-controlling the orientation of an azobenzene monolayer coated at one of the glass substrates or by an in-plane rotating field. We discuss the role of the surface parameters and the different distortions, and calculate light transmission using the Jones method. Three different regimes are identified: free twist, stick-slip twist, and constrained twist. The results obtained here may be relevant for liquid crystal active waveplates and for determining surface viscosity and the azimuthal anchoring energy.
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Affiliation(s)
- R F de Souza
- Departamento de Física, Universidade Estadual de Maringá, Avenida Colombo 5790, 87020-900 Maringá, Paraná, Brazil
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Aryasova N, Reznikov Y. Liquid crystal alignment at macroscopically isotropic polymer surfaces: Effect of an isotropic-nematic phase transition. Phys Rev E 2016; 94:032702. [PMID: 27739728 DOI: 10.1103/physreve.94.032702] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2015] [Indexed: 11/07/2022]
Abstract
We study the effect of an isotropic-nematic (I-N) phase transition on the liquid crystal alignment at untreated polymer surfaces. We demonstrate that the pattern at the untreated substrate in the planar cell where the other substrate is uniformly rubbed strongly depends on the temperature gradient across the cell during the I-N phase transition, being macroscopically isotropic if the untreated substrate is cooled faster, but becoming almost homogeneous along the rubbing direction in the opposite temperature gradient. We interpret the observed effect using complementary models of heat transfer and nematic elasticity. Based on the heat transfer model we show that the asymmetric temperature conditions in our experiments provide unidirectional propagation of the I-N interface during the phase transition and determine the initial director orientation pattern at the test's untreated surface. Using the Frank-Oseen model of nematic elasticity, we represent the three-dimensional director field in the nematic cell as a two-dimensional (2D) pattern at the untreated surface and perform 2D numeric simulations. The simulations explain the experimental results: Different initial director orientations at the untreated surface evolve into different stationary patterns.
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Affiliation(s)
- Natalie Aryasova
- Department of Physics of Crystals, Institute of Physics, National Academy of Sciences, prospekt Nauki 46, 03028 Kiev, Ukraine
| | - Yuri Reznikov
- Department of Physics of Crystals, Institute of Physics, National Academy of Sciences, prospekt Nauki 46, 03028 Kiev, Ukraine
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Poy G, Oswald P. Do Lehmann cholesteric droplets subjected to a temperature gradient rotate as rigid bodies? SOFT MATTER 2016; 12:2604-2611. [PMID: 26906249 DOI: 10.1039/c5sm02906f] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
We performed a Fluorescence Recovery After Photobleaching (FRAP) experiment during the Lehmann rotation of cholesteric droplets in thermodynamic coexistence with the isotropic liquid and subjected to a temperature gradient. By creating and tracking bleached spots near the surface of banded droplets (in which the cholesteric helix is perpendicular to the gradient) and concentric circle droplets oriented by an electric field (in which the helix is parallel to the gradient), we found that neither type of droplet rotates as a solid. This result shows that the texture rotation is mainly due to the local director rotation.
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Affiliation(s)
- Guilhem Poy
- Université de Lyon, École Normale Supérieure de Lyon, Laboratoire de physique, CNRS, UMR 5672, 46 Allée d'Italie, 69364 Lyon, France.
| | - Patrick Oswald
- Université de Lyon, École Normale Supérieure de Lyon, Laboratoire de physique, CNRS, UMR 5672, 46 Allée d'Italie, 69364 Lyon, France.
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Oswald P, Pirkl S. Lehmann rotation of the cholesteric helix in droplets oriented by an electric field. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 89:022509. [PMID: 25353491 DOI: 10.1103/physreve.89.022509] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/07/2014] [Indexed: 06/04/2023]
Abstract
We study the Lehmann rotation of the cholesteric helix in droplets of the liquid crystal N-(p-methoxybenzilidene)-p-butylaniline doped with a small amount of the chiral molecule R811 when they are subjected to a temperature gradient. We show that the helix rotates much faster when it is parallel to the temperature gradient than when it is perpendicular to it. The first configuration is obtained by submitting the droplets to an ac electric field parallel to the temperature gradient, whereas the second one is observed at zero field. We show that the rotation velocity of the helix strongly depends on the droplet radius, even when the helix is parallel to the temperature gradient. This observation supports the idea that the Leslie thermomechanical coupling cannot explain alone the Lehmann effect.
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Affiliation(s)
- Patrick Oswald
- Université de Lyon, Ecole Normale Supérieure de Lyon, Laboratoire de physique, 46 Allé d'Italie, 69364 Lyon, Cedex 07, France
| | - Slavomir Pirkl
- Université de Lyon, Ecole Normale Supérieure de Lyon, Laboratoire de physique, 46 Allé d'Italie, 69364 Lyon, Cedex 07, France and Department of Applied Physics and Mathematics, University of Pardubice, 53210 Pardubice, Czech Republic
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Nitoń P, Żywociński A, Fiałkowski M, Hołyst R. A "nano-windmill" driven by a flux of water vapour: a comparison to the rotating ATPase. NANOSCALE 2013; 5:9732-9738. [PMID: 23959109 DOI: 10.1039/c3nr03496h] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
We measure the frequency of collective molecular precession as a function of temperature in the ferroelectric liquid crystalline monolayer at the water-air interface. This movement is driven by the unidirectional flux of evaporating water molecules. The collective rotation in the monolayer with angular velocities ω ~ 1 s(-1) (at T = 312 K) to 10(-2) s(-1) (at T = 285.8 K) is 9 to 14 orders of magnitude slower than rotation of a single molecule (typically ω ~ 10(9) to 10(12) s(-1)). The angular velocity reaches 0 upon approach to the two dimensional liquid-to-solid transition in the monolayer at T = 285.8 K. We estimate the rotational viscosity, γ1, in the monolayer and the torque, Γ, driving this rotation. The torque per molecule equals Γ = 5.7 × 10(-8) pN nm at 310 K (γ1 = 0.081 Pa s, ω = 0.87 s(-1)). The energy generated during one turn of the molecule at the same temperature is W = 3.5 × 10(-28) J. Surprisingly, although this energy is 7 orders of magnitude smaller than the thermal energy, kBT (310 K) = 4.3 × 10(-21) J, the rotation is very stable. The potential of the studied effect lies in the collective motion of many (>10(12)) "nano-windmills" acting "in concerto" at the scale of millimetres. Therefore, such systems are candidates for construction of artificial molecular engines, despite the small energy density per molecular volume (5 orders of magnitude smaller than for a single ATPase).
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Affiliation(s)
- Patrycja Nitoń
- Institute of Physical Chemistry of the Polish Academy of Sciences, 44/52 Kasprzaka Street, 01-224 Warsaw, Poland.
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Oswald P. Microscopic vs. macroscopic origin of the Lehmann effect in cholesteric liquid crystals. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2012; 35:10. [PMID: 22350704 DOI: 10.1140/epje/i2012-12010-5] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/23/2011] [Revised: 01/17/2012] [Accepted: 01/20/2012] [Indexed: 05/31/2023]
Abstract
In a recent letter (EPL 97, 36006 (2012)), we have shown that the Leslie thermomechanical coupling cannot alone explain the Lehmann effect (namely the rotation of cholesteric droplets when they are subjected to a temperature gradient). This result was obtained by measuring in a compensated cholesteric mixture the "Lehmann coefficient" as a function of temperature both below and at the transition to the isotropic liquid. In this article, we detail these experiments and present new ones performed with other compensated mixtures and a diluted cholesteric mixture. The new results confirm the macroscopic origin of the Lehmann effect, in contrast to the Leslie thermomechanical effect that is clearly of microscopic origin.
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Affiliation(s)
- P Oswald
- Laboratoire de Physique, École Normale Supérieure de Lyon, UMR 5672 of the CNRS, Lyon, France.
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Oswald P. Lehmann rotation of cholesteric droplets subjected to a temperature gradient: role of the concentration of chiral molecules. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2009; 28:377-383. [PMID: 19308470 DOI: 10.1140/epje/i2008-10431-3] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/27/2008] [Accepted: 01/21/2009] [Indexed: 05/27/2023]
Abstract
We present a systematic study of the Lehmann rotation of cholesteric droplets subjected to a temperature gradient when the concentration of chiral molecules is changed. The liquid crystal chosen is an eutectic mixture of 8CB and 8OCB doped with a small amount of the chiral molecule R811. The angular velocity of the droplets strongly depend on their size and on the concentration of chiral molecules. The Lehmann coefficient is estimated by using three different methods. Our results are consistent with a Lehmann coefficient proportional to the concentration of chiral molecules. We additionally show the existence of a critical size of the droplets below which they change texture and stop rotating.
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Affiliation(s)
- P Oswald
- Laboratoire de Physique, Ecole Normale Supérieure de Lyon, UMR 5672 of the CNRS, 46 Allée d'Italie, 69364, Lyon, France.
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