Special Issue "Liquid Crystal Rheology"

A special issue of Fluids (ISSN 2311-5521).

Deadline for manuscript submissions: closed (15 March 2018)

Special Issue Editors

Guest Editor
Prof. Dr. Davide Marenduzzo

School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK
Website | E-Mail
Interests: DNA; chromatin and chromosome physics; active matter physics; liquid crystal physics; soft condensed matter
Guest Editor
Dr. Oliver Henrich

Department of Physics, University of Strathclyde, Glasgow, UK
Website | E-Mail
Interests: soft condensed matter physics; liquid crystals; mesoscopic simulation methods for fluid dynamics; high-performance computing; coarse-grained modelling of DNA

Special Issue Information

Dear Colleagues,

Liquid crystals are “crystals which flow”. Our understanding of the flow properties (rheology) of these materials commenced with the seminal work of Leslie, Ericksen and Parodi, who introduced the first continuum description of the coupling between flow and orientational order in nematics in the 1970s. Their theory was extended, among others, by Beris and Edwards during the 1990s, who proposed equations in which flow coupled to a tensorial order parameter, which is more adapt at capturing samples with a variable degree of order.

Since then, the field of liquid crystal rheology has seen a dramatic increase in richness and complexity, leading to a number of open research avenues. The complexity may arise for instance due to the presence of topological defects or of complex spatially varying director field patterns, to the coupling between flow and external fields, or to the presence of multiple phases, such as in colloid–liquid crystal composites or liquid crystalline emulsions. Active materials which are liquid crystalline, such as solutions of cytoskeletal filaments with molecular motors and microbial suspensions, are also a topical area of research nowadays, with possible application as new soft materials, and their rheological properties are currently still understudied.

Within this Special Issue, we would like to address open questions in topics such as those listed above, in the broad field of liquid crystal rheology.

Prof. Dr. Davide Marenduzzo
Dr. Oliver Henrich
Guest Editors

Manuscript Submission Information

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Keywords

  • liquid crystal rheology
  • nematics, cholesterics, smectics and blue phases
  • shear and Poiseuille flow
  • liquid crystal hydrodynamic

Published Papers (7 papers)

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Research

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Open AccessArticle Rheology of an Inverted Cholesteric Droplet under Shear Flow
Received: 20 May 2018 / Revised: 26 June 2018 / Accepted: 28 June 2018 / Published: 3 July 2018
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Abstract
The dynamics of a quasi two-dimensional isotropic droplet in a cholesteric liquid crystal medium under symmetric shear flow is studied by lattice Boltzmann simulations. We consider a geometry in which the flow direction is along the axis of the cholesteric, as this setup
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The dynamics of a quasi two-dimensional isotropic droplet in a cholesteric liquid crystal medium under symmetric shear flow is studied by lattice Boltzmann simulations. We consider a geometry in which the flow direction is along the axis of the cholesteric, as this setup exhibits a significant viscoelastic response to external stress. We find that the dynamics depends on the magnitude of the shear rate, the anchoring strength of the liquid crystal at the droplet interface and the chirality. While low shear rate and weak interface anchoring the system shows a non-Newtonian behavior, a Newtonian-like response is observed at high shear rate and strong interface anchoring. This is investigated both by estimating the secondary flow profile, namely a flow emerging along the out-of-plane direction (absent in fully-Newtonian fluids, such as water) and by monitoring defect formation and dynamics, which significantly alter the rheological response of the system. Full article
(This article belongs to the Special Issue Liquid Crystal Rheology)
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Open AccessArticle Flow and Nematic Director Profiles in a Microfluidic Channel: The Interplay of Nematic Material Constants and Backflow
Received: 13 March 2018 / Revised: 9 May 2018 / Accepted: 16 May 2018 / Published: 1 June 2018
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Abstract
We numerically and analytically study the flow and nematic order parameter profiles in a microfluidic channel, within the Beris–Edwards theory for nematodynamics, with two different types of boundary conditions—strong anchoring/Dirichlet conditions and mixed boundary conditions for the nematic order parameter. We primarily study
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We numerically and analytically study the flow and nematic order parameter profiles in a microfluidic channel, within the Beris–Edwards theory for nematodynamics, with two different types of boundary conditions—strong anchoring/Dirichlet conditions and mixed boundary conditions for the nematic order parameter. We primarily study the effects of the pressure gradient, the effects of the material constants and viscosities modelled by a parameter L 2 and the nematic elastic constant L , along with the effects of the choice of the boundary condition. We study continuous and discontinuous solution profiles for the nematic director and these discontinuous solutions have a domain wall structure, with a layered structure that offers new possibilities. Our main results concern the onset of flow reversal as a function of L and L 2 , including the identification of certain parameter regimes with zero net flow rate. These results are of value in tuning microfluidic geometries, boundary conditions and choosing liquid crystalline materials for desired flow properties. Full article
(This article belongs to the Special Issue Liquid Crystal Rheology)
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Open AccessArticle Electrorheological Model Based on Liquid Crystals Membranes with Applications to Outer Hair Cells
Received: 2 February 2018 / Revised: 8 May 2018 / Accepted: 16 May 2018 / Published: 22 May 2018
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Abstract
Liquid crystal flexoelectric actuation uses an imposed electric field to create membrane bending, this phenomenon is found in outer hair cells (OHC) located in the inner ear, whose role is to amplify sound through the generation of mechanical power. Oscillations in the OHC
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Liquid crystal flexoelectric actuation uses an imposed electric field to create membrane bending, this phenomenon is found in outer hair cells (OHC) located in the inner ear, whose role is to amplify sound through the generation of mechanical power. Oscillations in the OHC membranes create periodic viscoelastic flows in the contacting fluid media. A key objective of this work on flexoelectric actuation relevant to OHC is to find the relations and impact of the electro-mechanical properties of the membrane, the rheological properties of the viscoelastic media, and the frequency response of the generated mechanical power output. The model developed and used in this work is based on the integration of: (i) the flexoelectric membrane shape equation applied to a circular membrane attached to the inner surface of a circular capillary, and (ii) the coupled capillary flow of contacting viscoelastic phases, which are characterized by the Jeffreys constitutive equation with different material conditions. The membrane flexoelectric oscillations drive periodic viscoelastic capillary flows, as in OHCs. By applying the Fourier transform formalism to the governing equations and assuming small Mach numbers, analytical equations for the transfer function, associated to the average curvature, and for the volumetric rate flow as a function of the electrical field were found, and these equations can be expressed as a third-order differential equation which depends on the material properties of the system. When the inertial mechanisms are considered, the power spectrum shows several resonance peaks in the average membrane curvature and volumetric flow rate. When the inertia is neglected, the system follows a non-monotonic behavior in the power spectrum. This behavior is associated with the solvent contributions related to the retardation-Jeffreys mechanisms. The specific membrane-viscoelastic fluid properties that control the power response spectrum are identified. The present theory, model, and computations contribute to the evolving fundamental understanding of biological shape actuation through electromechanical couplings. Full article
(This article belongs to the Special Issue Liquid Crystal Rheology)
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Open AccessArticle Nonlinear Rheology and Fracture of Disclination Network in Cholesteric Blue Phase III
Received: 29 March 2018 / Revised: 11 May 2018 / Accepted: 15 May 2018 / Published: 17 May 2018
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Abstract
Nonlinear rheological properties of chiral crystal cholesteryl oleyl carbonate (COC) in blue phase III (BPIII) were investigated under different shear deformations: large amplitude oscillatory shear, step shear deformation, and continuous shear flow. Rheology of the liquid crystal is significantly affected by structural rearrangement
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Nonlinear rheological properties of chiral crystal cholesteryl oleyl carbonate (COC) in blue phase III (BPIII) were investigated under different shear deformations: large amplitude oscillatory shear, step shear deformation, and continuous shear flow. Rheology of the liquid crystal is significantly affected by structural rearrangement of defects under shear flow. One of the examples on the defect-mediated rheology is the blue phase rheology. Blue phase is characterized by three dimensional network structure of the disclination lines. It has been numerically studied that the rheological behavior of the blue phase is dominated by destruction and creation of the disclination networks. In this study, we find that the nonlinear viscoelasticity of BPIII is characterized by the fracture of the disclination networks. Depending on the degree of the fracture, the nonlinear viscoelasticity is divided into two regimes; the weak nonlinear regime where the disclination network locally fractures but still shows elastic response, and the strong nonlinear regime where the shear deformation breaks up the networks, which results in a loss of the elasticity. Continuous shear deformation reveals that a series of the fracture process delays with shear rate. The shear rate dependence suggests that force balance between the elastic force acting on the disclination lines and the viscous force determines the fracture behavior. Full article
(This article belongs to the Special Issue Liquid Crystal Rheology)
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Open AccessArticle Elementary Flow Field Profiles of Micro-Swimmers in Weakly Anisotropic Nematic Fluids: Stokeslet, Stresslet, Rotlet and Source Flows
Received: 13 December 2017 / Revised: 22 January 2018 / Accepted: 27 January 2018 / Published: 8 February 2018
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Abstract
Analytic formulations of elementary flow field profiles in weakly anisotropic nematic fluid are determined, which can be attributed to biological or artificial micro-swimmers, including Stokeslet, stresslet, rotlet and source flows. Stokes equation for a nematic stress tensor is written with the Green function
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Analytic formulations of elementary flow field profiles in weakly anisotropic nematic fluid are determined, which can be attributed to biological or artificial micro-swimmers, including Stokeslet, stresslet, rotlet and source flows. Stokes equation for a nematic stress tensor is written with the Green function and solved in the k-space for anisotropic Leslie viscosity coefficients under the limit of leading isotropic viscosity coefficient. Analytical expressions for the Green function are obtained that are used to compute the flow of monopole or dipole swimmers at various alignments of the swimmers with respect to the homogeneous director field. Flow profile is also solved for the flow sources/sinks and source dipoles showing clear emergence of anisotropy in the magnitude of flow profile as the result of fluid anisotropic viscosity. The range of validity of the presented analytical solutions is explored, as compared to exact numerical solutions of the Stokes equation. This work is a contribution towards understanding elementary flow motifs and profiles in fluid environments that are distinctly affected by anisotropic viscosity, offering analytic insight, which could be of relevance to a range of systems from microswimmers, active matter to microfluidics. Full article
(This article belongs to the Special Issue Liquid Crystal Rheology)
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Review

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Open AccessReview Exploring “Dormant” Opto-Mechanical Properties of the Isotropic Phase of Liquid Crystals and Revealing Hidden Elasticity of (Ordinary) Liquids
Received: 15 April 2018 / Revised: 28 May 2018 / Accepted: 6 June 2018 / Published: 13 June 2018
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Abstract
There is little literature on the flow properties of the isotropic phase of liquid crystalline fluids. However, this phase is an ideal tool to bridge the physics of liquid crystals with those of (ordinary) fluids. Optical and mechanical studies are presented, demonstrating that
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There is little literature on the flow properties of the isotropic phase of liquid crystalline fluids. However, this phase is an ideal tool to bridge the physics of liquid crystals with those of (ordinary) fluids. Optical and mechanical studies are presented, demonstrating that away from any phase transition, the isotropic phase of liquid crystalline molecules (LCs) and liquid crystalline polymers (LCPs) can work as an optical oscillator in response to low-frequency mechanical excitation, establishing the elastic origin of the flow birefringence and “visualizing” the very existence of the elastic nature of the liquid state. Additionally, mimicking the excellent anchoring ability of liquid crystals, an alternative rheological protocol optimizing the fluid/substrate interfaces is presented to access the low-frequency shear elasticity in various one-component liquids and salt-free aqueous solutions. Full article
(This article belongs to the Special Issue Liquid Crystal Rheology)
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Open AccessReview A Short Review on the Rheology of Twist Grain Boundary-A and Blue Phase Liquid Crystals
Received: 22 February 2018 / Revised: 30 March 2018 / Accepted: 4 April 2018 / Published: 9 April 2018
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Abstract
Topological defects are important in determining the properties of physical systems and are known varyingly depending on the broken symmetry. In superfluid helium, they are called vortices; in periodic crystals, one refers to dislocations; and in liquid crystals, they are disclinations. The defects
[...] Read more.
Topological defects are important in determining the properties of physical systems and are known varyingly depending on the broken symmetry. In superfluid helium, they are called vortices; in periodic crystals, one refers to dislocations; and in liquid crystals, they are disclinations. The defects and the inter-defect interaction in some highly chiral liquid crystals stabilize some intermediate complex phases such as Blue Phases (BPs) and Twist Grain Boundary-A (TGBA) phases. The defect dynamics of these phases contributes to the rheological properties. The temperature range of these intermediate phases usually are very small in pure liquid crystals; consequently, a detailed experiment has been difficult to achieve. However, the temperature range could be enhanced significantly in multicomponent systems. In this review article, we discuss some recent experimental progress made in understanding the rheological properties of the wide-temperature-range TGBA and BP liquid crystals. Full article
(This article belongs to the Special Issue Liquid Crystal Rheology)
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