Special Issue "Stimuli Responsive Liquid Crystals"

A special issue of Materials (ISSN 1996-1944).

Deadline for manuscript submissions: 30 September 2021.

Special Issue Editors

Prof. Dr. Uroš Tkalec
E-Mail Website
Guest Editor
University of Ljubljana, Ljubljana, Slovenia
University of Maribor, Maribor, Slovenia
Interests: nematic colloids; topological defects; liquid crystal microfluidics
Prof. Dr. Dong Ki Yoon
E-Mail Website
Guest Editor
Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea
Interests: soft matter fabrication; multi-functional surfaces; nanophotonics; liquid crystal textures

Special Issue Information

Dear Colleagues,

Liquid crystal elasticity drives the formation of structural patterns and topological defects that allow for the assembly of various mesophases as well as inclusions dispersed within the bulk of liquid crystalline materials. The ordering of constituent anisotropic molecules can be additionally altered by external stimuli, like temperature, electric and magnetic fields, flow, shape of confinement, molecular adsorbates, and structured or functionalized surfaces. Many recent studies in the field explore how to control, design, and engineer on demand the response of liquid-crystal-based forms of soft matter that could perform reconfigurable tasks on the microscale. Examples include colloidal crystallization processes within liquid crystal hosts, pattern formation in active and driven nematic fluids, autonomous responses to chemical or mechanical stimuli in microfluidic environments, release or selective self-assembly at fluid interfaces, and templated micropatterning of diverse regular textures for fundamental studies and future technologies. Addressing the dynamic processes, assembly, and collective behavior of the underlying molecular order is, thus, crucial to overcome fundamental challenges in the development of new soft materials.

The goal of this Special Issue is to bring together current views of experimental and theoretical physicists, material scientists, chemists, and chemical engineers who are active in the broad selection of topics addressed above. It is our pleasure to invite authors to contribute original research articles, reviews, short communications, and concept papers.

Prof. Dr. Uroš Tkalec
Prof. Dr. Dong Ki Yoon
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Materials is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • stimuli-responsive liquid crystal structures
  • liquid crystal emulsions
  • liquid crystal interfaces
  • active nematics
  • topological defects
  • non-equilibrium soft matter
  • photosensitive liquid crystals

Published Papers (4 papers)

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Research

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Open AccessArticle
Design of Chemoresponsive Soft Matter Using Hydrogen-Bonded Liquid Crystals
Materials 2021, 14(5), 1055; https://doi.org/10.3390/ma14051055 - 24 Feb 2021
Cited by 1 | Viewed by 422
Abstract
Soft matter that undergoes programmed macroscopic responses to molecular analytes has potential utility in a range of health and safety-related contexts. In this study, we report the design of a nematic liquid crystal (LC) composition that forms through dimerization of carboxylic acids and [...] Read more.
Soft matter that undergoes programmed macroscopic responses to molecular analytes has potential utility in a range of health and safety-related contexts. In this study, we report the design of a nematic liquid crystal (LC) composition that forms through dimerization of carboxylic acids and responds to the presence of vapors of organoamines by undergoing a visually distinct phase transition to an isotropic phase. Specifically, we screened mixtures of two carboxylic acids, 4-butylbenzoic acid and trans-4-pentylcyclohexanecarboxylic acid, and found select compositions that exhibited a nematic phase from 30.6 to 111.7 °C during heating and 110.6 to 3.1 °C during cooling. The metastable nematic phase formed at ambient temperatures was found to be long-lived (>5 days), thus enabling the use of the LC as a chemoresponsive optical material. By comparing experimental infrared (IR) spectra of the LC phase with vibrational frequencies calculated using density functional theory (DFT), we show that it is possible to distinguish between the presence of monomers, homodimers and heterodimers in the mixture, leading us to conclude that a one-to-one heterodimer is the dominant species within this LC composition. Further support for this conclusion is obtained by using differential scanning calorimetry. Exposure of the LC to 12 ppm triethylamine (TEA) triggers a phase transition to an isotropic phase, which we show by IR spectroscopy to be driven by an acid-base reaction, leading to the formation of ammonium carboxylate salts. We characterized the dynamics of the phase transition and found that it proceeds via a characteristic spatiotemporal pathway involving the nucleation, growth, and coalescence of isotropic domains, thus amplifying the atomic-scale acid-base reaction into an information-rich optical output. In contrast to TEA, we determined via both experiment and computation that neither hydrogen bonding donor or acceptor molecules, such as water, dimethyl methylphosphonate, ethylene oxide or formaldehyde, disrupt the heterodimers formed in the LC, hinting that the phase transition (including spatial-temporal characteristics of the pathway) induced in this class of hydrogen bonded LC may offer the basis of a facile and chemically selective way of reporting the presence of volatile amines. This proposal is supported by exploratory experiments in which we show that it is possible to trigger a phase transition in the LC by exposure to volatile amines emitted from rotting fish. Overall, these results provide new principles for the design of chemoresponsive soft matter based on hydrogen bonded LCs that may find use as the basis of low-cost visual indicators of chemical environments. Full article
(This article belongs to the Special Issue Stimuli Responsive Liquid Crystals)
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Open AccessArticle
Tropisms of the Dowser Texture
Materials 2020, 13(20), 4681; https://doi.org/10.3390/ma13204681 - 21 Oct 2020
Viewed by 455
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. Full article
(This article belongs to the Special Issue Stimuli Responsive Liquid Crystals)
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Open AccessArticle
Effect of Isomeric Amine Chain Extenders and Crosslink Density on the Properties of Liquid Crystal Elastomers
Materials 2020, 13(14), 3094; https://doi.org/10.3390/ma13143094 - 10 Jul 2020
Cited by 3 | Viewed by 785
Abstract
Among the various types of shape changing materials, liquid crystal elastomers (LCEs) have received significant attention as they can undergo programmed and reversible shape transformations. The molecular engineering of LCEs is the key to manipulating their phase transition, mechanical properties, and actuation performance. [...] Read more.
Among the various types of shape changing materials, liquid crystal elastomers (LCEs) have received significant attention as they can undergo programmed and reversible shape transformations. The molecular engineering of LCEs is the key to manipulating their phase transition, mechanical properties, and actuation performance. In this work, LCEs containing three different types of butyl groups (n-, iso-, and sec-butyl) in the side chain were synthesized, and the effect of isomeric amine chain extenders on the thermal, mechanical, and actuation properties of the resulting LCEs was investigated. Because of the considerably low reactivity of the sec-butyl group toward the diacrylate in the LC monomer, only a densely crosslinked LCE was synthesized. Most interestingly, the mechanical properties, actuation temperature, and blocking stress of the LCEs comprising isobutyl groups were higher than those of the LCEs comprising n-butyl groups. This difference was attributed to the presence of branches in the LCEs with isobutyl groups, which resulted in a tighter molecular packing and reduced the free volume. Our results suggest a facile and effective method for synthesizing LCEs with tailored mechanical and actuation properties by the choice of chain extenders, which may advance the development of soft actuators for a variety of applications in aerospace, medicine, and optics. Full article
(This article belongs to the Special Issue Stimuli Responsive Liquid Crystals)
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Review

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Open AccessReview
Role of Stimuli on Liquid Crystalline Defects: From Defect Engineering to Switchable Functional Materials
Materials 2020, 13(23), 5466; https://doi.org/10.3390/ma13235466 - 30 Nov 2020
Cited by 1 | Viewed by 525
Abstract
Achieving tunable physical properties is currently one of the most exciting research topics. In order to realize this goal, a medium that is responsive to external stimuli and can undergo a change in its physical property is required. Liquid crystal (LC) is a [...] Read more.
Achieving tunable physical properties is currently one of the most exciting research topics. In order to realize this goal, a medium that is responsive to external stimuli and can undergo a change in its physical property is required. Liquid crystal (LC) is a prominent candidate, as its physical and optical properties can be easily manipulated with various stimuli, such as surface anchoring, rubbing, geometric confinement, and external fields. Having broken away from the past devotion to obtaining a uniform domain of LCs, people are now putting significant efforts toward forming and manipulating ordered and oriented defect structures with a unique arrangement within. The complicated molecular order with tunability would benefit the interdisciplinary research fields of optics, physics, photonics, and materials science. In this review, the recent progress toward defect engineering in the nematic and smectic phases by controlling the surface environment and electric field and their combinational methods is introduced. We close the review with a discussion of the possible applications enabled using LC defect structures as switchable materials. Full article
(This article belongs to the Special Issue Stimuli Responsive Liquid Crystals)
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