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Petrov DA, Chupeev IA. Antiferromagnetic liquid-crystal suspensions of goethite nanorods: three mechanisms of magnetic field influence on orientational structure. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2024; 47:54. [PMID: 39134885 DOI: 10.1140/epje/s10189-024-00448-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/14/2023] [Accepted: 07/30/2024] [Indexed: 09/06/2024]
Abstract
The study looks into magnetically induced orientational transitions in suspensions of goethite nanorods based on a nematic liquid crystal. The study considers magnetically compensated suspension, which is a liquid-crystal analogue of an antiferromagnet. Unlike conventional magnetic particles, goethite nanorods have a remanent magnetic moment directed along the long axis of the particle and also they have negative diamagnetic anisotropy. Thus, it can be claimed that liquid-crystal composites of goethite nanorods have three mechanisms of interaction with an external magnetic field. The first two mechanisms are originally quadrupolar and are related to diamagnetic susceptibility anisotropies of liquid-crystal matrix and impurity goethite nanorods. The third mechanism is a dipolar one and is due to a remanent longitudinal magnetic moment of each dispersed particle. The magnetic-field-induced birefringence is used to show that the presence of three competing orientational mechanisms of interaction with an external magnetic field can both increase and decrease the Fréedericksz transition threshold compared to a pure liquid crystal. Diagrams of orientational phases of the suspension were constructed, and cases of various orientational mechanism predominance were analysed. Besides, a representation of the free energy of the suspension near the Fréedericksz transition in the form of the Landau expansion was obtained. This made it possible to establish that the Fréedericksz transition can occur as a phase transition of both the first and second order.
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Affiliation(s)
- Danil A Petrov
- Perm State University, Bukirev St. 15, Perm, Russia, 614990.
| | - Ilya A Chupeev
- Perm State University, Bukirev St. 15, Perm, Russia, 614990
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2
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Oropesa WGC, Nascimento ES, Vieira AP. Mean-field model for a mixture of biaxial nematogens and dipolar nanoparticles. Phys Rev E 2024; 109:054701. [PMID: 38907419 DOI: 10.1103/physreve.109.054701] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2023] [Accepted: 03/27/2024] [Indexed: 06/24/2024]
Abstract
We analyze a mean-field model for mixtures involving biaxial nematogens and dipolar nanoparticles, taking into account not only orientational and isotropic pair interactions between nematogens but also orientational nematogen-nanoparticle interactions. We determine bulk equilibrium phase diagrams for a wide range of interaction strengths, identifying in each case the effect of the nanoparticles on the stability of nematic phases and on the appearance of multicritical points. Special attention is given to the limit of a low concentration of nanoparticles, in which their effect on the temperatures of both the first-order uniaxial-isotropic and the continuous biaxial-uniaxial transitions is investigated in detail.
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Affiliation(s)
- William G C Oropesa
- Universidade de São Paulo, Instituto de Fisica, Rua do Matao, 1371, 05508-090 São Paulo, SP, Brazil
- ICTP South American Institute for Fundamental Research, Instituto de Física Teórica, UNESP-Universidade Estadual Paulista, Rua Doutor Bento Teobaldo Ferraz 271, 01140-070 São Paulo, SP, Brazil
| | - Eduardo S Nascimento
- Departamento de Física, PUC-Rio, Rua Marquês de São Vicente 225, 22453-900 Rio de Janeiro, Rio de Janeiro, Brazil
| | - André P Vieira
- Universidade de São Paulo, Instituto de Fisica, Rua do Matao, 1371, 05508-090 São Paulo, SP, Brazil
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3
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Rafael EM, Tonti L, Daza FAG, Patti A. Active microrheology of colloidal suspensions of hard cuboids. Phys Rev E 2022; 106:034612. [PMID: 36266794 DOI: 10.1103/physreve.106.034612] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2022] [Accepted: 08/31/2022] [Indexed: 06/16/2023]
Abstract
By performing dynamic Monte Carlo simulations, we investigate the microrheology of isotropic suspensions of hard-core colloidal cuboids. In particular, we infer the local viscoelastic behavior of these fluids by studying the dynamics of a probe spherical particle that is incorporated in the host phase and is dragged by an external force. This technique, known as active microrheology, allows one to characterize the microscopic response of soft materials upon application of a constant force, whose intensity spans here three orders of magnitude. By tuning the geometry of cuboids from oblate to prolate as well as the system density, we observe different responses that are quantified by measuring the effective friction perceived by the probe particle. The resulting friction coefficient exhibits a linear regime at forces that are much weaker and larger than the thermal forces, whereas a nonlinear, force-thinning regime is observed at intermediate force intensities.
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Affiliation(s)
- Effran Mirzad Rafael
- Department of Chemical Engineering, The University of Manchester, Manchester, M13 9PL, United Kingdom
| | - Luca Tonti
- Department of Chemical Engineering, The University of Manchester, Manchester, M13 9PL, United Kingdom
| | - Fabián A García Daza
- Department of Chemical Engineering, The University of Manchester, Manchester, M13 9PL, United Kingdom
| | - Alessandro Patti
- Department of Chemical Engineering, The University of Manchester, Manchester, M13 9PL, United Kingdom
- Department of Applied Physics, University of Granada, Avenida Fuente Nueva s/n, 18071 Granada, Spain
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4
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Kennedy CL, Sayasilpi D, Schall P, Meijer JM. Self-assembly of colloidal cube superstructures with critical Casimir attractions. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2022; 34:214005. [PMID: 35203069 DOI: 10.1088/1361-648x/ac5866] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/15/2021] [Accepted: 02/24/2022] [Indexed: 06/14/2023]
Abstract
The structure of self-assembled materials is determined by the shape and interactions of the building blocks. Here, we investigate the self-assembly of colloidal 'superballs', i.e. cubes with rounded corners, by temperature-tunable critical Casimir forces to obtain insight into the coupling of a cubic shape and short range attractions. The critical Casimir force is a completely reversible and controllable attraction that arises in a near-critical solvent mixture. Using confocal microscopy and particle tracking, we follow the self-assembly dynamics and structural transition in a quasi-2D system. At low attraction, we observe the formation of small clusters with square symmetry. When the attraction is increased, a transition to a rhombic Λ1-lattice is observed. We explain our findings by the change in contact area at faces and corners of the building blocks combined with the increase in attraction strength and range of the critical Casimir force. Our results show that the coupling between the rounded cubic shape and short-range attraction plays a crucial role for the superstructures that form and provide new insights for the active assembly control of micro and nanocubes.
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Affiliation(s)
- Chris L Kennedy
- Department of Applied Physics, Eindhoven University of Technology, Groene Loper 19, 5600 MB Eindhoven, The Netherlands
- Institute for Complex Molecular Systems, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands
| | - Daphne Sayasilpi
- Institute of Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
| | - Peter Schall
- Institute of Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
| | - Janne-Mieke Meijer
- Department of Applied Physics, Eindhoven University of Technology, Groene Loper 19, 5600 MB Eindhoven, The Netherlands
- Institute for Complex Molecular Systems, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands
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5
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Dos Santos CTG, Vieira AP, Salinas SR, Andrade RFS. Real-space renormalization-group treatment of the Maier-Saupe-Zwanzig model for biaxial nematic structures. Phys Rev E 2021; 103:032111. [PMID: 33862734 DOI: 10.1103/physreve.103.032111] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2020] [Accepted: 02/11/2021] [Indexed: 11/07/2022]
Abstract
The Maier-Saupe-Zwanzig model for the nematic phase transitions in liquid crystals is investigated in a diamond hierarchical lattice. The model takes into account a parameter to describe the biaxiality of the microscopic units. Also, a suitably chosen external field is added to the Hamiltonian to allow the determination of critical parameters associated with the nematic phase transitions. Using the transfer-matrix technique, the free energy and its derivatives are obtained in terms of recursion relations between successive generations of the hierarchical lattice. In addition, a real-space renormalization-group approach is developed to obtain the critical parameters of the same model system. Results of both methods are in excellent agreement. There are indications of two continuous phase transitions. One of them corresponds to a uniaxial-isotropic transition, in the class of universality of the three-state Potts model on the diamond hierarchical lattice. The transition between the biaxial and the uniaxial phases is in the universality class of the Ising model on the same lattice.
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Affiliation(s)
- Cícero T G Dos Santos
- Instituto de Física, Universidade Federal da Bahia, 40170-115 Salvador, Brazil.,Instituto Federal de Educação, Ciência e Tecnologia do Sertão Pernambucano, 56302-100 Petrolina, Brazil
| | - André P Vieira
- Universidade de Sao Paulo, Instituto de Fisica, Rua do Matao, 1371, 05508-090 Sao Paulo, SP, Brazil
| | - Silvio R Salinas
- Universidade de Sao Paulo, Instituto de Fisica, Rua do Matao, 1371, 05508-090 Sao Paulo, SP, Brazil
| | - Roberto F S Andrade
- Instituto de Física, Universidade Federal da Bahia, 40170-115 Salvador, Brazil.,Centre for Data and Knowledge Integration for Health (CIDACS), Instituto Gonçalo Moniz, Fundação Oswaldo Cruz, 41745-715 Salvador, Brazil
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Peroukidis SD, Klapp SHL, Vanakaras AG. Field-induced anti-nematic and biaxial ordering in binary mixtures of discotic mesogens and spherical magnetic nanoparticles. SOFT MATTER 2020; 16:10667-10675. [PMID: 33084728 DOI: 10.1039/d0sm01366h] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Using computer simulations we explore the equilibrium structure and response to external stimuli of complex magnetic hybrids consisting of magnetic particles in discotic liquid crystalline matrices. We show that the anisotropy of the liquid crystalline matrix (either in the nematic or in the columnar phase) promotes the collective orientational ordering of self-assembled magnetic particles. Upon applying an external homogeneous magnetic field in an otherwise isotropic state, the magnetic particles self-assemble into linear-rodlike-chains. At the same time structural changes occur in the matrix. The matrix transforms from an isotropic to a non-conventional anti-nematic state in which the symmetry axis of the discs is, on average, perpendicular to the magnetic field. In addition, a stable biaxial nematic state is found upon applying an external field to an otherwise uniaxial discotic nematic state. These observed morphologies constitute an appealing alternative to binary mixtures of rigid rod-disc system and indicate that non-trivial biaxial ordering can be obtained in the presence of a uniaxial external stimulus.
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Mirzad Rafael E, Corbett D, Cuetos A, Patti A. Self-assembly of freely-rotating polydisperse cuboids: unveiling the boundaries of the biaxial nematic phase. SOFT MATTER 2020; 16:5565-5570. [PMID: 32539067 DOI: 10.1039/d0sm00484g] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Colloidal cuboids have the potential to self-assemble into biaxial liquid crystal phases, which exhibit two independent optical axes. Over the last few decades, several theoretical works have predicted the existence of a wide region of the phase diagram where the biaxial nematic phase would be stable, but imposed rather strong constraints on the particle rotational degrees of freedom. In this work, we use molecular simulation to investigate the impact of size dispersity on the phase behaviour of freely-rotating hard cuboids, here modelled as self-dual-shaped nanoboards. This peculiar anisotropy, exactly in between the oblate and prolate geometry, has been proposed as the most appropriate to promote phase biaxiality. We observe that size dispersity radically changes the phase behaviour of monodisperse systems and leads to the formation of an elusive biaxial nematic phase, being found in a large region of the packing fraction vs. polydispersity phase diagram. Although our results confirm the tendencies reported in past experimental observations on colloidal dispersions of slightly prolate goethite particles, they cannot reproduce the direct isotropic-to-biaxial nematic phase transition observed in these experiments.
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Affiliation(s)
- Effran Mirzad Rafael
- Department of Chemical Engineering and Analytical Science, The University of Manchester, Manchester, M13 9PL, UK.
| | - Daniel Corbett
- Department of Chemical Engineering and Analytical Science, The University of Manchester, Manchester, M13 9PL, UK.
| | - Alejandro Cuetos
- Department of Physical, Chemical and Natural Systems, Pablo de Olavide University, 41013 Sevilla, Spain
| | - Alessandro Patti
- Department of Chemical Engineering and Analytical Science, The University of Manchester, Manchester, M13 9PL, UK.
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8
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Cuetos A, Patti A. Dynamics of hard colloidal cuboids in nematic liquid crystals. Phys Rev E 2020; 101:052702. [PMID: 32575326 DOI: 10.1103/physreve.101.052702] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2020] [Accepted: 03/18/2020] [Indexed: 06/11/2023]
Abstract
We perform dynamic Monte Carlo simulations to investigate the equilibrium dynamics of hard board-like colloidal particles in oblate and prolate nematic liquid crystals. In particular, we characterize the particles' diffusion along the nematic director and perpendicularly to it, and observe a structural relaxation decay that strongly depends on the particle anisotropy. To assess the Gaussianity of their dynamics and eventual occurrence of collective motion, we calculate two- and four-point correlation functions that incorporate the instantaneous values of the diffusion coefficients parallel and perpendicular to the nematic director. Our simulation results highlight the occurrence of Fickian and Gaussian dynamics at short and long times, locate the minimum diffusivity at the self-dual shape, the particle geometry that would preferentially stabilise biaxial nematics, and exclude the existence of dynamically correlated particles.
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Affiliation(s)
- Alejandro Cuetos
- Department of Physical, Chemical and Natural Systems, Pablo de Olavide University, 41013 Sevilla, Spain
| | - Alessandro Patti
- Department of Chemical Engineering and Analytical Science, The University of Manchester, Manchester M13 9PL, United Kingdom
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Cuetos A, Mirzad Rafael E, Corbett D, Patti A. Biaxial nematics of hard cuboids in an external field. SOFT MATTER 2019; 15:1922-1926. [PMID: 30756112 DOI: 10.1039/c8sm02283f] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
By computer simulation, we model the phase behaviour of colloidal suspensions of board-like particles under the effect of an external field and assess the still disputed occurrence of the biaxial nematic (NB) liquid crystal phase. The external field promotes the rearrangement of the initial isotropic (I) or uniaxial nematic (NU) phase and the formation of the NB phase. In particular, very weak field strengths are sufficient to spark a direct I-NB or NU-NB phase transition at the self-dual shape, where prolate and oblate particle geometries fuse into one. By contrast, forming the NB phase at any other geometry requires stronger fields and thus reduces the energy efficiency of the phase transformation. Our simulation results show that self-dual shaped board-like particles with moderate anisotropy are able to form NB liquid crystals under the effect of a surprisingly weak external stimulus and suggest a path to exploit low-energy uniaxial-to-biaxial order switching.
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Affiliation(s)
- Alejandro Cuetos
- Department of Physical, Chemical and Natural Systems, Pablo de Olavide University, 41013 Sevilla, Spain
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10
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Yang Y, Chen G, Thanneeru S, He J, Liu K, Nie Z. Synthesis and assembly of colloidal cuboids with tunable shape biaxiality. Nat Commun 2018; 9:4513. [PMID: 30375393 PMCID: PMC6207716 DOI: 10.1038/s41467-018-06975-8] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2018] [Accepted: 10/01/2018] [Indexed: 12/13/2022] Open
Abstract
The design and assembly of monodisperse colloidal particles not only advances the development of functional materials, but also provides colloidal model systems for understanding phase behaviors of molecules. This communication describes the gram-scale synthesis of highly uniform colloidal cuboids with tunable dimension and shape biaxiality and their molecular mesogen-like assembly into various mesophasic structures in pristine purity. The synthesis relies on the nanoemulsion-guided generation of ammonium sulfate crystals that template the subsequent silica coating. The shape of the cuboidal particles can be tuned from square platelike, to biaxial boardlike, and to rodlike by independently controlling the length, width and thickness of the particles. We demonstrated the assembly of the cuboidal colloids into highly pure mesoscopic liquid crystal phases, including smectic A, biaxial smectic A, crystal B, discotic, and columnar phases, as well as established a correlation between mesophasic formation and colloidal biaxiality in experiments.
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Affiliation(s)
- Yang Yang
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD, 20742, USA
- State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, 130012, China
| | - Guangdong Chen
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD, 20742, USA
| | - Srinivas Thanneeru
- Department of Chemistry, Institute of Materials Sciences, University of Connecticut, Storrs, CT, 06269, USA
| | - Jie He
- Department of Chemistry, Institute of Materials Sciences, University of Connecticut, Storrs, CT, 06269, USA
| | - Kun Liu
- State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, 130012, China.
| | - Zhihong Nie
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD, 20742, USA.
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11
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Tasios N, Dijkstra M. A simulation study on the phase behavior of hard rhombic platelets. J Chem Phys 2018; 146:144901. [PMID: 28411607 DOI: 10.1063/1.4979517] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Using Monte Carlo simulations, we investigate the phase behavior of hard rhombic platelets as a function of the thickness of the platelets, T. The phase diagram displays a columnar phase and a crystal phase in which the platelets are stacked in columns that are arranged in a two-dimensional lattice. We find that the shape of the platelets determines the symmetry of the two-dimensional lattice, i.e., rhombic platelets form an oblique columnar phase and a simple monoclinic crystal phase. For sufficiently thick platelets, i.e., for a thickness-to-length ratio T/L>0.17, we find only an isotropic fluid, an oblique columnar phase, and a monoclinic crystal phase. Surprisingly, for an intermediate plate thickness, 0.083<T/L<0.17, we also find a region in between the isotropic (or nematic) phase and the columnar phase, where the smectic phase is stable. For sufficiently thin platelets, T/L<0.13, the phase diagram displays a nematic phase. With the exception of the smectic phase, our results resemble the phase behavior of discotic particles. Our results may guide the synthesis and future experiments on rhombic nanoplatelets.
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Affiliation(s)
- N Tasios
- Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
| | - M Dijkstra
- Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
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12
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Azari A, Crassous JJ, Mihut AM, Bialik E, Schurtenberger P, Stenhammar J, Linse P. Directed Self-Assembly of Polarizable Ellipsoids in an External Electric Field. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2017; 33:13834-13840. [PMID: 29111755 PMCID: PMC5719464 DOI: 10.1021/acs.langmuir.7b02040] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/15/2017] [Revised: 10/30/2017] [Indexed: 05/29/2023]
Abstract
The interplay between shape anisotropy and directed long-range interactions enables the self-assembly of complex colloidal structures. As a recent highlight, ellipsoidal particles polarized in an external electric field were observed to associate into well-defined tubular structures. In this study, we systematically investigate such directed self-assembly using Monte Carlo simulations of a two-point-charge model of polarizable prolate ellipsoids. In spite of its simplicity and computational efficiency, we demonstrate that the model is capable of capturing the complex structures observed in experiments on ellipsoidal colloids at low volume fractions. We show that, at sufficiently high electric field strength, the anisotropy in shape and electrostatic interactions causes a transition from three-dimensional crystal structures observed at low aspect ratios to two-dimensional sheets and tubes at higher aspect ratios. Our work thus illustrates the rich self-assembly behavior accessible when exploiting the interplay between competing long- and short-range anisotropic interactions in colloidal systems.
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13
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Patti A, Cuetos A. Monte Carlo simulation of binary mixtures of hard colloidal cuboids. MOLECULAR SIMULATION 2017. [DOI: 10.1080/08927022.2017.1402307] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Affiliation(s)
- A. Patti
- School of Chemical Engineering and Analytical Science, The University of Manchester, Manchester, UK
| | - A. Cuetos
- Department of Physical, Chemical and Natural Systems, Pablo de Olavide University,
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14
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Martínez-Ratón Y, González-Pinto M, Velasco E. Biaxial nematic phase stability and demixing behaviour in monolayers of rod-plate mixtures. Phys Chem Chem Phys 2016; 18:24569-81. [PMID: 27539250 DOI: 10.1039/c6cp05022k] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We theoretically study the phase behaviour of monolayers of hard rod-plate mixtures using a fundamental-measure density functional in the restricted-orientation (Zwanzig) approximation. Particles can rotate in 3D but their centres of mass are constrained to be on a flat surface. In addition, we consider both species to be subject to an attractive potential proportional to the particle contact area on the surface and with adsorption strengths that depend on the species type. Particles have board-like shape, with sizes chosen using a symmetry criterion: same volume and same aspect ratio κ. Phase diagrams were calculated for κ = 10, 20 and 40 and different values of adsorption strengths. For small adsorption strengths the mixtures exhibit a second-order uniaxial nematic-biaxial nematic transition for molar fraction of rods 0 ≤x≲ 0.9. In the uniaxial nematic phase the particle axes of rods and plates are aligned perpendicular and parallel to the monolayer, respectively. At the transition, the orientational symmetry of the plate axes is broken, and they orient parallel to a director lying on the surface. For large and equal adsorption strengths the mixture demixes at low pressures into a uniaxial nematic phase, rich in plates, and a biaxial nematic phase, rich in rods. The demixing transition is located between two tricritical points. Also, at higher pressures and in the plate-rich part of the phase diagram, the system exhibits a strong first-order uniaxial nematic-biaxial nematic phase transition with a large density coexistence gap. When rod adsorption is considerably large while that of plates is small, the transition to the biaxial nematic phase is always of second order, and its region of stability in the phase diagram considerably widens. At very high pressures the mixture can effectively be identified as a two-dimensional mixture of squares and rectangles which again demixes above a certain critical point. We also studied the relative stability of uniform phases with respect to density modulations of smectic, columnar and crystalline symmetry.
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Affiliation(s)
- Yuri Martínez-Ratón
- Grupo Interdisciplinar de Sistemas Complejos (GISC), Departamento de Matemáticas, Escuela Politécnica Superior, Universidad Carlos III de Madrid, Avenida de la Universidad 30, E-28911, Leganés, Madrid, Spain.
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15
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Schroer MA, Westermeier F, Lehmkühler F, Conrad H, Schavkan A, Zozulya AV, Fischer B, Roseker W, Sprung M, Gutt C, Grübel G. Colloidal crystallite suspensions studied by high pressure small angle x-ray scattering. J Chem Phys 2016; 144:084903. [PMID: 26931722 DOI: 10.1063/1.4941563] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We report on high pressure small angle x-ray scattering on suspensions of colloidal crystallites in water. The crystallites made out of charge-stabilized poly-acrylate particles exhibit a complex pressure dependence which is based on the specific pressure properties of the suspending medium water. The dominant effect is a compression of the crystallites caused by the compression of the water. In addition, we find indications that also the electrostatic properties of the system, i.e. the particle charge and the dissociation of ions, might play a role for the pressure dependence of the samples. The data further suggest that crystallites in a metastable state induced by shear-induced melting can relax to a similar structural state upon the application of pressure and dilution with water. X-ray cross correlation analysis of the two-dimensional scattering patterns indicates a pressure-dependent increase of the orientational order of the crystallites correlated with growth of these in the suspension. This study underlines the potential of pressure as a very relevant parameter to understand colloidal crystallite systems in aqueous suspension.
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Affiliation(s)
- M A Schroer
- Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
| | - F Westermeier
- Max-Planck-Institut für Struktur und Dynamik der Materie, CFEL, Luruper Chaussee 149, 22761 Hamburg, Germany
| | - F Lehmkühler
- Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
| | - H Conrad
- Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
| | - A Schavkan
- Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
| | - A V Zozulya
- Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
| | - B Fischer
- Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
| | - W Roseker
- Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
| | - M Sprung
- Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
| | - C Gutt
- Department of Physics, University of Siegen, Walter-Flex-Str. 3, 57072 Siegen, Germany
| | - G Grübel
- Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
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16
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Aliabadi R, Moradi M, Varga S. Orientational ordering of confined hard rods: the effect of shape anisotropy on surface ordering and capillary nematization. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2015; 92:032503. [PMID: 26465486 DOI: 10.1103/physreve.92.032503] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/28/2015] [Indexed: 06/05/2023]
Abstract
We examine the ordering properties of rectangular hard rods with length L and diameter D at a single planar wall and between two parallel hard walls using the second virial density-functional theory. The theory is implemented in the three-state Zwanzig approximation, where only three mutually perpendicular directions are allowed for the orientations of hard rods. The effect of varying shape anisotropy is examined at L/D=10,15,and20. In contact with a single hard wall, the density profiles show planar ordering, damped oscillatory behavior, and a wall-induced surface ordering transition below the coexisting isotropic density of a bulk isotropic-nematic (I-N) phase transition. Upon approaching the coexisting isotropic density, the thickness of the nematic film diverges logarithmically, i.e., the nematic wetting is complete for any shape anisotropy. In the case of confinement between two parallel hard walls, it is found that the continuous surface ordering transition depends strongly on the distance between confining walls H for H<L, while it depends weakly on H for H>L. The minimal density at which a surface ordering transition can be realized is located at around H∼2D for all studied shape anisotropies due to the strong interference effect between the two hard walls. The first-order I-N phase transition of the bulk system becomes a surface ordered isotropic I_{B} to capillary nematic N_{B} phase transition in the slit pore. This first-order I_{B}-N_{B} transition weakens with decreasing pore width and terminates in a critical point for all studied shape anisotropies.
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Affiliation(s)
- R Aliabadi
- Department of Physics, College of Science, Shiraz University, Shiraz 71454, Iran
| | - M Moradi
- Department of Physics, College of Science, Shiraz University, Shiraz 71454, Iran
| | - S Varga
- Institute of Physics and Mechatronics, University of Pannonia, PO Box 158, Veszprém H-8201, Hungary
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Petukhov AV, Meijer JM, Vroege GJ. Particle shape effects in colloidal crystals and colloidal liquid crystals: Small-angle X-ray scattering studies with microradian resolution. Curr Opin Colloid Interface Sci 2015. [DOI: 10.1016/j.cocis.2015.09.003] [Citation(s) in RCA: 40] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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18
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González-Pinto M, Martínez-Ratón Y, Velasco E, Varga S. Effect of shape biaxiality on the phase behavior of colloidal liquid-crystal monolayers. Phys Chem Chem Phys 2015; 17:6389-400. [PMID: 25655742 DOI: 10.1039/c4cp04812a] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We extend our previous work on monolayers of uniaxial particles [J. Chem. Phys., 2014, 140, 204906] to study the effect of particle biaxiality on the phase behavior of liquid-crystal monolayers. Particles are modelled as board-like hard bodies with three different edge lengths σ1 ≥ σ2 ≥ σ3, and the restricted-orientation approximation (Zwanzig model) is used. A density-functional formalism based on the fundamental-measure theory is used to calculate phase diagrams for a wide range of values with the largest aspect ratio κ1 = σ1/σ3 ∈ [1,100]. We find that particle biaxiality in general destabilizes the biaxial nematic phase already present in monolayers of uniaxial particles. While plate-like particles exhibit strong biaxial ordering, rod-like ones with κ1 > 21.34 exhibit reentrant uniaxial and biaxial phases. As particle geometry is changed from uniaxial- to increasingly biaxial-rod-like, the region of biaxiality is reduced, eventually ending in a critical-end point. For κ1 > 60, a density gap opens up in which the biaxial nematic phase is stable for any particle biaxiality. Regions of the phase diagram, where packing-fraction inversion occurs (i.e. packing fraction is a decreasing function of density), are found. Our results are compared with the recent experimental studies on nematic phases of magnetic nanorods.
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Affiliation(s)
- Miguel González-Pinto
- Departamento de Física Teórica de la Materia Condensada, Facultad de Ciencias, Universidad Autónoma de Madrid, E-28049 Madrid, Spain.
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