Tuning biaxiality of nematic phases of board-like colloids by an external magnetic field

Reinink, Anke B.G.M. Leferink op; Belli, Simone; Roij, René van; Dijkstra, Marjolein; Petukhov, Andrei V.; Vroege, G. J.

doi:10.1039/c3sm52242c

Cited by 27 publications

(24 citation statements)

References 41 publications

(79 reference statements)

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“…These computational and experimental findings unambiguously indicate that self-dual shaped HBPs cannot self-assemble into an N B phase, unless their anisotropy and/or size dispersity are especially relevant. Vroege and coworkers investigated the effect of a magnetic field on the phase behavior of quasi-dual-shaped polydisperse goethite particles in suspension and observed a biaxial-to-uniaxial nematic transition above a certain magnetic field strength [9]. To explain the origin of these experimental results, the same authors formulated a mean-field theory to calculate the phase diagram of HBPs in the presence of a magnetic field.…”

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Biaxial nematics of hard cuboids in an external field

et al. 2019

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We investigate the phase behavior of colloidal suspensions of board-like particles under the effect of an external field and assess the still disputed occurrence of the biaxial nematic (N B ) liquid crystal phase. The external field promotes the rearrangement of the initial isotropic (I) or uniaxial nematic (N U ) phase and the formation of the N B phase. In particular, very weak field strengths are sufficient to spark a direct I-N B or N U -N B phase transition at the self-dual shape, where prolate and oblate particle geometries fuse into one. By contrast, forming the N B 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 N B liquid crystals under the effect of a surprisingly weak external stimulus and suggest a path to exploit low-energy uniaxial-to-biaxial order switching.It is well established that anisotropic colloidal particles can self-assemble into a plethora of fascinating liquid crystal (LC) phases in a solvent. Onsager showed that mere excluded volume effects can force hard-core particles to align along a common director at sufficiently large concentrations [1]. The resulting LC phases found at the thermodynamic equilibrium and their structural properties strongly depend on the particle geometry. In particular, prolate particles tend to orient along their major axis, while oblate particles along their minor axis. Although this tendency is regularly observed in systems of uniaxial particles, such as disks, whose orientation is determined by a single unit vector, it is less predictable for biaxial particles, such as cuboids, whose orientation is fully determined by two unit vectors. For instance, slightly oblate hard board-like particles (HBPs) have been shown to orient along their major axis and thus form prolate (rather than oblate) smectic LC phases [2]. This unusual arrangement was observed in suspensions of HBPs with length-to-thickness ratio L * ≡ L/T = 12 and width-tothickness ratio W * ≡ W/T ≈ √ L * , a geometry where oblate and prolate shapes fuse into one.Such an exclusive particle architecture, referred to as self-dual shape, was predicted to favour the formation of the biaxial nematic (N B ) phase in systems of HBPs with rounded [3] or square [4] corners. Nevertheless, these theoretical predictions were made within the context of the Zwanzig model, which does not allow free rotations of particles and restricts their orientations to only six. Computer simulations that explored the phase behavior of freely rotating HBPs highlighted the challenge of observing stable N B phases, even when an element of sizedispersity is incorporated [5]. The very recent and insightful simulation study by the Dijkstra's group showed that monodisperse HBPs, with a geometry close or equal to the self-dual shape, are able to stabilise the N B phase * Electronic address: alessandro.patti@manchester.ac.uk only if their anisotropy is signif...

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Biaxial nematics of hard cuboids in an external field

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“…Recognising this breakthrough has conferred to colloids a position in materials science in their own right [4]. Although current LC display technology is entirely based on molecular LCs, the appealing scenario of employing materials with high thermal stability, enhanced susceptibility to external fields and more accessible production costs, makes colloidal LCs excellent candidates for displays [5][6][7]. Additionally, and perhaps more interestingly, colloidal suspensions of board-like particles can form biaxial nematic (N B ) phases [8], whose existence, theoretically predicted by Freiser almost 50 years ago [9], is still an open question at the molecular scale.…”

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Monte Carlo simulation of binary mixtures of hard colloidal cuboids

Patti

Cuetos

2017

Molecular Simulation

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We perform extensive Monte Carlo simulations to investigate the phase behaviour of colloidal suspensions of hard board-like particles (HBPs). While theories restricting particle orientation or ignoring higher ordered phases suggest the existence of a stable biaxial nematic phase, our recent simulation results on monodisperse systems indicate that this is not necessarily the case, even for particle shapes exactly in between prolate and oblate geometries, usually referred to as self-dual shape. Motivated by the potentially striking impact of incorporating biaxial ordering into display applications, we extend our investigation to bidisperse mixtures of short and long HBPs and analyse whether size dispersity can further enrich the phase behaviour of HBPs, eventually destabilise positionally ordered phases and thus favour the formation of the biaxial nematic phase. Not only do our results indicate that bidisperse mixtures of self-dual shaped HBPs cannot self-assemble into biaxial nematic phases, but they also show that these particles are not able to form uniaxial nematic phases either. This surprising behaviour is also observed in monodisperse systems. Additionally, bidisperse HBPs tend to phase separate in coexisting isotropic and smectic phases or, at relatively large pressures, in a smectic phase of mostly short HBPs and a smectic phase of mostly long HBPs. We conclude that limiting the particle orientational degrees of freedom or neglecting the presence of positionally ordered (smectic, columnar and crystal) phases can dramatically alter the phase behaviour of HBPs and unrealistically enlarge the region of stability of the biaxial nematic phase. Molecular Simulation paper˙V2colloidal particles with precise symmetry and directional interactions sparked the discovery of a collective behaviour, key for the synthesis of photonic crystals [2] and macroporous solids [3], that is not observed in atomic and molecular systems. Recognising this breakthrough has conferred to colloids a position in materials science in their own right [4]. Although current LC display technology is entirely based on molecular LCs, the appealing scenario of employing materials with high thermal stability, enhanced susceptibility to external fields and more accessible production costs, makes colloidal LCs excellent candidates for displays [5][6][7]. Additionally, and perhaps more interestingly, colloidal suspensions of board-like particles can form biaxial nematic (N B ) phases [8], whose existence, theoretically predicted by Freiser almost 50 years ago [9], is still an open question at the molecular scale. The intriguing prospect of manufacturing biaxial LCDs has been enfeebled by the difficulty of obtaining a stable molecular N B phase, especially at convenient temperatures for display applications. The experimental findings by Vroege and coworkers, who observed a remarkably stable N B phase in systems of polydisperse goethite particles, have provided renewed expectations and perhaps an indication on how, at the molecular scale, the stability...

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“…In addition to this, binary mixtures consisting of board-shaped particles with added polymers can stabilize biaxial order very efficiently [28]. Even the biaxiality of the nematic phase can be tuned by applying an external magnetic field [29].…”

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confidence: 99%

Effect of shape biaxiality on the phase behavior of colloidal liquid-crystal monolayers

Gonzalez-Pinto

Martı́nez-Ratón

Velasco

et al. 2015

Phys. Chem. Chem. Phys.

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We extend our previous work on monolayers of uniaxial particles [J. Chem. Phys. 140, 204906 (2014)] 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 σ 1 ≥ σ 2 ≥ σ 3 , and use is made of the restricted-orientation approximation (Zwanzig model). A density-functional formalism based on the fundamental-measure theory is used to calculate phase diagrams for a wide range of values of the largest aspect ratio (κ 1 = σ 1 /σ 3 ∈ [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 κ 1 > 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 κ 1 > 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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Tuning biaxiality of nematic phases of board-like colloids by an external magnetic field

Cited by 27 publications

References 41 publications

Biaxial nematics of hard cuboids in an external field

Biaxial nematics of hard cuboids in an external field

Monte Carlo simulation of binary mixtures of hard colloidal cuboids

Effect of shape biaxiality on the phase behavior of colloidal liquid-crystal monolayers

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