Electric-field induced shape transition of nematic tactoids

The occurrence of new textures of liquid crystals is an important factor in tuning their optical and photonics properties. Here, we show, both experimentally and by numerical computation, that under an electric field chitin tactoids (i.e. nematic droplets) can stretch to aspect ratios of more than 15, leading to a transition from a spindle-like to a cigar-like shape. We argue that the large extensions occur because the elastic contribution to the free energy is dominated by the anchoring. We demonstrate that the elongation involves hydrodynamic flow and is reversible, the tactoids return to their original shapes upon removing the field.Comment: 8 pages, 6 figures; accepted for publication in Physical Review

Hybrid Nanocomposites with Tunable Alignment of the Magnetic Nanorod Filler

For many important applications, the performance of polymer-anisotropic particle nanocomposite materials strongly depends on the orientation of the nanoparticles. Using the very peculiar magnetic properties of goethite ({\alpha}-FeOOH) nanorods, we produced goethite-poly(hydroxyethyl methacrylate) nanocomposites in which the alignment direction and the level of orientation of the nanorods could easily be tuned by simply adjusting the intensity of a magnetic field applied during polymerization. Because the particle volume fraction was kept low (1-5.5 vol \%), we used the orientational order induced by the field in the isotropic phase rather than the spontaneous orientational order of the nematic phase. At the strongest field values (up to 1.5 T), the particles exhibit almost perfect antinematic alignment, as measured by optical birefringence and small-angle X-ray scattering. The results of these two techniques are in remarkably good agreement, validating the use of birefringence measurements for quantifying the degree of orientational order. We also demonstrate that the ordering induced by the field in the isotropic suspension is preserved in the final material after field removal. This work illustrates the interest, for such problems, of considering the field-induced alignment of anisotropic nanoparticles in the isotropic phase, an approach that is effective at low filler content, that avoids the need of controlling the nematic texture, and that allows tuning of the orientation level of the particles at will simply by adjusting the field intensity

A liquid-crystalline hexagonal columnar phase in highly-dilute suspensions of imogolite nanotubes

International audienceLiquid crystals have found wide applications in many fields ranging from detergents to information displays and they are also increasingly being used in the 'bottom-up' self-assembly approach of material nano-structuration. Moreover, liquid-crystalline organizations are frequently observed by biologists. Here we show that one of the four major lyotropic liquid-crystal phases, the columnar one, is much more stable on dilution than reported so far in literature. Indeed, aqueous suspensions of imogolite nanotubes, at low ionic strength, display the columnar liquid-crystal phase at volume fractions as low as B0.2%. Consequently, due to its low visco-elasticity, this columnar phase is easily aligned in an alternating current electric field, in contrast with usual columnar liquid-crystal phases. These findings should have important implications for the statistical physics of the suspensions of charged rods and could also be exploited in materials science to prepare ordered nanocomposites and in biophysics to better understand solutions of rod-like biopolymers

Optical and X-ray scattering studies of the electric field-induced orientational order in colloidal suspensions of pigment nanorods

© 2018 Elsevier B.V. Under pulsed or a.c. electric fields, colloidal suspensions of nanorods can show strong electro-optic effects, such as the Kerr effect, with fast response times (a few ms), which makes them good candidates for some commercial applications. For this purpose, suspensions of Pigment red 176 nanorods in dodecane were recently developed and their physical properties have been studied. We report here on the investigation of the orientational order induced by electric fields in isotropic suspensions of pigment nanorods by three different techniques: transient electric birefringence, transient electric dichroism, and in-situ small-angle X-ray scattering under electric field (“Electro-SAXS”). We show that, in spite of the apolar character of the solvent, the Maxwell-Wagner-O'Konski mechanism (i.e. the polarization of the counter-ion cloud around each particle) is responsible for the field-induced alignment of the nanorods. Although the particles are only weakly charged and the dielectric constant of dodecane is low, the pigment nanorods effectively behave as metallic particles in an insulating matrix and reach strong values (S ~0.5) of the induced nematic order parameter at moderate field amplitudes (~1 V/μm). This study confirms the feasibility of using suspensions of Pigment red 176 nanorods in dodecane for electro-optic applications

Temperature Dependence of the Electroclinic Effect in the Twist-Bend Nematic Phase

Funding Information: This research was funded by the Croatian Science Foundation (Grant No. IP-2019-04-7978); by the Agence Nationale pour la Recherche ANR (France) through Grant BESTNEMATICS, No. ANR-15-CE24-0012; by the French-Croatian bilateral program COGITO; by the Université de Picardie Jules Verne, Amiens, France. Publisher Copyright: © 2023 by the authors.Peer reviewedPublisher PD

Fréedericksz-Like Transition in a Biaxial Smectic- A Phase

The two main classes of liquid-crystal (LC) phases of rodlike molecules are nematics, where the rods align in the same direction (the nematic director n), and smectics, where the rods not only are aligned but also form layers. The electro-optic effects in LC devices that are a backbone in today’s display industry mainly use the Fréedericksz transition, which is the bulk reorientation of a surface-anchored nematic by an electric field. Conventional (uniaxial) smectics do not present a Fréedericksz transition, because, due to their layered structure, the director reorientation would distort the layers, which would cost too much energy. In a worldwide ongoing effort to extend the variety of LC compounds suitable for applications in the display industry, bent- shaped molecules have recently raised much attention, since they present multiple new LC phases with unusual properties. In this paper, we report on a structural and electro-optic study of the LC phases of a bent-shaped dimer. On cooling from the isotropic liquid, this compound shows a usual nematic (N), a twist- bend nematic (NTB), and a biaxial smectic-A phase (Sm Ab). Quite surprisingly, contrary to usual smectics, Sm Ab presents a remarkable electro-optic response, with low ( < 4 V) voltage threshold, no reorganization of the smectic layers, and low ( < 1 ms) response time (i.e., 30 times faster than the N phase at higher temperature). We interpret this unexpected electro-optic effect as a Fréedericksz transition affecting the secondary director m of the Sm Ab, and we model it by analogy with the usual Fréedericksz transition of the n director of the uniaxial N phase. Indeed, a Fréedericksz transition affecting only m in this biaxial fluid smectic does not alter its layered structure and costs little energy. From the point of view of applications, thanks to its low relaxation time, this “biaxial” Fréedericksz transition could be exploited in electro-optic devices that require fast switching

Setting things straight in ‘The twist-bend nematic: a case of mistaken identity’

Stimulated by the paper that precedes this, our aim is to correct an error made concerning the twist-bend nematic phase and to place its development, both theoretical and experimental, in their proper context. The prediction starts with R.B.Meyer in 1973 but then jumps to 2001 and a different direction with I. Dozov and to 2002 with R. Memmer. There was then a gap until 2011 when a team of thirteen European scientists, while studying a liquid crystal dimer formed from achiral bent molecules, identified a nematic phase as the missing twist-bend nematic. This was based on a detailed study of its defining characteristics such as the chirality of the phase and its helicoidal structure with its degenerate handedness. This discovery proved to be of considerable interest and has prompted numerous investigations resulting in the discovery of many other mesogens forming the twist-bend nematic phase. However, as we shall see this view has not been universally shared; indeed, amongst others, Samulski, Vanakaras and Photinos claim that cyanobiphenyl mesogens instead of producing the NTB nematic, form the polar twisted phase, NPT so that the twist-bend nematic has yet to be discovered. It is this error that we shall correct. The Doubly Degenerate Heliconical Structure The Doubly Degenerate Heliconical Structure.</p

Probing permanent dipoles in CdSe nanoplatelets with transient electric birefringence

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Electric-field induced twist-bend to splay-bend nematic phase transition?

International audience1. Introduction Spontaneously distorted nematic states have been theoretically predicted long ago for bent-shaped mesogens [1-3]. Recent experiments [4] confirmed the existence of the twist-bend nematic phase, NTB, with short-pitch heliconical precession of the director n. Another predicted state [3], the splay-bend nematic, NSB, with n oscillating in a plane, is still elusive. However, a recent birefringence study [5] suggested NSB structure of the defect wall separating two NTB domains with opposite chiralities.2. Twist-bend to splay-bend transition under strong electric fieldHere we investigate the NTB state of the bent-shaped dimer CB7CB under strong a.c. electric field, (E ~10 V/µm) parallel to the helix axis. This in-plane field is applied in a 2 µm cell using two ITO electrodes (Fig.1). Crossing the N- NTB transition under field, we observe in the interelectrode region slightly lower transition temperature and much stronger birefringence than that reported in the NTB phase [5], suggesting an electric-field induced NTB – NSB transition.3. ConclusionA strong electric field applied to the NTB phase induces a transition to the splay-bend nematic phase, instead to the usual uniform nematic N. We explain this in the framework of the elastic-instability model [3] of the NTB and NSB phases by taking into account the strong biaxiality of the bent-shape nematic.References[1]R. B. Meyer, in Molecular Fluids, edited by R. Balian and G. Weill (Gordon and Breach, New York 1976), pp. 273 [2]V. L. Lorman and B. Mettout, Phys. Rev. Lett. 82, 940 (1999).[3]I. Dozov, EPL (Europhysics Letters) 56, 247 (2001).[4]M. Cestari, S. Diez-Berart, D. A. Dunmur, A. Ferrarini, M. R. de la Fuente, D. J. B. Jackson, D. O. Lopez, G. R. Luckhurst, M. A. Perez-Jubindo, R. M. Richardson, J. Salud, B. A. Timimi and H. Zimmermann, Physical Review E 84, 031704 (2011).[5]C. Meyer, G. R. Luckhurst and I. Dozov, Journal of Materials Chemistry C 3, 318 (2015)

Optical filter based on Fabry-Perot structure using a suspension of goethite nanoparticles as electro-optic material

International audienceWe have investigated the feasibility of optical tunable filters based on a Fabry-Perot etalon that uses a suspension of goethite (α-FeOOH) nanorods as electro-optic material for application in optical telecommunications in the near IR range. These synthetic nanoparticles have a high optical anisotropy that give rise to a very strong Kerr effect in their colloidal suspensions. Currently, these particles are dispersed in aqueous solvent, with pH2 to ensure the colloidal electrostatic stability. However, the high conductivity of these suspensions requires using high-frequency electric fields (f > 1 MHz), which brings about a high power consumption of the driver. To decrease the field frequency, we have changed the solvent to ethylene glycol which has a lower electrical conductivity than the aqueous solvent. We have built a Fabry-Perot cell, filled with this colloidal suspension in the isotropic phase, and showed that a phase shift of 14 nm can be obtained in a field of 3V/μm. Therefore, the device can operate as a tunable filter. A key advantage of this filter is that it is, by principle, completely insensitive to the polarization of the input light. However, several technological issues still need to be solved, such as ionic contamination of the suspension from the blocking layers, and dielectrophoretic and thermal effects