Recent experiments have reported that ferroelectric nanoparticles have drastic effects on nematic liquid crystals--increasing the isotropic-nematic transition temperature by about 5 K, and greatly increasing the sensitivity to applied electric fields. To understand these effects, we develop a theory for the statistical mechanics of ferroelectric nanoparticles in liquid crystals. This theory predicts the enhancements of liquid-crystal properties, in good agreement with experiments. These predictions apply even when electrostatic interactions are partially screened by moderate concentrations of ions.
Recent discovery of the ferroelectric nematic phase N_F resurrects a question about stability of the uniform N_F state with respect to the formation of either standard for solid ferroelectrics domain structure, or often occurring in liquid crystals space modulation of the polarization vector P (and naturally coupled to P nematic director. In this work within Landau mean-field theory we investigate the linear stability of the minimal model admitting the conventional paraelectric nematic N and N_F phases. Our minimal model, (besides the standard terms of the expansion over P and director gradien…
Nematics with a broken polar symmetry is one of the fascinating recent discoveries in the field of soft matter. High spontaneous polarisation and the fluidity of the ferroelectric nematic $N_{\mathrm{F}}$ phase make such materials attractive for future applications and interesting for fundamental research. Here we explore the polar and mechanical properties of a room-temperature ferroelectric nematic and its behaviour in a magnetic field. We show that $N_{\mathrm{F}}$ is much less susceptible to the splay deformation than to the twist. The strong splay rigidity can be attributed to the electro…
Recent experimental realization of ferroelectric nematic liquid crystalline phases stimulated material development and numerous experimental studies of these new phases, guided by their fundamental and applicative interest. In this Perspective, we give an overview of this emerging field by linking history and theoretical predictions to a general outlook of the development and properties of the materials exhibiting ferroelectric nematic phases. We will highlight the most relevant observations up-to-date, e.g., giant dielectric permittivity values, polarization values an order of magnitude large…
A thermal gradient-induced circular motion of particles placed on ferroelectric nematic liquid crystal sessile drops is demonstrated and explained. Unlike hurricanes and tornadoes that are the prime examples for thermal motors and where turbulent flows are apparent, here the texture without tracer particles appears completely steady indicating laminar flow. We provide a simple model showing that the tangential arrangement of the ferroelectric polarization combined with the vertical thermal gradient and the pyroelectricity of the fluid drives the rotation of the tracer particles that become ele…
Ferroelectric ordering in liquids is a fundamental question of physics. Here, we show that ferroelectric ordering of the molecules causes formation of recently reported splay nematic liquid-crystalline phase. As shown by dielectric spectroscopy, the transition between the uniaxial and the splay nematic phase has the characteristics of a ferroelectric phase transition, which drives an orientational ferroelastic transition via flexoelectric coupling. The polarity of the splay phase was proven by second harmonic generation (SHG) imaging, which additionally allowed for determination of the splay m…
We present a generalization of the Maier-Saupe (MS) theory that incorporates the description of the ferroelectric nematic phase (NF) within its scope of application. The extension of the theory is carried out in a natural way, by adding to the nematic potential of MS a term proportional to cosθ and to the polar order parameter <cosθ >, where θ is the angle between the long molecular axis and the spontaneous polarization. The NF phase can be reached by cooling from the isotropic phase with or without the formation of an intermediate normal nematic phase, depending on the relative intensity of t…
We have structurally characterized the liquid crystal phase that appears as an intermediate state when a dielectric nematic, having polar disorder of its molecular dipoles, transitions to the almost perfectly polar-ordered ferroelectric nematic. This intermediate phase, which fills a 100-year-old void in the taxonomy of smectics and which we term the "smectic $Z_A$", is antiferroelectric, with the nematic director and polarization oriented parallel to smectic layer planes, and the polarization alternating in sign from layer to layer. The period of this polarization wave ($\sim$180 A) is mesosc…
We study a ferromagnetic suspension or a suspension of magnetic nanoparticles in an anisotropic nematic medium, in three different one-dimensional variational settings, ordered in terms of increasing complexity. The three models are featured by a nematic energy, a magnetic energy and a magneto-nematic coupling energy and the experimentally observed patterns are modelled as local or global energy minimizers. We numerically observe polydomains with distinct states of magnetization for weak to moderate magneto-nematic coupling in our models. We demonstrate that these polydomains are stabilised by…
Adiabatic scanning calorimetry (ASC) is capable of providing simultaneously the specific enthalpy h(T) and the specific heat capacity cp(T), and is an important tool to determine the order of transitions and to render high-resolution information on pretransitional thermal behavior. Here we report on ASC results on the compound DIO and on mixtures with RM734. Both compounds exhibit a low-temperature ferroelectric nematic phase (NF) and a high-temperature paraelectric nematic (N). In DIO these two phases are separated by an intermediate phase (Nx). Detailed data of h(T) and cp(T), indicated that…
Elasticity-Driven Periodic Polarization Patterns in Confined Chiral Ferroelectric Nematic Fluid
Jožef Stefan Institute（约瑟夫·斯特凡研究所）；Faculty of Mathematics and Physics, University of Ljubljana（卢布尔雅那大学数学与物理学院）；CENN Nanocenter（CENN纳米中心）；Department of Physics, Faculty of Science and Technology, University of the Basque Country UPV/EHU（巴斯克国立大学理学院物理系）；School of Chemistry, University of Leeds（利兹大学化学学院）；School of Physics and Astronomy, University of Leeds（利兹大学物理与天文学院）
The recent discovery of the ferroelectric splay nematic phase has opened the door to experimental investigation of one of the most searched liquid crystal phases in decades, with high expectations for future applications. However, at this moment, there are more questions than answers. In this work, we examine the formation and structure of large polar nematic domains of the ferroelectric splay nematic material RM734 in planar liquid crystals cells with different aligning agents and specifications. We observe that confining surfaces have a strong influence over the formation of different types …
Direct visualization of local electric fields in a layer of a ferroelectric nematic liquid
Jožef Stefan Institute（约瑟夫·斯特凡研究所）；Faculty of Mathematics and Physics, University of Ljubljana（卢布尔雅那大学数学与物理学院）；School of Chemistry, University of Leeds（利兹大学化学学院）；School of Physics and Astronomy, University of Leeds（利兹大学物理与天文学学院）
Realignment of ferroelectric nematic by photoinduced electric field
Kent State University（肯特州立大学）；Institute of Physics, National Academy of Sciences of Ukraine（乌克兰国家科学院物理研究所）；Institute of Physical Chemistry, Polish Academy of Sciences（波兰国家科学院物理化学研究所）
The electroviscous effect deals with the change in the viscosity of fluids due to an external electric field. Here, we report experimental studies on the electroviscous effects in a ferroelectric nematic liquid crystal. It was synthesised accomplishing a new synthetic route which provides higher yield than conventional one. We measure electric field-dependent viscosity under a steady shear at different temperatures. In the low field range, the increase in viscosity ($Δη=η(E)-η_0$) is proportional to $E^2$ and the corresponding viscoelectric coefficient ($f\approx10^{-9}$m\textsuperscript{2}/V\…
We study the optical properties of gold nanoparticles coated with a nematic liquid crystal whose director field is distributed around the nanoparticle according to the anchoring conditions at the surface of the nanoparticle. The distribution of the nematic liquid crystal is obtained by minimization of the corresponding Frank free-energy functional whilst the optical response is calculated by the discrete-dipole approximation. We find, in particular, that the anisotropy of the nematic liquid-crystal coating does not affect much the (isotropic) optical response of the nanoparticle. However, for …
We show by means of continuum theory and simulations that geometric percolation in uniaxial nematics of hard slender particles is fundamentally different from that in isotropic dispersions. In the nematic, percolation depends only very weakly on the density and is, in essence, determined by a distance criterion that defines connectivity. This unexpected finding has its roots in the non-trivial coupling between the density and the degree of orientational order that dictate the mean number of particle contacts. Clusters in the nematic are much longer than wide, suggesting the use of nematics for…
Calorimetric evidence for the existence of an intermediate phase between the ferroelectric nematic phase and the nematic phase in the liquid crystal RM734
本研究利用高分辨率量热测量证明，液晶 RM734 在铁电向列相 NF 与经典向列相 N 之间的狭窄温区内也存在中间相 Nx。
Nematic liquid crystals have been known for more than a century, but it was not until the 60s-70s that, with the development of room temperature nematics, they became widely used in applications. Polar nematic phases have been long-time predicted, but have only been experimentally realized recently. Synthesis of materials with nematic polar ordering at room temperature is certainly challenging and requires a deep understanding of its formation mechanisms, presently lacking. Here, we compare two materials of similar chemical structure and demonstrate that just a subtle change in the molecular s…
The pioneering realization of suspensions of ferroelectric nanoparticles in a nematic host was one of excellent contributions of Yuri Reznikov to the science of liquid crystals [Y. Reznikov et al., Appl. Phys. Lett. 82 (2003) 191]. This achievement created great excitement as entailed the enhancement of materials dielectric properties and increase of the phase transition temperature between nematic and isotropic phase. In this contribution, we examine the spectrum of fundamental fluctuations in one of his ferroelectric suspensions by dynamic light scattering measurements, which gives insight i…
We study experimentally the impact of spherical nanoparticles on the orientational order parameters of a host nematic liquid crystal. We use spherical core-shell quantum dots that are surface functionalized to promote homeotropic anchoring on their interface with the liquid crystal host. We show experimentally that the orientational order may be strongly affected by the presence of spherical nanoparticles even at low concentrations. The orientational order of the composite system is probed by means of polarised micro-Raman spectroscopy and by optical birefringence measurements as function of t…
We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxia…
Spontaneous electric polarization of solid ferroelectrics follows aligning directions of crystallographic axes. Domains of differently oriented polarization are separated by domain walls (DWs), which are predominantly flat and run along directions dictated by the bulk translational order and the sample surfaces. Here we explore DWs in a ferroelectric nematic (NF) liquid crystal, which is a fluid with polar long-range orientational order but no crystallographic axes nor facets. We demonstrate that DWs in the absence of bulk and surface aligning axes are shaped as conic sections. The conics bise…
Topological spin and polar textures have fascinated people in different areas of physics and technologies. However, the observations are limited in magnetic and solid-state ferroelectric systems. Ferroelectric nematic is the first liquid-state ferroelectric that would carry many possibilities of spatially distributed polarization fields. Contrary to traditional magnetic or crystalline systems, anisotropic liquid crystal interactions can compete with the polarization counterparts, thereby setting a challenge in understating their interplays and the resultant topologies. Here, we discover chiral…
