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Polarization flutter, produced by an applied AC electric field drives an equilibrium ferroelectric nematic ($\mathrm{N_F}$) liquid crystal (LC) through a transition into a dissipative active ferroelectric nematic state exhibiting strong elasto-hydrodynamic intermolecular interaction. In such a fluttering ferroelectric, the typical equilibrium $\mathrm{N_F}$ textural features adopted to reduce electrostatic energy, such as preferences for director bend, and alignment of polarization parallel to LC/air interfaces, are overcome, giving way to nonequilibrium conjugate structures in which director splay, and alignment of polarization normal to $\mathrm{N_F}$/air interfaces are preferred. Viewing the latter textures as those of an active nematic phase reveals that self-organization to reduce effective viscosity and resulting dissipation generates a flow-driven apparent nematic elasticity and interface structuring that dominates equilibrium LC elastic and surface forces., Comment: main paper (38 pages with 10 figures), supplement (6 pages)

We show that stable, freely suspended liquid crystal films can be made from the ferroelectric nematic ($\mathrm{N_F}$) phase and from the recently discovered polar, lamellar $\mathrm{SmZ_A}$ and $\mathrm{SmA_F}$ phases. The $\mathrm{N_F}$ films display two-dimensional, smectic-like parabolic focal conic textures comprising director/polarization bend that are a manifestation of the electrostatic suppression of director splay in the film plane. In the $\mathrm{SmZ_A}$ and $\mathrm{SmA_F}$ phases, the smectic layers orient preferentially normal to the film surfaces, a condition never found in typical thermotropic or lyotropic lamellar LC phases, with the $\mathrm{SmZ_A}$ films exhibiting focal-conic fan textures mimicking the appearance of typical smectics in glass cells when the layers are oriented normal to the plates, and the $\mathrm{SmA_F}$ films showing a texture of plaquettes of uniform in-plane orientation where both bend and splay are suppressed, separated by grain boundaries. The $\mathrm{SmA_F}$ phase can also be drawn into thin filaments, in which X-ray scattering reveals that the smectic layer planes are normal to the filament axis. Remarkably, the filaments are mechanically stable even if they break, forming free-standing, fluid filaments supported only at one end. The unique architectures of these films and filaments are stabilized by the electrostatic self-interaction of the liquid crystal polarization field, which enables the formation of confined, fluid structures that are fundamentally different from those of their counterparts made using previously known liquid crystal phases., Comment: Main paper 29 pages (5 figures); Supplement: 9 pages (9 figures)

We report the observation of a Twist Grain Boundary (TGB) phase of DNA, a striking motif of three dimensional (3D) equilibrium self-assembly of the DNA tetramer 5'-GTAC-3', a base sequence that is self-complimentary, pairing to form 4-base long, blunt-end Watson/Crick (WC) duplexes. Hydrophobic blunt ends and liquid crystal ordering enable these short duplexes to aggregate into long-DNA-like columns, even though the double helix has a break every 4 bases. A further step assembles these columns into monolayer sheets in which the columns are mutually parallel, and, finally, these sheets stack into lamellar arrays in which the column axis of each layer rotates in helical fashion, through a 60 degree angle with each passage to the next layer. This reorientation in a left-handed TGB helix enables each 2nm diameter WC column to be parallel to, and to partially enter, the major grooves of its neighbors., Comment: 21 pages, 3 figures

The idea that rod-like molecules possessing an electric dipole moment could exhibit a ferroelectric nematic phase was suggested more than a century ago. However, only recently such a phase has been reported for two quite different liquid crystals: RM734 (4-[(4-nitrophenoxy)carbonyl)]phenyl 2,4-dimethoxybenzoate) and DIO (2.3',4',5'-tetrafluoro[1,1'-biphenyl]-4-yl 2.6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate). For RM734 a direct ferroelectric nematic (NF) to classical nematic N transition was reported, whereas for DIO an intermediate phase Nx was discovered between the NF and the N phases. Here we present high-resolution calorimetric evidence that an intermediate Nx phase also exists in RM734 along a narrow temperature range between the NF and the N phases., Comment: 18 pages, 7 Figures

The collective behavior of the shortest DNA oligomers in high concentration aqueous solutions is an unexplored frontier of DNA science and technology. Here we broaden the realm of DNA nanoscience by demonstrating that single-component aqueous solutions of the DNA 4-base oligomer GCCG can spontaneously organize into three-dimensional (3D) periodic mesoscale frameworks. This oligomer can form B-type double helices by Watson-Crick (WC) pairing, into tiled brickwork-like duplex strands, which arrange into mutually parallel arrays and form the nematic and columnar liquid crystal phases, as is typical for long WC chains. However, at DNA concentrations above 400mg/mL, these solutions nucleate and grow an additional mesoscale framework phase, comprising a periodic network on a three dimensional body-centered cubic (BCC) lattice. This lattice is an array of nodes (valence-8, each formed by a pair of quadruplexes of GCCG terminal Gs), connected with a separation of 6.6 nm by struts (6-GCCG-long WC duplexes). This 3D-ordered DNA framework is of low density (DNA volume fraction ~0.2), but, due to its 3D crystal structure, is osmotically incompressible over its phase range. Atomistic simulations confirm the stability of such structures, which promise to form the basis of novel families of simply and inexpensively made nanoscale frameworks for templating and selection applications., Comment: 33 pages, 8 figures

The current intense study of ferroelectric nematic liquid crystals was initiated by the observation of the same ferroelectric nematic phase in two independently discovered organic rod-shaped mesogenic compounds, RM734 and DIO. We recently reported that the compound RM734 also exhibits a monotropic, low-temperature, antiferroelectric phase having reentrant isotropic symmetry (the $I_A$ phase), the formation of which is facilitated to a remarkable degree by doping with small (below $1\%$) amounts of the ionic liquid ${\rm BMIM-PF}_6$. Here we report similar phenomenology in DIO, showing that this reentrant isotropic behavior is not only a property of RM734 but is rather a more general, material-independent feature of ferroelectric nematic mesogens. We find that the reentrant isotropic phases observed in RM734 and DIO are similar but not identical, adding two new phases to the ferroelectric nematic realm. The two $I_A$ phases exhibit similar, strongly peaked, diffuse x-ray scattering in the WAXS range $(1

We report a transition from the ferroelectric nematic liquid crystal ($N_F$) phase to a lower-temperature, antiferroelectric fluid phase having reentrant isotropic symmetry ($I_A$), in the liquid crystal compound RM734 doped with small concentrations of the ionic liquids BMIM or EMIM. Even a trace amount of ionic liquid dopant facilitates the kinetic pathway for the transition from the $N_F$ to the $I_A$, enabling simple cooling to produce this isotropic fluid phase rather than resulting in crystallization. The $I_A$ was also obtained in the absence of specific ionic liquid doping by appropriate temperature cycling in three distinct, as-synthesized-and-purified batches of RM734, two commercial and one from our laboratory. An additional birefringent, lamellar-modulated, antiferroelectric phase with the director parallel to the layers, resembling the smectic $Z_A$, is found between the paraelectric and ferroelectric nematic phases in RM734/BMIM mixtures., Comment: 45 pages, including Supplementary Information

Mechanically quenching a thin film of smectic-C liquid crystal results in the formation of a dense array of thousands of topological defects in the director field. The subsequent rapid coarsening of the film texture by the mutual annihilation of defects of opposite sign has been captured using high-speed, polarized light video microscopy. The temporal evolution of the texture has been characterized using an object-detection convolutional neural network to determine the defect locations, and a binary classification network customized to evaluate the brush orientation dynamics around the defects in order to determine their topological signs. At early times following the quench, inherent limits on the spatial resolution result in undercounting of the defects and deviations from expected behavior. At intermediate to late times, the observed annihilation dynamics scale in agreement with theoretical predictions and simulations of the $2$D XY model., Comment: 11 pages, 10 figures

Polymorphism, the phenomenon that a species can exist in many discrete forms, is common in nature, such as hair colors in an animal species, flower colors in a tree species, and blood types in humans, etc. In materials science, it refers to a solid that can exist in multiple forms with different crystalline structures. In the liquid crystals field, however, polymorphism is hard to find because a discontinuous structural variation is basically impossible because of their fluid or partially fluid nature. Herein we show that the B4 and DC phases that for many years have been classified as distinctive phases are connected, in terms of their nano-architectures, based on the study of a single compound, a flexible bent-shaped dimer. The surrounding solvent is the key to assisting the dimeric molecules in morphing and adopting different supramolecular structures at the mesoscale. Furthermore, we accidentally find a novel nanotube-like structure that has not yet been reported in view of the B4/DC phases. Together with the known sponge (DC) and the helical filament (B4) structures, they are just some of the manifestations of the polymorphism in a class of low-birefringence, chiral phase from achiral liquid crystals., Comment: M: 16p, 5f; SI: 9p, 9f

The combination of large spontaneous polarization and fluidity makes the newly discovered ferroelectric nematic liquid crystalline phase (NF) responsive to electric fields in ways that have no counterpart in other materials. We probe this sensitive field response by confining a NF fluid in microchannels that connect electrodes through straight and curved paths. We find that by applying electric fields as low as E c.a. 0.5 V/mm, the NF phase orders with its polarization smoothly following the winding paths of the channels even when oriented antiparallel to the line connecting positive to negative electrodes, implying analogous behavior of the electric field. Upon inversion of E, the polar order undergoes a complex multistage switching process dominated by electrostatic interactions. Multistage polarization switching dynamics is also found in numerical simulations of a quasi-2D continuum model of NF liquid crystals in microchannels, which also clarify the conditions under which the electric field is guided by the microchannels. Experiments and theory indicate that all observations are direct consequences of the prompt effective screening of electric field components normal to the channel walls. This electric "superscreening" effect emerges as a distinctive property of the NF phase, capable of inducing conditions in which both the polarization and the electric field are guided by microchannels., Comment: Paper Submitted

Numerous measurements of the dielectric constant $\epsilon$ of the recently discovered ferroelectric nematic ($N_F$) liquid crystal (LC) phase report extraordinarily large values of $\epsilon^\prime$ (up to ~30,000). We show that what is in fact being measured in such experiments is the high capacitance of the non-ferroelectric, interfacial, insulating layers of nanoscale thickness that bound the $N_F$ material in typical cells. We analyze a parallel-plate cell filled with $N_F$ material of high-polarization $\mathbf{P}$, oriented parallel to the plates at zero applied voltage. Minimization of the dominant electrostatic energy renders $\mathbf{P}$ spatially uniform and orients it to make the electric field in the $N_F$ as small as possible, a condition under which the voltage applied to the cell appears almost entirely across the high-capacity interfacial layers. This coupling of orientation and charge creates a combined polarization-external capacitance (PCG) Goldstone reorientation mode requiring applied voltages orders of magnitude smaller than that of the $N_F$ layer alone to effectively transport charge across the $N_F$ layer. The $N_F$ layer acts as a low-value resistor and the interfacial capacitors as reversible energy storage reservoirs, lowering the restoring force (mass) of the PCG mode and producing strong reactive dielectric behavior. Analysis of data from several experiments on ferroelectric liquid crystals (chiral smectics C, bent-core smectics, and the $N_F$ phase supports the PCG model, showing that deriving dielectric constants from electrical impedance measurements of high-polarization ferroelectric LCs, without properly accounting for the self-screening effects of polarization charge and the capacitive contributions of interfacial layers, can result in overestimation of the $\epsilon^\prime$ values of the LC by many orders of magnitude., Comment: 30 pages of text, 11 figures, 59 references (47 pages total)

We report the smectic $A_F$, a new liquid crystal phase of the ferroelectric nematic realm. The smectic $A_F$ is a phase of small polar, rod-shaped molecules which form two-dimensional fluid layers spaced by approximately the mean molecular length. The phase is uniaxial, with the molecular director, the local average long-axis orientation, normal to the layer planes, and ferroelectric, with a spontaneous electric polarization parallel to the director. Polarization measurements indicate almost complete polar ordering of the $\sim 10$ Debye longitudinal molecular dipoles, and hysteretic polarization reversal with a coercive field of about $2 \times 10^5$ V/m is observed. The smectic $A_F$ phase appears upon cooling in two binary mixtures of partially fluorinated mesogens: 2N/DIO, exhibiting a nematic ($N$) -- smectic $Z_A$ (Sm$Z_A$) -- ferroelectric nematic ($N_F$) -- smectic $A_F$ (Sm$A_F$) phase sequence; and 7N/DIO, exhibiting an $N$ -- Sm$Z_A$ -- Sm$A_F$ phase sequence. The latter presents an opportunity to study a transition between two smectic phases having orthogonal systems of layers., Comment: 26 pages, 6 figures

We investigated the electrostatic behavior of ferroelectric liquid droplets exposed to the pyroelectric field of a lithium niobate ferroelectric crystal substrate. The ferroelectric liquid is a nematic liquid crystal in which almost complete polar ordering of the molecular dipoles generates an internal macroscopic polarization locally collinear to the mean molecular long axis. Upon entering the ferroelectric phase by reducing the temperature from the nematic phase, the liquid crystal droplets become electromechanically unstable and disintegrate by the explosive emission of fluid jets. These jets are mostly interfacial, spreading out on the substrate surface, and exhibit fractal branching out into smaller streams to eventually disrupt, forming secondary droplets. We understand this behavior as a manifestation of the Rayleigh instability of electrically charged fluid droplets, expected when the electrostatic repulsion exceeds the surface tension of the fluid. In this case the charges are due to the bulk polarization of the ferroelectric fluid which couples to the pyroelectric polarization of the underlying lithium niobate substrate through its fringing field and solid-fluid interface coupling. Since the ejection of fluid does not neutralize the droplet surfaces, they can undergo multiple explosive events as the temperature decreases., Comment: 15 pages, 10 figures