Polar in-plane surface orientation of a ferroelectric nematic liquid crystal: Polar monodomains and twisted state electro-optics

We show that surface interactions can vectorially structure the three-dimensional polarization field of a ferroelectric fluid. The contact between a ferroelectric nematic liquid crystal and a surface with in-plane polarity generates a preferred in-plane orientation of the polarization field at that interface. This is a route to the formation of fluid or glassy monodomains of high polarization without the need for electric field poling. For example, unidirectional buffing of polyimide films on planar surfaces to give quadrupolar in-plane anisotropy also induces macroscopic in-plane polar order at the surfaces, enabling the formation of a variety of azimuthal polar director structures in the cell interior, including uniform and twisted states. In a π-twist cell, obtained with antiparallel, unidirectional buffing on opposing surfaces, we demonstrate three distinct modes of ferroelectric nematic electro-optic response: intrinsic, viscosity-limited, field-induced molecular reorientation; field-induced motion of domain walls separating twisted states of opposite chirality; and propagation of polarization reorientation solitons from the cell plates to the cell center upon field reversal. Chirally doped ferroelectric nematics in antiparallel-rubbed cells produce Grandjean textures of helical twist that can be unwound via field-induced polar surface reorientation transitions. Fields required are in the 3-V/mm range, indicating an in-plane polar anchoring energy of w_P ∼3 × 10⁻³ J/m².

Nematic liquid crystals (LCs) are useful because of their facile collective response to applied fields and to surface forces (1). In a liquid crystal, the bulk response is the long-ranged deformation of a fluid, elastic field of molecular orientation, on which confining surfaces establish geometrical and topological structural constraints. In the realm of electro-optics, these two basic elements of LC phenomenology have been combined to create LC display technology (2), thereby enabling the portable computing revolution of the 20th century (3). In this development and until very recently, nematic electro-optics has been based on bulk dielectric alignment, in which a quadrupolar coupling to applied electric field induces polarization in a nonpolar LC to generate torque and molecular reorientation. Surface interactions employed to achieve desirable device structures are similarly quadrupolar, with common treatments such as buffing (4) or photo-alignment (5–8) described by the Rapini–Papoular (RP) model (9) and its variants.Nematic LCs are fluids having internal long-range orientational ordering. In statistical mechanical terms, because the isotropic-to-nematic phase transition breaks orientational symmetry, it yields Goldstone modes in the nematic, describing spatial variation of the director, n(r), the local average molecular orientation. Because of the full orientational symmetry of the isotropic phase, in the nematic phase the director has no globally preferred orientation and therefore the harmonic (elastic) energy cost of orientational variation of wavevector q decreases to zero as Kq² at long wavelengths, where K is the orientational (Frank) elastic constant. A bulk nematic sample can be oriented by providing an arbitrarily small force, for example an arbitrarily small applied electric or magnetic field, which couples to nematic orientation via quadrupolar dielectric or diamagnetic anisotropy. The nematic order is similarly infinitely responsive to boundary conditions imposed by surfaces on which there is orientational anisotropy. Because the nematic is a fluid, these conditions make it possible to put a nematic in a container, have it spontaneously anneal into a space-filling, three-dimensional (3D) director orientation structure dictated by the bounding surfaces, and have this structure respond in a predictable way to applied field. In practice, to be useful in applications, the surfaces and fields must be strong enough to eliminate defects and to produce sufficiently fast reorientations of the director.A novel nematic LC phase (10–13) has recently been shown to be a ferroelectric nematic (N_F) (14), offering a variety of opportunities to exploit LC field and surface phenomena in new ways. The ferroelectric nematic is a 3D liquid with a macroscopic electric polarization P(r) (14). On the nanoscale, each molecular dipole is constrained to be nearly parallel to its molecular steric long axis, which translates macroscopically into a strong orientational coupling making P(r) locally parallel to ±n(r), the local average molecular long axis orientation and uniaxial optic axis of the phase (14). The polarization thus endows the N_F with coupling between n(r) and applied electric field, E, that is linear and is dominant over the dielectric coupling at low E. The N_F phase exhibits self-stabilized, spontaneous polar ordering that is nearly complete (12, 14), with a polar order parameter, P = ⟨cos (β_i)⟩ ∼0.9, where β_i is the angle between a typical molecular dipole and the local average polarization density P. The resulting large spontaneous polarization (P ∼6 µC/cm²) enables field-induced nematic director reorientation and an associated electro-optic response with applied fields in typical cells as small as ∼1 V/cm, a thousand times smaller than those used to reorient dielectric nematics.The polar nature of the N_F also results in transformative changes in the interaction of the LC with bounding surfaces, a key aspect of nematic LC science and its potential for technology. Here, we demonstrate that structuring of the vectorial orientation distribution of a 3D volume of polar molecules can be achieved by controlling the polarity of its 2D bounding surfaces. In the simplest example, if the orientation of the preferred polarization is identical on the surfaces of the two parallel glass plates forming a cell, then the N_F volume polarization can be similarly oriented, that is, poled into a uniform orientation by the surfaces, without the need for an applied field.Materials with spontaneous vectorial order such as ferromagnets and ferroelectrics minimize their energy by breaking up into domains of different orientation of their magnetization, M(r), or polarization, P(r), respectively, with these fields aligned locally parallel to the domain boundaries in order to minimize the energy of the internal and external magnetic or electric fields they produce. Intentionally disrupting this domain structure by applying an external field (“field poling”) is a key process in the use of such materials, e.g., putting an iron rod in a magnetic field to create a bar magnet (15), or actively reversing the polarization of a ferroelectric nanocrystal that serves as a data element in nonvolatile solid-state memories (16). Field poling of soft materials, such as the corona poling of chromophore-containing polymers to generate poled monodomains for nonlinear optical and electronic electro-optical applications, has been less successful because of the high fields required (17, 18). In ferroelectric nematics, in contrast, we find that polar aligning surfaces can be used to achieve uniform alignment of their nearly saturated bulk polarization, in the absence of an applied electrical poling field.In a uniaxial (C_∞ symmetric) ferroelectric nematic, the reduction in symmetry associated with the appearance of the bulk polarization, P(r), yields a macroscopic coupling of P to applied field E, with a corresponding bulk energy density U_PE = −P⋅E. In materials where the net molecular dipole is nearly parallel to the steric long axis of the molecule, P(r) is macroscopically constrained to be parallel to ±n(r) by an orientational energy of the form U_Pn = −u_Pn(n⋅v)², where we define a unit vector polar director v(r) = P(r)/P. The energy density coefficient u_Pn can be estimated as u_Pn ∼k_BT/vol where vol is a molecular volume. If we apply a step change in the orientation of P(r) at x = 0, then n(r) will follow within a distance |x| = 𝜉 ∼√(K/u_Pn), which will be of molecular dimensions. v(r) and ±n(r) are therefore essentially locked together in orientation on the macroscopic scale, so that if a field is applied, the response is generally to reorient n(r) by reorienting P(r). An exception to this is the movement of pure polarization reversal (PPR) walls, which flip P by 180° with no reorientation of n (14).     

无创性定量左室心腔内涡流的方法及临床应用

定量观察左室腔内涡流,可以评价左室的收缩功能和舒张功能。涡流的形态及位置随心功能减低而发生变化。已经证实,MRI及超声心动图均能够准确观测左室内的涡流。虽然观测方法仍存在一些局限性,借助声学造影的超声心动图方法是能够用于临床的最为准确有效的技术。该方法利用声学微泡流动与涡流的一致性特征对心室重构及心肌做功进行定量研究。敏感参数包括涡流深度比(0.482±0.06)、长度比(0.467±0.05)、宽度比(0.128±0.06)、球形指数(3.66±0.6)、相对强度比(2.10±0.8)、涡流的相对强度(1.19±0.5)和脉动相关系数(1.31±0.5);舒张早期涡流半径(3±1mm)、舒张早期充盈血流前锋速度Vp(47±6cm/s)及Vp/E(0.59±0.07)。但是,由于临床试验研究对象的例数较少,临床应用效果仍缺乏相应的循证医学的依据。     

From the Cover: Tornado-inspired acoustic vortex tweezer for trapping and manipulating microbubbles

Spatially concentrating and manipulating biotherapeutic agents within the circulatory system is a longstanding challenge in medical applications due to the high velocity of blood flow, which greatly limits drug leakage and retention of the drug in the targeted region. To circumvent the disadvantages of current methods for systemic drug delivery, we propose tornado-inspired acoustic vortex tweezer (AVT) that generates net forces for noninvasive intravascular trapping of lipid-shelled gaseous microbubbles (MBs). MBs are used in a diverse range of medical applications, including as ultrasound contrast agents, for permeabilizing vessels, and as drug/gene carriers. We demonstrate that AVT can be used to successfully trap MBs and increase their local concentration in both static and flow conditions. Furthermore, MBs signals within mouse capillaries could be locally improved 1.7-fold and the location of trapped MBs could still be manipulated during the initiation of AVT. The proposed AVT technique is a compact, easy-to-use, and biocompatible method that enables systemic drug administration with extremely low doses.

Highly diverse drugs have been developed globally in recent years, providing the promise of better prevention, treatments, and cures for a broad range of diseases. When drugs are systemically administered, accurate targeted delivery of the drug to the diseased cells and tissues at sufficiently high doses is critical for drug function. For example, it has been reported that less than 0.7% of an injected drug actually gets into a targeted tumor (1). The high velocity of blood flow (∼1.5–33 cm/s in capillaries and venules) is the main obstacle for circulation-based drug delivery because drug leakage and retention in the targeted region are limited (2). Developing an approach that can noninvasively concentrate and manipulate circulating drugs to propel or navigate them against the flow could greatly improve treatment efficiencies, reduce required administered doses, and avoid off-target effects.Emerging techniques for contactless trapping and manipulation of biomolecules such as DNA, cells, nanoparticles, and microparticles with a high spatial resolution typically utilize optical, magnetic, or electrokinetic forces (3–11). However, each of the current techniques has its own potential drawbacks: 1) optical tweezers may cause physiological and heat damage to cells, high photon absorption in biological materials, and the formation of singlet oxygen (9, 12–14); 2) magnetic tweezers require targets to be prelabeled with magnetic materials, which likely affects cell viability (6, 8, 10, 15); and 3) electrokinetic tweezers can potentially affect cell physiology due to current-induced heating or direct exposure to an electric field (3–5, 16). In other words, these potent bioeffects combined with their short working distances (<1 cm) greatly limit the ability to translate these techniques into clinical applications. The safety and tissue-penetrating ability (∼20 cm) of ultrasound may provide an alternative option for developing so-called acoustic tweezers.A substantial number of acoustic-tweezer configurations have been explored previously for applications in science, engineering, and biomedical sciences. The three primary types of acoustic tweezers are standing-wave tweezers (17–19), single-beam acoustic tweezers (20–22), and acoustic-streaming tweezers (23–25). Standing-wave tweezers spatially form periodic pressure nodes to produce acoustic radiation forces that can be used to control the positions of particles (19). Unfortunately, a complex setup is needed because the trapped objects need to be located between one or more pairs of ultrasound transducers. Single-beam acoustic tweezers can produce a trapping effect to manipulate particles and cells under the conditions of Mie regime in a single-transducer setup (21, 26). However, high operating frequencies are required for this technique because the acoustic wavelength needs to be smaller than the size of the trapped particles or cells. Acoustic-streaming tweezers can be used to indirectly manipulate particles via acoustic-induced fluid flows (also termed acoustic streaming) with oscillating gas-filled microbubbles (MBs) or solid structures (25). The streaming flows generate regions of recirculation or pressure gradients that can be used to influence particle position. The oscillating MBs also produce acoustic radiation forces and streaming vortices to trap and rotate particles, respectively. The low spatial resolution is the main drawback of this type of tweezers, because MBs and microstructure-based phenomena are nonlinear. Although acoustic tweezers have been increasingly used for manipulating cells, particles, and organisms, there are very few reports of promising results in an in vivo environment. For example, the single-beam acoustic tweezers could penetrate through an ex vivo rat aorta and manipulate polystyrene microspheres of 3-µm size inside the vessel (22). However, in order to trap micrometer-size particles, it is necessary to operate such acoustic tweezers with high frequency (∼40 MHz; wavelength: tens of micrometers). The poor tissue penetration of such high-frequency ultrasound would largely limit the in vivo application of single-beam acoustic tweezers.Ideal acoustic tweezers for in vivo applications must meet the following conditions: long penetration depth, strong trapping force, simple setup, multiaxis manipulation, and tissue safety. With the aim of meeting these requirements, we focused on a tornado-inspired acoustic vortex tweezer (AVT) concept that we previously demonstrated is feasible (26, 27). AVT employ destructive interference to produce a ring-shaped beam pattern called a potential well. According to the force-potential mechanism operating in a Rayleigh regime, particles with high acoustic impedances comparing with blood will experience strong trapping forces that drive them toward the beam axis. Here we report on an acoustic-tweezer transducer based on AVT theory that aims to trap and manipulate bioparticles (lipid-shelled gaseous MBs) within the blood circulation in vivo (Fig. 1).Open in a separate windowFig. 1.Concept and paradigm of the study. Tornado-inspired AVT were used to generate a potential well for the noninvasive intravascular trapping of lipid-shelled gaseous MBs within the circulation in vivo. RBC, red blood cell.Several studies have demonstrated the diversity of uses for MBs in medical applications, such as their use as sonography contrast agents, for permeabilization of the blood–brain barrier, and as drug/gene carriers (28–31). A new technique should not only address the current shortcomings, but also improve the imaging contrast and the efficiency of drug delivery. This paper first demonstrates the trapping of MBs using the technique in a static condition. Then, the trapping performance and possible side effects in a flow condition are investigated in detail. Finally, the feasibility of in vivo trapping and safety issues are investigated in a dorsal window chamber model.     

From the Cover: Emergence of dynamic vortex glasses in disordered polar active fluids

In equilibrium, disorder conspires with topological defects to redefine the ordered states of matter in systems as diverse as crystals, superconductors, and liquid crystals. Far from equilibrium, however, the consequences of quenched disorder on active condensed matter remain virtually uncharted. Here, we reveal a state of strongly disordered active matter with no counterparts in equilibrium: a dynamical vortex glass. Combining microfluidic experiments and theory, we show how colloidal flocks collectively cruise through disordered environments without relaxing the topological singularities of their flows. The resulting state is highly dynamical but the flow patterns, shaped by a finite density of frozen vortices, are stationary and exponentially degenerated. Quenched isotropic disorder acts as a random gauge field turning active liquids into dynamical vortex glasses. We argue that this robust mechanism should shape the collective dynamics of a broad class of disordered active matter, from synthetic active nematics to collections of living cells exploring heterogeneous media.

From a physicist’s perspective, flocks are ensembles of living or synthetic motile units collectively moving along a common emerging direction (1–4). They realize one of the most robust ordered states of matter observed over five orders of magnitude in scale and in systems as diverse as motility assays, self-propelled colloids, shaken grains, and actual flocks of birds (3, 5–10). The quiet flows of flocks are in stark contrast with the spatiotemporal chaos consistently reported and predicted in active nematic liquid crystals, another abundant form of ordered active matter realized in biological tissues, swimming cells, cellular extracts, and shaken rods (2, 11). Active nematics do not support any form of long-range order (4, 12). Their structure is continuously bent and destroyed by the proliferation and annihilation of singularities in their local orientation: topological defects (11, 13–15). Unlike in active nematics, topological defects in flocking matter are merely transient excitations which annihilate rapidly and allow uniaxial order to extend over system-spanning scales (4).This idyllic view of the ordered phases of active liquids is limited, however, to pure systems. Disorder is known to profoundly alter the stability of topological defects and the corresponding ordered states in equilibrium condensed matter (16–18), but its role in active fluids remains virtually uncharted territory. All previous studies (19–26), including our own early experiments (22), have been limited to weak disorder and smooth perturbations around topologically trivial states. Unlike in equilibrium, no available experiment, simulation, or theory has ever demonstrated or predicted disorder-induced topological excitations in active matter.In this paper we show how isotropic disorder generically challenges the extreme robustness of flocking matter to topological defects. We map the full phase behavior of colloidal flocks navigating through disordered lattice of obstacles and reveal an unanticipated state of active matter: a dynamical vortex glass. In dynamical vortex glasses, millions of self-propelled particles can steadily cruise through disorder, maintaining local orientational order and without relaxing the topological singularities of their flows. The associated flow patterns are exponentially degenerated and shaped by amorphous ensembles of frozen topological defects, yielding a dynamical state akin to the static vortex-glass phase of dirty superconductors and random-gauge magnets (27–29). Building a theory of flock hydrodynamics beyond the spin-wave approximation, we elucidate the emergence and stabilization of topological vortices by quenched disorder. Finally, we discuss the universality of the dynamical vortex glass phase beyond the specifics of polar active matter and colloidal flocks.     

From the Cover: Helicity conservation by flow across scales in reconnecting vortex links and knots

The conjecture that helicity (or knottedness) is a fundamental conserved quantity has a rich history in fluid mechanics, but the nature of this conservation in the presence of dissipation has proven difficult to resolve. Making use of recent advances, we create vortex knots and links in viscous fluids and simulated superfluids and track their geometry through topology-changing reconnections. We find that the reassociation of vortex lines through a reconnection enables the transfer of helicity from links and knots to helical coils. This process is remarkably efficient, owing to the antiparallel orientation spontaneously adopted by the reconnecting vortices. Using a new method for quantifying the spatial helicity spectrum, we find that the reconnection process can be viewed as transferring helicity between scales, rather than dissipating it. We also infer the presence of geometric deformations that convert helical coils into even smaller scale twist, where it may ultimately be dissipated. Our results suggest that helicity conservation plays an important role in fluids and related fields, even in the presence of dissipation.In addition to energy, momentum, and angular momentum, ideal (Euler) fluids have an additional conserved quantity—helicity (Eq. 1)—which measures the linking and knotting of the vortex lines composing a flow (1). For an ideal fluid, the conservation of helicity is a direct consequence of the Helmholtz laws of vortex motion, which both forbid vortex lines from ever crossing and preserve the flux of vorticity, making it impossible for linked or knotted vortices to ever untie (1, 2). Because conservation laws are of fundamental importance in understanding flows, the question of whether this topological conservation law extends to real, dissipative systems is of clear and considerable interest. The general importance of this question is further underscored by the recent and growing impact knots and links are having across a range of fields, including plasmas (3, 4), liquid crystals (5, 6), optical (7), electromagnetic (8), and biological structures (9–11), cosmic strings (12, 13), and beyond (14). Determining whether and how helicity is conserved in the presence of dissipation is therefore paramount in understanding the fundamental dynamics of real fluids and the connections between tangled fields across systems.The robustness of helicity conservation in real fluids is unclear because dissipation allows the topology of field lines to change. For example, in viscous flows vorticity will diffuse, allowing nearby vortex tubes to “reconnect” (Fig. 1 A–C), creating or destroying the topological linking of vortices. This behavior is not unique to classical fluids: analogous reconnection events have also been experimentally observed in superfluids (15) and coronal loops of plasma on the surface of the sun (16). In general, these observed reconnection events exhibit divergent, nonlinear dynamics that makes it difficult to resolve helicity dynamics theoretically (4, 17, 18). On the other hand, experimental tests of helicity conservation have been hindered by the lack of techniques to create vortices with topological structure. Thanks to a recent advance (19), this is finally possible.Open in a separate windowFig. 1.(A) A sketch of the evolution of vortex tube topology in ideal (Euler) and viscous (Navier–Stokes) flow. Dissipative flows allow for reconnections of vortex tubes, and so tube topology is not conserved. (B) Two frames of a 3D reconstruction of a vortex reconnection in experiment, which turns an initially linked pair of rings into a single twisted ring. (C) A close-up view of the reconnection in B. (D) If a tube is subdivided into multiple tubes, linking between the two may be created by introducing a twist into the pair. (E) Similarly, if a coiled tube is subdivided, linking can result even without adding twist. This can be seen either by calculating the linking number for the pair, or imagining trying to separate the two. (F) In a continuum fluid, the vortex tube may be regarded as a bundle of vortex filaments, which may be twisted. In this case, a twist of Δθ ～ 0.7 × 2π results in a total helicity of ? ～ 0.7Γ². (G) If the vortex tube is coiled, linking will also be introduced, as in E. Conceptually, this coiling can be regarded as producing a net rotation of the vortex bundle even when it is everywhere locally untwisted.By performing experiments on linked and knotted vortices in water, as well as numerical simulations of Bose–Einstein condensates [a compressible superfluid (20)] and Biot–Savart vortex evolution, we investigate the conservation of helicity, in so far as it can be inferred from the center-lines of reconnecting vortex tubes. We describe a new method for quantifying the storage of helicity on different spatial scales of a thin-core vortex: a “helistogram.” Using this analysis technique, we find a rich structure in the flow of helicity, in which geometric deformations and vortex reconnections transport helicity between scales. Remarkably, we find that helicity can be conserved even when vortex topology changes dramatically, and identify a system-independent geometric mechanism for efficiently converting helicity from links and knots into helical coils.     

Spontaneous helielectric nematic liquid crystals: Electric analog to helimagnets

Recently, a type of ferroelectric nematic fluid has been discovered in liquid crystals in which the molecular polar nature at molecule level is amplified to macroscopic scales through a ferroelectric packing of rod-shaped molecules. Here, we report on the experimental proof of a polar chiral liquid matter state, dubbed helielectric nematic, stabilized by the local polar ordering coupled to the chiral helicity. This helielectric structure carries the polar vector rotating helically, analogous to the magnetic counterpart of helimagnet. The helielectric state can be retained down to room temperature and demonstrates gigantic dielectric and nonlinear optical responses. This matter state opens a new chapter for developing the diverse polar liquid crystal devices.

In nature , a new matter state usually arises as a result of unexpected combinations of hierarchical orderings. Helicity is one of the most essential nature of matter states for organizing superstructures in soft matters, spanning many length scales from the atomic to the macroscopic biological levels. When constructed from building blocks with inherent polarity, three hierarchical orderings could coexist in a helical structure: 1) the head-to-tail or polar symmetry of each building block (e.g., Fig. 1C), 2) the orientational order of a swarm of building blocks (Fig. 1A), and 3) the emergent helicity (Fig. 1B). While a simultaneous realization of these three orderings could lead to extraordinary material properties, such highly hierarchical structures are often challenging to achieve in man-made systems. Probably the most familiar example is the chiral magnet or helimagnet (Fig. 1B) in quantum systems, where the magnetic spins form two- or three-dimensional spiral structures (1, 2). The polar magnetic helical structures are considered mainly to originate from either the breaking of the space-inversion symmetry in crystal structures (3) or the magnetic frustration (1, 4, 5). Their strong magnetism-chirality coupling triggers enormous interests in condensed matter physics, leading to many unique quantum and information functionalities (6–9). From the mirror relationship between the magnetism and electricity, we anticipate the incidence of a possible electric version of the helimagnets, namely helielectrics. However, the diverse magnetic topological states rarely show up in electric systems, except a few recent breakthroughs (e.g., the observation of the electric skyrmions, polar vortices, and merons in metal-organic crystals) (10–12). The special electric states at nanoscale exhibit extraordinary properties such as local negative dielectric permittivity (13) and strain-polarization coupling (14, 15). Nevertheless, nearly all the aforementioned chiral magnet or electric-analog systems are based on elaborately fabricated inorganics. It is expected that the revolutionary realization of these topologies in a soft matter system would bring the advantages of flexibility, simple preparation, large-area film formation, and ease of integration into electric devices.Open in a separate windowFig. 1.Topological analogy: electric versus magnetic states. (A) Uniform magnetization or polarization. (B) Helimagnet or helielectric states. Possible helicoidal (top) and heliconical (bottom) textures are shown. (C) Molecular structure of the polar anisotropic entity, RM734. The molecular polar dipole is nearly parallel to the long molecular axis. (D) The ferroelectric nematic state with spontaneous polarization. (E) HN* state with heli realized by adding chiral generators into the polar chiral nematic state. One-dimensional polarization fields are also depicted in D and E for clarity. (F) The molecular structures of the chiral generators S1 and S2. (G) The state diagram of the two HN* materials by mixing RM734 with S1 or S2.Among the soft matter systems, liquid analogs of ferromagnet and helimagnet have been reported in liquid crystal (LC) colloids recently (16–20). For the electric versions, there already exist a category of materials possessing all the aforementioned three hierarchical orderings (i.e., the ferroelectric smectic LCs) (21–26). The smectic C* (SmC*) has layered heliconical structure with its local polarity aligning perpendicular to the long molecular axis. Confinement to thin LC cells leads to the unwound ferroelectric state of SmC* with microsecond switching time, thereby being a promising candidate for LC display applications. However, the unavoidable defect generation in the devices originated from the crystal-like structure has been one of the main technical difficulties. Moreover, the SmC* has intrinsically low fluidity and polarity (spontaneous polarization P_s < 1 μC). Here, we report a discovery of a helimagnetic analog state in polar LC materials, dubbed helielectric nematic (HN*). The spontaneous polar nematic ordering is coupled to the chiral orientational helicity (Fig. 1B), taking the form with a nearly helicoidal orientational field. Thanks to its much higher fluidity than the traditional SmC* ferroelectrics, uniform structures can be easily obtained by the typical thermal annealing process. The simultaneous observation of the traditional nonlinear second-harmonic generation (SHG) and SHG interferometry microscopies, as well as the optical observations of the selective reflection from HN* state, allow us to directly visualize the helical polar field. In contrast to the traditional nanoscopic helimagnetic or helielectric inorganics, a wide tunability of the periodic distance ranging from micrometers to near ultraviolet wavelength is achieved in the fluidic structure. Besides, the ability of switching between the polar and nonpolar helical LC states enables complementary physics study for the topology features in HN*. As gifts of the chirality–polarity interaction, the matter state uniquely expresses giant dielectric and SHG optical response, especially interesting SHG amplification when the SHG wavelength coincides with the reflection band of the HN* state.     

Optical vector field rotation and switching with near-unity transmission by fully developed chiral photonic crystals

State-of-the-art nanostructured chiral photonic crystals (CPCs), metamaterials, and metasurfaces have shown giant optical rotatory power but are generally passive and beset with large optical losses and with inadequate performance due to limited size/interaction length and narrow operation bandwidth. In this work, we demonstrate by detailed theoretical modeling and experiments that a fully developed CPC, one for which the number of unit cells N is high enough that it acquires the full potentials of an ideal (N → ∞) crystal, will overcome the aforementioned limitations, leading to a new generation of versatile high-performance polarization manipulation optics. Such high-N CPCs are realized by field-assisted self-assembly of cholesteric liquid crystals to unprecedented thicknesses not possible with any other means. Characterization studies show that high-N CPCs exhibit broad transmission maxima accompanied by giant rotatory power, thereby enabling large (>π) polarization rotation with near-unity transmission over a large operation bandwidth. Polarization rotation is demonstrated to be independent of input polarization orientation and applies equally well on continuous-wave or ultrafast (picosecond to femtosecond) pulsed lasers of simple or complex (radial, azimuthal) vector fields. Liquid crystal–based CPCs also allow very wide tuning of the operation spectral range and dynamic polarization switching and control possibilities by virtue of several stimuli-induced index or birefringence changing mechanisms.

Optical vector field (more commonly called polarization) rotators and switches are essential components of all modern optical and photonic systems for communications, ellipsometry, metrology, biological/chemical detection, and quantum processing/computing (1–10). There are, however, some inherent limitations. Wave plates made with birefringent crystals, for example, require strict alignment of the optic axis with respect to the polarization orientation of incident light and generally do not work with laser vector beams of complex polarization fields; Faraday rotators that do not have this requirement are generally too cumbersome and bulky due to their weak optical rotatory powers. One promising approach to circumvent these limitations is to employ chiral optical materials such as chiral photonic crystals and metasurfaces. Nevertheless, structural chirality, such as chiral metamaterials, metasurfaces, and photonic crystals that are capable of very large optical rotatory power (up to ∼100,000°/mm), are inevitably accompanied by large absorption losses (11–15). In metamaterials/surfaces, the intrinsic noncircular absorption and nanofabrication difficulty also add to the limitation of their practical scalability in the interaction length, resulting in small (<π) net polarization rotation angle, very small aperture, and narrow operating spectral bandwidth (11–13). Similar issues confront most chiral photonic crystals (CPCs) due to the limitations of molecular self-assembly or nanofabrication/processing technique and high transmission loss associated with operation near the Bragg reflection band (14, 15).Here, we show by theory and experimental corroborations that a fully developed liquid crystal–based CPC, one for which the number of unit cells N approaches that (N → ∞) of an ideal crystal, can circumvent all the aforementioned limitations and possess several advantageous characteristics impossible with conventional low-N thin counterparts. Such high-period–number chiral photonic crystals (HN-CPCs) are achieved by fabricating cholesteric liquid crystals (CLCs) to thicknesses several hundred times that of conventional ones using a refined field-assisted self-assembly (FASA) technique (16, 17; see SI Appendix, Note 1, for more details). Optical properties of CLCs as CPCs arise from complex “collective” responses from many unit cells. While thicker crystals obviously give rise to larger effects, the resulting properties as the crystal thickness or period number N evolves from low values to a very high value do not lend themselves to such simple linear extrapolation; as a function of N, pleasant surprises and new insights and possibilities abound. Our studies show that for N > 500, these CLCs exhibit simultaneously broad transmission maxima and large polarization rotation power in the off-Bragg-resonance spectral regime. Polarization rotation is independent of input polarization orientation and acts equally well on simple or complex vector fields (18–22) of continuous-wave (CW) or ultrafast pulsed laser beams. Liquid crystal–based CPCs also allow dynamic polarization switching and control by virtue of field–induced index/birefringence changing mechanisms at modest or ultrafast (picosecond to femtosecond) speeds (23–34).     

Experimental observation of the origin and structure of elastoinertial turbulence

Turbulence generally arises in shear flows if velocities and hence, inertial forces are sufficiently large. In striking contrast, viscoelastic fluids can exhibit disordered motion even at vanishing inertia. Intermediate between these cases, a state of chaotic motion, “elastoinertial turbulence” (EIT), has been observed in a narrow Reynolds number interval. We here determine the origin of EIT in experiments and show that characteristic EIT structures can be detected across an unexpectedly wide range of parameters. Close to onset, a pattern of chevron-shaped streaks emerges in qualitative agreement with linear and weakly nonlinear theory. However, in experiments, the dynamics remain weakly chaotic, and the instability can be traced to far lower Reynolds numbers than permitted by theory. For increasing inertia, the flow undergoes a transformation to a wall mode composed of inclined near-wall streaks and shear layers. This mode persists to what is known as the “maximum drag reduction limit,” and overall EIT is found to dominate viscoelastic flows across more than three orders of magnitude in Reynolds number.

Many fluids in nature and applications, such as paints, polymer melts, or saliva, have viscous as well as elastic properties, and their flow dynamics fundamentally differ from that of Newtonian fluids. A standard example of such viscoelastic fluids is solutions of long-chain polymers, and here, surprisingly even very dilute solutions show a drastic suppression of turbulence and significantly lower drag levels (1, 2), a phenomenon commonly exploited in pipeline flows to save pumping costs. In seeming contradiction to this stabilizing effect are observations at much lower Reynolds numbers (Re; the ratio of inertial to viscous forces), where polymers have the exact opposite effect; they initiate fluctuations and increase the flow’s drag. The resulting chaotic motion was first detected in a narrow Reynolds number interval, 1,000≲Re≲2,000, just below the onset of ordinary turbulence (3, 4) and interpreted as a form of early turbulence. However, it was later shown (5) that the corresponding elastoinertial instability can be traced to the polymer drag reduction regime at larger Re. The suggestion of a possible connection between these two seemingly opposing effects has sparked much recent interest in the phenomenon of elastoinertial turbulence (EIT) (6–13).It has additionally been speculated that EIT may be connected to purely “elastic turbulence,” a fluctuating state driven by a linear elastic instability in the inertialess limit (14). This instability requires curved streamlines (14–16) and is hence not to be expected in flows through smooth straight pipes. This does not, however, rule out the possibility that in planar flows, the instability may arise nonlinearly (17), and this possibility has been supported by recent experiments (18, 19).Although EIT was first observed in pipe flow experiments (3, 4), information on the structure and nature of the resulting state is almost exclusively based on simulations using polymer models. Such simulations and theoretical considerations have suggested a range of possible transition scenarios. In direct numerical simulations (employing the Finitely Extensible Nonlinear Elastic-Peterlin [FENE-P] model), the characteristic features of EIT include near-wall vortical structures oriented perpendicular to the mean flow direction (i.e., spanwise direction) and elongated sheets of constant polymer stretch inclined with respect to the wall. In these simulations, the transition leading to this state is nonlinear (i.e., subcritical) and requires perturbations of finite amplitude (5–7). In another study, the aforementioned spanwise vortical structures were suggested to be linked to the well-known Tollmien–Schlichting (TS) instability that occurs in channel flow of Newtonian fluids at substantially larger Reynolds numbers. Again, here the transition would be subcritical but linked to TS waves (8). Yet other studies reported a linear instability that gives rise to chevron-shaped streaks (9). The latter proposed that this supercritical transition may be the starting point of a sequence of instabilities that eventually lead to EIT. Even more recently, FENE-P simulations of a simplified two-dimensional (2D) variant of EIT identified a subcritical scenario linked to the aforementioned linearly unstable mode (12).In the present study, we visualize the onset of EIT in experiments and show that the flow pattern is in excellent agreement with the unstable mode predicted by linear stability analysis (9). However, in experiments, fluctuations are already present close to onset, suggesting that nonlinear effects cannot be neglected. Moreover, for increasing elasticity number, the instability can be pushed to Re an order of magnitude below the parameter regime predicted by linear analysis. Although a quantitative comparison between experiments and computations remains challenging due to model limitations and difficulties in characterizing fluids, overall these observations suggest that the instability scenario is subcritical, an aspect that is also in line with the recent model study of EIT limited to two dimensions (12). For increasing Re on the other hand, the dominant flow structures adjust from a center to a wall mode, and fluctuation levels strongly increase. The resulting three-dimensional EIT flow pattern persists to the so-called “maximum drag reduction” (MDR) regime at much larger Re. Structural features of EIT can hence be detected across more than three decades in Re.     

Symmetry and dynamics of molecular rotors in amphidynamic molecular crystals

Rotary biomolecular machines rely on highly symmetric supramolecular structures with rotating units that operate within a densely packed frame of reference, stator, embedded within relatively rigid membranes. The most notable examples are the enzyme FoF1 ATP synthase and the bacterial flagellum, which undergo rotation in steps determined by the symmetries of their rotators and rotating units. Speculating that a precise control of rotational dynamics in rigid environments will be essential for the development of artificial molecular machines, we analyzed the relation between rotational symmetry order and equilibrium rotational dynamics in a set of crystalline molecular gyroscopes with rotators having axial symmetry that ranges from two- to fivefold. The site exchange frequency for these molecules in their closely related crystals at ambient temperature varies by several orders of magnitude, up to ca. 4.46 × 10⁸ s^-1.     

Lyssaviruses and Bats: Emergence and Zoonotic Threat

The continued detection of zoonotic viral infections in bats has led to the microbial fauna of these mammals being studied at a greater level than ever before. Whilst numerous pathogens have been discovered in bat species, infection with lyssaviruses is of particular significance from a zoonotic perspective as, where human infection has been reported, it is invariably fatal. Here we review the detection of lyssaviruses within different bat species and overview what is understood regarding their maintenance and transmission following both experimental and natural infection. We discuss the relevance of these pathogens as zoonotic agents and the threat of newly discovered viruses to human populations.     

Electron crystallography of ultrathin 3D protein crystals: Atomic model with charges

Membrane proteins and macromolecular complexes often yield crystals too small or too thin for even the modern synchrotron X-ray beam. Electron crystallography could provide a powerful means for structure determination with such undersized crystals, as protein atoms diffract electrons four to five orders of magnitude more strongly than they do X-rays. Furthermore, as electron crystallography yields Coulomb potential maps rather than electron density maps, it could provide a unique method to visualize the charged states of amino acid residues and metals. Here we describe an attempt to develop a methodology for electron crystallography of ultrathin (only a few layers thick) 3D protein crystals and present the Coulomb potential maps at 3.4-Å and 3.2-Å resolution, respectively, obtained from Ca²⁺-ATPase and catalase crystals. These maps demonstrate that it is indeed possible to build atomic models from such crystals and even to determine the charged states of amino acid residues in the Ca²⁺-binding sites of Ca²⁺-ATPase and that of the iron atom in the heme in catalase.Protein atoms scatter electrons four to five orders of magnitude more strongly than they do X-rays, thus allowing individual protein molecules to be imaged by electron microscopy (1). Although not fully exploited so far, electron protein crystallography has great potential and indeed has yielded superb high-resolution (∼2.0-Å resolution) atomic structures from 2D crystals (2). However, electron crystallography of 3D crystals is problematic, as stacking of even a few layers makes diffraction patterns discrete in all directions, and methods developed for conventional electron crystallography of 2D crystals (3) are not useful (SI Appendix, Fig. S1A, Left). This problem can be overcome, however, as Gonen and coworkers demonstrated (4, 5), by rotating the crystal to spatially integrate the intensities of diffraction spots as in X-ray crystallography (SI Appendix, Fig. S1A, Right) or, in certain cases, even combining simple tilt series.Another important feature of electron scattering is that the diffraction pattern formed by elastically scattered electrons is directly related to the distribution of Coulomb potential. This is in marked contrast to X-rays, which, because they are scattered by electrons, yield an electron density map. Coulomb potential maps may be more difficult to interpret, compared with electron density maps by X-ray crystallography, as the appearance of the same residues may differ depending on their charged state, resolution, and surrounding environment (Fig. 1 and SI Appendix, Fig. S2), but they provide unique information, not attainable by X-rays (6). Theoretical potential maps (F_calc maps; Fig. 1 B–E) calculated from an atomic model of Ca²⁺-ATPase (7) using scattering factors for 300-keV electrons highlight these features. For instance, densities of acidic residues are absent or weak when lower-resolution data are included in the map calculation (Fig. 1 B and C). In contrast, when calculated using scattering factors for X-rays, the densities of the same residues appear more or less identical irrespective of the resolution range (Fig. 1 F and G). These features of Coulomb potential maps result from the fact that atomic scattering factors for electrons vary considerably over a range of spatial frequency depending on the charged state (Fig. 1A) and can become close to zero or even negative (e.g., for O⁻, Fig. 1A). An advantageous consequence is that it is possible to determine experimentally the charged states of protein residues and metals. As proteins use metals of different ionic states for many purposes, notably for catalysis and electron transfer, information on the charged state of metals and amino acid residues can be critical in understanding protein function.Open in a separate windowFig. 1.Atomic scattering factors and theoretical maps. (A) Atomic scattering factors for 300-keV electrons based on values from International Tables for Crystallography (14), except for H⁺ (taken from ref. 15). Scattering factors for X-rays are provided in SI Appendix, Fig. S2. (B–G) Theoretical Coulomb potential (B–E) and electron density (F and G) maps around the Ca²⁺-binding site of Ca²⁺-ATPase, calculated for 8- to 3.4-Å (B, D, and F) and 5- to 3.4-Å (C, E, and G) resolution and contoured at 1.0 σ (B–E) or 1.2 σ (F and G). Viewed from the cytoplasmic side approximately perpendicular to the membrane. Superimposed is the atomic model of Ca²⁺-ATPase derived from that determined by X-ray (PDB ID code 1SU4) (7) and refined in this study. Cyan spheres represent bound Ca²⁺ (I and II). For the potential maps (B–E), standard charges are assigned to all titratable residues (except for Asp-800 in D and E, treated as a neutral residue) and calculated using the scattering factors for 300-keV electrons. Note that appearances of some charged residues vary substantially depending on the resolution range in the Coulomb potential maps (compare B and C or D and E), but are nearly identical in the electron density maps (F and G).Here we present the Coulomb potential maps at 3.4-Å and 3.2-Å resolution, respectively, of Ca²⁺-ATPase and catalase obtained from ultrathin (just a few layers thick) crystals using a new electron diffractometer (SI Appendix, Fig. S1B). These maps demonstrate that it is indeed possible to build atomic models from such crystals and even to determine the charged states of amino acid residues in the Ca²⁺-binding sites of Ca²⁺-ATPase and that of the iron atom in the heme in catalase.     

From the Cover: Moving beyond the constraints of chemistry via crystal structure discovery with isotropic multiwell pair potentials

The rigid constraints of chemistry—dictated by quantum mechanics and the discrete nature of the atom—limit the set of observable atomic crystal structures. What structures are possible in the absence of these constraints? Here, we systematically crystallize one-component systems of particles interacting with isotropic multiwell pair potentials. We investigate two tunable families of pairwise interaction potentials. Our simulations self-assemble a multitude of crystal structures ranging from basic lattices to complex networks. Sixteen of the structures have natural analogs spanning all coordination numbers found in inorganic chemistry. Fifteen more are hitherto unknown and occupy the space between covalent and metallic coordination environments. The discovered crystal structures constitute targets for self-assembly and expand our understanding of what a crystal structure can look like.

Do we know all conceivable crystal structures? This question appears naïve at first, because crystallography is a mature field, but the list of reported inorganic crystal structures is not necessarily representative of all kinds of order that are possible on other scales. Atomic crystal structures are affected by the discreteness of the periodic table and the resulting constraints on chemical bonding (1). Molecular crystals (2), metal organic frameworks (3), nanoparticle superlattices (4), and other soft-matter assemblies (5) are free from these chemical constraints and can exhibit entirely new types of crystallographic order distinct from those found with atoms. A universal list of all plausible crystal structures in systems of particles ranging from the angstrom to the micrometer scale would benefit the search for—and design of—new materials.Crystal structures observed on the atomic scale are subject to the laws of quantum mechanics and to the discrete nature of the atom. The constraints of the chemical bond limit the ways in which atoms can be arranged; in particular, inorganic compounds display geometries that are specific to different kinds of bonding, for example, tetrahedral coordinations in the case of covalent bonding with sp3-hybridized orbitals, or high coordination numbers (CNs) in the case of metallic bonding. For example, in the case of water, angular information is usually directly encoded into the computational model (6, 7); however, structures with similar local arrangements have recently been observed in simulations with isotropic multiwell potentials (8, 9). In addition to known crystal structures, these tunable pair potentials can also be parametrized to model particles “in between” the discontinuous types of behaviors that are possible on the atomic scale, due to the quantized nature of the realm of electronic interactions. As a result, these interaction potentials can model systems on the mesoscopic soft-matter length scale, where particle properties and shapes are highly variable. How do these crystal structures differ from those observed on the atomic scale?Here we show that molecular dynamics simulations of single-component systems interacting via simple isotropic oscillating pair potentials (OPPs) can produce—via crystallization from disordered initial conditions—the majority of reported one-component as well as several multicomponent inorganic crystal structures (10). Our findings are obtained with pair potentials that encode coordination geometry solely via the shape of the radial function of the interaction, and contain no angular terms. We characterize simulation outcomes semiautomatically with the help of computational crystal structure identification techniques. Among the crystal structures we observe are many previously unknown structures, some resembling known ones and some surprisingly complex. The previously unknown crystal structures cluster near each other in parameter space in islands of complexity, demonstrating that certain pair potential features promote specific coordination environments, and that certain coordination environments are inherently prone to crystallographic diversity. Because the functional form of the potential is generic, chemical constraints do not play a role in our simulations, and the particles explore feasible geometries more freely. Such geometries should be accessible, if not to atomic crystals, then to colloidal crystals of nanoparticles where valence is easily tunable.     

SARS-CoV-2: Evolution and Emergence of New Viral Variants

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the etiological agent responsible for the coronavirus disease 2019 (COVID-19). The high rate of mutation of this virus is associated with a quick emergence of new viral variants that have been rapidly spreading worldwide. Several mutations have been documented in the receptor-binding domain (RBD) of the viral spike protein that increases the interaction between SARS-CoV-2 and its cellular receptor, the angiotensin-converting enzyme 2 (ACE2). Mutations in the spike can increase the viral spread rate, disease severity, and the ability of the virus to evade either the immune protective responses, monoclonal antibody treatments, or the efficacy of current licensed vaccines. This review aimed to highlight the functional virus classification used by the World Health Organization (WHO), Phylogenetic Assignment of Named Global Outbreak (PANGO), Global Initiative on Sharing All Influenza Data (GISAID), and Nextstrain, an open-source project to harness the scientific and public health potential of pathogen genome data, the chronological emergence of viral variants of concern (VOCs) and variants of interest (VOIs), the major findings related to the rate of spread, and the mutations in the spike protein that are involved in the evasion of the host immune responses elicited by prior SARS-CoV-2 infections and by the protection induced by vaccination.     

Aligned hemozoin crystals in curved clusters in malarial red blood cells revealed by nanoprobe X-ray Fe fluorescence and diffraction

The human malaria parasite Plasmodium falciparum detoxifies the heme byproduct of hemoglobin digestion in infected red blood cells by sequestration into submicron-sized hemozoin crystals. The crystal is composed of heme units interlinked to form cyclic dimers via reciprocal Fe─O (propionate) bonds. Templated hemozoin nucleation was envisaged to explain a classic observation by electron microscopy of a cluster of aligned hemozoin crystals within the parasite digestive vacuole. This dovetails with evidence that acylglycerol lipids are involved in hemozoin nucleation in vivo, and nucleation of β-hematin, the synthetic analogue of hemozoin, was consistently induced at an acylglycerol-water interface via their {100} crystal faces. In order to ascertain the nature of hemozoin nucleation in vivo, we probed the mutual orientations of hemozoin crystals in situ within RBCs using synchrotron-based X-ray nanoprobe Fe fluorescence and diffraction. The X-ray patterns indicated the presence of hemozoin clusters, each comprising several crystals aligned along their needle c axes and exposing {100} side faces to an approximately cylindrical surface, suggestive of nucleation via a common lipid layer. This experimental finding, and the associated nucleation model, are difficult to reconcile with recent reports of hemozoin formation within lipid droplets in the digestive vacuole. The diffraction results are verified by a study of the nucleation process using emerging tools of three-dimensional cellular microscopy, described in the companion paper.     

Insights into pilus assembly and secretion from the structure and functional characterization of usher PapC

Ushers constitute a family of bacterial outer membrane proteins responsible for the assembly and secretion of surface organelles such as the pilus. The structure at 3.15-Å resolution of the usher pyelonephritis-associated pili C (PapC) translocation domain reveals a 24-stranded kidney-shaped β-barrel, occluded by an internal plug domain. The dimension of the pore allows tandem passage of individual folded pilus subunits in an upright pilus growth orientation, but is insufficient for accommodating donor strand exchange. The molecular packing revealed by the crystal structure shows that 2 PapC molecules in head-to-head orientation interact via exposed β-strand edges, which could be the preferred dimer interaction in solution. In vitro reconstitution of fiber assemblies suggest that PapC monomers may be sufficient for fiber assembly and secretion; both the plug domain and the C-terminal domain of PapC are required for filament assembly, whereas the N-terminal domain is mainly responsible for recruiting the chaperone–subunit complexes to the usher. The plug domain has a dual function: gating the β-pore and participating in pilus assembly.     

Refractory cytopenia with t(l;7), + 8 abnormality and dysplastic eosinophils showing intranuclear Charcot-Leyden crystals: a fluorescence in situ hybridization study

A case of refractory cytopenia and marrow eosinophilia showing t(l;7) translocation and concomitant trisomy 8 is reported. The eosinophils were dysplastic, and showed the unique feature of intranuclear Charcot-Leyden crystal formation, giving rise to a'lip-like'appearance. We speculate that this unusual cytologic feature resulted from abnormal precipitation of Charcot-Leyden crystal protein in the eosinophils. By fluorescence in situ hybridization using a chromosome 8 specific a-satellite probe, the abnormal eosinophils were shown to have derived from the abnormal clone. We postulate that the dysplastic clone might have retained a differentiation potential and be responsive to normal haemopoietic stimuli.