ABSTRACT: In this review article, we discuss the rheological properties of the thermotropic smectic liquid crystal 8CB with focal conic domains (FCDs) from the viewpoint of structural rheology. It is known that the unbinding of the dislocation loops in the smectic phase drives the smectic-nematic transition. Here we discuss how the unbinding of the dislocation loops affects the evolution of the FCD size, linear and nonlinear rheological behaviors of the smectic phase. By studying the FCD formation from the perpendicularly oriented smectic layers, we also argue that dislocations play a key role in the structural development in layered systems. Furthermore, similarities in the rheological behavior between the FCDs in the smectic phase and the onion structures in the lyotropic lamellar phase suggest that these systems share a common physical origin for the elasticity.
Project description:Controlling topological defects in 3D liquid crystal phases is a crucial element in the development of novel devices, from blue-phase displays to passive biochemical sensors. However, it remains challenging to realize the 3D topological conditions necessary to robustly and arbitrarily direct the formation of defects. Here, using a series of short pillar arrays as topological templates, we demonstrate the hierarchical assembly of focal conic domains (FCDs) in smectic-A liquid crystals that break the underlying symmetry of the pillar lattice, exhibit tunable eccentricity, and together develop a nontrivial yet organized array of defects. The key to our approach lies in the selection of the appropriate ratio of the size of focal domain to the dimension of pillars such that the system favors the "pinning" of FCD centers near pillar edges while avoiding the opposing effect of confinement. Our study unequivocally shows that the arrangement of FCDs is strongly influenced by the height and shape of the pillars, a feature that promotes both a variety of nontrivial self-assembled lattice types and the attraction of FCD centers to pillar edges, especially at regions of high curvature. Finally, we propose a geometric model to reconstruct the smectic layer structure in the gaps between neighboring FCDs to estimate the energetic effects of nonzero eccentricity and assess their thermodynamic stability.
Project description:Electrocaloric materials have become a viable technology for solid state heat management applications. We demonstrate both theoretically and experimentally that liquid crystals (LCs) can be exploited as efficient electrocaloric materials. Numerical and experimental investigations determine the conditions under which the strongest electrocaloric effect (ECE) responses are expected in LCs. Specifically, we show that a large ECE can be expected at the isotropic-nematic and in particular at the isotropic-smectic A phase transition. In our theoretical study, LC ordering is modelled using a Landau - de Gennes - Ginzburg mesoscopic approach. The simulation results are in qualitative agreement with our high precision electrocaloric measurements conducted on 8CB and 12CB liquid crystals. In the latter, we obtained ?TEC?~?6.5?K, corresponding to the largest response measured so far in LCs. The fluid property of LC electrocaloric heat cooling elements could lead to the development of devices with a higher coefficient of performance and thus better cooling power yield per mass of the ECE-based device.
Project description:The phase diagrams of two mixtures of chemically similar smectogenic mesogens strongly differing in molecular length were investigated. In these mixtures the nematic phase present in the pure short mesogen disappeared rapidly on the addition of the longer mesogen, while the smectic state was preserved. In the smectic state the smectic A phase was the much more stable phase as the smectic C phase disappeared quite rapidly as well. In these compounds the loss of the smectic C phase is accompanied by a decrease in smectic translational order and very small tilt angles. This leads to a concentration induced smectic C to smectic A transition. Thus smectic A seems to be the most stable phase to accommodate mesogenic molecules of substantially different length. These surprising results are of general interest for the understanding of the structure and dynamics of smectic phases, as the structure of these bidisperse smectics is signified by extensive out-of-layer fluctuations.
Project description:OBJECTIVE:Focal cortical dysplasias (FCDs) often cause pharmacoresistant epilepsy, and surgical resection can lead to seizure-freedom. Magnetic resonance imaging (MRI) and positron emission tomography (PET) play complementary roles in FCD identification/localization; nevertheless, many FCDs are small or subtle, and difficult to find on routine radiological inspection. We aimed to automatically detect subtle or visually-unidentifiable FCDs by building a classifier based on an optimized cortical surface sampling of combined MRI and PET features. METHODS:Cortical surfaces of 28 patients with histopathologically-proven FCDs were extracted. Morphology and intensity-based features characterizing FCD lesions were calculated vertex-wise on each cortical surface, and fed to a 2-step (Support Vector Machine and patch-based) classifier. Classifier performance was assessed compared to manual lesion labels. RESULTS:Our classifier using combined feature selections from MRI and PET outperformed both quantitative MRI and multimodal visual analysis in FCD detection (93% vs 82% vs 68%). No false positives were identified in the controls, whereas 3.4% of the vertices outside FCD lesions were also classified to be lesional ("extralesional clusters"). Patients with type I or IIa FCDs displayed a higher prevalence of extralesional clusters at an intermediate distance to the FCD lesions compared to type IIb FCDs (p < 0.05). The former had a correspondingly lower chance of positive surgical outcome (71% vs 91%). CONCLUSIONS:Machine learning with multimodal feature sampling can improve FCD detection. The spread of extralesional clusters characterize different FCD subtypes, and may represent structurally or functionally abnormal tissue on a microscopic scale, with implications for surgical outcomes.
Project description:A system of two liquid-crystalline phenylpyrimidines differing strongly in molecular length was studied. The phase diagram of these two chemically similar mesogens, with a length ratio of 2, was investigated, and detailed X-ray diffraction and electrooptical measurements were performed. The phase diagram revealed a destabilization of the nematic phase, which is present in the pure short compound, while the smectic state was stabilized. The short compound forms smectic A and smectic C phases, whereas the longer compound forms a broad smectic C phase and a narrow higher-ordered smectic phase. Nevertheless, in the mixtures, the smectic C phase is destabilized and disappears rapidly, whereas smectic A is the only stable phase observed over a broad concentration range. In addition, the smectic translational order parameters as well as the tilt angles of the mixtures are reduced. The higher-ordered smectic phase of the longer mesogen was identified as a smectic F phase.
Project description:Smectic ordering in aqueous solutions of monodisperse stiff double-stranded DNA fragments is known not to occur, despite the fact that these systems exhibit both chiral nematic and columnar mesophases. Here, we show, unambiguously, that a smectic-A type of phase is formed by increasing the DNA's flexibility through the introduction of an unpaired single-stranded DNA spacer in the middle of each duplex. This is unusual for a lyotropic system, where flexibility typically destabilizes the smectic phase. We also report on simulations suggesting that the gapped duplexes (resembling chain-sticks) attain a folded conformation in the smectic layers, and argue that this layer structure, which we designate as smectic-fA phase, is thermodynamically stabilized by both entropic and energetic contributions to the system's free energy. Our results demonstrate that DNA as a building block offers an exquisitely tunable means to engineer a potentially rich assortment of lyotropic liquid crystals.
Project description:Chiral symmetry breaking in soft matter is a hot topic of current research. Recently, such a phenomenon was found in a fluidic phase showing orientational order of molecules-the nematic phase; although built of achiral molecules, the phase can exhibit structural chirality-average molecular direction follows a short-pitch helix. Here, we report a series of achiral asymmetric dimers with an odd number of atoms in the spacer, which form twisted structures in nematic as well as in lamellar phases. The tight pitch heliconical nematic (NTB) phase and heliconical tilted smectic C (SmCTB) phase are formed. The formation of a variety of helical structures is accompanied by a gradual freezing of molecular rotation. In the lowest temperature smectic phase, HexI, the twist is expressed through the formation of hierarchical structure: nanoscale helices and mesoscopic helical filaments. The short-pitch helical structure in the smectic phases is confirmed by resonant X-ray measurements.
Project description:Regional measurements of fixed charge densities (FCDs) of healthy human cartilage endplate (CEP) using a two-point electrical conductivity approach.The aim of this study was to determine the FCDs at four different regions (central, lateral, anterior, and posterior) of human CEP, and correlate the FCDs with tissue biochemical composition.The CEP, a thin layer of hyaline cartilage on the cranial and caudal surfaces of the intervertebral disc, plays an irreplaceable role in maintaining the unique physiological mechano-electrochemical environment inside the disc. FCD, arising from the carboxyl and sulfate groups of the glycosaminoglycans (GAG) in the extracellular matrix of the disc, is a key regulator of the disc ionic and osmotic environment through physicochemical and electrokinetic effects. Although FCDs in the annulus fibrosus (AF) and nucleus pulposus (NP) have been reported, quantitative baseline FCD in healthy human CEP has not been reported.CEP specimens were regionally isolated from human lumbar spines. FCD and ion diffusivity were concurrently investigated using a two-point electrical conductivity method. Biochemical assays were used to quantify regional GAG and water content.FCD in healthy human CEP was region-dependent, with FCD lowest in the lateral region (P?=?0.044). Cross-region FCD was 30% to 60% smaller than FCD in NP, but similar to the AF and articular cartilage (AC). CEP FCD (average: 0.12?±?0.03?mEq/g wet tissue) was correlated with GAG content (average: 31.24?±?5.06??g/mg wet tissue) (P?=?0.005). In addition, the cross-region ion diffusivity in healthy CEP (2.97?±?1.00?×?10?cm/s) was much smaller than the AF and NP.Healthy human CEP acts as a biomechanical interface, distributing loads between the bony vertebral body and soft disc tissues and as a gateway impeding rapid solute diffusion through the disc.N/A.
Project description:Using X-ray emission spectroscopy, we find appreciable local magnetic moments until 30 GPa to 40 GPa in the high-pressure phase of iron; however, no magnetic order is detected with neutron powder diffraction down to 1.8 K, contrary to previous predictions. Our first-principles calculations reveal a "spin-smectic" state lower in energy than previous results. This state forms antiferromagnetic bilayers separated by null spin bilayers, which allows a complete relaxation of the inherent frustration of antiferromagnetism on a hexagonal close-packed lattice. The magnetic bilayers are likely orientationally disordered, owing to the soft interlayer excitations and the near-degeneracy with other smectic phases. This possible lack of long-range correlation agrees with the null results from neutron powder diffraction. An orientationally disordered, spin-smectic state resolves previously perceived contradictions in high-pressure iron and could be integral to explaining its puzzling superconductivity.
Project description:Developing effective methods for the instant detection of Cu2+ and S2- is highly desired in the biological and environmental fields. Herein, a novel fluorescent nanoprobe was elaborately designed and synthesized by grafting a phenanthroline derivative onto the surface of carbon dots (CDs). The obtained functionalized CDs (FCDs) exhibited blue fluorescence (FL) with excellent photostability and possessed a mean diameter around 4 nm. Cu2+ can be selectively captured by the phenanthroline group of FCDs to generate an absorptive complex in situ, leading to obvious quenching of the FCDs' FL signal through an inner filter effect. Furthermore, the FL of the FCD?Cu2+ can be effectively recovered by S2- anions due to the release of FCDs from the FCD?Cu2+ complex owing to the formation of stable CuS (Ksp = 1.27 × 10-36) between S2- and Cu2+. The detection limits of the FCDs were determined to be 40.1 nM and 88.9 nM for Cu2+ and S2-, respectively. Moreover, this nanoprobe can also be used for the imaging of intracellular Cu2+ and S2-, which shows strong application prospects in the field of biology.