Dynamics and Topology of Symmetry Breaking with Skyrmions.

We observe that pretransitional order parameter fluctuations of a skyrmion-forming chiral nematic liquid crystal are slowed down for 4 orders of magnitude, if confined to ≲100 nm thin layers. Fluctuating fragments of half-skyrmions are observed in a narrow temperature interval and are explained by thermally activated hopping between the various energy states. Skyrmion fluctuations are accompanied by imbalanced topological charge: positive charges appear at higher temperatures and dominate in the fluctuating region until skyrmions fully condense and negative charges appear at lower temperatures.

Theoretical and experimental study of the nanoparticle-driven blue phase stabilisation.

We have studied theoretically and experimentally the effects of various types of nanoparticles (NPs) on the temperature stability range [Formula: see text] T (BP) of liquid-crystalline (LC) blue phases. Using a mesoscopic Landau-de Gennes type approach we obtain that the defect core replacement (DCR) mechanism yields in the diluted regime [Formula: see text] T (BP)(x) [Formula: see text] 1/(1 - xb) , where x stands for the concentration of NPs and b is a constant. Our calculations suggest that the DCR mechanism is efficient if a local NP environment resembles the core structure of disclinations, which represent the characteristic property of BP structures. These predictions are in line with high-resolution ac calorimetry and optical polarising microscopy experiments using the CE8 LC and CdSe or aerosil NPs. In mixtures with CdSe NPs of 3.5nm diameter and hydrophobic coating the BPIII stability range has been extended up to 20K. On the contrary, the effect of aerosil silica nanoparticles of 7.0nm diameter and hydrophilic coating is very weak.

Olfactory discrimination of complex mixtures of amino acids by the black bullhead Ameiurus melas.

On the basis of previous findings of behavioural discrimination of amino acids and on the knowledge of electrophysiology of the catfish (genera Ictalurus and Ameiurus) olfactory organs, behavioural experiments that investigated olfactory discrimination of amino acid mixtures were carried out on the black bullhead Ameiurus melas. Repeated presentations of food-rewarded mixtures released increased swimming activity measured by counting the number of turns >90° within 90 s of stimulus addition. Non-rewarded amino acids and their mixtures released little swimming activity, indicating that A. melas discriminated between the conditioned and the non-conditioned stimuli. Two questions of mixture discrimination were addressed: (1) Are A. melas able to detect components within simple and complex amino acid mixtures? (2) What are the smallest differences between two complex mixtures that A. melas can detect? Three and 13 component mixtures tested were composed primarily of equipotent amino acids [determined by equal electroolfactogram (EOG) amplitude] that contained L-Cys at ×100 the equipotent concentration. Ameiurus melas initially perceived the ternary amino acid mixture as its more stimulatory component alone [i.e. cysteine (Cys)], whereas the conditioned 13 component mixture containing the more stimulatory L-Cys was perceived immediately as different from L-Cys alone. The results indicate that components of ternary mixtures are detectable by A. melas but not those of more complex mixtures. To test for the smallest detectable differences in composition between similar multimixtures, all mixture components were equipotent. Initially, A. melas were unable to discriminate the mixtures of six amino acids from the conditioned mixtures of seven amino acids, whereas they discriminated immediately the mixtures of four and five amino acids from the conditioned mixture. Experience with dissimilar mixtures enabled the A. melas to start discriminating the seven-component conditioned mixture from its six-component counterparts. After fewer than five training trials, A. melas discriminated the mixtures of nine and 10 amino acids from a conditioned mixture of 12 equipotent amino acids; however, irrespective of the number of training trials, A. melas were unable to discriminate the 12 component mixture from its 11 component counterparts.

Liquid crystal-carbon nanotubes mixtures.

The self-organizing properties of nematic liquid crystals (LCs) can be used to align carbon nanotubes (CNTs) dispersed in them. In the previous paper [P. van der Schoot, V. Popa-Nita, and S. Kralj, J. Phys. Chem. B 112, 4512 (2008)], we have considered the weak anchoring limit of the nematic LC molecules at the nanotube's surface, where the CNT alignment is caused by the anisotropic interfacial tension of the nanotubes in the nematic host fluid. In this paper, we present the theoretical results obtained for strong enough anchoring at the CNT-LC interface for which the nematic ordering around nanotube is apparently distorted. Consequently, relatively strong long-range and anisotropic interactions can emerge within the system. In order to get insight into the impact of LC ordering on the alignment of nanotubes we treat the two mixture components on the same footing and combine Landau-de Gennes free energy for the thermotropic ordering of the liquid crystal and Doi free energy for lyotropic nematic ordering of carbon nanotubes caused by their mutually excluded volume. The phase ordering of the binary mixture is analyzed as a function of the volume fraction of the carbon nanotubes, the strength of coupling, and the temperature. We find that the degree of ordering of the nanorods can be tuned by raising or lowering the temperature or by increasing or decreasing their concentration.

The unique branching patterns of Deinococcus glycogen branching enzymes are determined by their N-terminal domains.

Glycogen branching enzymes (GBE) or 1,4-alpha-glucan branching enzymes (EC 2.4.1.18) introduce alpha-1,6 branching points in alpha-glucans, e.g., glycogen. To identify structural features in GBEs that determine their branching pattern specificity, the Deinococcus geothermalis and Deinococcus radiodurans GBE (GBE(Dg) and GBE(Dr), respectively) were characterized. Compared to other GBEs described to date, these Deinococcus GBEs display unique branching patterns, both transferring relatively short side chains. In spite of their high amino acid sequence similarity (88%) the D. geothermalis enzyme had highest activity on amylose while the D. radiodurans enzyme preferred amylopectin. The side chain distributions of the products were clearly different: GBE(Dg) transferred a larger number of smaller side chains; specifically, DP5 chains corresponded to 10% of the total amount of transferred chains, versus 6.5% for GBE(Dr). GH13-type GBEs are composed of a central (beta/alpha) barrel catalytic domain and an N-terminal and a C-terminal domain. Characterization of hybrid Deinococcus GBEs revealed that the N2 modules of the N domains largely determined substrate specificity and the product branching pattern. The N2 module has recently been annotated as a carbohydrate binding module (CBM48). It appears likely that the distance between the sugar binding subsites in the active site and the CBM48 subdomain determines the average lengths of side chains transferred.

Normal red blood cells' shape stabilized by membrane's in-plane ordering.

Red blood cells (RBCs) are present in almost all vertebrates and their main function is to transport oxygen to the body tissues. RBCs' shape plays a significant role in their functionality. In almost all mammals in normal conditions, RBCs adopt a disk-like (discocyte) shape, which optimizes their flow properties in vessels and capillaries. Experimentally measured values of the reduced volume (v) of stable discocyte shapes range in a relatively broad window between v ~ 0.58 and 0.8. However, these observations are not supported by existing theoretical membrane-shape models, which predict that discocytic RBC shape is stable only in a very narrow interval of v values, ranging between v ~ 0.59 and 0.65. In this study, we demonstrate that this interval is broadened if a membrane's in-plane ordering is taken into account. We model RBC structures by using a hybrid Helfrich-Landau mesoscopic approach. We show that an extrinsic (deviatoric) curvature free energy term stabilizes the RBC discocyte shapes. In particular, we show on symmetry grounds that the role of extrinsic curvature is anomalously increased just below the nematic in-plane order-disorder phase transition temperature.

Alignment of carbon nanotubes in nematic liquid crystals.

The self-organizing properties of nematic liquid crystals can be used to align carbon nanotubes dispersed in them. Because the nanotubes are so much thinner than the elastic penetration length, the alignment is caused by the coupling of the unperturbed director field to the anisotropic interfacial tension of the nanotubes in the nematic host fluid. In order to relate the degree of alignment of the nanotubes to the properties of the nematic liquid crystal, we treat the two components on the same footing and combine Landau-de Gennes free energies for the thermotropic ordering of the liquid crystal and for the lyotropic nematic ordering of carbon nanotubes caused by their mutually excluded volumes. The phase ordering of the binary mixture is analyzed as a function of the volume fraction of the carbon nanotubes, the strength of the coupling and the temperature. We find that the degree of ordering of the nanorods is enslaved by the properties of the host liquid and that it can be tuned by raising or lowering the temperature or by increasing or decreasing their concentration. By comparing the theory to recent experiments, we find the anchoring energy of multiwalled carbon nanotubes to be in the range from 10(-10) to 10(-7) N m(-1).

Impact of spherical nanoparticles on nematic order parameters.

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 polarized micro-Raman spectroscopy and by optical birefringence measurements as function of temperature and concentration. Our data show that the orientational order depends on the concentration in a nonlinear way, and the existence of a crossover concentration χ_{c}≈0.004pw. It separates two different regimes exhibiting pure-liquid crystal like (χ<χ_{c}) and distorted-nematic ordering (χ>χ_{c}), respectively. In the latter phase the degree of ordering is lower with respect to the pure-liquid crystal nematic phase.

A biocatalytic and thermoreversible hydrogel from a histidine-containing tripeptide.

We report the first histidine-containing self-assembling tripeptide devoid of capping groups that forms a thermoreversible hydrogel under physiological conditions and catalyses hydrolysis of an ester, providing a minimalist building block for functional soft materials.

Waves at the nematic-isotropic interface: thermotropic nematogen-non-nematogen mixtures.

We develop a theory for surface modes at the nematic-isotropic interface in thermotropic nematogen-non-nematogen mixtures. We employ the dynamical generalization of the Landau-de Gennes model for the orientational (nonconserved) order parameter, coupled with the Cahn-Hilliard equation for concentration (conserved parameter), and include hydrodynamic degrees of freedom. The theory uses a generalized form of the Landau-de Gennes free-energy density to include the coupling between the concentration of the non-nematogen fluid and the orientational order parameter. Two representative phase diagrams are shown. The method of matched asymptotic expansions is used to obtain a generalized dispersion relation. Further analysis is made in particular cases. Orientational order parameter relaxation dominates in the short-wavelength limit, while in the long-wavelength limit viscous damping processes become important. There is an intermediate region (depending on the temperature) in which the interaction between conserved parameter dynamics and hydrodynamics is important.

Effect of dispersed silica particles on the smectic-A-smectic-C* phase transition.

We study the influence of the dispersed aerosil particle concentration on the soft and Goldstone mode dynamics across the smectic-A-smectic-C* phase transition using dielectric spectroscopy. We use CE8 liquid crystal filled with aerosils of concentrations x=m(s)/(m(s)+m(LC))=0.025, 0.05, 0.1, and 0.15, where m(s) and m(LC) stand for masses of aerosil and liquid crystal, respectively. In dispersions with x<0.15 the characteristic Goldstone frequency temperature dependence f(G)(T) is almost bulklike in contrast to the corresponding soft mode dependence f(s)(T). For x>0 the degeneracy of the modes at the SmA-SmC* transition temperature is lifted. With increasing x the modes' intensities gradually decrease and the critical effective coefficient gamma tentatively approaches the value characterizing the three-dimensional XY universality class. For x=0.15, a drastic change in behavior is observed. The Goldstone mode is completely suppressed and the f(s)(T) dependence begins to deviate from the Arrhenius behavior. A simple phenomenological approach is used in the explanation of results.

Molecular dynamics study of the isotropic-nematic quench.

Effects of cylindrical and spherical confinement on the kinetics of the isotropic-nematic quench is studied numerically. The nematic liquid crystal structure was modeled by a modified induced-dipole--induced-dipole interaction. Molecules were allowed to wander around points of a hexagonal lattice. Brownian molecular dynamics was used in order to access macroscopic time scales. In the bulk we distinguish between the early, domain, and late stage regime. The early regime is characterized by the exponential growth of the nematic uniaxial order parameter. In the domain regime domains are clearly visible and the average nematic domain size xi(d) obeys the dynamical scaling law xi(d)-t(gamma). The late stage evolution is dominated by dynamics of individual defects. In a confined system the qualitative change of the scaling behavior appears when xi(d) becomes comparable to a typical linear dimension R of the confinement. In the confining regime (xi(d)>or=R) the scaling coefficient gamma depends on the details of the confinement and also the final equilibrium nematic structure. The domain growth is well described with the Kibble-Zurek mechanism.

Annihilation of edge dislocations in smectic-A liquid crystals.

This paper presents a theoretical study of the annihilation of edge dislocations in the same smectic plane in a bulk smectic-A phase. We use a time-dependent Landau-Ginzburg approach where the smectic ordering is described by the complex order parameter psi( r--> ,t) =eta e(iphi) . This quantity allows both the degree of layering and the position of the layers to be monitored. We are able to follow both precollision and postcollision regimes, and distinguish different early and late behaviors within these regimes. The early precollision regime is driven by changes in the phi ( r--> ) configuration. The relative velocity of the defects is approximately inversely proportional to the interdefect separation distance. In the late precollision regime the symmetry changes within the cores of defects also become influential. Following the defect collision, in the early postcollision stage, bulk layer order is approached exponentially in time. At very late times, however, there seems to be a long-time power-law tail in the order parameter fluctuation relaxation.

Surface depinning of smectic-A edge dislocations.

Using a Landau-de Gennes approach, we model the formation of an edge dislocation in a smectic-A cell initially in the bookshelf structure. The driving force is the mismatch between the layer thickness in a bulk smectic-A liquid crystal and that imposed by confining plates. The core structure of the dislocation is calculated taking into account spatial variations of the smectic translational order parameter. We numerically determine the critical condition for the surface-driven formation and depinning of the dislocation. By exploiting this phenomenon, we show how the value of the positional anchoring strength at the surface can be determined.

Symmetry breaking in nematic liquid crystals: analogy with cosmology and magnetism.

Universal behavior related to continuous symmetry breaking in nematic liquid crystals is studied using Brownian molecular dynamics. A three-dimensional lattice system of rod-like objects interacting via the Lebwohl-Lasher interaction is considered. We test the applicability of predictions originally derived in cosmology and magnetism. In the first part we focus on coarsening dynamics following the temperature driven isotropic-nematic phase transition for different quench rates. The behavior in the early coarsening regime supports predictions made originally by Kibble in cosmology. For fast enough quenches, symmetry breaking and causality give rise to a dense tangle of defects. When the degree of orientational ordering is large enough, well defined protodomains characterized by a single average domain length are formed. With time subcritical domains gradually vanish and supercritical domains grow with time, exhibiting a universal scaling law. In the second part of the paper we study the impact of random-field-type disorder on a range of ordering in the (symmetry broken) nematic phase. We demonstrate that short-range order is observed even for a minute concentration of impurities, giving rise to disorder in line with the Imry-Ma theorem prediction only for the appropriate history of systems.

Influence of a random field on particle fractionation and solidification in liquid-crystal colloid mixtures.

The influence of a random-anisotropy (RA) type disorder on the phase separation of nematogen-colloid mixtures is studied theoretically by combining the phenomenological Landau-de Gennes, Carnahan-Starling, and hard-sphere crystal theories. We assume that the colloids enforce the RA disorder on the surrounding thermotropic liquid-crystal (LC) molecules. We adopt the Imry-Ma argument according to which the lower-temperature phase exhibits a domain-type pattern. The colloids impose a finite degree of orientational ordering even in the isotropic (paranematic) phase. In the ordered phase they give rise to a domain-type structure, resulting in the distorted nematic (speronematic) phase. The RA field opposes the phase separation tendency. With increasing disorder the difference between the paranematic and speronematic ordering decreases. Consequently there is a critical disorder, above which both phases become identical from the orientation point of view, but have different concentrations of colloids. We have also estimated another characteristic value of disorder above which the isotropic phase can exist only in a liquid state, the crystal phase being suppressed completely.

Annihilation of nematic point defects: pre-collision and post-collision evolution.

The annihilation of the nematic hedgehog and anti-hedgehog within an infinite cylinder of radius R is studied. The semi-microscopic lattice-type model and Brownian molecular dynamics are used. We distinguish among the i) early pre-collision, ii) late pre-collision, iii) early post-collision, and iv) late post-collision stages. In the pre-collision stage our results agree qualitatively with the existing experimental observations and also continuum-type simulations. The core of each defect exhibits a ring-like structure, where the ring axis is set perpendicular to the cylinder symmetry axis. For xi(0)d/(2R) > 1 the interaction between defects is negligible, where xi(0)d describes the initial separation of defects. Consequently, the defects annihilate within the simulation time window for xi(0)d/(2R) < 1. For close enough defects their separation scales as xi(d) [see text] (t(c)- t)(0.4+/-0.1), where t(c) stands for the collision time. In elastically anisotropic medium the hedgehog is faster than the anti-hedgehog. In the early pre-collision stage the defects can be treated as point-like particles, possessing inherent core structure, that interact via the nematic director field. In the late pre-collision stage the cores reflect the interaction between defects. After the collision a charge-less ring structure is first formed. In the early post-collision stage the ring adopts an essentially untwisted circular structure of the radius xi(r). In the late post-collision stage we observe two qualitatively different scenarios. For mu = xi(r)/R < mu(c) approximately 0.25 the ring collapses leading to the escaped radial equilibrium structure. For mu > mu(c) the chargeless ring triggers the nucleation growth into the planar polar structure with line defects.