Mesogen polarity effects on biaxial nematics. Centrally located dipoles

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The significance of nanoparticle shape in chirality transfer to a surrounding nematic liquid crystal reporter medium

This perspective reports on recent progress toward the development of an approach to a priori predict - both chirality "strength" and efficacy of chirality transfer from a chiral nanoshape solute to an achiral nematic environment

Modeling and simulations of some anisotropic soft-matter systems: from biaxial to chiral materials

We have modeled various soft-matter systems with molecular dynamics (MD) simulations. The first topic concerns liquid crystal (LC) biaxial nematic (Nb) phases, that can be possibly used in fast displays. We have investigated the phase organization of biaxial Gay-Berne (GB) mesogens, considering the effects of the orientation, strength and position of a molecular dipole. We have observed that for systems with a central dipole, nematic biaxial phases disappear when increasing dipole strength, while for systems characterized by an offset dipole, the Nb phase is stabilized at very low temperatures. In a second project, in view of their increasing importance as nanomaterials in LC phases, we are developing a DNA coarse-grained (CG) model, in which sugar and phosphate groups are represented with Lennard-Jones spheres, while bases with GB ellipsoids. We have obtained shape, position and orientation parameters for each bead, to best reproduce the atomistic structure of a B-DNA helix. Starting from atomistic simulations results, we have completed a first parametrization of the force field terms, accounting for bonded (bonds, angles and dihedrals) and non-bonded interactions (H-bond and stacking). We are currently validating the model, by investigating stability and melting temperature of various sequences. Finally, in a third project, we aim to explain the mechanism of enantiomeric discrimination due to the presence of a chiral helix of poly(gamma-benzyl L-glutamate) (PBLG), in solution of dimethylformamide (DMF), interacting with chiral or pro-chiral molecules (in our case heptyl butyrate, HEP), after tuning properly an atomistic force field (AMBER). We have observed that DMF and HEP molecules solvate uniformly the PBLG helix, but the pro-chiral solute is on average found closer to the helix with respect to the DMF. The solvent presents a faster isotropic diffusion, twice as HEP, also indicating a stronger interaction of the solute with the helix

Can off-centre mesogen dipoles extend the biaxial nematic range?

We have investigated the possibility of extending the stability range of the biaxial nematic phase by adding an off-centre dipole of various strengths and orientations to elongated biaxial Gay-Berne (GB) mesogens yielding a relatively narrow biaxial nematic (Nbx) phase, and a smectic (Sbx) phase when dipole-less. The effect of dipoles is not easy to predict, and our previous investigations have shown the limited benefits of having a central dipole. Here we show, employing molecular dynamics (MD) simulations, that a not too strong off-centre dipole positioned along the longest axis of the nematogen can extend the temperature range of stability of the biaxial nematic phase, also shifting it towards lower temperatures

On the effects of different trans and cis populations in azobenzene liquid crystal elastomers: a Monte Carlo investigation

We investigate main-chain liquid crystal elastomers (LCEs) formed by photoresponsive azobenzene units with different populations of trans and cis conformers (from fully trans to fully cis). We study their macroscopic properties as well as their molecular organization using extensive Monte Carlo simulations of a simple coarse-grained model where the trans and cis conformers are represented by soft-core biaxial Gay−Berne particles with size and interaction energy parameters obtained by fitting a bare bone azobenzene moiety represented at atomistic level. We find that increasing the fraction of cis conformers, as could be obtained by near-UV irradiation, shifts the nematic−isotropic transition to a lower temperature, consistently with experiment, while generating internal stress in a clamped sample. An analysis of pair distributions shows that the immediate surroundings of a bent cis molecule are slightly less dense and more orientationally disordered in comparison with that of a trans conformer. Comparing nematic and smectic LCEs, actuation in the smectic phase proved less effective, disrupting the smectic layers to some extent but preserving orientational order of the azobenzene moieties

Can multi-biaxial mesogenic mixtures favour biaxial nematics? A computer simulation study

Extending the range of existence of biaxial nematic phases is key to their use in applications. Here, we have investigated using extensive molecular dynamics (MD) simulations of a coarse-grained model the possible advantages of using mesogenic mixtures. We have studied the phase organisation of five thermotropic mixtures of biaxial Gay-Berne (GB) ellipsoidal particles having the same volume, but different shapes and interactions, with aspect ratios ranging from rod-like to disc-like and, choosing fractional compositions so as to model a Gaussian dispersity of shapes. The parameterisation is based on a previous GB model with biaxialities of opposite sign for steric and attractive interactions which was shown to exhibit a stable biaxial nematic phase. We found that mixing different biaxial GB particles has an overall stabilising effect on the biaxial nematic phase with respect to temperature, layering and, to some extent also demixing. The mixtures show a decrease of ordering transition temperatures, a widening of nematic temperature ranges, and the formation of smectic phases at lower temperatures

Ionic Diffusion in Spherified Calcium Alginate Gels: A Laboratory Experiment

Dilute water solutions of sodium alginate are used in molecular cooking and the dairy industry as thickening and gelling agents and also as edible films. Typically, they are obtained from brown algae or seaweeds and form stable gels in the presence of calcium ions. Calcium alginate gels can be easily prepared in the form of small solid spheres by dropping, with a syringe or a pipette, the sodium alginate solution into a water bath of calcium ions (\u201cdirect method\u201d). After rinsing the calcium alginate spheres, fully or just partially jellified, their external pH can be easily regulated using ordinary acid and/or basic solutions. The chapter describes in detail the various steps of the procedure adopted for the preparation of calcium alginate spheres and for detecting the diffusion of protonated water inside them by observing the colour change of a suitable pH indicator contained in the spheres

Chirality amplification by desymmetrization of chiral ligand-capped nanoparticles to nanorods quantified in soft condensed matter

Induction, transmission, and manipulation of chirality in molecular systems are well known, widely applied concepts. However, our understanding of how chirality of nanoscale entities can be controlled, measured, and transmitted to the environment is considerably lacking behind. Future discoveries of dynamic assemblies engineered from chiral nanomaterials, with a specific focus on shape and size effects, require exact methods to assess transmission and amplification of nanoscale chirality through space. Here we present a remarkably powerful chirality amplification approach by desymmetrization of plasmonic nanoparticles to nanorods. When bound to gold nanorods, a one order of magnitude lower number of chiral molecules induces a tighter helical distortion in the surrounding liquid crystal-a remarkable amplification of chirality through space. The change in helical distortion is consistent with a quantification of the change in overall chirality of the chiral ligand decorated nanomaterials differing in shape and size as calculated from a suitable pseudoscalar chirality indicator

Reversible water driven chirality inversion in cellulose-based helices isolated from Erodium awns

Among the movements observed in some cellulosic structures produced by plants are those that involve the dispersion and burial of seeds, as for example in Erodium from the Geraniaceae plant family. Here we report on a simple and efficient strategy to isolate and tune cellulose-based hygroscopic responsive materials from Erodium awns’ dead tissues. The stimuli-responsive material isolated forms left-handed (L) or right-handed (R) helical birefringent transparent ribbons in the wet state that reversibly change to R helices when the material dries. The humidity-driven motion of dead tissues is most likely due to a composite material made of cellulose networks of fibrils imprinted by the plant at the nanoscale, which reinforces a soft wall polysaccharide matrix. The inversion of the handedness is explained using computational simulations considering filaments that contract and expand asymmetrically. The awns of Erodium are known to present hygroscopic movements, forming R helices in the dry state, but the possibility of actuating chirality via humidity suggests that these cellulose-based skeletons, which do not require complicated lithography and intricate deposition techniques, provide a diverse range of applications from intelligent textiles to micro-machines.info:eu-repo/semantics/publishedVersio