Playing the blues, the greens and the reds with cellulose-based structural colours

POCI- 01-0145-FEDER-007688 (Reference UIDB/50025/2020-2023) PTDC/CTM-BIO/6178/2014 M-ERA-NET2/0007/2016 PTDC/CTM-REF/30529/2017 EUTOPIA CA17139 Slovenian Research Agency Grant Z1-5441 P1-0099Structural vivid colours can arise from the interference of light reflected from structures exhibiting periodicity on scales in the range of visible wavelengths. This effect is observed with light reflected from cell-walls of some plants and exoskeletons of certain insects. Sometimes the colour sequence observed for these structures consists of nearly circular concentric rings that vary in colour from Red, Orange, Yellow, Green, Cyan to Blue, from the periphery to the centre, similarly to the colour scheme sequence observed for the rainbow (ROYGB). The sequence of colours has been found for solid films obtained from droplets of aqueous cellulose nanocrystals (CNCs) suspensions and attributed to a "coffee ring"effect. In this work, coloured lyotropic solutions and solid films obtained from a cellulose derivative in the presence of trifluoroacetic acid (TFA), which acts as a "reactive solvent", are revisited. The systems were investigated with spectroscopy, using circularly and linearly polarised light, coupled with a polarised optical microscope (POM) and scanning electron microscopy (SEM). The lyotropic cholesteric liquid crystalline solutions were confined in capillaries to simplify 1D molecular diffusion along the capillary where an unexpected sequence of the structural colours was observed. The development and reappearance of the sequence of vivid colours seem consistent with the reaction-diffusion of the "reactive solvent"in the presence of the cellulosic chains. The strong TFA acts as an auto-catalyst for the chemical reaction between TFA and the hydroxyl groups, existing along the cellulosic chain, and diffuses to the top and bottom along the capillaries, carrying dissolved cellulosic chains. Uncovering the precise mechanism of colour sequence and evolution over time in cellulosic lyotropic solutions has important implications for future optical/sensors applications and for the understanding of the development of cellulose-based structures in nature. This journal isauthorsversionpublishe

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

Smectic-A edge dislocations in a very thin cell

We study stable “bookshelf” smectic-A structures within a very thin plane-parallel cell of thickness L in which the mismatch between surface preferred (d s) and intrinsic (d0) smectic layer thicknesses occurs. The Landau-Ginzburg approach based on a complex smectic order parameter is used. For a weak enough smectic positional anchoring strength W smectic layers adopt the modified bookshelf profile. In a thick enough cell with increasing W a lattice of edge dislocations is continuously formed at the confining surfaces and then depinned from them. The structure with dislocations is formed when the condition d 0/(ζ s |d 0/d s − 1|) ∼ 2 is fulfilled, where ζ s is the positional surface anchoring extrapolation length. If the cell is thin enough the dislocations formed at opposite cell plates annihilate and consequently the smectic layers adopt a locked bookshelf structure. This transition is discontinuous and takes place when d 0/(L|d 0/d s − 1|) ∼ 5 is realized. To observe these transitions in a cell of thickness L∼ 1μm the conditions W∼ 10-6 J/m 2 and |d 0/d s − 1| ∼ 5 · 10−4 have to be fulfilled. All the three qualitatively different structures coexist at the triple point

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 ξ(0) d/(2R) > 1 the interaction between defects is negligible, where ξ(0) d describes the initial separation of defects. Consequently, the defects annihilate within the simulation time window for ξ(0) d/(2R) < 1. For close enough defects their separation scales as ξd $\varpropto$ (t c - t)0.4±0.1, where tc 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 ξr. In the late post-collision stage we observe two qualitatively different scenarios. For μ = ξr/R μc the chargeless ring triggers the nucleation growth into the planar polar structure with line defects

NMR spectra and relaxation in incommensurate systems in the presence of a devil's staircase

The possibility of discriminating true incommensurate phases from long period commensurate phases via NMR lineshape and spin-lattice relaxation measurements is discussed The theoretical results are compared with experimental data in Rb2ZnCl4, Rb2ZnBr4, and [N(CH3)4]2ZnCl4. Devil's staircase effects seem to be in the majority of cases masked by defects.La possibilité de faire la différence entre des phases vraiment incommensurables et des structures à longue période est discutée. Les résultats théoriques sont comparés avec ceux d'expériences effectuées sur Rb2ZnCl 4, Rb2ZnBr4, et [N(CH3)4] 2ZnCl4. Dans la plupart des cas, les effets dus à la présence d'un escalier du diable sont masqués par ceux des défauts

Dynamics of Nematic Liquid Crystal Disclinations: The Role of the Backflow

International audienceWe measure the electric-field-driven annihilation of nematic disclination pairs with strength ±1/2 in the 4−cyano−4′−n-pentylbiphenyl (5CB) liquid crystal. The use of a very weak azimuthal anchoring ensures a two-dimensional director field. The relaxation is governed by the formation of a π wall connecting the two opposite charge defects. The +1/2 disclinations move almost twice as fast as the −1/2 disclinations. The simple used geometry allows a quantitative comparison with numerical studies based on the hydrodynamics of the tensorial order parameter. The simulations show that in the π wall regime the symmetry breaking is due to the backflow and not to the elastic anisotropy

Calorimetric study of the Paranematic-to-Nematic transition of polydomain side-chain liquid-crystalline elastomers with different mesogen composition

The phase transition behaviour of various nematic side-chain liquid-crystalline elastomers with different mesogen composition has been explored by means of high-resolution ac calorimetry. Polydomain samples of the same crosslinking density and different type of mesogens have been investigated. The results show a strong dependence of the phase transition features upon the composition of the mesogen. The distance from the critical point, reflected in the sharpness of the heat capacity anomalies, increases when adding a shorter-length mesogen. The results provide new insight for the impact of mesogens on the thermodynamic behaviour and, thus, on the thermomechanical response of nematic liquid-crystalline elastomers