Tropisms of the dowser texture

UID/CTM/50025/2019 M-ERA-NET2/0007/2016 Action EUTOPIA CA17139Due to its low symmetry C2υ, the dowser texture is characterised by a 2D unitary vector field or alternatively by a unitary complex field. For the same symmetry reasons, the dowser texture is sensitive, in first order, to perturbations such as thickness gradients, electric fields or flows. We will focus on corresponding properties called respectively: Cuneitropism, electrotropism and rheotropism. In particular we will show that topological defects, known as dowsons or monopoles, can be manipulated by means of these tropisms.publishersversionpublishe

Dislocations and other topological oddities

AbstractWe will show that the book Dislocations by Jacques Friedel, published half a century ago, can still be recommended, in agreement with the author's intention, as a textbook “for research students at University and for students at engineering schools as well as for research engineers”. Indeed, today dislocations are known to occur not only in solid crystals but also in many other systems discovered more recently such as colloidal crystals or liquid crystals having periodic structures. Moreover, the concept of dislocations is an excellent starting point for lectures on topological defects occurring in systems equipped with order parameters resulting from broken symmetries: disclinations in nematic or hexatic liquid crystals, dispirations in chiral smectics or disorientations in lyotropic liquid crystals. The discussion of dislocations in Blue Phases will give us an opportunity to call on mind Sir Charles Frank, friend of Jacques Friedel since his Bristol years, who called these ephemeral mesophases “topological oddities”. Being made of networks of disclinations, Blue Phases are similar to Twist Grain Boundary (TGB) smectic phases, which are made of networks of screw dislocations and whose existence was predicted by de Gennes in 1972 on the basis of the analogy between smectics and superconductors. We will stress that the book by Jacques Friedel contains seeds of this analogy

On Generation, Motions, and Collisions of Dowsons

PTDC/CTM-BIO/6178/2014. M-ERA-NET2/0007/2016 (CellColor).Dowsons are ±2π point singularities of the unitary complex order parameter eiφ characterizing the so-called dowser texture in a thin nematic layer with homeotropic boundary conditions. Dowsons are therefore similar to disclinations in freely-standing smectic C films or to vortices in two-dimensional superfluids or superconductors. Using especially tailored setups called dowsons' colliders, pairs of dowsons of opposite signs are generated and set into motion on counter-rotating trajectories leading to collisions. In a first approximation, the velocity of dowsons is orthogonal and proportional to the local phase gradient (Formula presented.). The outcome of collisions, i.e., either annihilation or bypass, depends on the distance of trajectories Δφ in terms of the phase: for Δφ π the dowsons are passing by. This rule is valid only for quasi-static stationary wound up textures and can be easily broken by application of a Poiseuille flow in an appropriate direction.publishersversionpublishe

Travelling colourful patterns in self-organized cellulose-based liquid crystalline structures

Publisher Copyright: © 2021, The Author(s)Cellulose-based systems are useful for many applications. However, the issue of self-organization under non-equilibrium conditions, which is ubiquitous in living matter, has scarcely been addressed in cellulose-based materials. Here, we show that quasi-2D preparations of a lyotropic cellulose-based cholesteric mesophase display travelling colourful patterns, which are generated by a chemical reaction-diffusion mechanism being simultaneous with the evaporation of solvents at the boundaries. These patterns involve spatial and temporal variation in the amplitude and sign of the helix´s pitch. We propose a simple model, based on a reaction-diffusion mechanism, which simulates the observed spatiotemporal colour behaviour.publishersversionpublishe

Mind the microgap in iridescent cellulose nanocrystal films

info:eu-repo/semantics/publishedVersio

Cristaux liquides : l’univers métastable des monopôles nématiques

Les cristaux liquides permettent d’illustrer facilement le concept universel de défauts topologiques, incontournable en physique de la matière condensée. Nous décrivons ici une expérience permettant de générer la texture métastable dite pseudo-planaire des cristaux liquides nématiques : cette texture est particulièrement adaptée à l’étude des défauts ponctuels de ceux-ci. On peut les y créer par paires monopôle-antimonopôle et étudier leurs collisions et leur section efficace d’annihilation. Cette texture est aussi douée de cunéitropisme, c’est-à-dire de la faculté de s’orienter dans un gradient d’épaisseur, dont une des conséquences est la formation de parois qui ont toutes les caractéristiques de solitons

Physics of Free-Standing Lyotropic Films

We explore the structures and properties of stable, free-standing films of lyotropic mesophases drawn on apertures of various shapes in an atmosphere of controlled humidity. New phenomena are uncovered and interpreted

Flexo-electricity of the dowser texture

POCI-01-0145-FEDER-007688 ,PTDC/CTM-BIO/6178/2014 and M-ERA-NET2/0007/2016 (CellColor).The persistent quasi-planar nematic texture known also as the dowser texture is characterized by a 2D unitary vector field d. We show here that the dowser texture is sensitive, in first order, to electric fields. This property is due to the flexo-electric polarisation P collinear with d expected from R. B. Meyer's considerations on flexo-electricity in nematics. It is pointed out that due to the flexo-electric polarisation nematic monopoles can be manipulated by electric fields of appropriate geometry.publishe