Guilhem Poy

Guilhem Poy
Université de Montpellier | UM1 · Laboratoire Charles Coulomb (L2C)

PhD

About

41
Publications
4,992
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
430
Citations
Citations since 2016
36 Research Items
416 Citations
2016201720182019202020212022020406080
2016201720182019202020212022020406080
2016201720182019202020212022020406080
2016201720182019202020212022020406080
Introduction
My current research interests include fluctuating chiral soft matter environments such as cholesteric liquid crystals or granular media with twisted objects. My CNRS project focuses on how chirality can be used to tune the diffusive transports of guest objects inside such chiral media, thanks to combined experimental and theoretical approaches. I am also working with collaborators on the theoretical modeling of the optical properties of novel liquid crystals devices and structures.
Additional affiliations
October 2021 - present
Université de Montpellier
Position
  • Researcher
March 2021 - September 2021
University of Ljubljana
Position
  • Research Associate
April 2019 - March 2021
University of Ljubljana
Position
  • Fellow
Education
September 2012 - July 2014
September 2011 - July 2012

Publications

Publications (41)
Article
Full-text available
The structure of the nematic (cholesteric) drops that form at the clearing temperature of a mixture of the bent-core molecule CB7CB and the rodlike molecule 8CB doped with a surfactant is optically determined. Using experimental observations and numerical simulations, it is demonstrated that the director field inside these drops is not escaped conc...
Article
Full-text available
Solitons attract a great deal of interest in many fields, ranging from optics to fluid mechanics, cosmology, particle physics and condensed matter. However, solitons of these very different types rarely coexist and interact with each other. Here we develop a system that hosts optical solitons coexisting with topological solitonic structures localiz...
Chapter
L’ouvrage Cristaux liquides : nouvelles perspectives porte principalement sur les phases nématiques et cholestériques. Il traite d’abord de l’interaction entre lumière et défauts topologiques, puis examine différents effets électrocinétiques non linéaires permettant de déplacer des particules sous champ électrique alternatif ainsi que la dynamique...
Chapter
This chapter describes two crossed effects observed in nematic and cholesteric liquid crystals (LC) subjected to a temperature gradient G. The first effect couples the rotation of the molecules to the temperature gradient (thermomechanical [TM] effect), while the second one couples the flows to the temperature gradient (thermohydrodynamical effect)...
Article
Full-text available
Photo-alignment is a versatile tool to pattern the alignment at the confining substrates in a liquid crystal (LC) cell. Arbitrary alignment patterns can be created by using projection with a spatial light modulator (SLM) for the illumination. We demonstrate that a careful design of the alignment patterns allows the stabilization of topological soli...
Article
Full-text available
In practically all branches of physics, different types of solitons, with a number of them enjoying topological protection, are found. Here we explore how one- and two-dimensional topological solitons formed by spatially localized continuous orientational patterns of optical axis in uniaxial birefringent media interact with light. These solitons, i...
Article
Full-text available
We experimentally and numerically show that chirality can play a major role in the nonlinear optical response of soft birefringent materials, by studying the nonlinear propagation of laser beams in frustrated cholesteric liquid crystal samples. Such beams exhibit a periodic nonlinear response associated with a bouncing pattern for the optical field...
Article
Full-text available
We present a unified theoretical framework for paraxial and wide-angle beam propagation methods in inhomogeneous birefringent media based on a minimal set of physical assumptions. The advantage of our schemes is that they are based on differential operators with a clear physical interpretation and easy numerical implementation based on sparse matri...
Article
Patterned liquid crystal (LC) configurations find widespread applications in functional devices such as lenses, gratings, displays and soft-robots. In combination with external stimuli such as an applied electric field, photo-alignment at the surfaces offers an attractive way to stabilize different LC structures in the bulk of a device. Herein, a p...
Article
Yielding behavior is well known in attractive colloidal suspensions. Adhesive non-Brownian suspensions, in which the interparticle bonds are due to finite-size contacts, also show yielding behavior. We use a combination of steady-state, oscillatory, and shear reversal rheology to probe the physical origins of yielding in the latter class of materia...
Preprint
Full-text available
Yielding behavior is well known in attractive colloidal suspensions. Adhesive non-Brownian suspensions, in which the interparticle bonds are due to finite-size contacts, also show yielding behavior. We use a combination of steady-state, oscillatory and shear-reversal rheology to probe the physical origins of yielding in the latter class of material...
Article
Full-text available
The Lehmann effect is the continuous rotation of cholesteric droplets subjected to a temperature gradient. Discovered by Otto Lehmann in 1900, this effect was re-observed recently by several authors not only in cholesterics but also in nematics when the director field is twisted inside the droplets. In most experiments the droplets coexist with the...
Article
Full-text available
We propose an efficient method to simulate light propagation in lossless and non-scattering uniaxial birefringent media, based on a standard ray-tracing technique supplemented by a newly-derived transport equation for the electric field amplitude along a ray and a tailored interpolation algorithm for the reconstruction of the electromagnetic fields...
Article
Full-text available
The Lehmann effect is the rotation of cholesteric droplets when they are submitted to a temperature gradient. So far, this effect was only observed in the coexistence region between the cholesteric phase and its isotropic liquid. This zone of coexistence is due to the presence in the LC of impurities. In this paper, we show that the rotation veloci...
Article
Full-text available
New creep experiments under sinusoidal compression/dilation deformation of a homeotropic sample of smectic-A liquid crystal (8CB) show that its response is nonlinear at very small amplitude of deformation. This behavior is explained by taking into account the crossing between the edge dislocations that climb parallel to the layers and the screw dis...
Article
Full-text available
We revisit a theoretical paper of Akopyan and Zel’dovich about the thermomechanical coupling terms in nematic liquid crystals. We show that the expressions of these terms given by these authors must be corrected to satisfy the Onsager reciprocity relations, a point already stressed by Pleiner and Brand in 1987. We then extend this calculation to th...
Thesis
Full-text available
Cette thèse porte sur l’effet Lehmann, un effet hors d’équilibre couplant un gradient de température avec la rotation de la texture de gouttes cholestériques ou nématiques coexistant avec la phase isotrope. Nous avons d’abord caractérisé les couplages thermomécaniques de Leslie, Akopyan et Zel’dovich en mesurant en phase cholestérique la vitesse de...
Article
Full-text available
We present a numerical method to compute defect-free textures inside cholesteric domains of arbitrary shape. This method has two interesting properties, namely a robust and fast quadratic convergence to a local minimum of the Frank free energy, thanks to a trust region strategy. We apply this algorithm to study the texture of cholesteric droplets i...
Article
Full-text available
We study the action of a rotating magnetic field on the Fréedericksz instability under electric field of a homeotropic nematic sample sandwiched between two parallel electrodes. The liquid crystal (LC) is of negative dielectric and magnetic anisotropies and the magnetic field is parallel to the electrodes used to apply the electric field. We show t...
Article
Full-text available
We show that the Leslie and texture-dependent Akopyan and Zel’dovich thermomechanical coupling terms of nematodynamics cannot explain the thermal Lehmann rotation of the twisted bipolar droplets observed in the coexistence region between a cholesteric phase and the isotropic liquid. On the other hand, these terms are pertinent below the transition...
Article
Full-text available
We propose a general method to calculate the drift velocity of cholesteric textures subjected to a temperature gradient when the backflow effects are negligible. The textures may be Translationally Invariant Configurations (TICs) or localized structures such as cholesteric droplets or cholesteric fingers. For the TICs and for the droplets, the drif...
Article
Full-text available
We present experimental results on dense corn-starch suspensions as examples of non-Brownian, nearly hard particles that undergo continuous and discontinuous shear thickening (DST) at intermediate and high densities, respectively. Our results offer strong support for recent theories involving a stress-dependent effective contact friction among part...
Article
Full-text available
Suspended droplets of cholesteric (chiral nematic) liquid crystals spontaneously rotate in the presence of a heat flux due to a temperature gradient, a phenomenon known as Lehmann effect. So far, it is not clear whether this effect is due to the chirality of the phase and the molecules or only to the chirality of the director field. Here, we report...
Article
Full-text available
We performed a Fluorescence Recovery After Photobleaching (FRAP) experiment during the Lehmann rotation of cholesteric droplets in thermodynamic coexistence with the isotropic liquid and subjected to a temperature gradient. By creating and tracking bleached spots near the surface of banded droplets (in which the cholesteric helix is perpendicular t...
Data
We show that the Leslie and texture-dependent Akopyan and Zel’dovich thermomechanical coupling terms of nematodynamics cannot explain the thermal Lehmann rotation of the twisted bipolar droplets observed in the coexistence region between a cholesteric phase and the isotropic liquid. On the other hand, these terms are pertinent below the transition...
Data
We show that the Leslie and texture-dependent Akopyan and Zel’dovich thermomechanical coupling terms of nematodynamics cannot explain the thermal Lehmann rotation of the twisted bipolar droplets observed in the coexistence region between a cholesteric phase and the isotropic liquid. On the other hand, these terms are pertinent below the transition...
Article
Full-text available
Shape measurements after the coalescence of isotropic droplets embedded in a thin sample of a homeotropic nematic phase provides a tool to measure the nematic-isotropic surface tension. In addition, this experiment allows us to check the scaling laws recently given by Brun et al. [P.-T. Brun, M. Nagel, and F. Gallaire, Phys. Rev. E 88, 043009 (2013...
Article
Full-text available
We present experimental results on dense corn-starch suspensions as examples of non-Brownian, nearly-hard particles that undergo continuous and discontinuous shear thickening (CST and DST) at intermediate and high densities respectively. Our results offer strong support for recent theories involving a stress-dependent effective contact friction amo...
Article
Full-text available
We study the role of the sample thickness d and of the concentration C of chiral molecules during the Lehmann rotation of cholesteric droplets of radius R subjected to a temperature gradient G[over ⃗]. Two configurations are studied depending on how the helix is oriented with respect to G[over ⃗]. The first result is that, at fixed C and R, the rot...
Article
Full-text available
Systematic measurements of the surface viscosity of several nematic liquid crystals in contact with a polymercaptan layer show that the latter scales like the bulk rotational viscosity . This result is interpreted in the framework of a ‘delocalized model’ for the surface viscosity.

Network

Cited By

Projects

Project (1)
Project
We try to understand the mechanism behind the Lehmann rotation of twisted nematic droplets in coexistence with the istotropic fluid when submitted to a temperature gradient. So far, we have found that the classical thermomechanical model (Leslie and Akopyan/Zeldo'vich) cannot explain the Lehmann effect because it predicts rotation velocity a thousand times smaller in comparison to the experiments. We also have shown with a photobleaching experiments that rotational hydrodynamic flows can be neglected inside the droplet.