Patrick Oswald

Patrick Oswald
Ecole normale supérieure de Lyon | ENS Lyon · Département de Physique

About

188
Publications
16,995
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3,820
Citations
Citations since 2016
37 Research Items
1356 Citations
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2016201720182019202020212022050100150200
2016201720182019202020212022050100150200
2016201720182019202020212022050100150200

Publications

Publications (188)
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...
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...
Article
Full-text available
We show experimentally and theoretically that the heliconical fluctuations that develop in a cholesteric phase (Ch) close to a transition to a chiral twist-bend nematic phase (N_{TB}) may lead to the appearance of a compensation point. At this point, the equilibrium twist of the cholesteric phase vanishes and changes sign. Mixtures of the flexible...
Article
Full-text available
In 1968, de Gennes and Meyer independently predicted that a cholesteric phase can form a stable oblique helicoidal (or heliconical) structure provided that K_{3}<K_{2} where K_{3} (K_{2}) is the bend (twist) constant. This structure usually develops under electric field when the material is of positive dielectric anisotropy and was observed for the...
Preprint
Full-text available
In 1968, P.G. de Gennes and R.B. Meyer independently predicted that a cholesteric phase can form a stable oblique helicoidal (or heliconical) structure provided that K3 < K2 where K3 (K2) is the bend (twist) constant. This structure usually develops under electric field when the material is of positive dielectric anisotropy and was observed for the...
Article
Full-text available
Capacitance measurements have been extensively used to measure the anchoring extrapolation length L at a nematic–substrate interface. These measurements are extremely delicate because the value found for L often critically depends on the sample thickness and the voltage range chosen to perform the measurements. Several reasons have been proposed to...
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)...
Preprint
Capacitance measurements have been extensively used to measure the anchoring extrapolation length L at a nematic-substrate interface. These measurements are extremely delicate because the value found for L often critically depends on the sample thickness and the voltage range chosen to perform the measurements. Several reasons have been proposed to...
Article
Full-text available
We propose to measure the ratio κ31 of the bend elastic constant K3 over the splay elastic constant K1 of a nematic phase by using an optical method based on the measurement of the birefringence of homeotropic/planar hybrid samples. This method is compared to the classical measurements using the Freedericksz transition under AC electric field and i...
Article
Full-text available
This paper deals with the Lehmann rotation of banded cholesteric droplets subjected to a temperature gradient when they coexist with their own isotropic liquid. I show that their angular rotation velocity increases --in absolute value-- when they are subjected to an additional AC electric field in the conducting regime. This velocity increase is co...
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
Edge dislocations are linear defects that locally break the positional order of the layers in smectic A liquid crystals. As in usual solids, these defects play a central role for explaining the plastic properties of the smectic A phase. This work focuses on the dynamical properties of dislocations in bulk samples prepared between two glass plates a...
Article
Full-text available
We show experimentally and theoretically that the Lehmann effect recently observed by Yoshioka and Araoka (Nature Com. 9, 432 (2018)) in emulsified cholesteric liquid crystal droplets under temperature gradients is due to Marangoni flows rather than to the thermomechanical or chemomechanical couplings often invoked to explain the phenomenon. Using...
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 propose a complete characterization of the chemical Leslie effect in a Langmuir monolayer of a chiral liquid crystal. In this goal, we developed new experimental techniques using an electric field and an humidifier to prepare large monodomains in which the molecules can freely rotate. We also designed six independent experiments to precisely mea...
Article
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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...
Article
Full-text available
In a recent letter (P. Oswald et al., EPL 103, 46004 (2013)), we have shown that a smectic-A phase hardens in compression normal to the layers when the liquid crystal is doped with gold nanoparticles. This is due to the formation of Cottrell clouds nearby the core of the edge dislocations and the appearance of an additional drag force that reduces...
Article
Full-text available
The performance of light-controlled liquid crystal anchoring surfaces depends on the nature of the photosensitive moieties and on the concentration of spacer units. Here, we study the kinetics of photosensitive liquid crystal cells that incorporate an azobenzene-based self-assembled monolayer. We characterize the photoinduced homeotropic-to-planar...
Article
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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
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
The viscoelastic behavior of a homeotropic nematic slab is studied when it is subjected to a (dilation-compression) sinusoidal deformation of small amplitude (linear regime). I show that the nematic phase behaves as an isotropic liquid of viscosity eta(c) (nu(3)) at low (high) frequency, where eta(c) is the third Miesowicz viscosity and nu(3) a sma...
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
I measure the Leslie thermomechnical coefficient ν in diluted cholesteric liquid crystals. The chiral molecules are R811 and cholesteryl chloride (CC) and the host nematic liquid crystals are 7CB and MBBA. I show that ν is proportional to the concentration of chiral molecules C when . This allows me to define the Leslie thermomechanical power as by...
Article
I show how to measure with a rotating magnetic field the azimuthal anchoring energy in a nematic liquid crystal. This technique is used to study the dynamics of memorization of an easy axis when a sample treated for planar sliding anchoring with a liquid polymer is maintained during a long time in a fixed magnetic field parallel to the glass plates...
Article
Full-text available
We performed measurements of the dielectric constants, splay elastic constant, and rotational viscosity of the nematic phase of mixtures of 4,4'-n-octyl-cyanobiphenyl (8CB) and biphenyl (BP). In contrast with previous results of DasGupta et al. [Phys. Rev. E 63, 041703 (2001); Phys. Lett. A 288, 323 (2001)], we do not find any anomaly of these cons...
Article
Full-text available
We study the Lehmann rotation of the cholesteric helix in droplets of the liquid crystal N-(p-methoxybenzilidene)-p-butylaniline doped with a small amount of the chiral molecule R811 when they are subjected to a temperature gradient. We show that the helix rotates much faster when it is parallel to the temperature gradient than when it is perpendic...
Article
Full-text available
We measure the mobility of edge dislocations in a smectic A liquid crystal doped with gold nanoparticles. The mobility is found to decrease when the concentration of nanoparticles increases, which leads to a hardening of the smectic phase. It is found that the closer the temperature to the nematic phase, the weaker the hardening. The critical behav...
Article
Full-text available
We study the behavior under rotating magnetic field of a nematic liquid crystal sandwiched between two parallel glass plates treated for partly sliding planar degenerate anchoring. This behavior is explained within a mean-field model taking into account the volume elasticity and the random planar anchoring of the molecules on the surfaces.
Article
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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.
Article
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We show how to measure with a rotating magnetic field the surface viscosity of a nematic liquid crystal in contact with a thin polymercaptan layer allowing for a planar sliding anchoring of the molecules.
Article
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In a recent letter (EPL 97, 36006 (2012)), we have shown that the Leslie thermomechanical coupling cannot alone explain the Lehmann effect (namely the rotation of cholesteric droplets when they are subjected to a temperature gradient). This result was obtained by measuring in a compensated cholesteric mixture the "Lehmann coefficient" as a function...
Article
By measuring as a function of temperature the Leslie thermomechnical coefficient in a compensated cholesteric mixture, we arrived at the conclusion that the Lehmann effect, namely the rotation of cholesteric droplets under the action of a temperature gradient, cannot be explained only by the Leslie thermomechanical coupling effect.
Article
By doping a nematic phase with a chiral molecule one obtains a cholesteric phase. Each chiral molecule is characterised by its helical twisting power (HTP) which is defined as HTP = q/(2πC) where q is the equilibrium pitch of the cholesteric phase and C the concentration (in wt%) of chiral molecules. In a similar way, we define the Lehmann rotatory...
Article
Recently, we have shown the existence of an electro-capillary instability of a nematic-isotropic interface stabilized by a temperature gradient (P. Oswald, EPL 90, 16005 (2010)). This instability results from a competition between the destabilizing action of the electric Maxwell stress and the stabilizing action of the thermal and capillary forces....
Article
We review the different dynamical patterns that cholesteric fingers of the first type (CF-1) and of the second type (CF-2) form in an a.c. electric field near the coexistence line with the homeotropic nematic phase. Videomicroscopy and computer image analysis were used for investigation of the patterns in polarized light. We show that CF-1s can for...
Article
We have studied the morphological instability under AC electric field of a nematic-isotropic interface confined between two electrodes set at different temperatures. The electrodes are treated for homeotropic anchoring so that, when the field is increased, the Ericksen elastic stress vanishes. In this limit, the instability resembles that observed...
Article
We study the behaviour of cholesteric fingers of the first and second types (CF1 and CF2, respectively) which form in cholesteric samples treated for homeotropic anchoring when they are subjected to the combined action of an AC electric field and a temperature gradient. We investigate under which conditions each type of finger can drift and form a...
Article
Full-text available
In 1900 Otto Lehmann observed that the internal texture of cholesteric droplets, when submitted to a thermal gradient, was constantly rotating. This phenomenon was explained phenomenologically in 1968 from symmetry arguments by F.M. Leslie in the framework of the nematodynamics. Six years later, Pierre-Gilles de Gennes noted in a premonitory way, i...
Article
Preface; Foreword; Acknowledgements; 1. General points on materials and their rheophysical behaviour; 2. Overview of the mechanics of continuous media; 3. Hydrodynamics of simple liquids; 4. Solid elasticity; 5. Defects, plasticity and fracture solids; 6. Rheology of isotropic viscoelastic materials (1); 7. Rheology of isotropic viscoelastic materi...
Article
Full-text available
We present a systematic study of the Lehmann rotation of cholesteric droplets subjected to a temperature gradient when the concentration of chiral molecules is changed. The liquid crystal chosen is an eutectic mixture of 8CB and 8OCB doped with a small amount of the chiral molecule R811. The angular velocity of the droplets strongly depend on their...
Book
Why is it necessary to strike while the iron is hot? What makes a good liquid crystal display? Why is rubber so elastic? These questions can be answered through rheophysics, using the mechanics of continuous media at the macroscopic scale, and statistical mechanics and the physics of defects at the microscopic level. This book addresses problems in...
Article
The thermomechanical Lehmann coefficient ν is directly measured as a function of temperature in a compensated cholesteric liquid crystal. The method consists of observing the continuous rotation of the director in samples treated for planar sliding anchoring when a temperature gradient is applied perpendicularly to the director. The main result is...
Article
Full-text available
We propose a new surface treatment allowing to obtain a sliding planar anchoring of nematic (or cholesteric) liquid crystals. It consists of depositing a thin layer of the polymercaptan hardener of an epoxy resin on an isotropic substrate (bare or ITO-coated glass plates). Microscopic observations of defect annihilations and capacitance measurement...
Article
Full-text available
In 1900, Otto Lehmann observed the continuous rotation of cholesteric drops when subjected to a temperature gradient. This thermomechanical phenomenon was predicted 68 years later by Leslie from symmetry arguments but was never reobserved to our knowledge. In this Letter, we present an experiment allowing quantitative analysis of the Lehmann effect...
Article
Full-text available
We show that the continuous cholesteric fingers, which form in homeotropic samples at the unwinding temperature of the cholesteric phase, drift and spiral when they are subjected to a temperature gradient. This phenomenon is attributed to the appearance of a Lehmann thermomechanical torque. Measurements of the finger drift velocity on both sides of...
Article
Full-text available
In a recent letter (Europhys. Lett. 80, 26001 (2007)), we have shown that a compensated cholesteric liquid crystal (in which the macroscopic helix completely unwinds) may be subjected to a thermomechanical torque (the so-called Lehmann effect), in agreement with previous findings of Eber and Janossy (Mol. Cryst. Liq. Cryst. Lett. 72, 233 (1982)). T...
Article
Chiral additives in the nematic liquid crystal can alter the dynamics of point defects moving on a disclination line. They exert a constant force on defects, leading to the bimodal distribution of distances between them at long times. The evolution of the system of defects in the presence of chiral additives provides a very direct proof of the exis...
Article
Full-text available
Experiments have shown that cholesteric droplets or cholesteric fingers may be put into motion by the action of an electric field. The former rotate whereas the latter drift perpendicularly to their axes. In all cases, the texture moves without visible material transport. The electric Lehmann effect was initially used to interpret these observation...
Article
Full-text available
In mixtures of thermotropic liquid crystals with spherical poly(methyl methacrylate) particles, self-supporting networklike structures are formed during slow cooling past the isotropic-to-nematic phase transition. Experimental results support the hypothesis that a third component, alkane remnants slowly liberated from the particles, plays a crucial...
Article
Cholesteric liquid cristals usually form a macroscopic helicoidal structure due to the presence of chiral molecules. In these particular systems, a linear thermomechanical coupling can be observed, called the Lehmann effect: a temperature gradient is able to generate a torque on the local orientation of the molecules. To explain this effect, Leslie...