About me

I am a french PhD student working with Patrick Oswald in the Matter & Complexity team of the Physics Laboratory at the ENS de Lyon. We are interested in the thermal Lehmann effect, an out-of-equilibrium effect which couples a temperature gradient with the rotation of the internal texture of liquid crystal droplets in equilibrium with the isotropic phase, in condition that this texture is chiral.

My PhD project includes experimental, numerical and theoretical aspects. In particular, I have participated in the design of a Fluorescence Recovery After Photobleaching (FRAP) experimental setup, and used this setup to show that there is no flow in the Lehmann experiment. I also developped a Finite Element code using the Deal.II library to solve the nematoelasticity equations with finite anchoring boundary conditions. This code was useful to compute the texture inside cholesteric and nematic droplets and test the validity of particular theoretical models.

Master 2 internship - Rheology

of cornstarch suspensions

Rheology correspond to the study of the flow of matter. The simplest case of study is the shear experiment, where you submit a flowing sample to a shear flow by imposing either a force or a velocity on one of the two plates delimiting the sample (schema on the left). We can then define the strain rate \(\dot\gamma\) as the plate velocity \(v\) divided by the thickness \(d\), the stress \(\sigma\) as the place force \(F\) divided by the plate surface \(S\), and the viscosity \(\eta\) as the proportionality factor between the stress and the strain rate:

When the viscosity of a material is independant of the strain rate or the stress, this material is called newtonian (e.g. water). On the other hand, many materials exhibit a dependance of the viscosity with the strain rate ; these materials are called complex fluids. Suspensions (particles suspended in a fluid) are a perfect exemple of a complex fluid. For low packing fraction of particles, the suspension stays newtonian if the carrier-fluid is newtonian, but with an increase of viscosity due to the hydrodynamic interactions between particles. For high packing fraction, the friction and electrostatic interactions becomes relevant, and interesting dependance between the viscosity and the the strain rate appears. One of these behaviours, called shear-thickening, is the increase of viscosity with the strain rate, which can lead to spectacular effects: for example, one can run across a pool filled with a suspension of cornstarch in water

During my internship, I studied two types of suspensions: supensions of cornstarch in water (repulsive interaction between particles) and suspensions of cornstarch in oil (attractive interaction between particles). The repulsive suspensions are found to be shear-thickening, and a comparison with a recent theoretical model for shear-thickening suspensions is proposed. The attractive suspensions display an interesting dynamic behaviour in Large Amplitude Oscillatory Shear (LAOS) experiment, which is interpreted in terms of shear-reversal and structures forming in the compression direction.

Shear Thickening in Dense Suspensions

Report Abstract

Unsteady flow and particle migration in dense, non-Brownian suspensions

M. Hermes, B. M. Guy, W. C. K. Poon, G. Poy, M. E. Cates, M. Wyart, Journal of Rheology, 60, 905–916 (2016)

DOI Article Abstract/Bibtex

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 particles. We show, however, that in the DST regime, where theory might lead one to expect steady-state shear bands oriented layerwise along the vorticity axis, the real flow is unsteady. To explain this, we argue that steady-state banding is generically ruled out by the requirement that, for hard non-Brownian particles, the solvent pressure and the normal-normal component of the particle stress must balance separately across the interface between bands. (Otherwise, there is an unbalanced migration flux.) However, long-lived transient shear bands remain possible.

@article{hermes_unsteady_2016,
	title = {Unsteady flow and particle migration in dense, non-Brownian suspensions},
	volume = {60},
	issn = {0148-6055, 1520-8516},
	doi = {10.1122/1.4953814},
	number = {5},
	journal = {Journal of Rheology},
	author = {Hermes, Michiel and Guy, Ben M. and Poon, Wilson C. K. and Poy, Guilhem and Cates, Michael E. and Wyart, Matthieu},
	year = {2016},
	pages = {905--916},
}

Master 1 internship - Stability

of relativistic jets

Astrophysical jets correspond to well-calibrated flows of matter and energy whose source can be a galactic object (for example a microquasar, i.e. a binary system with one compact object and an accretion disk) or an extra-galactic object (for example an active galactic nuclei, i.e. a compact region at the center of a glaxy with a high luminosity). These jets can propagate over considerable distances (up to millions of light-year), which raises the following question: how can we explain the remarkable stability of these jets? This question is relevant, because many magnetohydrodynamic effects (such as the Kelvin-Helmholtz instability) can destabilize the jet.

During my internship, I studied theoretically the mechanisms of stability and instability for a simple model of jet, the beam model. Using the linearized equations of the magnetohydrodynamics for a jet, I computed the dispersion relation of the perturbation of a relativistic and compressible jet, and solved this relation under special limit cases. I showed that there is instable modes at all frequencies when there is no magnetic field, and that the effect of a magnetic field is to shift the most unstable modes to lower frequencies.

Stabilité des jets ultrarelativistes

Report Abstract

Bachelor internship - Rotational

viscosity measurement

Liquid crystals are products which possess a mesophase, i.e. a phase intermediate between a solid and a liquid. The most simple mesophase is the nematic phase, with no positional order and an orientational order. The latter allows us to define the director \({\bf n}\) as the mean direction of the molecules on a mesoscopic volume. Similarly to an isotropic fluid, where a shear is associated with a viscous stress, a rotation of the molecules without flow in a nematic phase is associated with a bulk viscous torque: \[ {\bf\Gamma}_{vb} = \gamma_1 {\bf n} \times \frac{\partial{\bf n}}{\partial t}, \] where \(\gamma_1\) is called the bulk rotational viscosity. When the nematic phase is confined within two parallel plates treated with a sliding planar anchoring (the molecules are parallel to the plates and can freely rotate on the plate surface), we can similaryly describe the rotational friction on the surface with a surface viscous torque: \[ {\bf\Gamma}_{vs} = \gamma_s {\bf n} \times \frac{\partial{\bf n}}{\partial t}, \] where \(\gamma_s\) is called the surface rotational viscosity.

If a rotating magnetic field \({\bf B}\) is applied parallel to the plates, the director will also rotates at the same angular velocity \(\omega\), but will make a small angle \(\alpha\) with \({\bf B}\) due to the dissipation by \(\gamma_1\) and \(\gamma_s\). By measuring this delay angle for different \(\omega\) and \(B\), it is possible to measure both \(\gamma_1\) and \(\gamma_s\). During my internship, I interfaced with Labview the experimental setup designed by Patrick Oswald to apply the rotating magnetic field, and systematically measured the bulk and surface rotational viscosities of a typical liquid crystal in function of the temperature.

Experimental relationship between surface and bulk rotational viscosities in nematic liquid crystals

P. Oswald, G. Poy, F. Vittoz, V. Popa-Nita, Liquid Crystals, 40, 734–744 (2013)

DOI Article Abstract/Bibtex

Systematic measurements of the surface viscosity \(\gamma_s\) of several nematic liquid crystals in contact with a polymercaptan layer show that the latter scales like the bulk rotational viscosity \(\gamma_1\). This result is interpreted in the framework of a ‘delocalized model’ for the surface viscosity.

@article{oswald_experimental_2013,
	title = {Experimental relationship between surface and bulk rotational viscosities in nematic liquid crystals},
	volume = {40},
	issn = {0267-8292, 1366-5855},
	doi = {10.1080/02678292.2013.783936},
	number = {6},
	journal = {Liquid Crystals},
	author = {Oswald, Patrick and Poy, Guilhem and Vittoz, Franck and Popa-Nita, Vlad},
	year = {2013},
	pages = {734--744},
}

Peer-reviewed articles

Unsteady flow and particle migration in dense, non-Brownian suspensions

M. Hermes, B. M. Guy, W. C. K. Poon, G. Poy, M. E. Cates, M. Wyart, Journal of Rheology, 60, 905–916 (2016)

DOI Article Abstract/Bibtex

@article{hermes_unsteady_2016,
	title = {Unsteady flow and particle migration in dense, non-Brownian suspensions},
	volume = {60},
	issn = {0148-6055, 1520-8516},
	doi = {10.1122/1.4953814},
	number = {5},
	journal = {Journal of Rheology},
	author = {Hermes, Michiel and Guy, Ben M. and Poon, Wilson C. K. and Poy, Guilhem and Cates, Michael E. and Wyart, Matthieu},
	year = {2016},
	pages = {905--916},
}

Generalized drift velocity of a cholesteric texture in a temperature gradient

A. Dequidt, G. Poy, P. Oswald, Soft Matter, 12, 7529–7538 (2016)

DOI Article Abstract/Bibtex

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 drift is rotational while for the fingers, the drift is translational. We show that for the TICs, the drift is only due to the thermomechanical coupling terms of Leslie (classical term) and of Akopyan and Zel’dovich (which are additional texture-dependent terms). For the localized structures, we show that another mechanism involving the temperature variations of the elastic constants and the existence of a transverse temperature gradient can lead to a drift which adds to the one due the classical thermomechanical effects.

@article{dequidt_generalized_2016,
	title = {Generalized drift velocity of a cholesteric texture in a temperature gradient},
	volume = {12},
	issn = {1744-683X, 1744-6848},
	doi = {10.1039/C6SM01359G},
	number = {36},
	journal = {Soft Matter},
	author = {Dequidt, Alain and Poy, Guilhem and Oswald, Patrick},
	year = {2016},
	pages = {7529--7538},
}

Continuous Rotation of Achiral Nematic Liquid Crystal Droplets Driven by Heat Flux

J. Ignés-Mullol, G. Poy, P. Oswald, Physical Review Letters, 117, 057801 (2016)

DOI Article Abstract/Bibtex

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 the 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 the continuous rotation in a temperature gradient of nematic droplets of a lyotropic chromonic liquid crystal featuring a twisted bipolar configuration. The achiral nature of the molecular components leads to a random handedness of the spontaneous twist, resulting in the coexistence of droplets rotating in the two senses, with speeds proportional to the temperature gradient and inversely proportional to the droplet radius. This result shows that a macroscopic twist of the director field is sufficient to induce a rotation of the droplets, and that the phase and the molecules do not need to be chiral. This suggests that one can also explain the Lehmann rotation in cholesteric liquid crystals without introducing the Leslie thermomechanical coupling—only present in chiral mesophases. An explanation based on the Akopyan and Zeldovich theory of thermomechanical effects in nematics is proposed and discussed.

@article{ignes-mullol_continuous_2016,
	title = {Continuous Rotation of Achiral Nematic Liquid Crystal Droplets Driven by Heat Flux},
	volume = {117},
	issn = {0031-9007, 1079-7114},
	doi = {10.1103/PhysRevLett.117.057801},
	number = {5},
	journal = {Physical Review Letters},
	author = {Ignés-Mullol, Jordi and Poy, Guilhem and Oswald, Patrick},
	year = {2016},
	pages = {057801},
}

Do Lehmann cholesteric droplets subjected to a temperature gradient rotate as rigid bodies?

G. Poy, P. Oswald, Soft Matter, 12, 2604–2611 (2016)

DOI Article Abstract/Bibtex

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 to the gradient) and concentric circle droplets oriented by an electric field (in which the helix is parallel to the gradient), we found that neither type of droplet rotates as a solid. This result shows that the texture rotation is mainly due to the local director rotation.

@article{poy_lehmann_2016,
	title = {Do Lehmann cholesteric droplets subjected to a temperature gradient rotate as rigid bodies?},
	volume = {12},
	issn = {1744-683X, 1744-6848},
	doi = {10.1039/C5SM02906F},
	number = {9},
	journal = {Soft Matter},
	author = {Poy, Guilhem and Oswald, Patrick},
	year = {2016},
	pages = {2604--2611},
}

Droplet relaxation in Hele-Shaw geometry: Application to the measurement of the nematic-isotropic surface tension

P. Oswald, G. Poy, Physical Review E, 92, 062512 (2015)

DOI Article Abstract/Bibtex

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)] to explain the relaxation of ellipsoidal droplets in a Hele-Shaw cell.

@article{oswald_droplet_2015,
	title = {Droplet relaxation in Hele-Shaw geometry: Application to the measurement of the nematic-isotropic surface tension},
	volume = {92},
	issn = {1539-3755, 1550-2376},
	doi = {10.1103/PhysRevE.92.062512},
	number = {6},
	journal = {Physical Review E},
	author = {Oswald, Patrick and Poy, Guilhem},
	year = {2015},
	pages = {062512},
}

Lehmann rotation of cholesteric droplets: Role of the sample thickness and of the concentration of chiral molecules

P. Oswald, G. Poy, Physical Review E, 91, 032502 (2015)

DOI Article Abstract/Bibtex

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 \(\vec{G}\). Two configurations are studied depending on how the helix is oriented with respect to \(\vec{G}\). The first result is that, at fixed \(C\) and \(R\), the rotation velocity \(\omega\) increases with \(d\) when the helix is parallel to \(\vec{G}\), whereas it is independent of \(d\) when the helix is perpendicular to \(\vec{G}\). The second result is that, for a given \(C\), \(\omega_0=\lim_{R\rightarrow0}\omega(R)\) is the same for the two types of droplets independently of \(d\). This suggests that the, as yet unknown, physical mechanism responsible for the droplet rotation is the same in the two types of droplets. The third result is that the Lehmann coefficient \(\bar\nu\) defined from the Leslie-like relation \(\omega_0=−\bar\nu G/\gamma_1\) (with \(\gamma_1\) the rotational viscosity) is proportional to the equilibrium twist \(q\). Last, but not least, the ratio \(\bar R=\bar\nu/q\) depends on the liquid crystal chosen but is independent of the chiral molecule used to dope the liquid crystal.

@article{oswald_lehmann_2015,
	title = {Lehmann rotation of cholesteric droplets: Role of the sample thickness and of the concentration of chiral molecules},
	volume = {91},
	issn = {1539-3755, 1550-2376},
	doi = {10.1103/PhysRevE.91.032502},
	number = {3},
	journal = {Physical Review E},
	author = {Oswald, Patrick and Poy, Guilhem},
	year = {2015},
	pages = {032502},
}

Experimental relationship between surface and bulk rotational viscosities in nematic liquid crystals

P. Oswald, G. Poy, F. Vittoz, V. Popa-Nita, Liquid Crystals, 40, 734–744 (2013)

DOI Article Abstract/Bibtex

@article{oswald_experimental_2013,
	title = {Experimental relationship between surface and bulk rotational viscosities in nematic liquid crystals},
	volume = {40},
	issn = {0267-8292, 1366-5855},
	doi = {10.1080/02678292.2013.783936},
	number = {6},
	journal = {Liquid Crystals},
	author = {Oswald, Patrick and Poy, Guilhem and Vittoz, Franck and Popa-Nita, Vlad},
	year = {2013},
	pages = {734--744},
}

Conferences (talks & posters)

Lehmann effect: The end of the Leslie paradigm

G. Poy, J. Ignés-Mullol, P. Oswald, International Liquid Crystal Conference, Kent, Ohio, 2016 (Invited talk)

Website Abstract Slides

Heat-flux-driven rotation of nematic and cholesteric twisted bipolar droplets

G. Poy, F. Bunel, J. Ignés-Mullol, P. Oswald, StatPhys, Lyon, France, 2016 (Poster)

Website Poster

Du nouveau sur l'effet Lehmann thermique dans des gouttes cholestériques

G. Poy, J. Ignés-Mullol, P. Oswald, 17ème Colloque sur les systèmes anisotropes auto-organisés, Autrans, France, 2015 (Oral presentation)

Website Abstract Slides

Phone:	+33 (0)4 72 72 85 34
Email:	guilhem.poy@ens-lyon.fr
Address:	Laboratoire de Physique (UMR CNRS 5672) ENS de Lyon 46, allée d’Italie F-69364 Lyon Cedex 07, France

About me

Academic Curriculum

PhD Student

Research intern

Master 2 in Physics

Research intern

Master 1 in Physics

Research intern

Bachelor in Physics

"Classes Preparatoires" in Science

Scientific high school diploma

Master 2 internship - Rheology

of cornstarch suspensions

Master 1 internship - Stability

of relativistic jets

Bachelor internship - Rotational

viscosity measurement

Peer-reviewed articles

Conferences (talks & posters)

Master 1 - Signal processing

License 3 - Lab Work

Professional