About me

I am a Marie Skłodowska Curie Research Fellow working with Slobodan Žumer in the Soft Matter team of the Faculty of Physics and Mathematics at the University of Ljubljana. We want to develop a new research direction by combining knowledge from photonics and topological soft matter to find novel ways of guiding light using chiral birefringent media. The non-linear optical response of these media allows a laser beam to be self-confined and to propagate over long distances, leading to what is called spatial optical solitons. Our primary objective is to develop a complete model of optical solitons in chiral birefringent media and examine how these light solitons can be steered and controlled using topological solitons — localized and tunable perturbations of the molecular orientational field which cannot be continuously deformed into the uniform state. Our methodology is based on the theoretical and numerical modeling of the non-linear equations for the propagation of light in chiral birefringent media, combined with collaborative experiments.

PhD - Lehmann effect

You can download my PhD thesis with the following link:

PhD thesis

This thesis is focused on the Lehmann effect, an out-of-equilibrium effect which couples a temperature gradient with the rotation of the internal texture of liquid crystal droplets in coexistence with the isotropic phase.

First, we characterized the thermomechanical coupling terms of Leslie, Akopyan and Zel’dovich by measuring the rotation velocity of the molecules in two translationally invariant configura- tions with different orientations, below the cholesteric/isotropic transition.

Then, we characterized the texture of the droplets observed in the Lehmann experiment, both using optical observations and numerical simulations. More important, we showed for the first time that it is possible to observe the Lehmann effect in achiral nematic droplets, providing that the internal texture is chiral. We also used a photobleaching experiment to show that there is no visible flow in the vicinity of the droplet, which implies that the texture rotation is due to a local rotation of the molecules – not to a solid rotation of the droplet.

Finally, we proposed a theoretical model of the Lehmann effect based on the thermome- chanical coupling of Leslie, Akopyan and Zel’dovich. By applying this model to the numerically computed textures, we fitted our experimental data and found values for the thermomechanical coupling constants much bigger than those measured below the cholesteric/isotropic transition. This model is therefore incomplete.

SM1 — (-1/2) disclination line observed under crossed polarizers just below the cholesteric/isotropic temperature in a planar sliding sample of the mixture CCN-37 + 3 % CC. \(d=\) 10.6 µm and \(\Delta T=\) 40 °C. The bar represents 50 µm.

SM2 — Pair of \(\pm\) 1 disclination lines observed under crossed polarizers just below the cholesteric/isotropic temperature in a mixed sample of the mixture CCN-37 + 3 % CC. \(d=\) 10.8 µm and \(\Delta T=\) 40 °C. The bar represents 100 µm.

SM3 — Lehmann rotation of banded droplets observed in a sample of 7CB + 0.6 \% R811. The temperature gradient is oriented towards the internaut. \(d=\) 50 µm and \(\Delta T=\) 5 °C. The bar represents 50 µm.

SM4 — Lehmann rotation of cholesteric twisted bipolar droplets observed in a sample of CCN-37 + 5.9 \% CC. The temperature gradient is oriented towards the internaut. \(d=\) 50 µm and \(\Delta T=\) 1.25 °C. The bar represents 50 µm.

SM5 — Lehmann rotation of nematic twisted bipolar droplets observed in a sample of water + 28 \% SSY. The temperature gradient is oriented towards the internaut. \(d=\) 110 µm and \(\Delta T=\) 20 °C. The bar represents 10 µm.

SM6 — Fluorescence signal after a laser shot near a banded droplet of 7CB + 0.25 % R811 + 0.05 % NBD C6-ceramide. The droplet rotates at \(\omega_g=\) 0.139 rad/s when \(\Delta T=\) -10 K. The white point represents the center of the gaussian fit and the white circle represents the circle of equation \(|\vec{x}-\vec{x}_0|=\sigma/2\). The bar represents 50 µm.

SM7 — Same as SM6 with a droplet more distorted rotating at \(\omega_g=\) 0.036 rad/s when \(\Delta T=\) -10 °C. Mixture of 7CB + 0.25 % R811 + 0.05 % NBD C6-ceramide. The bar represents 50 µm.

SM8 — Fluorescence signal after a laser shot (left) and images in transmission between crossed polarizers under white light illumination (right) of a cholesteric twisted bipolar droplet. The black circle represents the outer contour of the droplets. The tilted black line gives the orientation of the bipole. The white point and circle have the same meaning as in SM6 and SM7. Mixture of CCN-37 + 5.9 % CC + 0.05 % NBD C6-ceramide, \(\Delta T=\) 2.5 °C. The bar represents 10 µm.

Master 2 - 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)

Email:	guilhem.poy@fmf.uni-lj.si
Address:	Faculty of Mathematics and Physics Jadranska 19 1000 Ljubljana, Slovenia

Marie Curie Fellow

Postdoctoral researcher

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