Research

Please refer to my CV and my Google Scholar page for a comprehensive, up-to-date list of my publications.

Active Matter hydrodynamics

Measuring the Stokes’ drag in an active fluid

Active fluids spontaneously flow without the help of an external force, often developing turbulent-like patterns (right). Their hydrodynamic properties are vastly different from those of passive fluids. With Pr. Kenny Breuer, I have been working on measuring the drag force applied on a 50 µm sphere sedimenting under gravity through a 3D kinesin-microtubule active fluid. I built from scratch a 3D Lagrangian tracking microscope based on a piezo-driven objective, that records bead trajectories with micron accuracy (left).


Chiral liquid crystal shells

My Ph.D. was conducted under the supervision of Dr. Teresa Lopez-Leon at the Gulliver laboratory of ESPCI Paris, at the intersection of soft matter physics and topology, using capillary microfluidics and polarized microscopy.

My research focused on the production, observation and manipulation of patterns in liquid crystals confined to spherical shells. Nematic liquid crystals are anisotropic fluids of rod-like molecules that align locally with each other. Cholesterics are chiral nematic phases, which develop a spontaneous twisted organization. Liquid crystals exhibit preferential orientation at interfaces, a phenomenon called anchoring.

On the surface of a sphere coated with a nematic under parallel anchoring, such an alignment cannot be achieved everywhere, leading to the unavoidable presence of topological defects. This situation can be realized experimentally by creating water-in-LC-in-water double emulsions.

Anchoring transitions at liquid crystal / water interfaces

I first uncovered a new mechanism to induce a transition from parallel to perpendicular anchoring at the interface between a liquid crystal and water, using temperature as a control parameter. Contrary to the traditional technique, which relies on the adsorption of surfactants at said interface, this new mechanism is reversible, homogeneous, and can affect the two interfaces of a shell. We can use it to our advantage to transform an initially polydisperse group of nematic shells (top left corner) into a monodisperse population of bivalent shells (bottom right corner).

G. Durey, Y. Ishii & T. Lopez-Leon, “Temperature-driven anchoring transitions at water / liquid crystal interfaces.”
Langmuir, 36, 9368-9376 (2020) [PDF]

Bands and stripes on cholesteric shells

I then applied this mechanism to cholesteric shells. Under parallel anchoring, they can be seen as simple twisted nematic shells. But perpendicular anchoring is incompatible with the spontaneous tendency of a cholesteric to twist. Thus complex structures arise from the competition between those two parameters. We identified and explained two types of patterns, having either thin or thick stripes, described the transitions between the two, and the organization of the thin stripes into double spirals, in a collaboration with Randall Kamien's group at the University of Pennsylvania.

L. Tran, M.O. Lavrentovich, G. Durey, A. Darmon, M.F. Haase, N. Li, D. Lee, K.J. Stebe, R.D. Kamien & T. Lopez-Leon,
“A change in stripes for cholesteric shells via anchoring in moderation.” Physical Review X, 7, 41029 (2017). [PDF]

Topological solitons on Janus liquid crystal shells

Lastly, I investigated cholesteric shells under strong perpendicular anchoring and high confinement. Under these conditions, the liquid crystal splits into a cholesteric and a nematic domain, giving a Janus nature to the shells. The frustration of the cholesteric is resolved through the creation of structures of localized twist, some of them topologically protected. I studied dynamical transformations between linear cholesteric fingers (left) and point-like torons (right) as well as their packing on a curved surface, in a collaboration with Ivan Smalyukh's group at the University of Colorado Boulder.

G. Durey, H.R.O. Sohn, P.J. Ackerman, É. Brasselet, I.I. Smalyukh & T. Lopez-Leon, “Topological solitons,
cholesteric fingers and singular defect lines in Janus liquid crystal shells.” Soft Matter, 16, 2669-2682 (2020) [PDF].

Fluid Dynamics / Wave Physics

My M.Sc. thesis was conducted under the supervision of Pr. Emmanuel Fort at the Langevin Institute of ESPCI Paris. We revisited the Faraday instability – when the horizontal surface of a liquid destabilizes into a pattern of standing waves due to a vertical oscillation of the bath – by showing its ability to control wave propagation.

A phase-conjugate mirror using the Faraday instability

The phase-conjugate of any propagating wave has the same wavefront, but propagates backwards in time. Using the dependency of the Faraday instability on the depth of vibrated liquid, I created a phase-conjugate mirror for surface water waves. It generates the counter-propagating wave corresponding to any excitation, regardless of the shape of the mirror, and is fully analogous to the four-wave mixing technique used in nonlinear optics.

V. Bacot, G. Durey, A. Eddi, M. Fink & E. Fort, “Phase-conjugate mirror for water waves driven by the Faraday instability.”
Proceedings of the National Academy of Sciences, 116, 201818742 (2019). [PDF]

History of science

As a side project of my Ph.D., I was involved in the Historical Preservation Committee of ESPCI Paris. Supervised by Dr. Brigitte Leridon, it is tasked with inventorying, safeguarding and exhibiting the historical scientific apparatuses of ESPCI – an essential task in light of upcoming extensive renovation works to the school premises.

Re-discovering the Féry pyrheliometric telescope

In a forgotten cupboard, my colleagues and I found Charles Féry's pyrheliometer: a reflecting telescope outfitted with a thermocouple, designed to determine the surface temperature of the Sun according to the Stefan-Boltzmann law. It was carried to the top of the Mont Blanc in 1906 and gave a result accurate to 2% of the modern value. While the instrument was refurbished, I researched its history and its principle of operation, leading to the publication of a peer-reviewed article.

G. Durey, A. P. Legrand, D. Beaudouin & B. Leridon, “Le télescope pyrhéliométrique de Charles Féry
et la mesure de la température du Soleil.” Nuncius (Journal of the Material and Visual History of Science) , 33, 345 (2018). [PDF]