ED Sciences Chimiques
Stimuli-Responsive Supramolecular Microgels: from Microstructure Control to the Study of their Interfacial Behavior
by Antoine BREZAULT (Centre de Recherche Paul Pascal)
The defense will take place at 10h00 - Amphithéâtre de l'IPGG ESPCI 10 rue Vauquelin 75 005 Paris
in front of the jury composed of
- Véronique SCHMITT - Directrice de recherche - CRPP, Université de Bordeaux - Directeur de these
- Thomas HELLWEG - Full professor - Université de Bielefeld - Rapporteur
- Nicolas HUANG - Professeur des universités - Institut Galien Paris-Saclay (CNRS UMR 8612) - Université Paris Saclay - Rapporteur
- Patrice WOISEL - Professeur des universités - UMET, UMR CNRS 8207 - Examinateur
- Marie LE MERRER - Chargée de recherche - Institut Lumière Matière, CNRS-Univ. Lyon 1 - Examinateur
- Valérie RAVAINE - Professeure - Institut des Sciences Moléculaires (ISM - UMR 5255) - CoDirecteur de these
- Patrick PERRIN - Professeur des universités - ESPCI - Sorbonne Université - Examinateur
- Nicolas SANSON - Maître de conférences - ESPCI - Sorbonne Université - Examinateur
Stimuli-responsive microgels with well-controlled size and structure are gaining increasing interest for both fundamental research and applications. This work aims at synthesizing supramolecular microgels with tailored properties and at studying their behavior in dispersion and at interfaces. To enhance their versatility, a supramolecular crosslinker (SC) based on a Fe(II)-bis terpyridine complex was incorporated into poly(N-isopropylacrylamide) (PNIPAM) microgels synthesized via dispersion polymerization. The challenge of incorporating cationic SCs into the microgels was addressed by using an anionic surfactant, sodium dodecyl sulfate (SDS), during synthesis. The development of a general "limited aggregation" model explaining the PNIPAM microgel synthesis mechanism in the presence of SDS revealed that the surfactant acts both as a stabilizer for growing particles and as a hydrophobic counter-ion to the cationic crosslinker. Thus, the added amount of SDS allows a control over both the size of the microgels and their crosslinking density. Continuous addition of SC during microgel synthesis allows a good control of the spatial distribution of the supramolecular crosslinkers within the microgels. A wide range of structures was obtained, from “ultra” core–shell microgels to more homogeneous ones. The SC is also a powerful tool for easily quantifying microgel structure, even if in a dried state, using transmission electron microscopy. In general, the structural characterization remains challenging, even with advanced scattering techniques. Moreover, the synthesized supramolecular microgels are not only sensitive to salt—due to the presence of charges on the crosslinker—but also degradable by chemical oxidation. SC cleavage leads to disassembly of the polymer network. We showed that the kinetics of SC cleavage and microgel degradation are accelerated in more homogeneous microgels. A post-mortem analysis of the free chains obtained after microgel disassembly, performed via size exclusion chromatography, enabled us to correlate the cleaved chain length distribution with the original microgel structure. In addition, PNIPAM supramolecular microgels adsorb at liquid interfaces. Using a Langmuir trough, we studied the effects of microgel size, crosslinking density, and SC spatial distribution on the conformation of microgels adsorbed at a model flat air–water interface. These results were then applied to revisit microgel behavior in the stabilization of oil-in-water Pickering/Ramsden emulsions, both in the so-called "Limited Coalescence" regime and in microgel-rich regimes, which remain relatively unexplored. This study marks significant progress, as it highlights the key role of microgel structure. Similarly, the stabilization of Pickering/Ramsden foams by microgels was found to be closely dependent on particle structure. Finally, on-demand destabilization of Pickering/Ramsden emulsions was achieved via controlled degradation of the adsorbed microgels. A correlation between the degradation kinetics of microgels in the bulk and the kinetics of emulsion destabilization was established. Stabilization of emulsions using partially cleaved microgels further confirmed the link between microgel structure, their interfacial organization, and emulsion properties.
ED Sciences Physiques et de l'Ingénieur
A QUANTUM NANOMECHANICAL OSCILLATOR COUPLED TO A TWO-LEVEL SYSTEM IN THE STRONG-COUPLING REGIME
by Guillaume BERTEL (Laboratoire Ondes et Matière d'Aquitaine)
The defense will take place at 14h00 - Amphithéâtre 3 351 Cr de la Libération, Université de Bordeaux, Bâtiment A9, 33400 Talence
in front of the jury composed of
- Fabio PISTOLESI - Directeur de recherche - LOMA - Directeur de these
- Anja METELMANN - Professor - Karlsruhe Institute of Technology - Rapporteur
- Gary STEELE - Professor - Kavli Institute of Nanoscience - Rapporteur
- Jérémie VIENNOT - Chargé de recherche - Institut Néel - Examinateur
- Brahim LOUNIS - Professeur - LP2N - Examinateur
- Clément DUTREIX - Chargé de recherche - LOMA - CoDirecteur de these
Among the various nanoscale systems operating in the quantum limit, nanomechanical oscillators have emerged as particularly powerful platforms. They constitute highly sensitive sensors and can mediate interactions between light and localized quantum systems, playing a central role in various quantum information processing architectures. Due to their relatively large mass, they also serve as ideal platforms for investigating the transition between classical and quantum behavior. This thesis examines the dynamics of a nanomechanical oscillator coupled to a driven two-level system. The analysis focuses on the resolved sideband limit, where the two-level system's damping rate $Gamma$ is much smaller than the mechanical frequency $omega_m$, and explores both the strong coupling ($omega_m gg g_0 gg Gamma$) and ultra-strong coupling ($g_0 sim omega_m gg Gamma$) regimes, where $g_0$ denotes the coupling strength. These regimes enable coherent manipulation of the mechanical state while preserving its quantum coherence. Under blue-detuned driving of the two-level system and assuming low mechanical damping, the system exhibits a dynamical instability marked by the emergence of limit cycles in the mechanical oscillator. The onset of this regime is accompanied by pronounced fluctuations in the photon emission from the two-level system, indicated by a large deviation from Poissonian statistics. The phonon statistics display a similar behavior, offering an additional experimental signature. As the coupling strength approaches $omega_m$, the mechanical oscillator reaches a steady state with non-classical feature. The phonon number distribution in this regime resembles that of Fock states, and the corresponding quasi-probability distribution in phase space displays negative regions, a signature of non-classicality. A method is proposed to detect this non-classicality experimentally through measurements of the emitted light, from which the full quantum state of the coupled system can be reconstructed. The robustness of this approach is analyzed in the presence of realistic noise sources. The thesis further investigates the response of the system under two-tone driving. A tailored model is developed to describe this regime at strong coupling. When the driving frequencies are tuned to the first blue and red sidebands, the mechanical oscillator reaches a steady state exhibiting quadrature squeezing. In the case where both driving frequencies are red-detuned, the oscillator evolves into a superposition of coherent states, analogous to the behavior of a parametrically driven resonator. In both configurations, non-classical features become prominent when $g_0$ approaches $omega_m$. These results provide new insights into the generation and stabilization of non-classical and squeezed mechanical states. We outline feasible strategies for their detection, offering potential applications in hybrid quantum systems and foundational tests of quantum mechanics.
Global approach on optimisation of the intensity of attosecond pulses
by Sylvain PRAWDZIAK (Centre Lasers Intenses et Applications)
The defense will take place at 14h00 - Amphithéâtre 1 Bât 29 Université de Bordeaux, Bâtiment A29, 351 cours de la Libération, 33400 Talence
in front of the jury composed of
- Constance VALENTIN - Chargée de recherche - Université de Bordeaux - Directeur de these
- Boris VODUNGBO - Maître de conférences - Sorbonne Université - Rapporteur
- Sophie KAZAMIAS - Professeure des universités - Université Paris-Saclay - Rapporteur
- David GAUTHIER - Chargé de recherche - CEA-Saclay - Examinateur
- Eric MEVEL - Professeur des universités - Université de Bordeaux - CoDirecteur de these
- Olivier UTEZA - Directeur de recherche - Aix Marseille Université - Examinateur
- Philippe TAMARAT - Professeur des universités - Université de Bordeaux - Examinateur
High harmonic generation (HHG) in rare gases is a prevalent method to generate XUV (200 -10 nm) beams useable as table-top sources of attosecond pulse (1 as = 10-18 s). These pules are notably used to probe ultrafast electronic dynamics in matter. However, HHG is not a very efficient process (10-5 in argon). In order to optimize the intensity of attosecond pulses and to extend the applicability of the XUV beam, characterisation and control of the spatial properties of said beams proves to be critical. Indeed, spatial-temporal couplings, such as chromatic aberrations inherent to the HHG process, can lead to perturbation of the resulting attosecond pulse train. We use two characterisation methods of the spatial properties: the “Spectral Wavefront Optical Reconstruction by Diffraction” (SWORD) technique and an XUV Hartman wavefront sensor. To remove chromatic aberrations in HHG we structure the driving laser to achieve a “flat-top” beam in the spatial and temporal domain, in order to limit the intensity dependant phase term thus limiting chromaticity and improving temporal resolution and intensity of the resulting attosecond pulse train.
ED Sciences et environnements
interactions between canopy structure and understory microclimate: implications for forest regeneration
by Klara BOUWEN (ISPA - Interaction Sol-Plante-Atmosphère)
The defense will take place at 13h30 - Salle de réunion ISPA 71 Avenue Edouard Bourlaux, Bâtiment C1, 33140 Villenave-d'Ornon, France
in front of the jury composed of
- Jérôme OGéE - Directeur de recherche - Université de Bordeaux - Directeur de these
- Catherine COLLET - Chargée de recherche - INRAE - Rapporteur
- Juan Pedro FERRIO - Directeur de recherche - EEAD-CSIC - Rapporteur
- Nadine RÜHR - Professeure - Karlsruhe Institute of Technology - Rapporteur
- Jean-Christophe DOMEC - Professeur - Bordeaux Sciences Agro - CoDirecteur de these
- Annabel PORTé - Directrice de recherche - INRAE - Examinateur
Forest canopy plays a crucial role in buffering understory microclimate and explains understory biodiversity, forest regeneration and forest resilience to future climate change. Forest management practices shape understory microclimate by modifying the structure and composition of the canopy through thinnings, tree species selection, understory control or forest fragmentation. Yet currently, forest managers have no tools to quantify the impact of their practices on microclimate. Using micrometeorological measurements, airborne LiDAR data and model simulations of the physics-based biometeorological model MuSICA, I investigated how variations in canopy structure influence the understory microclimate, with a particular focus on summertime climate extremes and their potential impact on seedling survival. Performing sensitivity analyses using physics-based models poses a challenge as they require meteorological input data (e.g. air temperature, humidity, wind speed) above the canopy, that are in turn modified by changes in canopy structure. Using the same climate forcing across a range of canopy structures can therefore lead to misleading conclusions. To this end, I codeveloped a new algorithm to account for dynamical feedbacks between changes in canopy structure and the climate conditions just above it. This algorithm relies on existing theories of turbulent flux-gradient similarity relationships within the atmospheric surface boundary layer that I tested against datasets collected in various forest ecosystems across Europe. I found that the influence of canopy roughness clearly affects the shape of the vertical profiles of windspeed and air temperature above the canopy, and that different theoretical framework used to represent these effects performed equally well. Building on this result, I then implemented the new algorithm into MuSICA and investigated how a change in canopy density or vertical complexity shapes understory microclimate during summer extremes. Based on experimental data from a range of forests across Europe, I first identified the existence of a threshold of canopy density below which the microclimate near the ground was becoming warmer than in a nearby open field, meaning that the temperature extremes in summer were amplified, not buffered, in these sparse canopy configurations. This threshold was confirmed by model simulations, and the consequences for the survival of young seedling was also evaluated. Model simulations showed however that the threshold in canopy density was site specific and varied also between years at a given location. Using experimental data and model simulations, I showed that the canopy vertical complexity had limited effect at mitigating this amplification of climate extremes below sparse canopies, unless a dense understory or shrub layer was present. By linking canopy structure to microclimate and seedling survival, this research advances our understanding of forest–atmosphere interactions and highlights the potential for forest management to either mitigate or amplify summertime climate extremes. The findings lay the groundwork for new decision-support tools that could help forest managers evaluate the microclimatic consequences of silvicultural practices and design strategies that support biodiversity and climate resilience.