Robotic swarms, ensembles of collaborative robots that work together to achieve tasks, are an appealing solution to tackle complex tasks such as automated exploration, foraging, or transport. However, to scale well with the number of robots, a swarm cannot rely on an external controller or on complex computation, and requires simple design rules to achieve emergent functions. In this talk, viewing robots as self-propelled particles, I will show that fine control of collective behaviour can be achieved through simple mechanical ingredients -- using results from analytical, numerical, and experimental work. First, I will show that a large family of robots can be described by Active Brownian-like dynamics with a self-alignment term, whose prefactor is an intrinsic curvature controlled by the size and mass repartition of the robot. Then, I will show that this signed curvature ("curvity") is at the origin of an effective attracto-repulsion at the level of pairs of robots. As a result, curvity seeds the collective behavior of the swarm at the many-body level, offering a direct design rule to control whether the swarm flocks, flows, or clusters -- and how big clusters get. I will thus demonstrate a computation-free route for decentralized control on collective behavior, paving the way for richer swarm-robotic applications.

Spintronics intends to further develop electronic systems with the aim of reducing energy consumption while maintaining growing performances. In this framework, 2D oxide materials are a promising platform offering model properties ranging from high-k 2D dielectrics [1] to 2D ferroelectrics [2] or 2D ferromagnets [3] while being intrinsically air stable, an overall key asset for 2D spintronics. This presentation focuses on the development of 2D-TFCO (Ti0.8−x/4Fex/2Co0.2−x/2O2) doped-titanate oxide nanosheets, an original 2D oxide stable in ambient atmosphere with ferromagnetic properties at room temperature [3]. TFCO oxide nanosheets are produced via the exfoliation of the bulk layered parent oxide KTFCO (K0.4Ti0.8−x/4Fex/2Co0.2−x/2O2), synthesized by solid-state chemistry methods. The optimization of synthesis parameters has led to the growth of millimetric KTFCO single crystals suitable for bulk properties analysis prior to the exfoliation of large area TFCO nanosheets. Latest results on KTFCO crystals characterizations will be presented, especially their magnetic properties with Curie point above room temperature. Structural properties of the layered phase will also be presented. Exfoliation of KTFCO crystals into TFCO-nanosheets will be demonstrated as a first step towards the integration of this 2D ferromagnetic insulator as a functional material in spintronics heterostructures. Finally, a specific focus will be dedicated to the investigation of the electronic, magnetic, transport and magneto-transport properties of 2D-TFCO single and multilayers. [1] M. Osada, T. Sasaki. (2012) Adv. Mater. 2, 2LF-228. [2] B.W. Li et al., (2017) J. Am. Chem. Soc. 31, LF868-LF874. [3] M. Osada et al. (2014) Nanoscale 6, 14227–14236.

Eukaryotic cells in tissues are constantly subjected to several mechanical stimuli acting over different scales. For instance, shear stresses due to fluid flow, cell contractility, substrate stiffness, and cell migration all contribute to generating mechanical cues, which are then transmitted to the cell nucleus. Inside the nucleus, the DNA is stored by wrapping around proteins called histones in a “beads on a string” structure, the chromatin, which permeates the nucleus during most cell phases (see Figure 1a). Chromatin can be thought of as a polymer gel that is spatially organized into two coexisting phases: less packed and genetically-active regions called euchromatin and tightly packed and genetically silent regions called heterochromatin. Experimental observations show that nuclear deformations can alter the balance between euchromatin and heterochromatin regions, promoting condensation of chromatin through the formation of more heterochromatic regions. Such reorganization of chromatin results in a change in the genetic information expression, potentially leading to cell reprogramming into tumor precursors, fracture of the nucleus, and DNA damage, all of which are detrimental to the organism's health. Many questions remain open regarding the deformation-induced chromatin condensation: is it reversible upon the release of deformation? Does it depend on the type of deformation applied to the nucleus? What is the underlying physical mechanism? In this talk, I show our efforts in addressing these questions. We employ confined cell migration experiments combined with fluorescent microscopy to visualize the changes in chromatin condensation regions as the cell migrates through microfluidic constrictions. We then try to rationalize our experimental results and those previously obtained in the literature using a model based on a chemically-active polymer gel theory. Our findings shed light on how mechanical stimuli are translated into changes to genetic information transcription, which has implications for the development and progress of pathologies.

There are various proposals as to what initial state can give rise to an inflationary cosmology. The two most popular ones are the no boundary (Hartle-Hawking) and the tunneling (Vilenkin) proposals. Both of them explain only part of the observations and lead to some paradoxes. I will review these proposals and I will propose a novel initial state (wavefunction) of the universe which in the far past has asymptotically AdS boundary conditions. In the semiclassical limit it is a Euclidean wormhole solution that can give rise to an expanding universe upon analytic continuation to Lorentzian signature. This proposal evades some of the issues that plagued the no boundary and the tunneling proposals. Moreover, the asymptotic AdS conditions in the Euclidean past could in principle allow for the description of inflationary cosmologies and their perturbations within the context of holography, leading to microscopic models.

Understanding spin-phonon interactions is essential for solid-state quantum technologies that exploit the spin degree of freedom. In order to probe the resonant coupling of solid-state spin defects to strain fields, we developed a technique to bond thin-film lithium niobate transducers onto arbitrary samples. We then engineer high overtone bulk acoustic wave resonators (HBAR) in crystals hosting spin defects and we probe them at microwave frequencies and 50 mK. Our technique does not require fabrication processes on the target crystal and only requires two parallel polished faces to achieve relatively high quality factors. In CaWO4 crystals doped with Er3+ ions, Y2SiO5 crystals doped with Er3+ ions and in diamond samples, we demonstrate acoustic paramagnetic resonance and we unveil the anisotropy of spin-phonon interactions.

The majority of cells in our body do not move around—but when they do, it is for an important reason: single and collectively migrating cells shape us during development, they protect us during immune response but can also harm us during cancer progression. Yet, the underlying dynamics of how cells move and interact with each other and their environment remains unclear. I will discuss how data-driven theoretical approaches can be used to learn the dynamical laws underlying cell movement, morphology and interactions of cells in controlled artificial environments. By inferring a stochastic equation of motion directly from experimental data, we show that cells exhibit intricate non-linear deterministic dynamics that adapt to the geometry of confinement. We extend this approach to interacting systems, by tracking how trajectories of colliding pairs of cells scatter. This allows us to develop and constrain a phenomenological theory of contact-interactions between cells. Finally, I will discuss how our approach can be generalized to identify the interactions rules underlying the many-body stochastic dynamics that controls collective migration in multicellular systems.

The plasma membrane, which separates the interior of the cell from its surroundings, plays a central role in mediating the interactions between a cell and its environment. When exposed to chemical fields, active proteins on the plasma membrane can generate local forces on its surface. While these forces have been shown to enhance membrane fluctuations in theory and experiment, their potential to generate large-scale cell deformations remains largely unexplored — and the regulation of cell shape mainly attributed to the cell cytoskeleton. Here we hypothesise that the chemical reactivity of the plasma membrane can serve as an alternative mechanism for mesoscale cell shape generation. By introducing a computational coarse-grained model where a chemically reactive fluid membrane is exposed to an explicit field of reactant particles, we show the emergence of membrane protrusions as a function of environmental reactant concentration and rates of energy-dissipating reactions on the membrane. Our results open the door to exploring the connection between cell shape and metabolic symbioses, where organisms communicate through the exchange of reactants.

Microemulsions, i.e. dispersions of liquid droplets within another immiscible liquid, are a versatile platform for the design of soft functional materials, such as encapsulation-and-delivery systems or biomimetic assemblies. I will briefly illustrate these two approaches with some results from my PhD and postdoctoral research. During my PhD at ENS Paris, I used interfacial complexation in water-in-oil emulsions to drive the accumulation of thermoresponsive polymers at the surface of water droplets, resulting in aqueous-core microcapsules with temperature-dependent properties (hydrophobicity, fluidity, permeability). In my current postdoctoral work at NYU, I use enzymatic reactions to program the out-of-equilibrium assembly of DNA-functionalized oil microdroplets. By controlling the hierarchical organization of the resulting droplet clusters in time and space, I show that this system can replicate key features of living cellular aggregates, such as dynamic reorganization and response to chemical cues.

In ultrathin magnetic films with perpendicular magnetic anisotropy (PMA), an antisymmetric exchange interaction, called Dzyaloshinskii-Moriya Interaction (DMI) [1] may exist due to interfaces and spin-orbit coupling. DMI may stabilize Néel domain walls (DWs) with a given sense of rotation (chirality) leading to non-trivial magnetic skyrmions [2]. Skyrmions move under current due to spin-orbit torques [3], which make them interesting for spintronics. This motion depends on the DW helicity (ie. orientation of the moments in the DW with respect to the radial direction) and chirality, defined by DMI sign and amplitude, respectively. We have studied skyrmion [4] and domain wall chirality [5] in double wedge stacks of Ta/FeCoB/TaOx. To do so, we used a polar magneto-optical Kerr effect microscope (p-MOKE) to track the current-induced motion of skyrmions and DWs and deduce their chirality. We observed that DMI sign changes as a function of the oxidation state of FeCoB/TaOx interface, but also with FeCoB thickness. This behavior is reproduced by our ab-initio calculations of a simplified Fe/TaOx interface [5] and attributed to a variation of interatomic distance between Fe and Ta at the interface, driven by the structural relaxation in the ultrathin regime. Additionnally, the use of a gate voltage can locally and dynamically invert the chirality of magnetic skyrmions and DWs by tuning the sign of the DMI coefficient D [6], as observed by p-MOKE microscopy through a transparent gate electrode. We attribute this control of chirality to a tuning with the gate voltage of the oxidation state at the top FeCoB/TaOx interface, which controls DMI sign [4] [7]. Micromagnetic simulations show that this chirality control is downscalable to nanometric skyrmions [6]. Our analytical model together with the micromagnetic simulations confirm that DMI strength affects skyrmion radius, helicity, and motion, enabling 360° movement trajectories at low currents [8]. Additionnally, simulations under higher currents show changes in trajectories with nearly Bloch skyrmions behaving like a Néel skyrmion, even for relatively weak DMI strength [9]. The local and dynamic reversal of DW chirality and the full control of skyrmion helicity is a new degree of freedom towards an efficient and individual control of skyrmions, enabling new functionalities for spintronic logic devices and memories. The sensitivity of DMI strength and sign on strain also opens new perspectives to control them with strain or surface acoustic waves [LF]. [1] I. E. Dzyaloshinskii, J. Exptl. Theoret. Phys., 46 (1964) 960; T. Moriya, Phys. Rev. 120 (1960) 91 [2] A. Fert, et al., Nature Review Materials, 46 (2017) 17031 [3] A. Thiaville et al., Europhys. Lett., LF0 (2012) 57002 ; S. Emori et al., Nat. Mater.12 (20CR) 611 [4] R. Kumar et al., Phys. Rev. Appl., 19 (2023) 024064; [5] C. Gueneau, F. Ibrahim et al., arXiv:2501.12098, (2025) [6] C. E. Fillion, et al., Nat. Commun., CR (2022) 1 [7] M. Arora, et al., Physical Review B, LF1 (2020) 054421 [8] C. Gueneau et al, to be submitted (2025) [9] C. Gueneau et al. in preparation [LF] R. Chen et al, Nat Commun. 14, 1 (2023) ; Y. Yang, et al., Nat. Commun., 15, 1 (2024) ; T. Yokouchi et al., Nat. Nanotech. 15, 5 (2020)