The fruit fly larva is a maggot which looks like a dull white cylinder. Within a few days, and without any changes in its genome sequence, it metamorphoses. It gets its sophisticated adult fly shape with wings, legs, antennas, and compound eyes. How do cells migrate, deform, and rearrange to shape a tissue ? To approach step by step the dynamics of this morphogenesis, we will journey from developmental biology to mechanics, from discrete description of cellular material to continuum mechanics quantification, and from experiments to modeling. We will investigate flows within geometries specifically designed to discriminate between models.

The most common numeral system is decimal, based on our ten fingers, but countless other systems exist and have been used throughout history. They use a different "base" or "radix". Traces of these alternative systems are still present today: when we buy a dozen eggs, use base 24 and 60 for timekeeping, or hear a French speaker say ``four-twenty four'' to mean eighty-four, a vestige of a vigesimal (base-20) system. We use our fingers to count in the same way that we may use our feet to measure lengths. We rely on these anthropocentric standards to communicate, but we expect natural phenomena to be independent of the language we speak, the metrics we choose, and even the kind of numbers we use. Both numbers and units should be informed by nature, not human conventions. To illustrate this radical idea, I will explore examples from the capillary dynamics of droplets and bubbles, and from explosions—two classical subjects of scaling. Starting with a single power law, I will show that its eventual breakdown is actually a necessity if the associated phenomenon is to be independent of our human imprint. No power law can extend indefinitely.

Immune cells, particularly lymphocytes, must move, interface with other cell types, extract information from these interactions, and, if necessary, respond rapidly through endocytosis, status changes, proliferation, or cell killing. These critical biological processes rely on cytoskeletal rearrangement, mechanical sensing, and force production. In our research, we use microfluidics and micro-fabricated tools to investigate the forces at cell-cell interfaces and the cellular rearrangements triggered by antigen recognition. Recently, we introduced functionalized oil droplets as a novel antigen-presenting tool in the context of B cells, uncovering an unexpected role of microtubules in limiting F-actin polymerization, which facilitates the formation and maintenance of a distinct immune synapse. Additionally, I will present new applications of these methods, focusing on lymphocyte interactions with the microenvironment of lymph nodes and examining how mechanical factors influence their immune functions.

Our work is characterised by its interdisciplinarity, at the interface of Physics with Biology and Computer Sciences. It aims at highlighting how in silico experiments based on statistical Physics principles and intensive simulations allow answering targeted applied questions of high impact in domains ranging from plant biotechnology to human metabolism. We focus on complex coarse-grained macromolecular systems and non-equilibrium and non-linear phenomena. With our approach based on kinetic Monte Carlo simulations, we mainly investigate two types of systems: i) one-dimensional and unidirectional transport processes (e.g. the cytoskeletal transport, and the translation of mRNAs by ribosomes), and ii) the emergence of complex three-dimensional polysaccharidic structures (e.g. starch, glycogen, and lignocellulose) resulting from either synthesising or degrading enzymatic activities.

Propriétés électroniques et magnétiques de graphène Kagomé Alain Rochefort Polytechnique Montréal, Département de génie physique, Montréal, Canada Résumé La formation de polymères Kagomé avec des propriétés qui s’apparentent à celles du graphène a récemment généré de nombreux travaux sur ce type de matériau 2D aux propriétés parfois exotiques telles que des phases topologiques, des structures de bandes plates ainsi que des phases magnétiques liées à la structure des monomères. Dans cette présentation, je montrerai différents exemples de matériaux pouvant être produits par une approche de synthèse de surface (OSS) qui possèdent des propriétés électroniques et magnétiques prometteuses et pour lesquelles nous proposons également différentes approches pour moduler ces propriétés. Je montrerai également comment la symétrie des unités de base du polymère est au centre du contrôle de ces propriétés.

In complex or disordered media, standing waves can undergo a strange phenomenon that has intrigued physicists and mathematicians for over 60 years, known as “wave localization.” It consists in a concentration of the wave energy in a very restricted sub-region of the complete domain. This localization has major consequences not only on the vibration but also the propagation properties of the system, in particular electronic transport in disordered alloys. It has been demonstrated experimentally in mechanics, acoustics, electromagnetics and quantum physics. In this talk, we will present a theory that highlights the existence of an underlying and universal structure, the localization landscape, obtained by solving a problem associated with the wave equation [1]. In quantum systems, this landscape makes it possible to define an “effective potential” that predicts the localization subregions, the energies of localized modes, the density of states and the long-range decay of wave functions, regardless of the dimension, both in continuous and discrete settings. Finally, we will provide an overview of the applications of this theory to mechanics, semiconductor physics, as well as for molecular and cold atom systems. [1] M. Filoche & S. Mayboroda, PNAS (2012) LF9:14761-14766.

The citation for the 2014 Nobel Prize in Physics was for “the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources”, thus suggesting that visible LEDs for lighting lead to global energy savings. As is well known among economists (but perhaps less among physicists, engineers and policy makers), the various rebound effects can severely affect the overall energy-consumption gains (if any) when a more energy-efficient technology comes to market. It is an open question to assess whether the III-N LED technology actually leads to energy savings. I will discuss the notion of rebound effect (notably direct and indirect rebound effects) in general, for semiconductor technologies and for the case of the III-N LED technology, notably in light of the surprisingly scarce literature on the topic. I will then discuss what I think this implies when discussing the impacts of our research practices and mostly the potential (social, economic and environmental) impacts of the technologies developed in academic laboratories.

There has been a growing realization that the properties of a material can be modified just by placing it in an optical cavity. The quantum vacuum fields surrounding the material inside the cavity can cause nonintuitive modifications of electronic states through ultrastrong vacuum–matter coupling, producing a vacuum-dressed material with novel properties. Existing theoretical predictions include cavity-enhanced, cavity-induced, and cavity-mediated enhancement of electron–phonon coupling and superconductivity, electron pairing, anomalous Hall effect, ferroelectric phase transitions, quantum spin liquids, and photon condensation. Achieving the so-called ultrastrong coupling (USC) regime is a prerequisite for observing these effects, which arise when the interaction energy becomes a significant fraction of the bare photonic mode and matter excitation frequencies. Most intriguingly, when a material is ultrastrongly coupled with cavity-enhanced vacuum electromagnetic fields, its ground state will contain virtual photons. This nonperturbative virtual driving without external fields can lead to phase transitions in thermal equilibrium. This talk will describe our recent studies of USC phenomena in various solid-state cavity quantum electrodynamics systems in search of such vacuum-induced phases of matter. We utilize the phenomenon of Dicke cooperativity, i.e., many-body enhancement of light–matter interaction, to explore quantum-optical strategies for creating, controlling, and utilizing novel phases in condensed matter enabled by the quantum vacuum.