Πάντα χωρεῖ καὶ οὐδὲν μένει
(everything flows and nothing stands still)
Understanding how complex systems move, flow, get deformed is certainly not a novel question, but it plays a central role in several major challenges of the XXIst century.
In a world always on the move, mobility issues (whether they deal with pedestrians, vehicles, ...) will, and should, shape our cities and it is therefore a matter of paramount interest to be able to describe and predict the associated traffic. We claim that Physics has its say for these issues. Indeed, the way these systems flow results from collective effects that emerge from the interaction of elementary entities, whether they be pedestrians or cars. We strive to understand, describe and model traffic at the multiple scales that are relevant in a city, starting from simple elementary entities that mutually interact. This is achieved by notably leveraging tools from Statistical Physics and exploiting them on a new scientific frontier, while paying due attention to the idiosyncrasies and the specific issues of interest for each of them.
On a topically distinct front, we adopt a similar approach to understand and model the deformation of complex materials. These materials abound both in Nature and as synthetic materials, thanks to their peculiar mechanical properties; they are typically deformed and made to flow before they reach their final shapes and will often endure mechanical stresses in their 'lifetime'. Accordingly, better understanding their deformation is often a prerequisite to grasping their global response (for geological flows, earthquakes, ...) or improving their performances (foams with various functionalities, cosmetics, metallic glasses, ...).