Alexandre Morin (Leiden University)

Title: Flocks in the Wind

Abstract:

How do flocks navigate when it’s windy? By combining model experiments and theory, I investigate the response of flocks, or active ferromagnets, to external fields. I will demonstrate their hysteric response and show how orientational elasticity and confinement act together to protect the direction of collective motion up to a limit where flocks are forced to fly along the wind.

Alma Dal Co (Harvard University)

Title: Interaction networks in microbial communities

AbstractCommunities of interacting microbes perform fundamental processes on Earth, such as cycling the elements and shaping the health of animals and humans. The processes that these microbial communities perform arise from a dense network of interactions between individual cells. Most microbial communities are spatially structured systems, where cells move little, thus interactions occur mostly between cells close in space. Therefore, the spatial arrangement of different species can affect the processes that the whole community performs. My goal is to uncover how the local interactions between cells determine community-level processes. To do so, I look at synthetic bacterial communities under the microscope at a resolution that allows me to observe both the individual cells and the community as a whole. I measure properties of the single cells, like their growth and their phenotype, and I use mathematical modeling to uncover how these individual-level properties determine community-level properties.

Anton Souslov (University of Bath)

Active elastocapillarity

Active solids use elastic coupling between energy-consuming elements to achieve functionality inaccessible at equilibrium. For two-dimensional active surfaces, powerful experimental techniques allow for exquisite control over spatial patterning and fuel delivery. However, achieving such control in three-dimensional objects presents a challenge. I will present a continuum theory that describes an active surface wrapped around a passive soft solid. The competition between active surface stresses and bulk elasticity leads to a broad range of previously unexplored phenomena, which we have dubbed active elastocapillarity. In passive materials, positive surface tension rounds out corners and drives every shape towards a sphere. By contrast, activity can send the surface tension negative, which allows us to tune the target shape using elasticity. We discover that in these reconfigurable objects, material nonlinearity controls reversible switching and snap-through transitions between anisotropic shapes. Even for stable surfaces, a signature of activity arises in the negative group velocity of surface Rayleigh waves. These phenomena offer insights into living cellular membranes and underpin novel design principles across scales from robotic metamaterials down to shape-shifting nanoparticles.