Vincent Ouazan-Reboul (LPTMS)

Salle 523, couloir 12-13, 5è étage

Complex interactions in and out of equilibrium

One of the main challenges in the modeling of biological systems is that their physical behavior at all scales is
dictated by intricate interactions between many different complex objects. In this talk, I will present theoretical
results on two different systems where complex interactions play a key role, one equilibrium and the other active.

I will first discuss the equilibrium self-assembly of proteins, which can be seen as particles with short-range
anisotropic interactions. Strikingly, proteins with vastly different physico-chemical properties tend to form into
similar fibrous pathological aggregates. By performing lattice Monte-Carlo simulations of three-dimensional particles, I
will show that complex anisotropic iteractions lead to a great morphological diversity in the resulting assemblies. In
particular, many choices of interactions lead to the formation of fibers, which are found to result from geometrical
frustration. On the other hand, I will also demonstrate that anisotropy is a useful design tool for controlling the size
and shape of equilibrium aggregates.

In a second part, I will discuss the self-organization of mixtures of enzyme-like active particles. As opposed to the
previous system, these objects are intrinsically out of equilibrium, and develop isotropic, long-ranged, non-reciprocal
interactions. By using a combination of linear stability analysis and Brownian dynamics simulations, I will show that
catalytically active particles can self-organize into droplet-like structures. My focus will be on the case where
different species of enzymes participate in a biochemical reaction network. This different type of complexity, which
stems from the existence of an intricate interaction network between different species instead of structural anisotropy,
can be an intrinsic driver of self-organization and lead to novel collective dynamics.