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Abstract

Active matter is everywhere, from macroscopic to microscopic scales, we find systems such
as human crowd or flock of birds as well as bacterial colonies. These systems composed of
particles are able to convert their surrounding energy into motion, and naturally exist out of
thermodynamic equilibrium. At the microscopic scale, a specific class of active particles is
particularly interesting: called microswimmers, these are biological or artificial micro-sized
particles able to move in a fluid, such as bacteria or chemically driven Janus particles. In
nature, these microswimmers rarely swim alone and can exhibit intriguing collective behavior at interfaces such as cluster formation, as well as swarming, swirling, raft and biofilm
formation. The fundamental mechanisms of the emergence of collective behavior for living
and inanimate active systems is not yet understood, especially because these systems are far
from equilibrium, where our experimental and theoretical understanding is limited.
This thesis aims to elucidate the impact of the activity on the emergence of collective behavior in an active system, at a microscopic level, by using a stochastic approach, over three
works, from active sedimenting particles to early biofilm formation in the case of the bacteria
Pseudomonas aereginosa, via the aggregation formation for the micro-algae Chlamydomonas
reinhardtii.