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Modeling and Simulation of Individual and Collective Swimming Mechanisms in Active Suspensions

27 juillet 2015

Modeling and Simulation of Individual and Collective Swimming Mechanisms in Active Suspensions

Thèse Blaise Delmotte

Lundi 21 Septembre à 10 h 00 - Amphithéâtre Nougaro

Sous réserve d’autorisation de soutenance par les rapporteurs

Abstract :

We have all witnessed the flocking of starlings in the sky and the schools of fish that form in the ocean. This kind of organization of living creatures is not limited to those that we see, but also occurs for those that we don’t : swimming microorganisms. Suspen- sions of micro-swimmers exhibit a rich dynamics. Their behaviors can play an important role in the survival of the group, its development, the balance between species, their trophic strategies and even animal fertility. They can form coherent structures due to collective motion, mix the surrounding fluid or modify its rheological properties. Such diversity results from the complex interplay between swimming strategies, physiological processes, chemical reactions and hydrodynamic interactions. Fluid Mechanics is there- fore essential to understand and master the mechanisms involved in these phenomena. While experimental studies bring out new findings and, sometimes, provide physical ex- planations, modeling remains essential. Yet, including an accurate description of the micro-swimmers in a suspension containing thousands (nay millions) individuals, requires considering a wide range of coupled scales (from one micron 10−6m to several millimeters 10−3m). What happens on large scales depends on sophisticated mechanisms occurring two or three orders of magnitude below. Therefore, the multiscale modeling of such phe- nomena is still a major challenge for the state-of-the-art numerical methods.

This thesis aims at providing a contribution in that direction. In a first part, we will show that reproducing swimming mechanisms at the scale of the micro-swimmer can be achieved with various models spanning different levels of complex- ity. We will then present our developments to incorporate these models in an efficient framework for large scale simulations. We will show how to simultaneously account for the Brownian motion of the smallest particles (10−6m). Our code reproduces known results from the literature with the same accuracy, but at lower cost and at larger scales, thus bridging a gap between particle-based models, experiments and continuum formulations from kinetic theory. Using the capabilities afforded by our method, we eventually address two open problems in the experimental literature : the origins of orientational correla- tions between interacting self-propelled micro-droplets and the mechanisms at play in the nonlinear enhancement of Brownian particle diffusion in active suspensions.

Keywords : Micro-organisms, Active Suspensions, Numerical Modeling, Low Reynolds
number flows, Brownian motion.

Raymond E. GOLDSTEIN - Professor (Cambridge, DAMTP) - Rapporteur

Ignacio PAGONABARRAGA - Professor (Universitat de Barcelona) - Rapporteur

Bertrand MAURY - Professeur (Université Paris Sud) - Président

Arezoo M. ARDEKANI - Assistant Professor (Purdue University) - Examinatrice

Eric E. KEAVENY - Lecturer (Imperial College) - Co-encadrant

Franck PLOURABOUÉ - Directeur de Recherche (CNRS) - Co-encadrant

Eric CLIMENT - Professeur (IMFT/INPT) - Directeur de thèse

Pierre DEGOND - Chair Professor (Imperial College) - Co-directeur

(Thèse INPT)

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