26 Novembre – Thesis defense - Maxime Lavaud

10 h Building A4N - University of Bordeaux (Talence campus)

Confined brownian motion.

In this manuscript, I present the work done during my PhD on confined Brownian motion. Brownian motion is the erratic movement of microscopic particles when immersed into a fluid. Thanks to Einstein and his successors, it is generally possible to describe Brownian motion using simple equations. However, in the last two decades a scientific revolution has taken place with the advent of miniaturization and in particular microfluidics, enabling the creation of complex networks of pipes at the micrometer scale. Microfluidics makes it possible to sort out particles, such as drops, cells or bubbles, but also to distribute drugs in cells and observe their effect on thousands of them. Regarding Brownian motion, it has been observed that once confined near a wall, a particle moves much slower due to non-slip boundary conditions at the wall. The mobility is thus modified by confinement-induced effects.
My thesis work consists in experimentally measuring, analyzing and modeling the movement of micrometric colloids diffusing near a wall. To track the motion of confined Brownian microparticles, I use Lorenz-Mie holography. The Lorenz-Mie framework allows me to record the thermally-induced three-dimensional trajectories of individual microparticles, within salty aqueous solutions, in the vicinity of a rigid wall, and in the presence of surface charge with a nanometric resolution. From the recorded trajectory, I construct the time-dependent position and displacement probability density functions, and analyze the non-Gaussian character of the latter which is a direct signature of the hindered mobility near the wall. Based on these distributions, I implement a novel, robust and self-calibrated multifitting method, allowing thermal-noise-limited inference of diffusion coefficients spatially-resolved at the nanoscale, equilibrium potentials, and forces at the femtonewton resolution. Moreover, I use this novel tool to deduce non-conservative forces and study long-time higher-order statistical properties. Our objective for the future is to use this novel tool to have a new approach in various problems relevant to nanophysics and microbiology.

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