Sommaire:

The dynamics of colloidal particles at the interface between two fluids plays a central role in micro-rheology, encapsulation, emulsification, biofilms formation and water remediation. Moreover, this subject is also challenging from a theoretical point of view because of the complexity of hydrodynamics at the interface and of the role of the contact line. Despite this great interest, the behavior of a single particle at a fluid interface was never directly characterized. In this thesis, we study the Brownian motion of micrometric spherical silica beads and anisotropic polystyrene spheroids at a flat air-water interface. We fully characterize and control all the experimentally relevant parameters. The bead contact angle is finely tuned in the range 30-140° by surface treatments and measured in situ by a homemade Vertical Scanning Interferometer. The spheroid aspect ratio varies in the range 1 – 10 by stretching of commercial beads. The translational and the rotational dynamics are followed by particle tracking. Counter-intuitively, and against all hydrodynamic models, the diffusion is much slower than expected. To explain this extra dissipation we devised a model considering the contribution of thermally activated fluctuations of the interface at the triple line. Such fluctuations couple with the lateral movement of the particle via random forces that add to the ones due to the shocks of surroundings molecules. Fluctuation-dissipation theorem allows obtaining the extra friction associated to this additional mechanism. The fitting values of the total friction are discussed in term of the typical scales of particle surface heterogeneities and of surface capillary waves.

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