Résumé
In industrial and biological applications, we often handle colloids at a fluid-fluid interface. Some examples are removal of contaminants from wastewater, the study of cellular signaling or the stabilization of Pickering emulsion. For a more fundamental point of view, because of their range in size, shapes and material, colloids are also suitable probes to test interfacial hydrodynamics. In this contribution we study the diffusion of ellipsoidal beads at an air-fluid interface. We stretch commercial spherical polystyrene beads (diameter 1.95 µm) using a homemade apparatus, in order to obtain uniaxial prolate ellipsoids with aspect ratio in the range 1 - 10. We use Vertical Scanning Interferometry (VSI) to measure the profile of the particle and of the interface, revealing how much the colloid is immersed in water (mean contact angle = 54°) and the interface deformation. The ellipsoidal dynamics is followed by particle tracking. The trajectory of an individual ellipsoid is used to calculate both translational and rotational diffusion coefficients. This measurement is then made on all the accessible ellipsoidal aspect ratios. In apparently contrast with the partial wetting, we found the dynamics at the interface is slowed down with respect to the dynamics in the bulk. Translational diffusion coefficients are 2-3 times smaller and rotational diffusion coefficient is reduced by a factor of 10 with respect to the ones predicted by the Stokes-Einstein theory. We will discuss this surprising slowing down by taking into account the extra-dissipation due to the movement of the triple line on the colloid surface.