Brownian motion of spherical colloidal particles at the air-water interface
Résumé
The dynamics of colloids at the interface between two fluids is a crucial and timeliness research topic as it governs the behavior of kinetically arrested colloidal gels in interfacial microrheology, the formation of bacteria based biofilm and the cellular signaling via membrane proteins. Moreover, this subject is also challenging from a theoretical point of view because of the complexity of hydrodynamics at the interface and of the unexplored 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 contribution, the Brownian motion of micrometric spherical silica beads at a flat air-water interface is addressed. We fully characterize and control all the experimentally relevant parameters. The particle contact angle is finely tuned in the range 30-140° by surface treatments and measured in situ at 1° resolution by a homemade Vertical Scanning Interferometer. The particle dynamics and translational diffusion coefficients are obtained by particle tracking. Counter-intuitively, and against all hydrodynamical models, the diffusion is much slower than expected; the drag exerted on the particle increases when the particle is less immersed in water. To explain this extra dissipation we devised a model considering the effect related to thermally activated fluctuations of the interface at the triple line. Such fluctuations couple with the lateral movement of the particle via tiny 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 obtained total friction is thus in agreement with the measured particle diffusion, and in particular with its slowing down at large contact angles. The physical origin of the fluctuations is discussed in the two extreme limits: of a moving line using the generally accepted Blake molecular kinetic theory and of a pinned line taking into account the role of capillary fluctuation at the interface. Both of them lead to the right order of magnitude for the extra dissipation and can capture the measured dynamics.