Résumé
Dynamics of partially wetted colloids at the interface between two fluids is a crucial and timeliness topic: for instance, it governs the behavior of kinetically arrested colloidal gels, 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 [1]. Despite this great interest, the behavior of a single particle at a fluid interface was never directly characterized. In this contribution, the dynamics of micrometric spherical silica particles trapped at a flat air-water interface is fully addressed. The particle contact angle θ is finely tuned in the range 30°-140° by surface treatments and measured in situ at 0.5° resolution by a homemade Vertical Scanning Interferometer [2]. The particle dynamics and translational diffusion coefficients are obtained by particle tracking. Counter-intuitively, and against all hydrodynamical models [3], 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 surface waves. Such waves are generated by the thermally activated triple line jumps between hydrophilic sites of the particle surface [4]. The total friction is thus coherent with the measured particle diffusion, and in particular with its slowing down at large contact angles. The jump lengths λ are consistent with the Cassie - Baxter model of an heterogeneous surface characterized by an intrinsic roughness and by hydrophobic and hydrophilic patches. [1] D. M. Kaz, R. McGorty, M. Mani, M. P. Brenner, V. N. Manoharan, Nature materials, 11.2, 138-142 (2011). [2] C. Blanc et al., Phys.Rev. Lett.,111, 058302 (2013). [3] T. M. Fischer, P. Dhar, P. Heinig, T, J.Fluid. Mech., 558, 451-475 (2006). [4] T. Blake, J.Colloid Interface Sci., 299, 1-13 (2006).