Résumé
The physical behavior of colloids suspended in a fluid close to a solid wall has been well characterized in the past[1]. Conversely the dynamical behavior of a particle close to a fluid interface is a relatively new field with strong implications in green and food processing [2]. Previous focus was limited to spherical particle’s viscous drag close to the interface [3] and wetting dynamics after interface breaching [4]. In this paper we present experimental results on the dynamics of micrometric colloids(spheres and ellipsoids)close to an interface between water based solution and air prior to the particle’s interfacebreaching. In order to study such dynamics we use a home-made pending-drop setup where particles dispersed in water based solutions (mixture with heavy water and/or with salt at different molarity) sediment towards the interface with air of the drop.The drop curvature is adjusted via the water solution pressure in order to obtain a flat interface at the particle scale. All the particle’s degreesof freedom (i.e. distance from the interface, position in a plane parallel to the interface and, for ellipsoids,orientation) are measured tracking the particle-interface interference patterns using incoherent ledswith three different wavelengths. The typical dynamics of a particle approaching the interface shows an initial sedimentation regimedriven by gravity and viscous drag. Closer to the interface a second regime arises which strongly depend on the solution molarity and density and particle morphology. For both particles morphologies the sedimentation dynamics stops at a distance from the interface of the order of hundreds nanometers. At small molarity (0.5 mM) both type of particles fluctuate around this position during the entire experimental observation time interval (10^3s). Increasing the molarity, ellipsoids more easily then spheres start to escape from this position approaching rapidly the interface till a distance of few nanometers. Such observations point out the importance of the electrostatic and morphology in the interaction with the interface. A calibration of thisinteraction will be presented by taking advantage of the buoyancy driving force changing the solution density. Building up on all these experimental observations, amodel considering the interplay between all the contributing interactions (i.e. gravity with buoyancy, electrostatics and Van der Walls) will be discussed and compared to the experimental results. [1]M. Hosoda, K. Sakai, and K. Takagi, Phys. Rev. E 58, 6275 (1998).[2] V. Sarrot, Z. Huang, D. Legendre, P. Guiraud, Chem. Eng. Sci. 62, 7359–7369 (2007).[3] G. M. Wang, R. Prabhakar and E. M. Sevick, Phys. Rev. Lett. 103, 248303 (2009).[4] A. Wang, W.B. Rogers, V.N. Manoharan, arXiv:1607.08167 [cond-mat.soft].