The first story is motivated by the collision of grains in a fluid. The complex dynamics involved in such collisions are highly influenced by the micro-roughness of the solid surfaces. I will present experimental results on the collision in a viscous fluid between a solid smooth sphere and a wetted textured wall consisting of micro-pillars . The dynamics of the sphere in the near-wall region is measured using a high-frequency interferometric technique. For negligible sphere inertia, the experiments reveal a texture-modified lubrication drag force, that can be interpreted in terms of an effective slip. When the sphere inertia is non-neglible, the sphere will either stick or bounce off the wall. The observed dynamics are well described using texture-modifed drag and contact forces in the equation of motion of the sphere.
The second story stems from the collection by gravity of water droplets condensing on inclined surfaces. On a smooth surface, small droplets remain pinned to the surface, which limits the yield of collected water. On a micro-grooved surface, the growth and shedding of a few drops are promoted by a long-range coalescence mechanism mediated by groove imbibition . This accelerated "coarsening" effect can be used to enhance the passive harvesting of dew water from atmospheric
 T. Chastel and A. Mongruel, Sticking collision between a sphere and a textured wall in a viscous
fluid, Phys. Rev. Fluids 4, 014301 (2019).
 P.B. Bintein, H. Lhuissier, A. Mongruel, L. Royon and D. Beysens, Grooves accelerate dew shedding, Phys. Rev. Letters 122, 098005 (2019).