Particle deposition after droplet evaporation on ultra-hydrophobic micro-textured surfaces

We study the size and shape of the final deposit obtained when a drop with colloidal particles has dried on a ultra-hydrophobic surface made of micro-posts. As expected, most of the particles lie inside a circular area, whose radius roughly corresponds to the Laplace pressure threshold for liquid impalement inside the structure (Cassie-Wenzel transition), inducing a coffee-stain deposit due to contact-line pinning. Less expected is the observation of tiny deposits on top of posts in the area external to the main ring, despite the low macroscopic liquid/solid friction. Experiments are carried out varying the concentration in particles and initial volume of drops, in order to determine the influence of these parameters on the size distribution of deposits. A microscopic insight of the tiny deposits is proposed, based on recent experiments of non-volatile liquid sliding drops.