Forming nanostructured surfaces through Janus colloidal silica particles with nanowrinkles: A new strategy to superhydrophobicity

Proper nano-structuring coupled with low surface energy properties is exploited to obtain super-hydrophobic surfaces. A biomimicking hierarchically structured coating upon a material surface is obtained by depositing Janus nano-structured wrinkled colloidal particles. The morphology of the surface is thus ruled by different scales: the size of the wrinkles (few tens of nanometer), particle radius (several hundreds of nanometer) and interparticle distance. Janus wrinkled silica particles have one side exposing hydroxyl groups while the other side is partially silanized with dichlorodimethylsilane. To obtain these Janus particles, wax colloidosomes with surface covered by starting rough particles were successfully prepared. The exposed surface of rough wrinkled silica particles was then silanized, thus obtaining a Janus structure. The Janus character was prooved in different manners: the simple so called "visual test", through Energy Filtered Transmission Electron Microscopy (was proven in different EFTEM) and by further functionalization with polypropylene grafted maleic anhydride (PPgMA). A glass microscope slide was covered by drop casting the particles obtaining super-hydrophobic features of the surface characterized by a high water contact angle (149 degrees) and, more interestingly, by a very low water contact angle hysteresis (CAH = 2 degrees) and roll-off angle (ROA = 1.8 degrees). Superhydrophobicity is confirmed by the very low values of apparent surface free energy estimated with the OWRK (Owens-Wendt-Rabel e Kaelble) method. SEM and AFM analyses prove the multiple scale hierarchical roughness, from nano to micro size, that resembles the one present at the surface of the lotus leaves.