Liquid-solid nano-interactions of ceramic and metals

Wetting behaviour of a low surface tension fluid as hydrophobic paraffin oil on metal and ceramic surfaces was studied at nano-scale by SPFM non contact probe analysis and at micro-scale by optical microscopy. Solid surfaces were obtained by sputtering molybdenum and aluminum over silicon wafer substrates. Bottom-up method based on vapor condensation technique was used to deposit the fluid upon the evaluated solid surfaces. Force-Distance curves showed that liquid initially disposed over all the substrates as an extremely thin, full coating, film with an apparent variable depth depending upon the evaluated solid substrates. This layer appeared to be extremely adherent and showed the features of a gel instead behaving as a proper liquid. By going on with the fluid condensation process and increasing the total amount of deposited fluid we observed the formation of nanodrops lying on and coexisting with the previously deposited film. Drop contact angles values were found characteristic of the specific solid substrate and appeared indeed to be inversely proportional to the first deposited film thickness. The fluid deposition processes were prolonged up to reach micrometric sized dimensions where the gel-like features disappeared and typical liquid features appeared. In order to reveal the status of the original nanometric liquid layer several micrometric sized drops were therefore partially or totally removed from the substrate with AFM tips. It was found that no apparent mixing occurred between these two different conditions of the same deposited fluid. To deeply explore the features of this ice-like early deposited film and to detect if the observed behavior should be more dependent upon the surface roughness or surface energetic the same samples underwent fluid vapor exposures after being treated with low surface energy silanes that created a monolayer coating. These surfaces showed a completely different behaviour. A huge formation of very similar nano-drops was imm- ediately observed. The performed analysis shows that at the nano-scale the Young Equation does not apply satisfactorily, even in the case of extremely flat inert surfaces in contact with an inert and non polar liquid featured as a quite common basis for biological applications.