WETTING PROPERTIES AND AIR ENTRAPMENT AT SUPERHYDROPHOBIC SURFACES IN N-ALKANOLS SOLUTIONS

From self-cleaning materials to microfluidic devices many applications can exploit the wetting characteristic of nature inspired surfaces showing contact angle above 150° and a very small hysteresis (superhydrophobic-SH). The influence of short chain n-alcohols on wetting properties of superhydrophobic surface was investigated by measuring the advancing and receding contact angles using a sessile drop technique. The superhydrophobic surfaces used in the study were prepared by a mixed inorganic-organic coating. In order to check how short chain surface active agents can affect surface energy of such surfaces their wettability (static conditions) and kinetics of the three phase contact formation (dynamic conditions) were studied. The contact angle variation with respect to the surface tension of the solutions has shown that the decrease in the contact angle values is mainly due to preferential adsorption of the surfactant molecules at liquid/vapour interface. Adsorption at the solid/liquid interface also takes place, but it is a secondary effect due to the limited area available for surfactant molecules to interact with the surface coating. As high affinity to air is a characteristic feature of the SH surfaces so air can be entrapped (inside cavities) during the SH surface immersion into aqueous phase and affect the the kinetics of the three phase contact (TPC) formation by the colliding bubble. It was observed that kinetics of TPC formation was strongly dependent on amount of the gas entrapped at SH surface by different immersion modes. The presence of "air pockets", due to non-homogenous wetting, facilitated the bubble attachment upon collison over the SH surface within milliseconds. Acknowledgement. The study was initiated within the COST P21 and has been continued within the COST D43 Action. Corresponding author: m.ferrari@ge.ieni.cnr.it