Effect of Surface Hydrophobicity on Critical Pinning Concentration of Nanoparticles To Trigger the Coffee Ring Formation during the Evaporation Process of Sessile Drops of Nanofluids

The evaporation process of sessile drops of water/nanofluid on hydrophilic/hydrophobic surfaces was observed experimentally. The evaporation process of nanofluid drops generally consists of four stages in the following sequence: (1) the constant contact radius mode, (2) the constant contact angle mode, (3) the mixed mode, and (4) the second pinned mode. The contact angle of nanofluid drops at the occurrence of the second pinning is consistently lower than the receding contact angle of the surface. The critical pinning concentration, at which the contact line is pinned again to initiate the second pinned mode, is independent of the initial concentration of silica nanoparticles. The critical pinning concentration increases along with the surface hydrophobicity, which can be identified by the receding contact angle of the surface. The critical pinning concentration is linearly dependent on the receding contact angle. It is interesting to find out that there exists a critical receding contact angle of 33 degrees in this study. A droplet of nanofluid containing silica nanoparticles placed on self-assembled silane monolayer coated silicon wafer with its receding contact angle smaller than the critical receding contact angle (33 degrees) would always pin immediately to trigger the coffee ring formation.