Relative humidity effects on contact angle and water drop penetration time of hydrophobized fine sand

The purpose of this study was to determine the effects of ambient relative humidity (RH) on contact angle and water drop penetration time (WDPT) using a series of sand samples hydrophobized with stearic acid. The contact angle was estimated using the sessile drop method. The contact angle increased first sharply and then slightly with increasing stearic acid content up to about 5.0 g kg(-1). The contact angle did not change with increasing stearic acid content above 5.0 g kg(-1). The contact angle increased with increasing RH from 33 to 94%. The RH did not affect the contact angle of samples with a stearic acid content above 5.0 g kg(-1), where the particles were considered to be completely coated by hydrophobic organic material. The contact angle increased with increasing RH in sands partially coated with hydrophobic material, which might have resulted from an increase in adsorbed water molecules on the remaining high-energy mineral surfaces. The WDPT increased with increasing RH. The higher the RH, the lower the stearic acid content at which extreme repellency (WDPT >= 3600 s) appeared. Samples with a contact angle below 75 degrees or with surface free energy above 72 mN m(-1) (equivalent to the surface tension of water) were non-repellent (WDPT < 1 s). Samples with a contact angle above 90 degrees or with surface free energy below 50 mN m(-1) were extremely repellent. There was no effect of RH on the relationship between WDPT and contact angle (or surface free energy), indicating that the effect of RH was comparable on both measurements.