Characterizing water repellency indices: Contact angle and water drop penetration time of hydrophobized sand

Abstract The accuracy and the reproducibility of water repellency measurements can be improved with the use of proper measurement techniques. The purpose of this study is to compare the contact angles obtained using three different methods and to examine the relationship between contact angle and water drop penetration time (WDPT) using non-repellent to extremely repellent sands. Fine silica sand, artificially hydrophobized with stearic acid, was used for the measurements. Contact angles were estimated using the molarity of an ethanol droplet (MED) test, the capillary rise method (CRM) and the sessile drop method (SDM). The CRM was effective for soils with contact angles < 90°. The MED test was suitable for soils with contact angles ≥ 90° using 10 s as the ethanol drop penetration time. The SDM was effective for soils with contact angles ranging from very low to very high (11–105°). Directly measured contact angles using the SDM were in good agreement with indirectly obtained contact angles using the MED test and the CRM. The WDPT was < 1 s for contact angles ranging from 11 to 69°, gradually increased from 69 to 93°, and exceeded 3600 s beyond that. The WDPT was most sensitive for contact angles within the range of 88–93°. Extreme repellency was observed where the solid surface free energy was ≤ 45 mN m−1.