Comparative analysis of the dynamic contact angles for two types of superhydrophobic layered double hydroxide film surfaces

The wetting properties of two types of superhydrophobic layered double hydroxide (LDH) films modified by treatment with laurate (La) anions, NiAl–CO32-–LDH/La film and ZnAl–La–LDH film, have been investigated using the sessile drop and Wilhelmy plate methods. The spreading kinetics of water droplets on ZnAl–La–LDH film surfaces was fitted by a power equation, and an effective lubrication coefficient was calculated. Unlike one-cycle measurements reported in a number of earlier studies, cycling contact angle measurements using the Wilhelmy plate method provide more information about the mechanism of contact angle hysteresis. Both the advancing contact angles (θa) and the receding contact angles (θr) of NiAl–CO32-–LDH/La film—in which the La anions are adsorbed on the surface of the LDH—obtained in different cycles were found to be independent of cycle number. In contrast, for the ZnAl–La–LDH film—in which the La anions are intercalated into the interlayer galleries of the LDH—the advancing contact angles (θa) and the receding contact angles (θr) obtained in different cycles were found to vary. It was found that both θa and θr decreased with increasing number of cycles. The different contact angle hysteresis behaviors were qualitatively and quantitatively investigated by means of dynamic contact angle analysis. The results suggest that contact angle hysteresis of NiAl–CO32-–LDH/La and ZnAl–La–LDH films can be ascribed to dynamic hysteresis and intrinsic hysteresis, respectively.