Understanding the wettability of rough surfaces using simultaneous optical and electrochemical analysis of sessile droplets.

The interaction of a droplet with a solid surface is characterized by two parameters, the contact angle and the wetted area under the droplet. The Cassie-Baxter and the Wenzel modes make predictions on the interfacial area by comparing the contact angles on smooth surfaces (the intrinsic wettability) with those on rough surfaces (the apparent wettability). In these models, the actual wetted area is used as a fitting parameter. In this work, we highlight the significance of determining the actual wetted area under the droplet and the limitation of using only the contact angle to represent the wetting behavior of a surface. Our experimental studies were performed on hydrophilic carbon surfaces where a combination of optical measurements (contact angle and hysteresis) along with an electrochemical approach was employed. An electrochemical method was used to determine the true wetted area using a droplet of aqueous electrolyte on the surface. The interfacial area was then used to correlate wetting behavior to that of the model predictions. We examined the impact of electrolyte concentration and potential sweep rate in our evaluation of the wetted area. Our results show that, for a rough hydrophilic surface, the decrease in contact angles with increasing solid-liquid interfacial areas is not always valid, as generally predicted by the Wenzel and the Cassie-Baxter models.