Journal of Colloid and Interface Science, Vol.565, 582-591, 2020
Understanding wetting dynamics and stability of aqueous droplet over superhydrophilic spot surrounded by superhydrophobic surface
Patterned superhydrophilic-superhydrophobic (SHL - SHB) surfaces have shown promise in droplet-based biochemical assays. However, fundamental understanding of the behavior of liquid droplets on such patterned surfaces has not received much attention. Here, we report wetting dynamics and stability of an aqueous droplet placed over a superhydrophilic spot (theta - 0 degrees) surrounded by a superhydrophobic surface (theta similar to 160 degrees). We study the shape evolution (contact angle (theta) and contact line diameter (d(c))) of an aqueous droplet placed over a horizontal SHL - SHB surface with its volume (V-d), using experiments and analytical modeling. The results showed that depending upon the Bond number (Bo) and spot diameter (d(s)), three different regimes: spherical cap with fixed d(c) and varying theta (Regime I), oblate spheroid with fixed d(c) and varying theta (Regime II), and oblate spheroid with varying d(c) and fixed theta (Regime III), are observed. The transition from Regime I to Regime II occurs for Bo similar to 1 whereas that from Regime II to Regime III occurs at Bo(CT) similar to 0.33d(s)(1.30). Analysis of the present case wherein the contact line lies at the boundary of SHL - SHB surfaces, revealed anomaly with respect to the statements of Wenzel, Cassie-Baxter and McCarthy. Further, the stability of a droplet placed over the superhydrophilic spot on an SHL - SHB angular surface is studied using experiments and analytical modeling, which showed that the competition between contact line pinning force (F-p) and gravitational force (F-g) governs its stability. The stable and unstable regimes are identified based on the Bond number (Bo) and spot diameter (d(s)) and the critical Bond number for stable - unstable transition depends on spot diameter as Bo(CT) similar to 0.5d(s)(-0.93). (C) 2020 Elsevier Inc. All rights reserved.