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 (θ~0°) surrounded by a superhydrophobic surface (θ~160°). We study the shape evolution (contact angle (θ) and contact line diameter (dc)) of an aqueous droplet placed over a horizontal SHL - SHB surface with its volume (Vd), using experiments and analytical modeling. The results showed that depending upon the Bond number (Bo) and spot diameter (ds), three different regimes: spherical cap with fixed dc and varying θ (Regime I), oblate spheroid with fixed dc and varying θ (Regime II), and oblate spheroid with varying dc and fixed θ (Regime III), are observed. The transition from Regime I to Regime II occurs for Bo~1 whereas that from Regime II to Regime III occurs at Bocr~0.33ds1.30. 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 (Fp) and gravitational force (Fg) governs its stability. The stable and unstable regimes are identified based on the Bond number (Bo) and spot diameter (ds) and the critical Bond number for stable - unstable transition depends on spot diameter as Bocr~0.5ds-0.93.