Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India.
Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India.
J Colloid Interface Sci. 2020 Apr 1;565:582-591. doi: 10.1016/j.jcis.2020.01.056. Epub 2020 Jan 17.
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 (θ0) surrounded by a superhydrophobic surface (θ160). We study the shape evolution (contact angle (θ) and contact line diameter (d)) of an aqueous droplet placed over a horizontal SHL - SHB surface with its volume (V), using experiments and analytical modeling. The results showed that depending upon the Bond number (Bo) and spot diameter (d), three different regimes: spherical cap with fixed d and varying θ (Regime I), oblate spheroid with fixed d and varying θ (Regime II), and oblate spheroid with varying d and fixed θ (Regime III), are observed. The transition from Regime I to Regime II occurs for Bo1 whereas that from Regime II to Regime III occurs at Bo0.33d. 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) and gravitational force (F) governs its stability. The stable and unstable regimes are identified based on the Bond number (Bo) and spot diameter (d) and the critical Bond number for stable - unstable transition depends on spot diameter as Bo~0.5d.
具有图案的超亲水-超疏水（SHL-SHB）表面在基于液滴的生化分析中显示出了前景。然而，对于液滴在这种图案化表面上的行为，人们并没有给予太多关注。在这里，我们报告了在超亲水斑点（θ0）上放置的液滴的润湿动力学和稳定性，该斑点被超疏水表面（θ160）包围。我们通过实验和分析模型研究了水平 SHL-SHB 表面上液滴的形状演化（接触角（θ）和接触线直径（d））与其体积（V）之间的关系。结果表明，取决于邦数（Bo）和斑点直径（d），可以观察到三种不同的状态：具有固定 d 且θ变化的球形帽（状态 I）、具有固定 d 且θ变化的扁球形（状态 II）以及具有固定θ且 d 变化的扁球形（状态 III）。从状态 I 到状态 II 的转变发生在 Bo1 时，而从状态 II 到状态 III 的转变发生在 Bo0.33d 时。对本案例的分析表明，由于接触线位于 SHL-SHB 表面的边界上，因此与 Wenzel、Cassie-Baxter 和 McCarthy 的陈述存在异常。此外，还通过实验和分析模型研究了在 SHL-SHB 角状表面上放置在超亲水斑点上的液滴的稳定性，结果表明，接触线固定力（F）和重力（F）之间的竞争决定了其稳定性。基于邦数（Bo）和斑点直径（d）确定了稳定和不稳定状态，并且稳定-不稳定转变的临界邦数取决于斑点直径，即 Bo~0.5d。
