Wang Yanbin, Andrews Joseph Eugene, Hu Liangbing, Das Siddhartha
Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA.
Phys Chem Chem Phys. 2017 Jun 7;19(22):14442-14452. doi: 10.1039/c7cp01777d.
In this study, we employ molecular dynamics (MD) simulations to probe the spreading of a drop on a superhydrophobic (SH) surface. The SH surface consists of nanopillars and the drop spreads while being in the Cassie-Baxter (CB) state on the nanopillared surface. Most remarkably, unlike the spreading on non-SH surfaces, we witness that the spreading on SH surfaces is not dominated by the motion of the three-phase contact line (TPCL). Rather, the TPCL remains pinned at the edge of a nanopillar and the spreading is ensured by the liquid surface or the liquid-vapor interface (of this pinned TPCL) bending down and wetting the solid adjacent to the TPCL. Such bending may actually enforce a progressive temporal increase in the instantaneous local contact angle eventually making it equal to or more than 180°. This is in sharp contrast to the classical spreading dynamics, where, with the spreading being dictated by the TPCL motion, the local contact angle always decreases with time. We carry out simulations where the solids supporting the nanopillars have vastly different wettabilities; however, this principle of bending-driven spreading is invariably witnessed. In fact, given the recent experimental study on the rolling of drops on SH surfaces manifesting exactly identical liquid-surface-bending-driven drop motion, we can infer that regardless of the drop size (e.g., nanoscopic or millimetric) or the nature of drop motion (spreading or rolling), the motion of drops in the CB state on SH surfaces is universally driven by the bending of liquid surfaces and not by the motion of the TPCL.
在本研究中，我们采用分子动力学（MD）模拟来探究液滴在超疏水（SH）表面上的铺展情况。该超疏水表面由纳米柱组成，液滴在纳米柱表面处于 Cassie-Baxter（CB）状态时发生铺展。最值得注意的是，与在非超疏水表面上的铺展不同，我们观察到在超疏水表面上的铺展并非由三相接触线（TPCL）的运动主导。相反，三相接触线固定在纳米柱的边缘，铺展是通过液体表面或（该固定三相接触线的）液-气界面向下弯曲并润湿三相接触线附近的固体来实现的。这种弯曲实际上可能会使瞬时局部接触角随时间逐渐增大，最终使其等于或大于180°。这与经典的铺展动力学形成鲜明对比，在经典铺展动力学中，铺展由三相接触线的运动主导，局部接触角总是随时间减小。我们进行了模拟，其中支撑纳米柱的固体具有截然不同的润湿性；然而，这种弯曲驱动铺展的原理总是能观察到。事实上，鉴于最近关于液滴在超疏水表面上滚动的实验研究表明存在完全相同的液-表面-弯曲驱动的液滴运动，我们可以推断，无论液滴大小（例如，纳米级或毫米级）或液滴运动的性质（铺展或滚动）如何，超疏水表面上处于CB状态的液滴运动普遍由液体表面的弯曲驱动，而不是由三相接触线的运动驱动。
