Aurbach Franziska, Wang Fei, Nestler Britta
Institute of Applied Materials-Microstructure Modelling and Simulation, Karlsruhe Institute of Technology, Straße am Forum 7, 76131 Karlsruhe, Germany.
Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
J Chem Phys. 2025 Jul 21;163(3). doi: 10.1063/5.0275605.
This study extends the investigation of the wetting phenomenon to larger droplets on a smooth solid substrate, where gravitational effects become significant. We examine three distinct droplet geometries: a spherical cap, an oblate spheroidal cap, and a pancake shape, and we observe that the approximated droplet shape depends on the droplet size and the interfacial tensions. For small droplets on a smooth solid substrate, the droplet shape is assumed to be a spherical cap. However, as the droplet size increases, the equilibrium shape of the droplet becomes deformed due to the effect of gravity. The actual equilibrium contact angles, here referred to as microscopic contact angles, are determined by Young's law and are independent of droplet size or axisymmetric shape. Macroscopic contact angles are obtained by approximating the droplet shape with a geometric model and minimizing the free energy without assuming the validity of Young's law. We extend this concept to larger droplets and systematically compare these macroscopic angles with the microscopic ones. Our results demonstrate a correlation between the macroscopic contact angle and droplet size, with significant deviations from Young's law, increasing with droplet size. These findings are consistent with the Allen-Cahn model. Furthermore, we highlight the impact of the chosen measurement method on the accuracy of the contact angle, which varies with droplet size. In addition, the contact angle hysteresis-caused by varying liquid-solid and solid-gas interfacial tensions, which describes an intermolecular rearrangement of the liquid and gas species in an adsorption layer on the microscopic scale-is also influenced by the droplet size.
本研究将对润湿现象的研究扩展到光滑固体基底上的较大液滴，此时重力效应变得显著。我们研究了三种不同的液滴几何形状：球冠形、扁球形帽形和平饼形，并且观察到近似的液滴形状取决于液滴大小和界面张力。对于光滑固体基底上的小液滴，假定液滴形状为球冠形。然而，随着液滴尺寸增大，由于重力作用，液滴的平衡形状会发生变形。实际的平衡接触角，这里称为微观接触角，由杨氏定律确定，且与液滴大小或轴对称形状无关。宏观接触角是通过用几何模型近似液滴形状并在不假定杨氏定律有效性的情况下使自由能最小化而获得的。我们将这一概念扩展到更大的液滴，并系统地将这些宏观角度与微观角度进行比较。我们的结果表明宏观接触角与液滴大小之间存在相关性，与杨氏定律存在显著偏差，且偏差随液滴大小增加。这些发现与艾伦 - 卡恩模型一致。此外，我们强调了所选测量方法对接触角精度的影响，其随液滴大小而变化。另外，由液 - 固和固 - 气界面张力变化引起的接触角滞后，它描述了微观尺度上吸附层中液体和气体物种的分子间重排，也受液滴大小影响。
