Tzitzilis Dimitrios, Tsekeridis Charalampos, Ntakoumis Ioannis, Papadopoulos Periklis
Department of Physics, University of Ioannina, 45110 Ioannina, Greece.
University Research Center of Ioannina, Institute of Materials Science and Computing, Ioannina 45110, Greece.
Langmuir. 2024 Jul 2;40(26):13422-13427. doi: 10.1021/acs.langmuir.4c00618. Epub 2024 Jun 2.
Low adhesion of liquids on solid surfaces can be achieved with protrusions that minimize the contact area between the liquid and the solid. The wetting state where an air cushion forms under the drop is known as the Cassie-Baxter state. Surfaces where liquids form macroscopic contact angles above 150° are called superhydrophobic and superhygrophobic, if we refer to water or any liquid, respectively. The Cassie state is desirable for applications, but it is usually unstable compared to the Wenzel state, where the drop is in direct contact with the rough surface. The Cassie-to-Wenzel transition can be triggered by an increase in pressure and vibrations, but the inverse Wenzel-to-Cassie is much more difficult to observe. Here, we examine under what conditions the Wenzel-to-Cassie transition is triggered when the microscopic contact angle changes abruptly. For this, we applied a lubricant of low surface tension around drops that were in the Wenzel state on microstructured surfaces. The increase of the microscopic contact angle lifted the drop from the rough surface, when the pillar height and spacing are large and small, respectively. Numerical calculations for the drop-lubricant interface showed that the surface geometry requirements for the Wenzel-to-Cassie transition are stricter than the ones for the stability of the Cassie state. A surface geometry where the Cassie state is more stable than the Wenzel for a given Laplace pressure of the drop may not always allow the Wenzel-to-Cassie transition to take place. Therefore, the stability of the Cassie state is a necessary but insufficient condition for the inverse transition.
通过使液体与固体之间的接触面积最小化的凸起，可以实现液体在固体表面上的低附着力。液滴下方形成气垫的润湿状态被称为卡西 - 巴克斯特状态。如果分别针对水或任何液体而言，液体形成大于150°的宏观接触角的表面被称为超疏水和超憎水表面。卡西状态对于应用来说是理想的，但与液滴直接接触粗糙表面的文策尔状态相比，它通常不稳定。卡西到文策尔的转变可以由压力增加和振动触发，但反向的文策尔到卡西转变则更难观察到。在这里，我们研究当微观接触角突然变化时，在什么条件下文策尔到卡西的转变会被触发。为此，我们在微结构表面上处于文策尔状态的液滴周围施加了低表面张力的润滑剂。当柱体高度和间距分别较大和较小时，微观接触角的增加会使液滴从粗糙表面抬起。液滴 - 润滑剂界面的数值计算表明，文策尔到卡西转变的表面几何要求比卡西状态稳定性的要求更严格。对于给定的液滴拉普拉斯压力，卡西状态比文策尔状态更稳定的表面几何结构可能并不总是允许文策尔到卡西的转变发生。因此，卡西状态的稳定性是反向转变的必要但不充分条件。
