Slippery liquid-infused porous surfaces (SLIPS) are an innovation that reduces droplet-solid contact line pinning and interfacial friction. Recently, it has been shown that a liquid analogue of Young’s law can be deduced for the apparent contact angle of a sessile droplet on SLIPS despite there never being contact by the droplet with the underlying solid. Since contact angles on solids are used to characterize solid–liquid interfacial interactions and the wetting of a solid by a liquid, it is our hypothesis that liquid–liquid interactions and the wetting of a liquid surface by a liquid can be characterized by apparent contact angles on SLIPS. Here, we first present a theory for deducing liquid–liquid interfacial tensions from apparent contact angles. This theory is valid irrespective of whether or not a film of the infusing liquid cloaks the droplet–vapor interface. We show experimentally that liquid–liquid interfacial tensions deduced from apparent contact angles of droplets on SLIPS are in excellent agreement with values from the traditional pendant drop technique. We then consider whether the Zisman method for characterizing the wettability of a solid surface can be applied to liquid surfaces created using SLIPS. We report apparent contact angles for a homologous series of alkanes on Krytox-infused SLIPS and for water–IPA mixtures on both the Krytox-infused SLIPS and a silicone oil-infused SLIPS. The alkanes on the Krytox-infused SLIPS follow a linear relationship in the liquid form of the Zisman plot provided that the effective droplet–vapor interfacial tension is used. All three systems follow a linear relationship on a modified Zisman plot. We interpret these results using the concept of the critical surface tension (CST) for the wettability of a solid surface introduced by Zisman. In our liquid surface case, the obtained critical surface tensions were found to be lower than the infusing liquid–vapor surface tensions.

中文翻译：

---

SLIPS 上的液体杨氏定律：液-液界面张力和 Zisman 图

光滑的液体注入多孔表面 (SLIPS) 是一项创新，可减少液滴-固体接触线的钉扎和界面摩擦。最近，已经表明，尽管液滴从未与下面的固体接触，但对于 SLIPS 上固着液滴的表观接触角，可以推断出杨氏定律的液体类似物。由于固体上的接触角用于表征固-液界面相互作用和液体对固体的润湿，因此我们假设液-液相互作用和液体对液体表面的润湿可以通过表观接触角来表征在 SLIPS 上。在这里，我们首先提出了一种从表观接触角推导出液-液界面张力的理论。无论注入液体的薄膜是否覆盖液滴-蒸汽界面，该理论都是有效的。我们通过实验表明，从 SLIPS 上液滴的表观接触角推导出的液-液界面张力与传统悬滴技术的值非常一致。然后，我们考虑是否可以将用于表征固体表面润湿性的 Zisman 方法应用于使用 SLIPS 创建的液体表面。我们报告了注入 Krytox 的 SLIPS 上的同源系列烷烃的表观接触角，以及注入 Krytox 的 SLIPS 和注入硅油的 SLIPS 上的水-IPA 混合物的表观接触角。如果使用有效液滴-蒸汽界面张力，注入 Krytox 的 SLIPS 上的烷烃在 Zisman 图的液体形式中遵循线性关系。所有三个系统都遵循修改后的 Zisman 图上的线性关系。我们使用 Zisman 引入的固体表面润湿性临界表面张力 (CST) 的概念来解释这些结果。在我们的液体表面情况下，发现获得的临界表面张力低于注入的液体 - 蒸汽表面张力。