The Liquid Young’s Law on SLIPS: Liquid–Liquid Interfacial Tensions and Zisman Plots

Abstract

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.

Utilizing Lubricant Loss from Omniphobic Coatings as a Multifunctional Delivery Mechanism

“…The MDA-MB-468 breast cancer cell line and HEK-293 cell line were used for the proliferation assays and accompanying microscopy images. The cells were cultured per standard protocol 21 before seeding on 48-well plates (Corning). Briefly, the MDA and HEK cells were cultured in RPMI1640 (Gibco) medium and Dulbecco's Modified Eagle Medium (Gibco), respectively, with 10% fetal bovine serum (FBS) and 1% penicillin−streptomycin (PS).…”

“…21 However, lubricant is depleted from exposure to the incoming liquids, primarily through the droplet sliding mechanisms of full or partial cloaking. 2,21 When the incoming liquid contacts the LIS, the droplet can be fully enveloped by the oil layer or partially cloaked along the contacting interface, which then allows the droplet to slide across the surface oil layer, taking the small quantity of lubricant as the droplet is removed. 2 Likewise, the lubricant layer can be gradually removed via diffusion when wholly immersed in liquids for extended periods or washed with suitable solvents or surfactants, e.g., ethanol, n-hexane, dichloromethane, etc.…”

“…The retention of lubricant is driven by the minimization of system energy, namely that the combination of the substrate (gel matrix or etched and chemically treated surface) and the incoming liquid results in greater system energy than the substrate and lubricant and is rejected . However, lubricant is depleted from exposure to the incoming liquids, primarily through the droplet sliding mechanisms of full or partial cloaking. , When the incoming liquid contacts the LIS, the droplet can be fully enveloped by the oil layer or partially cloaked along the contacting interface, which then allows the droplet to slide across the surface oil layer, taking the small quantity of lubricant as the droplet is removed . Likewise, the lubricant layer can be gradually removed via diffusion when wholly immersed in liquids for extended periods or washed with suitable solvents or surfactants, e.g., ethanol, n -hexane, dichloromethane, etc.…”

Engulfment of a drop on solids coated by thin and thick fluid films

“…Over the last few years, wetting on soft materials has emerged as a topical field in fluid mechanics, where pronounced deformations of the soft substrate near the contact line greatly affect the spreading dynamics (Semprebon, McHale & Kusumaatmaja 2017; Sadullah, Semprebon & Kusumaatmaja 2018; McHale et al. 2019, 2022; Andreotti & Snoeijer 2020). A drop contacting a substrate coated with an immiscible fluid is one type of soft wetting problem that has received far less attention (Smith et al.…”

“…These surfaces rely on a stable viscous fluid film to separate drops and other particles from the supporting solid, offering a relatively slippery, pressure-stable surface compared to their solid counterparts (Bohn & Federle 2004;Solomon et al 2016;Huang & Guo 2019;Semprebon et al 2021). Over the last few years, wetting on soft materials has emerged as a topical field in fluid mechanics, where pronounced deformations of the soft substrate near the contact line greatly affect the spreading dynamics (Semprebon, McHale & Kusumaatmaja 2017;Sadullah, Semprebon & Kusumaatmaja 2018;McHale et al 2019McHale et al , 2022Andreotti & Snoeijer 2020). A drop contacting a substrate coated with an immiscible fluid is one type of soft wetting problem that has received far less attention (Smith et al 2013;Daniel et al 2017Daniel et al , 2018.…”