title: Wetting Mechanisms on Silicone Surfaces with Liquid, Frozen, and Vaporous Water
Creators: name: Hauer, Lukas
Publisher: name: UNSPECIFIED
Year: 2023

description: Silicone coatings show low contact friction, water repellency, and high mobility of wetting drops - features that are necessary for self-cleaning, lubrication, anti-icing/-biofouling, drag-reduction, and enhanced heat and mass transfer. For this, silicone coatings are found in numerous fields, such as life science, aerospace, automotive, and electronic industry. Silicones comprise polymeric chains; the most abundant silicone is polydimethylsiloxane (PDMS). PDMS is benign, environmentally friendly, safe, and easy to process, while its material properties are highly tunable. In the following, three distinct PDMS coatings are considered, which are distinguished by chain reticulation: PDMS oil coatings (mobile chains), PDMS elastomer coatings (crosslinked chains), and PDMS chain coatings (single-grafted chains). Contacting drops (and solids, too) induce shape and/or composition adaptations on the PDMS coating. Such wetting-induced coating responses alter the contact friction temporally and may cause irreversible structural degradation, over time. When phase change (condensation/sublimation/frosting) is involved the coating response (contact friction and surface deterioration) can be different. Understanding the coating responses to imposed wetting scenarios (e.g. forced wetting or phase change) is pivotal for smart coating designs with optimized and lasting surface functionality. Gaining insight, however, is challenging as the surface responses scatter over orders of magnitude on time and length scales, a multitude of physical and chemical concepts are involved, and the wetting interplayers are highly coupled. This brings even modern experimental and numerical methods to their limits and the full spectrum of surface responses is still elusive. In this cumulative dissertation, I explore wetting-associated mechanisms (i.e., drop sliding, condensation, frosting, and sorption) on PDMS coatings (i.e., oil, elastomer). I utilize micro- and macroscopic visualization (e.g. confocal microscopy) of different wetting scenarios. I focus on non-equilibrium wetting processes (e.g. drop sliding or frost formation) to explore the dynamic response of the PDMS coating. I use continuum mechanical and thermodynamical concepts to model experimental observations analytically and numerically. This complementary approach delivers a fundamental understanding of the wetting interactions on a theoretical level, supported by experimental evidence. This understanding allows coating and application to be matched, providing optimized surface functionality. In cases of mismatches, the surface may malfunction and degrade over time. In particular, I illustrate this with moving drops on PDMS elastomers: fast drop movement yields high surface dissipation, thus, poor drop mobility. However, slow movement yields strong surface deterioration as the coating material is (partially) entrained by the drop. The optimal operation point is met at intermediate sliding speeds, where surface dissipation and deterioration are low. In another demonstration, I show this optimal operation point for PDMS oil coatings under frost conditions: when frost forms fast and spiky, and the oil retention on the surface is poor, rapid oil depletion is unavoidable. The optimal operating point is found in warmer, very dry, or very humid atmospheres while the surface is equipped with nanometric scaffold structures to increase oil retention.