Fluorine- and Silicon-Free Water- and Oil-Repellent Surfaces based on a New Density-Control Concept

What is it about?

In this paper, we suggest a new approach to controlling the wetting behavior of liquids on solid surfaces by modulating the material’s density, rather than relying on the conventional strategies of altering chemical species and/or surface topography. Conventional techniques typically involve chemical methods that require fluorine or silicon, or physical methods that introduce surface roughness to reduce liquid wettability. A well-known example is the water-repellent behavior observed on lotus leaves. However, research and development based on these conventional approaches have a long history, and solving the remaining challenges increasingly requires fundamentally different material concepts. In particular, growing concerns over PFAS have imposed significant restrictions on fluorine-based methods.

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Why is it important?

We revisited the fact that wetting phenomena are strongly governed by van der Waals forces, which constitute the origin of the attractive interactions between solids and liquids. The van der Waals force of a substance is determined by its chemical composition and by how densely those chemical units are packed within a unit volume. Therefore, even when the chemical composition remains the same, reducing the number density should weaken the van der Waals force and consequently lower liquid wettability. To realize this concept, we employed polyurethane, a type of polymer. Taking advantage of the restricted mobility of chain-like polymers near the surface, we hypothesized that a low-density region could be formed. Indeed, the our engineered polyurethane exhibits high water repellency despite containing highly polar urethane groups, and it also demonstrates excellent oil repellency despite the majority of its molecular backbone being composed of hydrocarbon segments that typically show strong affinity with oils.

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