Questions and Answers on the Wettability of Nano‐Engineered Surfaces

Abstract

Surface roughening has long been used to confer materials remarkable wetting properties, such as super hydrophobicity. When roughness belongs to the microscale, existing models can, to some extent, explain and predict real‐world wetting states. With the advent of nanotechnology, however, a multitude of nano‐engineered materials are being developed and unexpected wetting behaviors have been observed which cannot be described by conventional theories. While it is clear that advanced nanotextured materials are essential to a vast range of ground breaking technologies, the intricate mechanisms governing the wetting of such surfaces—at the limit of validity of continuum theories—also need to be clarified. This review aims at presenting what is known and what remains to be discovered about the wettability of nanoengineered materials. The advantages and uniqueness of nanostructured substrates is highlighted first, with a focus on practical applications benefitting from the distinctive wetting properties of nanorough substrates. Classic wetting theories and their limitations are briefly discussed, before describing with a top down vision, why the wetting of nanostructured substrates differ from that of microtextured surfaces. Recent experimental and theoretical studies which contributed to the progress in this field are considered, before outlining potential future areas of research.

Micro‐/Nanostructured Interface for Liquid Manipulation and Its Applications

“…Here, r is the roughness factor, which is defined as the ratio of the actual surface area to the geometric area, θ* and θ refer to the apparent CAs of a liquid droplet on the rough substrate and its corresponding flat substrate, respectively …”

“…Two important factors including micro‐/nanoscale hierarchical structures and materials with suitable surface energy are co‐related to the construction of superhydrophobic surface. There are two approaches to construct superhydrophobic surface, the first one is combining multiple‐scale roughness with inherently hydrophobic coatings, and the second one is the combining of re‐entrant geometry with hydrophilic surfaces . For example, Zhang et al fabricated CuO nanoneedle‐covered copper surfaces by anodization and then chemical modification with fluoroalkyl‐silane (FAS‐17) ( Figure a).…”

“…Here, r is the roughness factor, which is defined as the ratio of the actual surface area to the geometric area, θ* and θ refer to the apparent CAs of a liquid droplet on the rough substrate and its corresponding flat substrate, respectively. [19] For a rough surface, r is usually larger than 1, which means micro-/nanostructures can amplify the hydrophobicity or hydrophilicity depending on the intrinsic nature of the surface. For flat substrates, the wettability is largely determined by the chemical composition of the surface.…”

“…There are two approaches to construct superhydrophobic surface, the first one is combining multiple-scale roughness with inherently hydrophobic coatings, and the second one is the combining of re-entrant geometry with hydrophilic surfaces. [19,138] For example, Zhang et al fabricated CuO nanoneedle-covered copper surfaces by anodization and then chemical modification with fluoroalkyl-silane (FAS-17) (Figure 5a). The surfaces exhibited superhydrophobicity, which provided a promising interface for metal anticorrosion.…”

“…Although it is common to marvel at these remarkable phenomena in nature, it wasn't until the invention of high‐resolution microscopy did people start to study and reveal the mechanisms underlying all these phenomena . With a closer view of these creatures, it is realized that the morphology, or the microscale and nanoscale interfacial structure, is usually responsible for the unconventional interactions between liquid and the solid surfaces . When it comes to the micro‐ and nanoscales, the interfacial structures can exhibit properties that cannot be observed at the macroscale, such as (super)wettability, which is the driving factor for liquid manipulation on some surfaces …”