Amphiphobic Nanostructured Coatings for Industrial Applications.

The search for surfaces with non-wetting behavior towards water and low-surface tension liquids affects a wide range of industries. Surface wetting is regulated by morphological and chemical features interacting with liquid phases under different ambient conditions. Most of the approaches to the fabrication of liquid-repellent surfaces are inspired by living organisms and require the fabrication of hierarchically organized structures, coupled with low surface energy chemical composition. This paper deals with the design of amphiphobic metals (AM) and alloys by deposition of nano-oxides suspensions in alcoholic or aqueous media, coupled with perfluorinated compounds and optional infused lubricant liquids resulting in, respectively, solid⁻liquid⁻air and solid⁻liquid⁻liquid working interfaces. Nanostructured organic/inorganic hybrid coatings with contact angles against water above 170°, contact angle with n-hexadecane (surface tension γ = 27 mN/m at 20 °C) in the 140⁻150° range and contact angle hysteresis lower than 5° have been produced. A full characterization of surface chemistry has been undertaken by X-ray photoelectron spectroscopy (XPS) analyses, while field-emission scanning electron microscope (FE-SEM) observations allowed the estimation of coatings thicknesses (300⁻400 nm) and their morphological features. The durability of fabricated amphiphobic surfaces was also assessed with a wide range of tests that showed their remarkable resistance to chemically aggressive environments, mechanical stresses and ultraviolet (UV) radiation. Moreover, this work analyzes the behavior of amphiphobic surfaces in terms of anti-soiling, snow-repellent and friction-reduction properties-all originated from their non-wetting behavior. The achieved results make AM materials viable solutions to be applied in different sectors answering several and pressing technical needs.

Is a Knowledge of Surface Topology and Contact Angles Enough to Define the Drop Impact Outcome?

It is well known that a superhydrophobic surface may not be able to repel impacting droplets because of the so-called Cassie-to-Wenzel transition. It has been proven that a critical value of the receding contact angle (θR) exists for the complete rebound of water, recently experimentally measured to be 100° for a large range of impact velocities. On the contrary, in the present work, no rebound was observed when low-surface-tension liquids such as hexadecane (σ = 27.5 mN/m at 25 °C) are concerned, even for very low impact velocities and very high values of θR and low contact angle hysteresis. Therefore, the critical threshold of θR ≈ 100° does not sound acceptable for all liquids and for all hydrophobic surfaces. For the same Weber numbers, a Cassie-to-Wenzel state transition occurs after the impact as a result of the easier penetration of low-surface-tension fluids in the surface structure. Hence, a criterion for the drop rebound of low-surface-tension liquids must consider not only the contact angle values with surfaces but also their surface tension and viscosity. This suggests that, even if it is possible to produce surfaces with enhanced static repellence against oils and organics, generally the realization of synthetic materials with self-cleaning and antisticking abilities in dynamic phenomena, such as spray impact, remains an unsolved task. Moreover, it is demonstrated that the chemistry of the surface, the physicochemical interactions with the liquid drops, and the possible wettability gradient of the surface asperity also play important roles in determining the critical Weber number above which impalement occurs. Therefore, the classical numerical simulations of drop impact on dry surfaces are definitively not able to capture the final outcomes of the impact for all possible fluids if the surface topology and chemistry and/or the wettability gradient in the surface structure are not properly reflected.

A Mechanistic Explanation of the Peculiar Amphiphobic Properties of Hybrid Organic-Inorganic Coatings by Combining XPS Characterization and DFT Modeling.

We report a combined X-ray photoelectron spectroscopy and theoretical modeling analysis of hybrid functional coatings constituted by fluorinated alkylsilane monolayers covalently grafted on a nanostructured ceramic oxide (Al2O3) thin film deposited on aluminum alloy substrates. Such engineered surfaces, bearing hybrid coatings obtained via a classic sol-gel route, have been previously shown to possess amphiphobic behavior (superhydrophobicity plus oleophobicity) and excellent durability, even under simulated severe working environments. Starting from XPS, SEM, and contact angle results and analysis, and combining it with DFT results, the present investigation offers a first mechanistic explanation at a molecular level of the peculiar properties of the hybrid organic-inorganic coating in terms of composition and surface structural arrangements. Theoretical modeling shows that the active fluorinated moiety is strongly anchored on the alumina sites with single Si-O-Al bridges and that the residual valence of Si is saturated by Si-O-Si bonds which form a reticulation with two vicinal fluoroalkylsilanes. The resulting hybrid coating consists of stable rows of fluorinated alkyl chains in reciprocal contact, which form well-ordered and packed monolayers.