Coating-free approaches to achieve liquid repellent, or omniphobic, surfaces could exploit inexpensive intrinsically wetting materials, such as polyethylene terephthalate and nylon, for applications such as liquid-vapor extraction and drag reduction. However, it is not clear whether the existing criteria for assessing coating-based omniphobicity, based on contact angles, would be reliable for coating-free approaches, especially considering localized defects/damages during manufacturing and usage. We assessed the omniphobicity of silica surfaces adorned with arrays of doubly reentrant pillars, cavities, and hybrid designs with sessile drops and on immersion in water and hexadecane through contact angle goniometry and confocal microscopy. We demonstrate that the assessment of omniphobicity of surfaces derived from intrinsically wetting materials can be misleading, if solely based on the measurement of contact angles. Specifically, localized defects in microtextures consisting of pillars may lead to the spontaneous loss of omniphobicity and detecting them through contact angles can be difficult. We also demonstrate that the immersion of those surfaces into probe liquids may serve as a simple and quick 'litmus' test for omniphobicity. Thus, immersion as the additional criterion for omniphobicity might prove itself useful in the context of large-scale manufacturing.

Superomniphobic surfaces, which repel droplets of polar and apolar liquids, are used for reducing frictional drag, packaging electronics and foods, and separation processes, among other applications. These surfaces exploit perfluorocarbons that are expensive, vulnurable to physical damage, and have a long persistence in the environment. Thus, new approaches for achieving superomniphobicity from common materials are desirable. In this context, microtextures comprising “mushroom-shaped” doubly reentrant pillars (DRPs) have been shown to repel drops of polar and apolar liquids in air irrespective of the surface make-up. However, it was recently demonstrated that DRPs get instantaneously infiltrated by the same liquids on submersion because while they can robustly prevent liquid imbibition from the top, they are vulnerable to lateral imbibition. Here, we remedy this weakness through bio-inspiration derived from cuticles of Dicyrtomina ornata, soil-dwelling bugs, that contain cuboidal secondary granules with mushroom-shaped caps on each face. Towards a proof-of-concept demonstration, we created a perimeter of biomimicking pillars around arrays of DRPs using a two-photon polymerization technique; another variation of this design with a short wall passing below the side caps was investigated. The resulting gas-entrapping microtextured surfaces (GEMS) robustly entrap air on submersion in wetting liquids, while also exhibiting superomniphobicity in air. To our knowledge, this is the first-ever microtexture that confers upon intrinsically wetting materials the ability to simultaneously exhibit superomniphobicity in air and robust entrapment of air on submersion. These findings should advance the rational design of coating-free surfaces that exhibit ultra-repellence (or superomniphobicity) towards liquids.

20 years since the first report on the biofouling potential of chemicals used for scale control, still, antiscalants with high bacterial growth potential are used in practice. Evaluating the bacterial growth potential of commercially available antiscalants is therefore essential for a rational selection of these chemicals. Previous antiscalant growth potential tests were conducted in drinking water or seawater inoculated with model bacterial species which do not represent natural bacterial communities. To reflect better on the conditions of desalination systems, we investigated the bacterial growth potential of eight different antiscalants in natural seawater and an autochthonous bacterial population as inoculum. The antiscalants differed strongly in their bacterial growth potential varying from ≤ 1 to 6 μg easily biodegradable C equivalents/mg antiscalant. The six phosphonate-based antiscalants investigated showed a broad range of growth potential, which depended on their chemical composition, whilst the biopolymer and the synthetic carboxylated polymers-based antiscalants showed limited or no significant bacterial growth. Moreover, nuclear magnetic resonance (NMR) scans enabled antiscalant fingerprinting, identifying components and contaminants, providing a rapid and sensitive characterization, and opening opportunities for rational selection of antiscalants for biofouling control.