Slippery Surfaces and Materials for Skin-Friction Drag Reduction

Over the past two decades, passive drag reduction technologies have emerged as promising solutions to enhance surface performance, offering advantages in efficiency, environmental compatibility, and long-term sustainability. The growing demand for integrated strategies that improve the performance of engineering systems has stimulated extensive research into innovative approaches to minimize energy consumption and mitigate environmental impact. Within the maritime sector, this need has become particularly urgent. In recent years, increasingly stringent international regulations have been introduced, reflecting a global awareness of environmental protection and the necessity to reduce emissions and fuel consumption. These regulations have intensified the pressure on researchers and industrial and technological development to implement advanced solutions that can be effectively integrated into existing infrastructures and current technologies. Consequently, significant efforts are being directed toward the adaptation and optimization of surface treatment methods and drag reduction techniques that are both compliant with modern regulatory standards. In this context, passive technologies represent a key area of investigation, as they offer the potential to achieve substantial performance improvements without additional energy input or complex system modifications. Inspired by the natural world and by the evolution of animals and plants over millions of years, researchers have increasingly investigated biological designs and mechanisms, drawing from them to develop and enhance technologies aimed at reducing drag. This field of study is referred to as biomimetics. Numerous natural systems highlight surface adaptations, demonstrating the relevance of fluid-surface interactions: the lotus leaf and the Coleoptera’s wings (e.g., lady beetles) exhibit pronounced water-repellent properties, thanks to its micro and nanostructure; the leading-edge structures of owls enable stabilized airflow and reduced aerodynamic noise; whale tubercles are known to delay stall, enhance lift, and reduce drag; the pitcher plant (Nepenthes) generates a highly slippery interface through the formation of a stable liquid–liquid boundary with an immiscible fluid; and shark skin, characterized by microstructured riblets-like, facilitates efficient motion through water by reducing frictional resistance. These examples, among many others, highlight the diversity of evolutionary solutions that inspire drag-reduction technologies. Starting from these biological designs, rough and regular patterned surfaces, superhydrophobic coatings, and liquid-impregnated materials have become a wide range of physical mechanisms and technical complexities to be investigated. This research aims to provide a comprehensive overview of slippery surfaces, progressing from idealized, regularly textured geometries to more realistic hierarchical materials, thereby enabling a deeper understanding and detailed analysis. The study focuses on small-scale phenomena occurring in the near-wall region, investigated through an integrated theoretical, numerical, and experimental approach. First, a research activity on riblet surfaces has been conducted, leading to the development of a novel model capable of describing the behavior of patterned walls under turbulent flow conditions beyond the viscous regime. In this context, a multiscale homogenization approach has been formulated to derive effective boundary conditions that capture the influence of surface roughness on the overlying flow. The predictive capability of the proposed model has to be subsequently assessed and validated through direct numerical simulations. Extending this framework, the investigation has been extended to more complex configurations through the design and implementation of a high-precision Taylor–Couette experimental apparatus. This facility enabled systematic hydrodynamic resistance measurements and flow visualization studies. Experiments have been conducted on riblet-textured surfaces, progressively modified through the application of superhydrophobic coatings and subsequent impregnation with lubricant oils. These investigations provided insight into the influence of surface functionalization on drag reduction as well as the onset of flow instabilities, such as the formation of Taylor-Couette vortical structures. Moreover, a linear stability analysis has been developed to theoretically and numerically define the critical parameters of the study, through the definition of effective slippery boundary conditions. Finally, the study has been further advanced toward realistic, complex, and anisotropic surfaces by addressing the critical issue of lubricant depletion. A custom-built mesoscale Taylor–Couette device has been designed, built, and then employed to investigate the progressive loss of lubricant and the associated degradation of surface performance. Despite its practical relevance, the underlying physical mechanisms governing lubricant depletion and the consequent loss of functionality remain only partially understood. This work, therefore, aims to provide a comprehensive analysis of lubricant loss processes, identifying the key governing parameters and describing their role in the long-term effectiveness of slippery surfaces.