Thermodynamic analysis on wetting states and wetting state transitions of rough surfaces

Elucidating the lotus and rose-petal effects on hierarchical surfaces: Study of the effect of topographical scales on the contact angle hysteresis

In nature, superhydrophobicity is almost systematically associated with a multiscale topography. Nevertheless, multiscale-textured natural surfaces can either produce water-repellent properties such as on the sacred lotus leaf or high liquid-to-solid adhesion such as on the rose petal. To conceive bio-inspired surfaces with self-cleaning properties, the proper contributions of each topographical scale to the wetting behavior need to be investigated. Conditions for the equilibrium of menisci produced at a given topographical scale are derived, yielding a recursion relation between each topographical scale. We introduce the equilibrium anchorage depth to quantify the penetration of water at equilibrium. To study the contact angle hysteresis (CAH), we thoroughly describe the mechanisms driving the advancing and receding motions of the triple line. Both phenomena depend on what we define as precursor advancing and receding motions. Eventually, the equilibrium, advancing and receding anchorage depths are related to the CAH. Topographical heterogeneities at a topographical subscale i are always associated with a reduced equilibrium anchorage depth and an enhanced robustness at all topographical scales of higher orders of magnitude. Eventually, it is demonstrated that advancing and receding anchorage depths are bounded by the equilibrium anchorage depth, elucidating how rose-petal-like surfaces systematically produce a high CAH.

The Challenge of Superhydrophobicity: Environmentally Facilitated Cassie-Wenzel Transitions and Structural Design

Superhydrophobic materials can be used in various fields to optimize production and life due to their unique surface wetting properties. However, under certain pressure and perturbation conditions, the droplets deposited on superhydrophobic materials are prone to change from Cassie state to Wenzel state, which limits the practical applications of the materials. In recent years, a large number of works have investigated the transition behavior, transition mechanism, and influencing factors of the wetting transition that occurs when a superhydrophobic surface is under a series of external environments. Based on these works, in this paper, the phenomenon and kinetic behavior of the destruction of the Cassie state and the mechanism of the wetting transition are systematically summarized under external conditions that promote the wetting transition on the material surface, including pressure, impact, evaporation, vibration, and electric wetting. In addition, superhydrophobic surface morphology has been shown to directly affect the duration of the Cassie state. Based on the published work the effects of specific morphology on the Cassie state, including structural size, structural shape, and structural level, are summarized in this paper from theoretical analyses and experimental data.

Theoretical Investigation of Wenzel and Cassie Wetting States on Porous Films and Fiber Meshes

In this study, the wetting phenomenon on a thin, porous film covering a substrate was investigated on the basis of a classical equilibrium theory. The equilibrium contact angles of the Wenzel, hydrophobic Cassie, and hydrophilic Cassie states and the transition points between them were derived as functions of parameters, such as the porosity and specific surface area of the porous film. These expressions were applied to describe wetting on fiber mats/meshes. The equilibrium contact angles and transition points of the three wetting states on the fiber mesh covering the substrate are expressed as functions of parameters, such as fiber radius, roughness factor, and volume porosity of the fiber mesh. A fiber mesh can attain superhydrophobicity when it is composed of thin hydrophobic fibers with surface roughness and has an appropriate volume porosity.

The Reduction in the Deformation of HDPE Composites Using Self-Lubricating Fillers in an Aqueous Environment

Reducing the deformation of polymer matrix materials can decrease the fluctuation of coefficient of friction (COF), and friction-induced vibration and its amplitudes. HDPE composites with T-ZnOw as a fixed strengthening filler were modified with the addition of Si₃N₄ particles at different concentrations. The COFs, wear rates, micro-morphologies, and friction-induced vibrations were obtained by conducting sliding tests against carbon steel balls in an aqueous environment at a low velocity and high load. The mechanism of the reduction in frictional fluctuation due to the addition of Si₃N₄ particles was revealed through the frictional responses. The results demonstrated that 4 wt% addition of Si₃N₄ in HDPE can enhance the strain–stress property and improve the lubrication by forming a lubricating film. Therefore, the surface deformation and the fluctuations of COFs and its vibrations were reduced. The aggregation phenomenon and reduced strain–stress response at a high concentration of Si₃N₄ disrupted the positive fluctuating reduction, and resulted in a rough surface with severe tearing and cracking deformations. Additionally, it led to fluctuating wear behaviors with high COF and vibrations. The results obtained in this study can elucidate the effects of adding Si₃N₄ particles to enhance lubrication in polymer composites. Additionally, the results provide a new research method for designing and manufacturing polymer-based composites with low friction-induced fluctuations.

Influence of sterilization on the performance of anodized nanoporous titanium implants

Towards clinical translation of bioactive nano-engineered titanium implants, achieving appropriate sterilization and understanding its influence on the modified implant characteristics is essential. With limited studies exploring the influence of sterilization techniques on electrochemically anodized titanium with TiO₂ nanostructures, we aimed to advance this domain by performing an in-depth evaluation of the influence of common sterilization techniques (ethanol immersion, various UV irradiation times, gamma irradiation, and dry/wet autoclaving) on TiO₂ nanopores fabricated on micro-rough Ti surfaces (dual micro-nano) via single step anodization. Various sterilized surfaces were systematically compared in terms of topographical, chemical, crystalline, wettability and mechanical characteristics. Next, we investigated the protein adhesion capacity and functions of primary gingival fibroblasts (proliferation, adhesion/alignment and spreading morphology) to compare the bioactivity of the sterilized nanopores. Ethanol immersion, gamma irradiation and UV irradiation conserved the topography of the fabricated nanopores, while autoclave sterilization (both dry and wet) compromised the nanoporous structures. Various duration of UV-sterilization resulted in no significant changes in the surface topography and chemistry of the fabricated TNPs. Our findings revealed that UV irradiation is the most appropriate technique to sterilize nano-engineered titanium implants for appropriate wettability, protein adhesion capacity and enhanced metabolism and proliferation of human gingival fibroblasts (hGFs). This study systematically investigated the influence of sterilization on anodized nano-engineered titanium implants towards achieving reproducible outcomes (in terms of topography, chemistry and bioactivity), and found that UV irradiation holds great promise for application across different nano-engineered metal surfaces.

Innovative fouling-resistant materials for industrial heat exchangers: a review

Fouling of heat exchangers (HEs) has become a major concern across the industrial sector. Fouling is an omnipresent phenomenon but is particularly prevalent in the dairy, oil, and energy industries. Reduced energy performance that results from fouling represents significant operating loss in terms of both maintenance and impact on product quality and safety. In most industries, cleaning or replacing HEs are currently the only viable solutions for controlling fouling. This review examines the latest advances in the development of innovative materials and coatings for HEs that could mitigate the need for costly and frequent cleaning and potentially extend their operational life. To better understand the correlation between surface properties and fouling occurrence, we begin by providing an overview of the main mechanisms underlying fouling. We then present selected key strategies, which can differ considerably, for developing antifouling surfaces and conclude by discussing the current trends in the search for ideal materials for a range of applications. In our presentation of all these aspects, emphasis is given wherever possible to the potential transfer of these innovative surfaces from the laboratory to the three industries most concerned by HE fouling problems: food, petrochemicals, and energy production.

Robust superhydrophobicity: mechanisms and strategies

Superhydrophobic surfaces hold great prospects for extremely diverse applications owing to their water repellence property. The essential feature of superhydrophobicity is micro-/nano-scopic roughness to reserve a large portion of air under a liquid drop. However, the vulnerability of the delicate surface textures significantly impedes the practical applications of superhydrophobic surfaces. Robust superhydrophobicity is a must to meet the rigorous industrial requirements and standards for commercial products. In recent years, major advancements have been made in elucidating the mechanisms of wetting transitions, design strategies and fabrication techniques of superhydrophobicity. This review will first introduce the mechanisms of wetting transitions, including the thermodynamic stability of the Cassie state and its breakdown conditions. Then we highlight the development, current status and future prospects of robust superhydrophobicity, including characterization, design strategies and fabrication techniques. In particular, design strategies, which are classified into passive resistance and active regeneration for the first time, are proposed and discussed extensively.

Free-energy landscapes of intrusion and extrusion of liquid in truncated and inverted truncated conical pores: Implications for the Cassie-Baxter to Wenzel transition

As the simplest model of transition between the superhydrophobic Cassie-Baxter (CB) and Wenzel (W) states of a macroscopic droplet sitting on a microscopically rough or corrugated substrate, a substrate whose surface is covered by identical truncated or inverted truncated conical pores is considered. The free-energy landscapes of the intrusion and extrusion processes of a liquid into single pore are analyzed when the liquid is compressed or stretched so that the liquid phase is either stable or metastable relative to the vapor phase. Therefore, this model is also relevant to the stability of the superhydrophobic submerged substrates. In this study, the macroscopic classical capillary theory is adopted. Even within this simplified model, two simple geometries of truncated and inverted truncated cones lead to completely different free-energy landscapes. A simple criterion for the stability of the CB state based on Laplace pressure is shown not to be sufficient to understand the destruction and recovery of the CB state. The free-energy landscapes indicate that a gradual and an abrupt destruction of CB state is possible, which depends on the orientation of the conical pore and whether the liquid is compressed or stretched. The extensions of these theoretical results to more complex geometries are briefly discussed.