Temporal Evolution of Surface Contamination under Ultra-high Vacuum

Attaining Tailored Wicking Behavior with Additive Manufacturing

Additive manufacturing (AM) has opened a new pathway to create customized wicking materials. With lower manufacturing costs and a larger design space than many alternatives for wicking, AM is of particular value in fields such as thermal management and microfluidics. Fluid propagation during wicking in porous media, however, has largely remained limited to Washburnian (t ) behavior, and optimizing these materials for wicking in a variety of use cases presents a challenge. In this work, we present a method of tailoring wicking behavior to an arbitrary target function of propagation distance versus time, achieved through the use of AM to create nonuniform porous materials. Layers of parallel lines, each successive layer rotated 90° from the last, form a gridded structure with a spatially varying unit cell size for which analytical models for the capillary pressure and solid fraction and a semianalytical model for permeability were found. These models were validated with capillary rise experiments for spatially uniform porous materials over a range of solid fractions from 0.4 to 0.9. Leveraging these models and representing a nonuniform porous material as a series of Ohmic fluidic resistors, we created an inverse design algorithm that generates a wicking material with spatially varying parameters to achieve a specified target function for fluid propagation as a function of time. These materials can exhibit atypical wicking behavior, including fluid propagation displaying simple linear and piecewise linear relationships with time rather than the conventional Washburn relationship.

Conversion of Layered WS2 Crystals into Mixed-Domain Electrochemical Catalysts by Plasma-Assisted Surface Reconstruction

Electrocatalytic water splitting is crucial to generate clean hydrogen fuel, but implementation at an industrial scale remains limited due to dependence on expensive platinum (Pt)-based electrocatalysts. Here, an all-dry process to transform electrochemically inert bulk WS2 into a multidomain electrochemical catalyst that enables scalable and cost-effective implementation of the hydrogen evolution reaction (HER) in water electrolysis is reported. Direct dry transfer of WS2 flakes to a gold thin film deposited on a silicon substrate provides a general platform to produce the working electrodes for HER with tunable charge transfer resistance. By treating the mechanically exfoliated WS2 with sequential Ar-O2 plasma, mixed domains of WS2, WO3, and tungsten oxysulfide form on the surfaces of the flakes, which gives rise to a superior HER with much greater long-term stability and steady-state activity compared to Pt. Using density functional theory, ultraefficient atomic sites formed on the constituent nanodomains are identified, and the quantification of atomic-scale reactivities and resulting HER activities fully support the experimental observations.

Surface (bio)-functionalization of metallic materials: How to cope with real interfaces?

Metallic materials are an important class of biomaterials used in various medical devices, owing to a suitable combination of their mechanical properties. The (bio)-functionalization of their surfaces is frequently performed for biocompatibility requirements, as it offers a powerful way to control their interaction with biological systems. This is particularly important when physicochemical processes and biological events, mainly involving proteins and cells, are initiated at the host-material interface. This review addresses the state of "real interfaces" in the context of (bio)-functionalization of metallic materials, and the necessity to cope with it to avoid frequent improper evaluation of the procedure used. This issue is, indeed, well-recognized but often neglected and emerges from three main issues: (i) ubiquity of surface contamination with organic compounds, (ii) reactivity of metallic surfaces in biological medium, and (iii) discrepancy in (bio)-functionalization procedures between expectations and reality. These disturb the assessment of the strategies adopted for surface modifications and limit the possibilities to provide guidelines for their improvements. For this purpose, X-ray photoelectrons spectroscopy (XPS) comes to the rescue. Based on significant progresses made in methodological developments, and through a large amount of data compiled to generate statistically meaningful information, and to insure selectivity, precision and accuracy, the state of "real interfaces" is explored in depth, while looking after the two main constituents: (i) the bio-organic adlayer, in which the discrimination between the compounds of interest (anchoring molecules, coupling agents, proteins, etc) and organic contaminants can be made, and (ii) the metallic surface, which undergoes dynamic processes due to their reactivity. Moreover, through one of the widespread (bio)-functionalization strategy, given as a case study, a particular attention is devoted to describe the state of the interface at different stages (composition, depth distribution of contaminants and (bio)compounds of interest) and the mode of protein retention. It is highlighted, in particular, that the occurrence or improvement of bioactivity does not demonstrate that the chemical schemes worked in reality. These aspects are particularly essential to make progress on the way to choose the suitable (bio)-functionalization strategy and to provide guidelines to improve its efficiency.

Ambient-mediated wetting on smooth surfaces

A consensus was built in the first half of the 20th century, which was further debated more than 3 decades ago, that the wettability and condensation mechanisms on smooth solid surfaces are modified by the adsorption of organic contaminants present in the environment. Recently, disagreement has formed about this topic once again, as many researchers have overlooked contamination due to its difficulty to eliminate. For example, the intrinsic wettability of rare earth oxides has been reported to be hydrophobic and non-wetting to water. These materials were subsequently shown to display dropwise condensation with steam. Nonetheless, follow on research has demonstrated that the intrinsic wettability of rare earth oxides is hydrophilic and wetting to water, and that a transition to hydrophobicity occurs in a matter of hours-to-days as a consequence of the adsorption of volatile organic compounds from the ambient environment. The adsorption mechanisms, kinetics, and selectivity, of these volatile organic compounds are empirically known to be functions of the substrate material and structure. However, these mechanisms, which govern the surface wettability, remain poorly understood. In this contribution, we introduce current research demonstrating the different intrinsic wettability of metals, rare earth oxides, and other smooth materials, showing that they are intrinsically hydrophilic. Then we provide details on research focusing on the transition from wetting (hydrophilicity) to non-wetting (hydrophobicity) on somooth surfaces due to adsorption of volatile organic compounds. A state-of-the-art figure of merit mapping the wettability of different smooth solid surfaces to ambient exposure as a function of the surface carbon content has also been developed. In addition, we analyse recent works that address these wetting transitions so to shed light on how such processes affect droplet pinning and lateral adhesion. We then conclude with objective perspectives about research on wetting to non-wetting transitions on smooth solid surfaces in an attempt to raise awareness regarding this surface contamination phenomenon within the engineering, interfacial science, and physical chemistry domains.

Mitigating Contamination with Nanostructure-Enabled Ultraclean Storage

Airborne hydrocarbon contamination hinders nanomanufacturing, limits characterization techniques, and generates controversies regarding fundamental studies of advanced materials; consequently, we urgently need effective and scalable clean storage techniques. In this work, we propose an approach to clean storage using an ultraclean nanotextured storage medium as a getter. Experiments show that our proposed approach can maintain surface cleanliness for more than 1 week and can even passively clean initially contaminated samples during storage. We theoretically analyzed the contaminant adsorption-desorption process with different values of storage medium surface roughness, and our model predictions showed good agreement with experiments for smooth, nanotextured, and hierarchically textured surfaces, providing guidelines for the design of future clean storage systems. The proposed strategy offers a promising approach for portable and cost-effective storage systems that minimize hydrocarbon contamination in applications requiring clean surfaces, including nanofabrication, device storage and transportation, and advanced metrology.

The Surface Energy of Hydrogenated and Fluorinated Graphene

The surface energy of graphene and its chemical derivatives governs fundamental interfacial interactions like molecular assembly, wetting, and doping. However, quantifying the surface energy of supported two-dimensional (2D) materials, such as graphene, is difficult because (1) they are so thin that electrostatic interactions emanating from the underlying substrate are not completely screened, (2) the contribution from the monolayer is sensitive to its exact chemical state, and (3) the adsorption of airborne contaminants, as well as contaminants introduced during transfer processing, screens the electrostatic interactions from the monolayer and underlying substrate, changing the determined surface energy. Here, we determine the polar and dispersive surface energy of bare, fluorinated, and hydrogenated graphene through contact angle measurements with water and diiodomethane. We accounted for many contributing factors, including substrate surface energies and combating adsorption of airborne contaminants. Hydrogenating graphene raises its polar surface energy with little effect on its dispersive surface energy. Fluorinating graphene lowers its dispersive surface energy with a substrate-dependent effect on its polar surface energy. These results unravel how changing the chemical structure of graphene modifies its surface energy, with applications for hybrid nanomaterials, bioadhesion, biosensing, and thin-film assembly.

A universal approach to recover the original superhydrophilicity of micro/nano-textured metal or metal oxide surfaces

Micro/nano-textured metal or metal oxide surfaces that are naturally superhydrophilic will spontaneously transform into hydrophobic even superhydrophobic after being exposed to ambient air due to the adsorption of airborne organics. This fast wettability transition not only affects the true evaluation of surface wettability but also deteriorates the application performance. Albeit the mechanisms responsible for the wettability transition have been clarified, there is no universal method to recover the initial superhydrophilicity, and how the surface morphology affects the wettability transition is still unclear. Herein, we observe and compare the wettability transition of a wide variety of micro/nano-textured metal or metal oxide surfaces and propose a solvent cleaning method to recover their original superhydrophilicity. We prove that the spontaneously adsorbed organics can be removed by our proposed cleaning method while maintaining the original surface morphology and composition. Our proposed cleaning method is valid for both micro/nano-textured metal and metal oxide surfaces. We also prove that the rate of the wettability transition is not primarily affected by the specific area of surface structures but by the closeness of structural arrangement. Densely packed surface nanostructures can significantly delay the wettability transition by suppressing the diffusion of organic molecules. Our results help the true evaluation of surface wettability and provide a route for the design and preparation of long-lasting superhydrophilic surfaces.