Ambient-mediated wetting on smooth surfaces

A Scalable Methodology for Reinstating the Superhydrophilicity of Ambient-Contaminant Compromised Surfaces

The degradation of the hemiwicking property of superhydrophilic high-energy surfaces due to contaminant adsorption from the ambient atmosphere is well-documented. This degradation compromises the performance of such surfaces, thus affecting their efficacy in real-world applications where hemiwicking is critical. In this work, the role of surface micro- and nanostructure morphology of laser-textured metallic surfaces on superhydrophilicity degradation is studied. We explore intrinsic contact angle variations of superhydrophilic surfaces via the adsorption of organics from the surroundings, which brings about the associated changes in surface chemistry. Furthermore, we explore condensation from humid air as a scalable and environmentally friendly methodology that can reinstate surface superhydrophilicity to a considerable extent (64% recovery of intrinsic wettability after 3 h of condensation) due to the efficient removal of physisorbed contaminants from the surface texture features. The present results strengthen the argument that contact-line movement at fine scales can be used for depinning and removal of adsorbed organic molecules from contaminated surfaces.

Wetting Behavior of Zn Droplets on Fe Surfaces: Insights from Molecular Dynamics Simulations

The liquid metal embrittlement (LME) induced by Zn melts in advanced high strength steels has seriously hindered their wide application in various industries. Microscopically, wetting is the precursor for LME; it is therefore crucial to understand the wetting of Zn melts on Fe surfaces. Molecular dynamics simulations were conducted to investigate the wetting behavior of Zn droplets on Fe(001), Fe(110), and Fe(111) surfaces from both thermodynamics and dynamics aspects. The simulation results reveal that the surface energy of solid Fe is significantly greater than the surface tension of liquid Zn and the interfacial energy of Fe-Zn solid-liquid interface at the pertinent temperatures. Consequently, Zn droplets tend to completely envelop the Fe substrates as they spread toward the equilibrium state. Specifically, Fe(111) surfaces possess the highest surface energy, whereas Fe(110) surfaces have the lowest surface energy. Meanwhile, the solid-liquid interfacial energy is minimal for Fe(111)/Zn and maximal for Fe(110)/Zn. These differences contribute to the strongest spreading driving force for Zn droplets on Fe(111) surfaces and the weakest on Fe(110). During the initial spreading stage, Zn droplets form precursor films on all Fe surfaces. Nonetheless, on Fe(111), the dissolution reaction between the substrates and the droplets destabilizes the precursor films, ultimately resulting in complete wetting. Conversely, no dissolution is observed between Zn droplets and the Fe(001) or Fe(110) surface. As a result, the equilibrium Zn droplet consists of a prefreezing precursor film that grows epitaxially on the substrate and a main body of the droplet exhibiting a convex hull shape corresponding to pseudopartial wetting. These findings provide new insights into the wetting behavior of metal droplets on metal surfaces, particularly for understanding the liquid metal embrittlement induced by Zn melts in steels.

Slippery liquid-like surfaces as a promising solution for sustainable drag reduction

Drag reduction is crucial for many industries, ranging from aerospace to microfluidics, to enhance the energy efficiency and reduce costs. This work is the first to study drag reduction enabled by novel slippery liquid-like surfaces fabricated from flexible polymers. We experimentally characterized the drag reduction performance of slippery liquid-like surfaces in the laminar flow regime. Our results indicate that liquid-like surfaces can reduce fluid drag regardless of surface wettability and have achieved nearly 20% drag reduction. Furthermore, the durability tests show that these surfaces can maintain slipperiness over a month when exposed to air or water and the drag reduction capability for at least one week under a fluid flow. These findings highlight the potential of slippery liquid-like surfaces as a promising solution for sustainable drag reduction.

Mask-Enabled Topography Contrast on Aluminum Surfaces

Patterned solid surfaces with wettability contrast can enhance liquid transport for applications such as electronics thermal management, self-cleaning, and anti-icing. However, prior work has not explored easy and scalable blade-cut masking to impart topography patterned wettability contrast on aluminum (Al), even though Al surfaces are widely used for thermal applications. Here, we demonstrate mask-enabled topography contrast patterning and quantify the resulting accuracy of the topographic pattern resolution, spatial variations in surface roughness, wettability, drop size distribution during dropwise condensation, and thermal emissivity of patterned Al surfaces. The method uses blade-cut vinyl mask templates and a commercially available lacquer resin that serves as a polymer resist against etching. Programmable mask templates enable complex patterning of wettability and emissivity contrast with feature sizes down to ∼1.5 mm. As-fabricated patterned samples show a water contact angle (θ) contrast from <5° to 80° between etched and smooth zones, while patterned samples that are further coated with a hydrophobic promoter show θ contrast between 150° and 120° on etched and smooth zones, respectively. In addition to measuring this wettability contrast via contact angle goniometry, we use condensation visualization experiments to study the spatially controlled condensate morphologies and drop size distributions. These condensation studies demonstrate enhanced droplet shedding on the superhydrophobic regions of striped patterned surfaces compared to homogeneous superhydrophobic surfaces. Motivated by the role of thermal radiation in many phase change processes, we use infrared thermography to map topography-mediated thermal emissivity (ε) contrast between etched (ε ≈ 0.65) and smooth (ε ≈ 0.26) regions. Thus, our study provides a route for researchers to readily create complex and scalable topography-patterned Al surfaces for potential applications in vapor chamber thermal rectification, radiative cooling condensation heat transfer, and high-temperature Leidenfrost or film boiling processes.

Ultrahigh Subcooling Dropwise Condensation Heat Transfer on Slippery Liquid-like Monolayer Grafted Surfaces

Rapid and continuous droplet shedding is crucial for many applications, including thermal management, water harvesting, and microfluidics, among others. Superhydrophobic surfaces, though effective, suffer from droplet pinning at high subcooling temperature (Tsub). Conversely, slippery liquid-like surfaces covalently bonded with flexible hydrophobic molecules show high stability and low droplet adhesion attributed to their dense and ultrasmooth water repellent polymer chains, enhancing dropwise condensation and rapid shedding. In this work, linear poly(dimethylsiloxane) chains of various viscosities are covalently bonded onto silicon substrates to form thin and smooth monolayer coated surfaces. The formation of the monolayer is characterized by cryogenic transmission electron microscopy. On these surfaces a very low contact angle hysteresis is reported within wide surface temperature ranges as well as continuous dropwise condensation at ultrahigh Tsub of 60 K. In particular, one of the highest condensation heat fluxes of 1392.60 kW·m-2 and a heat transfer coefficient of 23.21 kW·m-2·K-1 at ultrahigh Tsub of 60 K is reported. The experimental heat transfer performance is further compared to the theoretical heat transfer via the individual droplets with the droplet distribution elucidated via both macroscopic observations as well as environmental scanning electron microscopy. Finally, only a mild decrease in the heat transfer coefficient of 20.3% after 100 h of condensation test at Tsub of 60 K is reported. Slippery liquid-like surfaces promote droplet shedding and sustain dropwise condensation at high Tsub without flooding empowered by the lower frictional forces, addressing challenges in heat transfer performance and durability.

Fundamentals and Applications of Surface Wetting

In an era defined by an insatiable thirst for sustainable energy solutions, responsible water management, and cutting-edge lab-on-a-chip diagnostics, surface wettability plays a pivotal role in these fields. The seamless integration of fundamental research and the following demonstration of applications on these groundbreaking technologies hinges on manipulating fluid through surface wettability, significantly optimizing performance, enhancing efficiency, and advancing overall sustainability. This Review explores the behavior of liquids when they engage with engineered surfaces, delving into the far-reaching implications of these interactions in various applications. Specifically, we explore surface wetting, dissecting it into three distinctive facets. First, we delve into the fundamental principles that underpin surface wetting. Next, we navigate the intricate liquid-surface interactions, unraveling the complex interplay of various fluid dynamics, as well as heat- and mass-transport mechanisms. Finally, we report on the practical realm, where we scrutinize the myriad applications of these principles in everyday processes and real-world scenarios.