Graphene, an atomically thin two-dimensional material, has received significant attention due to its extraordinary electronic, optical, and mechanical properties. Studies focused on understanding the wettability of graphene for thermo-fluidic and surface-coating applications, however, have been sparse. Meanwhile, wettability results reported in literature via static contact angle measurement experiments have been contradictory and highlight the lack of clear understanding of the underlying physics that dictates wetting behavior. In this work, dynamic contact angle measurements and detailed graphene surface characterizations were performed to demonstrate that the defects present in CVD grown and transferred graphene coatings result in unusually high contact angle hysteresis (16-37°) on these otherwise smooth surfaces. Hence, understanding the effect of the underlying substrate based on static contact angle measurements as reported in literature is insufficient. The advancing contact angle measurements on mono-, bi-, and trilayer graphene sheets on copper, thermally grown silica (SiO2), and glass substrates were observed to be independent of the number of layers of graphene and in good agreement with corresponding molecular dynamics simulations and theoretical calculations. Irrespective of the number of graphene layers, the advancing contact angle values were also in good agreement with the advancing contact angle on highly ordered pyrolytic graphite (HOPG), reaffirming the negligible effect of the underlying substrate. These results suggest that the advancing contact angle is a true representation of a graphene-coated surface while the receding contact angle is significantly influenced by intrinsic defects introduced during the growth and transfer processes. These observations, where the underlying substrates do not affect the wettability of graphene coatings, is shown to be due to the large interlayer spacing resulting from the loose interlamellar coupling between the graphene sheet and the underlying substrate. The fundamental insights on graphene-water interactions reported in this study is an important step towards developing graphene-assisted surface coatings for heat transfer and microfluidics devices.
Understanding the complexities associated with contact line dynamics on chemically heterogeneous and superhydrophobic surfaces is important for a wide variety of engineering problems. Despite significant efforts to capture the behavior of a droplet on these surfaces over the past few decades, modeling of the complex dynamics at the three-phase contact line is needed. In this work, we demonstrate that contact line distortion on heterogeneous and superhydrophobic surfaces is the key aspect that needs to be accounted for in the dynamic droplet models. Contact line distortions were visualized and modeled using a thermodynamic approach to develop a unified model for contact angle hysteresis on chemically heterogeneous and superhydrophobic surfaces. On a surface comprised of discrete wetting defects on an interconnected less wetting area, the advancing contact angle was determined to be independent of the defects, while the relative fraction of the distorted contact line with respect to the baseline surface was shown to govern the receding contact angle. This behavior reversed when the relative wettability of the discrete defects and interconnected area was inverted. The developed model showed good agreement with the experimental advancing and receding contact angles, both at low and high solid fractions. The thermodynamic model was further extended to demonstrate its capability to capture droplet shape evolution during liquid addition and removal in our experiments and those in literature. This study offers new insight extending the fundamental understanding of solid-liquid interactions required for design of advanced functional coatings for microfluidics, biological, manufacturing, and heat transfer applications.
Understanding liquid dynamics on surfaces can provide insight into nature's design and enable fine manipulation capability in biological, manufacturing, microfluidic and thermal management applications. Of particular interest is the ability to control the shape of the droplet contact area on the surface, which is typically circular on a smooth homogeneous surface. Here, we show the ability to tailor various droplet contact area shapes ranging from squares, rectangles, hexagons, octagons, to dodecagons via the design of the structure or chemical heterogeneity on the surface. We simultaneously obtain the necessary physical insights to develop a universal model for the three-dimensional droplet shape by characterizing the droplet side and top profiles. Furthermore, arrays of droplets with controlled shapes and high spatial resolution can be achieved using this approach. This liquid-based patterning strategy promises low-cost fabrication of integrated circuits, conductive patterns and bio-microarrays for high-density information storage and miniaturized biochips and biosensors, among others.
The subnanometer pore structure of zeolites and other microporous materials has been proposed to act as a molecular sieve for various water separation technologies. However, due to the increased interaction between the solid and water in these nanoconfined spaces, it is unclear which type of interface, be it hydrophilic or hydrophobic, offers an advantageous medium for enhancing transport properties. In this work, we probe the role of hydrophilic defects on the transport of water inside the microporous hydrophobic MFI zeolite pore structure via combined sorption and high-pressure infiltration experiments. While the inclusion of defects was observed to increase the amount of water within the zeolite pore network by up to 7 times at the saturation pressure, the diffusivity of this infiltrated water was lowered by up to 2 orders of magnitude in comparison to that of water within the nearly defect-free hydrophobic MFI zeolite. Subsequently, the permeability of water within the more defective MFI zeolite was an order of magnitude lower than that of the nearly defect-free zeolite. The results from these experiments suggest that the intrinsic hydrophobic pore structure of MFI zeolites can facilitate faster water transport due to the decreased attraction between the water and the defect-free surface. While the strong attraction of water to the defects allows for water to infiltrate the porous network at lower pressures, the results suggest that this strong attraction decreases the mobility of the infiltrated water. The insights gained from this study can be utilized to improve the design of future membranes for water desalination and other separation techniques.
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