Characterization of the Intrinsic Water Wettability of Graphite Using Contact Angle Measurements: Effect of Defects on Static and Dynamic Contact Angles

An Amphiphilic Surface with Improved Thermal Radiation for Water Harvesting

Water scarcity poses a significant challenge for people living in arid areas. Despite the effectiveness of many bioinspired surfaces in promoting vapor condensation, their water-harvesting efficiency is insufficient. This is often exacerbated by overheating, which decreases the performance in terms of the micro-droplet concentration and movement on surfaces. In this study, we used a spotted amphiphilic surface to enhance the surfaces' water-harvesting efficiency while maintaining their heat emissivity. Through hydrophilic particle screening and hydrophobic groove modifying, the coalescence and sliding characteristics of droplets on the amphiphilic surfaces were improved. The incorporation of boron nitride (BN) nanoparticles further enhanced the surfaces' ability to harvest energy from condensation. To evaluate the water-harvesting performance of these amphiphilic surfaces, we utilized a real-time recording water-harvesting platform to identify microscopic weight changes on the surfaces. Our findings indicated that the inclusion of glass particles in hydrophobic grooves, combined with 1.0 wt.% BN nanoparticles, enhanced the water-harvesting efficiency of the amphiphilic surfaces by more than 20%.

Trace polymer coated clarithromycin spherulites: Formation mechanism, improvement in pharmaceutical properties and development of high-drug-loading direct compression tablets

Clarithromycin (CLA) is a high dose antibiotic drug exhibiting poor flowability and tabletability, making the tablet development challenging. This study aims to develop spherulitic CLA by introducing trace amount of polymer in crystallization solution. Its formation mechanism, physicochemical properties and potential for the direct compression (DC) tablets development were also investigated. Morphological analyses and the in situ observation on crystallization process revealed that the CLA spherulites are formed by fractal branching growth from both sides of the threadlike precursor fibers. 1H NMR analysis and nucleation time monitoring indicated that the existence of hydroxypropyl cellulose in solution slowed down the crystal nucleation and growth rate by forming hydrogen bonding interactions with CLA molecules, making the system maintain high supersaturation, providing high driving forces for CLA spherulitic growth. In comparison to commercial CLA, the CLA spherulites exhibit profoundly improved flowability, tabletability and dissolution behaviors. XPS, contact angle and Raman mapping analysis confirmed the presence of a thin HPC layer on the surfaces and interior of CLA spherulitic particles, resulting in increasing powder plasticity, interparticulate bonding strength and powder wettability, thus better tabletability and dissolution performances. The improved flowability and tabletability of CLA spherulites also enabled the successful development of DC tablet formulation with a high CLA loading (82.8 wt%) and similar dissolution profiles to reference listed drug. This study provides a novel solid form of CLA with superior manufacturability for further development.

Understanding the Intrinsic Water Wettability of Hexagonal Boron Nitride

The water wettability of hexagonal boron nitride (hBN) has attracted a lot of research interest in the past 15 years. Experimentally, the static water contact angle (WCA) has been widely utilized to characterize the intrinsic water wettability of hBN. In the current study, we have investigated the effect of airborne hydrocarbons and defects on both static and dynamic WCAs of hBN. Our results showed that the static WCA is impacted by defects, which suggests that previously reported static WCAs do not characterize the intrinsic water wettability of hBN since the state-of-the-art hBN samples always have relatively high defect density. Instead, we found that the advancing WCA of freshly exfoliated hBN is not affected by the defects and airborne hydrocarbons. As a result, the advancing WCA on freshly exfoliated hBN, determined to be 79 ± 3°, best represents the intrinsic water wettability of hBN. A qualitative model has been proposed to describe the effect of airborne hydrocarbons and defects on the static and dynamic WCA of hBN, which is well supported by the experimental results. The finding here has important implications for the water wettability of 2D materials.

Water Electric Field Induced Modulation of the Wetting of Hexagonal Boron Nitride: Insights from Multiscale Modeling of Many-Body Polarization

Understanding the behavior of water contacting two-dimensional materials, such as hexagonal boron nitride (hBN), is important in practical applications, including seawater desalination and energy harvesting. Water, being a polar solvent, can strongly polarize the hBN surface via the electric fields that it generates. However, there is a lack of molecular-level understanding about the role of polarization effects at the hBN/water interface, including its effect on the wetting properties of water. In this study, we develop a theoretical framework that introduces an all-atomistic polarizable force field to accurately model the interactions of water molecules with hBN surfaces. The force field is then utilized to self-consistently describe the water-induced polarization of hBN using the classical Drude oscillator model, including predicting the hBN-water binding energies which are found to be in excellent agreement with diffusion Monte Carlo (DMC) predictions. By carrying out molecular dynamics (MD) simulations, we demonstrate that the polarizable force field yields a water contact angle on multilayered hBN which is in close agreement with the recent experimentally reported values. Conversely, an implicit modeling of the hBN-water polarization energy utilizing a Lennard-Jones (LJ) potential, a commonly utilized approximation in previous MD simulation studies, leads to a considerably lower water contact angle. This difference in the predicted contact angles is attributed to the significant energy-entropy compensation resulting from the incorporation of polarization effects at the hBN-water interface. Our work highlights the importance of self-consistently modeling the hBN-water polarization energy and offers insights into the wetting-related interfacial phenomena of water on polarizable materials.

Elastocapillarity-driven 2D nano-switches enable zeptoliter-scale liquid encapsulation

Biological nanostructures change their shape and function in response to external stimuli, and significant efforts have been made to design artificial biomimicking devices operating on similar principles. In this work we demonstrate a programmable nanofluidic switch, driven by elastocapillarity, and based on nanochannels built from layered two-dimensional nanomaterials possessing atomically smooth surfaces and exceptional mechanical properties. We explore operational modes of the nanoswitch and develop a theoretical framework to explain the phenomenon. By predicting the switching-reversibility phase diagram-based on material, interfacial and wetting properties, as well as the geometry of the nanofluidic circuit-we rationally design switchable nano-capsules capable of enclosing zeptoliter volumes of liquid, as small as the volumes enclosed in viruses. The nanoswitch will find useful application as an active element in integrated nanofluidic circuitry and could be used to explore nanoconfined chemistry and biochemistry, or be incorporated into shape-programmable materials.

Size Matters: Free-Energy Calculations of Amino Acid Adsorption over Pristine Graphene

There is still growing interest in graphene interactions with proteins, both for its possible biological applications and due to concerns over detrimental effects at the cellular level. As with any process involving proteins, an understanding of amino acid composition is desirable. In this work, we systematically studied the adsorption process of amino acids onto pristine graphene via rigorous free-energy calculations. We characterized the free energy, potential energy, and entropy of the adsorption of all proteinogenic amino acids. The energetic components were further separated into pair interaction contributions. A linear correlation was found between the free energy and the solvent accessible surface area change during adsorption (ΔSASAads) over pristine graphene and uncharged amino acids. Free energies over pristine graphene were compared with adsorption onto graphene oxide, finding an almost complete loss of the favorability of amino acid adsorption onto graphene. Finally, the correlation with ΔSASAads was used to successfully predict the free energy of adsorption of several penta-l-peptides in different structural states and sequences. Due to the relative ease of calculating the ΔSASAads compared to free-energy calculations, it could prove to be a cost-effective predictor of the free energy of adsorption for proteins onto nonpolar surfaces.

Modified nano-SiO2/PU hydrophobic composite film prepared through in-situ coupling by KH550 for oil-water separation

In this study, hydrophobic polymer composite films based on polyurethane (PU) were prepared for oil-water separation. Hydrophilic fumed silica (nano-SiO₂) was introduced as reinforcing filler, and silane coupling agent (KH550) was used to crosslink PU with nano-SiO₂ in situ for enhancing the nano-SiO₂ dispersion in the films. The microscopic morphology, crystalline structure, and hydrophobic properties of the films were characterized by using scanning electron microscopy, X-ray diffraction, FTIR spectroscopy, water contact angle, and water absorption tests. The results showed that the hydrophobicity of the nano-SiO₂/PU composite films increased with the addition of nano-SiO₂. KH550 not only significantly promoted the crosslink action between PU and nano-SiO₂ but also enhanced the dispersion of nano-SiO₂ in the composite films. Moreover, the pore structure of the prepared films was changed with the addition of nano-SiO₂ and KH550, which greatly improved the hydrophobicity. The test results for oil-water separation performance showed that the prepared composite films can efficiently separate the oil from oil-water mixtures with good repeatability.

Controllable generation of interfacial gas structures on the graphite surface by substrate hydrophobicity and gas oversaturation in water

Spherical nanobubbles and flat micropancakes are two typical states of gas aggregation on solid-liquid surfaces. Micropancakes, which are quasi-two-dimensional gaseous structures, are often produced accompanied by surface nanobubbles. Compared with surface nanobubbles, the intrinsic properties of micropancakes are barely understood due to the challenge of the highly efficient preparation and characterization of such structures. The hydrophobicity of the substrate and gas saturation of solvents are two crucial factors for the nucleation and stability of interfacial gas domains. Herein, we investigated the synergistic effect of the surface hydrophobicity and gas saturation on the generation of interfacial gas structures. Different surface hydrophobicities were achieved by the aging process of highly oriented pyrolytic graphite (HOPG). The results indicated that higher surface hydrophobicity and gas oversaturation could create surface nanobubbles and micropancakes with higher efficiency. Strong surface hydrophobicity could promote nanobubble nucleation and higher gas saturation would induce bigger nanobubbles. Degassed experiments could remove most of these structures and prove that they are actually gaseous domains. Finally, we draw a region diagram to describe the formation conditions of nanobubbles, micropancakes based on observations. These results would be very helpful for further understanding the formation of interfacial gas structures on the hydrophobic surface under different gas saturation.

Organosilicon-Based Plasma Nanocoating on Crust Leather for Water-Repellent Footwear

In this study, functional nanocoatings for water-repellent footwear leather materials were investigated by chemical plasma polymerisation by implanting and depositing the organosilicon compound hexamethyldisiloxane (HMDSO) using a low-pressure plasma system. To this end, the effect of monomers on leather plasma deposition time was evaluated and both the resulting plasma polymers and the deposited leather samples were characterised using different experimental techniques, such as: Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). In addition, leather samples were tested by standard tests for color change, water resistance, surface wetting resistance and dynamic water contact angle (DWCA). The resulting polysiloxane polymers exhibited hydrophobic properties on leather. Furthermore, these chemical surface modifications created on the substrate can produce water repellent effects without altering the visual leather appearance and physical properties. Both plasma coating treatments and nanocoatings with developed water-repellency properties can be considered as a more sustainable, automated and less polluting alternative to chemical conventional processing that can be introduced into product-finishing processes in the footwear industry.

On the effects of induced polarizability at the water-graphene interface via classical charge-on-spring models

Molecular models of the water-graphene interaction are essential to describe graphene in condensed phases. Different challenges are associated with the generation of these models, in particular π-π and dispersion interactions; thus quantum and classical models have been developed and due to the numerical efficiency of the latter, they have been extensively employed. In this work, we have systematically studied, via molecular dynamics, two polarizable graphene models, denominated CCCP and CCCPD, employing the charge-on-spring model of the GROMOS forcefield, both being compatible with the polarizable water models COS/G2 and COS/D2, respectively. These models were compared with non-polarizable graphene and SPC water models. We focused the study on the water-graphene interface in two distinct systems and under the influence of an electric field: one composed of graphene immersed in water and the other composed of graphene with a water droplet above it. In the former, the orientation of water close to the graphene layer is affected by polarizable graphene in comparison to non-polarizable graphene. This effect is emphasised when an electric field is applied. In the latter, carbon polarizability reduced water contact angles, but graphene retained its hydrophobicity and the computed angles are within the experimental data. Given the significant extra computational cost, the use of polarizable models instead of the traditional fixed-charged approach for the graphene-water interaction may be justified when polarizability effects are relevant, for example, when applying relatively strong fields or in very anisotropic systems, such as the vacuum-bulk interface, as these models are more responsive to such conditions.

Investigation of the Drag-Reduction Phenomenon on Plasma-Modified Surface

Drag is one of the main energy-dissipating phenomena in engineering applications. Drag-reduction mechanisms have been studied to reduce this cost. Superhydrophobic surfaces (SHS) have high water repellency and have been studied as an alternative mechanism for reducing drag. The high level of repellency is due to the hierarchical structures in the micro- and nano-scales, making these surfaces able to trap air layers that impose the condition of slipping. The present work investigated the phenomenon of drag reduction on surfaces made of Sylgard® 184 elastomer and modified by low-pressure plasma treatments. Atmospheres with 40% Argon and 60% Acetylene, and 20% Argon and 80% Acetylene were used, varying the treatment times from 10 to 15 min of exposure to Acetylene. The surface, morphological and chemical modifications were confirmed by XPS and AFM analyses, showing the impression of a rough structure on the nanometric scale with deposition of chemical elements from the gas plasma. Furthermore, the obtained SHS showed lower resistance to flow, tested by the imposition of flow in channels.

Hydrophilic, Clean Graphene for Cell Culture and Cryo-EM Imaging

The wettability of graphene is critical for numerous applications but is very sensitive to its surface cleanness. Herein, by clarifying the impact of intrinsic contamination, i.e., amorphous carbon, which is formed on the graphene surface during the high-temperature chemical vapor deposition (CVD) process, the hydrophilic nature of clean graphene grown on single-crystal Cu(111) substrate was confirmed by both experimental and theoretical studies, with an average water contact angle of ∼23°. Furthermore, the wettability of as-transferred graphene was proven to be highly dependent on its intrinsic cleanness, because of which the hydrophilic, clean graphene exhibited improved performance when utilized for cell culture and cryoelectron microscopy imaging. This work not only validates the intrinsic hydrophilic nature of graphene but also provides a new insight in developing advanced bioapplications using CVD-grown clean graphene films.

Deaggregation and Crystallization Inhibition by Small Amount of Polymer Addition for a Co-Amorphous Curcumin-Magnolol System

Different from previously reported co-amorphous systems, a co-amorphous curcumin-magnolol (CUR-MAG CM) system, as compared with its crystalline counterparts, exhibited decreased dissolution due to its aggregation during dissolution. The main purpose of the present study is to deaggregate CUR-MAG CM to optimize drug dissolution and explore the deaggregation mechanism involved. Herein, a small amount of polymer (HPMC, HPC, and PVP K30) was co-formulated at 5% (w/w) with CUR-MAG CM as ternary co-amorphous systems. The polymer addition changed the surface properties of CUR-MAG CM including improved water wettability enhanced surface free energy, and hence exerted a deaggregating effect. As a result, the ternary co-amorphous systems showed faster and higher dissolution as compared with crystalline CUR/MAG and CUR-MAG CM. In addition, the nucleation and crystal growth of dissolved CUR and MAG molecules were significantly inhibited by the added polymer, maintaining a supersaturated concentration for a long time. Furthermore, polymer addition increased the T_g of CUR-MAG CM, potentially involving molecular interactions and inhibiting molecular mobility, resulting in enhanced physical stability under 25 °C/60% RH and 40 °C/75% RH conditions. Therefore, this study provides a promising strategy to optimize the dissolution and physical stability of co-amorphous systems by deaggregation and crystallization inhibition via adding small amounts of polymers.

An Approach to the Use of Glycol Alkoxysilane-Polysaccharide Hybrids in the Conservation of Historical Building Stones

Stone consolidants have been widely used to protect historical monuments. Consolidants and hydrophobic formulations based on the use of tetraethoxysilane (TEOS) and alkylalkoxysilanes as precursors have been widely applied, despite their lack of solubility in water and requirement to be applied in organic media. In the search for a “greener” alternative based on silicon that has potential use in this field, the use of tetrakis(2-hydroxyethyl)silane (THEOS) and tris(2-hydroxyethyl)methyl silane (MeTHEOS) as precursors, due their high water solubility and stability, is proposed in this paper. It is already known that THEOS and MeTHEOS possess remarkable compatibility with different natural polysaccharides. The investigated approach uses the water-soluble silanes THEOS–chitosan and MeTHEOS–chitosan as a basis for obtaining hybrid consolidants and hydrophobic formulations for the conservation of siliceous and calcareous stones. In the case of calcareous systems, their incompatibility with alkoxysilanes is known and is expected to be solved by the developed hybrid consolidant. Their application in the conservation of building stones from historical and archeological sites from Guanajuato, México was studied. The evaluation of the consolidant and hydrophobic formulation treatment was mainly conducted by determining the mechanical properties and contact angle measurements with satisfactory results in terms of the performance and compatibility with the studied stones.

Recent advances in femtosecond laser-structured Janus membranes with asymmetric surface wettability

Janus wettability membranes have received much attention because of their asymmetric surface wettability. On the basis of this distinctiveness from traditional symmetrical membranes, relevant scholars have been inspired to pursue many innovations utilizing such membranes. Femtosecond laser microfabrication shows many advantages, such as precision, short time, and environmental friendliness, over traditional fabrication methods. Now this has been applied in structuring Janus membranes by researchers. This review covers recent advances in femtosecond laser-structured Janus membranes with asymmetric surface wettability. The background in femtosecond laser-structured Janus membranes is first discussed, focusing on the Janus wettability membrane and femtosecond laser microfabrication. Then the applications of Janus membranes are introduced, which are divided into unidirectional fluid transport, oil-water separation, fog harvesting, and seawater desalination. Finally, based on femtosecond laser-structured Janus membranes, some existing problems are pointed out and future perspectives proposed.

Graphene oxide/reduced graphene oxide films as protective barriers on lead against differential aeration corrosion induced by water drops

Graphene-based materials have demonstrated high chemical stability and are very promising for protection against the corrosion of metal surfaces. For this reason, in this work, protective layers composed of graphene oxide, reduced graphene oxide and their mixtures were investigated, respectively, against the corrosion of the surface of lead induced by water drops. The materials were deposited on a Pb surface from their suspensions using a Meyer rod. The surface chemical composition, morphology and structure of the coatings were studied by X-ray photoemission spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and stylus profilometry. Moreover, a specific methodology based on the evolution of the water contact angle with time was used to evaluate the reactivity of the lead surface. The results show that the graphene-based materials can form an efficient barrier layer against the degradation of the Pb surface and that the degradation process induced by water is reduced by more than 70%. Moreover, unexpectedly, the best protective performance was obtained using graphene oxide as the coating.

Study of High-Density Polyethylene (HDPE) Kinetics Modification Treated by Dielectric Barrier Discharge (DBD) Plasma

In this work, the plasma was used in the dielectric barrier discharge (DBD) technique for modifying the high-density polyethylene (HDPE) surface. The treatments were performed via argon or oxygen, for 10 min, at a frequency of 820 Hz, voltage of 20 kV, 2 mm distance between electrodes, and atmospheric pressure. The efficiency of the plasma was determined through the triple Langmuir probe to check if it had enough energy to promote chemical changes on the material surface. Physicochemical changes were diagnosed through surface characterization techniques such as contact angle, attenuated total reflection to Fourier transform infrared spectroscopy (ATR-FTIR), X-ray excited photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Plasma electronics temperature showed that it has enough energy to break or form chemical bonds on the material surface, impacting its wettability directly. The wettability test was performed before and after treatment through the sessile drop, using distilled water, glycerin, and dimethylformamide, to the profile of surface tensions by the Fowkes method, analyzing the contact angle variation. ATR-FTIR and XPS analyses showed that groups and bonds were altered or generated on the surface when compared with the untreated sample. The AFM showed a change in roughness, and this directly affected the increase of wettability.

Low-Humidity Sensing Properties of Multi-Layered Graphene Grown by Chemical Vapor Deposition

Humidity sensing is fundamental in some applications, as humidity can be a strong interferent in the detection of analytes under environmental conditions. Ideally, materials sensitive or insensitive towards humidity are strongly needed for the sensors used in the first or second case, respectively. We present here the sensing properties of multi-layered graphene (MLG) upon exposure to different levels of relative humidity. We synthesize MLG by chemical vapor deposition, as shown by Raman spectroscopy, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Through an MLG-based resistor, we show that MLG is scarcely sensitive to humidity in the range 30%–70%, determining current variations in the range of 0.005%/%relative humidity (RH) well below the variation induced by other analytes. These findings, due to the morphological properties of MLG, suggest that defective MLG is the ideal sensing material to implement in gas sensors operating both at room temperature and humid conditions.

Nanomolar Surface-Active Charged Impurities Account for the Zeta Potential of Hydrophobic Surfaces

The electrification of hydrophobic surfaces is an intensely debated subject in physical chemistry. We theoretically study the ζ potential of hydrophobic surfaces for varying pH and salt concentration by solving the Poisson-Boltzmann and Stokes equations with individual ionic adsorption affinities. Using the ionic surface affinities extracted from the experimentally measured surface tension of the air-electrolyte interface, we first show that the interfacial adsorption and repulsion of small inorganic ions such as H₃O⁺, OH^-, HCO₃^-, and CO₃^2- cannot account for the ζ potential observed in experiments because the surface affinities of these ions are too small. Even if we take hydrodynamic slip into account, the characteristic dependence of the ζ potential on pH and salt concentration cannot be reproduced. Instead, to explain the sizable experimentally measured ζ potential of hydrophobic surfaces, we assume minute amounts of impurities in the water and include the impurities' acidic and basic reactions with water. We find good agreement between our predictions and the reported experimental ζ potential data of various hydrophobic surfaces if we account for impurities that consist of a mixture of weak acids (pK_a = 5-7) and weak bases (pK_b = 12) at a concentration of the order of 10^-7 M.

Thin Polymer Film Force Spectroscopy: Single Chain Pull-out and Desorption

Atomic force microscopy (AFM) was utilized to investigate the force associated with chain pull-out and single chain desorption of poly(styrene-co-butadiene) random copolymer thin films on mica, silicon, and graphite substrates. Chain pull-out events were common and produced a force of 20-25 pN. The polymer desorption force was strongest on the graphite substrate and weakest on the mica, which agreed with the calculated work of adhesion for each system and the substrate hydrophobicity. Furthermore, it was demonstrated that there was a systematic order to when each of these phenomena occurred during the tip retraction from the surface, which provided information about the structure of the thin films.

Investigating Electrode Flooding in a Flowing Electrolyte, Gas-Fed Carbon Dioxide Electrolyzer

Managing the gas-liquid interface within gas-diffusion electrodes (GDEs) is key to maintaining high product selectivities in carbon dioxide electroreduction. By screening silver-catalyzed GDEs over a range of applied current densities, an inverse correlation was observed between carbon monoxide selectivity and the electrochemical double-layer capacitance, a proxy for wetted electrode area. Plotting current-dependent performance as a function of cumulative charge led to data collapse onto a single sigmoidal curve indicating that the passage of faradaic current accelerates flooding. It was hypothesized that high cathode alkalinity, driven by both initial electrolyte conditions and cathode half-reactions, promotes carbonate formation and precipitation which, in turn, facilitates electrolyte permeation. This mechanism was reinforced by the observations that post-test GDEs retain less hydrophobicity than pristine materials and that water-rinsing and drying electrodes temporarily recovers peak selectivity. This knowledge offers an opportunity to design electrodes with greater carbonation tolerance to improve device longevity.

Transition of interfacial capacitors in electrowetting on a graphite surface by ion intercalation

The low voltage electrowetting response of a LiCl aqueous solution on a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG) is presented. For applied voltages below 1 V, the energy stored in the electrical double layer (EDL) is insufficient to drive the spreading of the drop due to the pinning of the three phase contact line at the step edges. Electrochemical impedance spectroscopy shows a dramatic increase in capacitance above 1 V, which provides a sufficient electrowetting force for depinning the contact line, resulting in a subsequent decrease of the contact angle. The transition of the interfacial capacitance from the EDL to the many-fold high capacitance of the pseudocapacitor drives the electrowetting transition on the HOPG surface. The observed changes in the capacitances above 1 V are correlated with the cyclic voltammetry and atomic force microscopy results, which show that the Cl- ions intercalate into the graphite galleries upon acquiring sufficient energy to overcome the van der Waals attraction between the graphene layers through the side of the step edge of the basal planes. To the best of our knowledge, this is the first study on the voltage dependent intercalation mediated transition of interfacial capacitance driving the spreading of an aqueous electrolyte drop on the HOPG surface, which provides a fundamental understanding of the mechanism and opens up potential applications in microfluidics and charge storage technologies.

Water Confined in Hydrophobic Cup-Stacked Carbon Nanotubes beyond Surface-Tension Dominance

Water confined in carbon nanotubes (CNTs) can exhibit distinctly different behaviors from the bulk. We report transmission electron microscopy (TEM) observation of water phases inside hydrophobic cup-stacked CNTs exposed to high vacuum. Unexpectedly, we observed stable water morphologies beyond surface-tension dominance, including nanometer thin free water films, complex water-bubble structures, and zigzag-shaped liquid-gas interface. The menisci of the water phases are complex and inflected, where we measured the contact angles on the CNT inner wall to be 68-104°. The superstability of the suspended ultrathin water films is attributed to the strong hydrogen-bonded network among water molecules and adsorption of water molecules on the cup-structured inner wall. The complex water-bubble structure is a result of the stability of free water films and interfacial nanobubbles, and the zigzag edge of the liquid-gas interface is explained by the pinning effect. These experimental findings provide valuable knowledge for the research on fluids under nanoscale confinement.

Determining the Surface Tension of Two-Dimensional Nanosheets by a Low-Rate Advancing Contact Angle Measurement

Because of their atomic thinness, two-dimensional (2D) nanosheets need be bound to a substrate or be dispersed in material in various applications. The surface tension (ST) of a 2D nanosheet is critical for analyzing the physicochemical interactions between 2D nanosheets and other materials. To date, the determination of the ST of 2D nanosheets has relied mainly on the contact angle (CA) method. However, because of the difficulty in measuring the thermodynamically significant Young?s CA, which is the only meaningful CA that can be used to determine the ST, significant differences exist in reported STs of 2D nanosheets. In this study, we obtained such unique Young?s CAs on graphene, boron nitride, molybdenum disulfide, and tungsten disulfide nanosheets by a low-rate advancing contact angle measurement using a rigorously designed experimental setup. By interpreting the CA with Neumann?s equation of state, we determined the STs of these four nanosheets to be 29.7 ? 0.6, 30.9 ? 0.7, 27.8 ? 0.7, and 29.1 ? 0.8 mJ/m², respectively. The surface energies of these 2D nanosheets were estimated to be in the range 95?120 mJ/m² by considering the contribution of ST and surface entropy. The accuracy of these determined STs was validated by the exfoliation and dispersion of 2D nanosheets in liquids with a series of STs. The study may have important implications for understanding the physicochemical interactions between 2D nanosheets and other materials and the development of 2D nanosheet-based devices.

Accessibility of the pores in highly porous alumina films synthesized via sequential infiltration synthesis

Inorganic nanoporous materials with highly accessible pores are of great interest for the design of efficient catalytic, purification and detection systems. Limited access to the pores is a common problem associated with traditional approaches for the synthesis of porous materials, affecting the functionality of the low-density structure. Recently, infiltration of a nanoporous polymer template with inorganic precursors followed by oxidative annealing was proposed as a new and efficient approach to creating porous inorganic structures with controlled thickness, composition and pore sizes. Here, we report an ultra-high accessibility of the pores in porous films prepared via polymer-swelling-assisted sequential infiltration synthesis (SIS). Using a quartz crystal microbalance technique, we show the increased solvent adsorbing capabilities of highly porous alumina films as a result of high interconnectivity of the pores in such structures. The directionality and highly interconnected nature of the pores are demonstrated in experiments with the partial blocking of pore access by the deposition of a single-layer graphene that is not transparent to solvent. 60% of the pores remain accessible when only 20% of the surface is exposed to solvent. Using humidity detection as an example, we also show that highly porous alumina produced by polymer-swelling-assisted SIS is a promising candidate for sensing applications.

Graphene oxide-dye nanocomposites: effect of molecular structure on the quality of laser-induced graphene

The nanocomposite coatings made using graphene oxide (GO) and six different organic dyes were used to produce the laser-induced graphene (LIG) coatings by means of near infrared picosecond laser irradiation. The coatings were investigated by means of contact angle measurement with three liquids (1-bromonaphtalene, glycerol and water), Raman spectroscopy, scanning electron microscopy, Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis. It was found that the more hydrophilic is the precursor surface the more hydrophobic LIG surface is produced after the laser treatment. Contact angle values obtained on LIG produced from pure GO reached 143°. FTIR spectra have shown that the interaction between GO and dye molecules is realized through the nitrogen atoms. Raman spectra have shown that the best quality LIG coating is obtained using a GO-neutral red nanocomposite precursor. A correlation among contact angle, Raman spectra and topological indices of dye molecules was found, and will serve for the further investigation of the mechanism of LIG production and development of low-defect coatings.

Meta-Analysis Comparing Wettability Parameters and the Effect of Wettability on Friction Coefficient in Lubrication

This work presents a meta-analysis that compares the suitability of various parameters used to characterize wettability in tribological systems. It also examines the relationship between wettability and the friction factor for multiple lubricant-surface pairings. The characterization of wetting behavior was similar when using the contact angle between a lubricant and surface and various dimensional and dimensionless formulations of a spreading parameter. It was possible to identify hydrodynamic, boundary, and mixed lubrication regimes by combining a dimensionless wettability parameter with the specific film thickness for a variety of neat ionic liquids and magnetorheological fluids in contact with metallic, thermoplastic, and elastic surfaces. This characterization was possible using multiple dimensionless wettability parameters, but those that can be fully determined using only the contact angle may be preferred by experimentalists. The use of dimensional and dimensionless wettability parameters that included polar and disperse components of surface tension and surface energy did not appear to provide additional insight into the wettability or frictional performance for the tribological system examined here.

Atomic Force Microscopy Force Mapping Analysis of an Adsorbed Surfactant above and below the Critical Micelle Concentration

Force curves collected using an atomic force microscope (AFM) in the presence of adsorbed surfactants are often used to draw conclusions about adsorbed film packing, rigidity, and thickness. However, some noteworthy features of such force curve characteristics have yet to be thoroughly investigated and explained. In this work, we collected force curves from tetradecyltrimethylammonium bromide films adsorbed on highly oriented pyrolytic graphite (HOPG), silica, and silica that had been hydrophobized by functionalization with dichlorodimethyl silane. Breakthrough events in the force curves from several different trials were compared to show that the breakthrough distance, often reported as the adsorbed film thickness, increased with concentration below the critical micelle concentration (CMC) but was approximately 3.5 nm on all surfaces between 2× and 10× CMC; an unexpected result because of the different surface chemistries for the three surfaces. We employed an AFM probe with a different force constant ( k) value as well as a colloidal probe and the breakthrough distance remained approximately 3.5 nm in all cases. Gradient mapping, a variant of force mapping, was also implemented on the three surfaces and resulted in a new technique for visualizing adsorbed surfactant in situ. The resulting maps showed patches of adsorbed surfactant below the CMC and revealed that with increasing concentration, the size of the patches increased resulting in full coverage near and above the CMC. These results are, to our knowledge, the first time force mapping has been used to spatially track patches of adsorbed surfactant. Finally, layers of surfactants on an AFM tip were investigated by collecting a force map on a single AFM tip using the tip of a separate AFM probe. A breakthrough event was observed between the tips, indicating that a layer of surfactant was present on at least one, if not both tips.

On the equilibrium contact angle of sessile liquid drops from molecular dynamics simulations

We present a new methodology to estimate the contact angles of sessile drops from molecular simulations by using the Gaussian convolution method of Willard and Chandler [J. Phys. Chem. B 114, 1954-1958 (2010)] to calculate the coarse-grained density from atomic coordinates. The iso-density contour with average coarse-grained density value equal to half of the bulk liquid density is identified as the average liquid-vapor (LV) interface. Angles between the unit normal vectors to the average LV interface and unit normal vector to the solid surface, as a function of the distance normal to the solid surface, are calculated. The cosines of these angles are extrapolated to the three-phase contact line to estimate the sessile drop contact angle. The proposed methodology, which is relatively easy to implement, is systematically applied to three systems: (i) a Lennard-Jones (LJ) drop on a featureless LJ 9-3 surface; (ii) an SPC/E water drop on a featureless LJ 9-3 surface; and (iii) an SPC/E water drop on a graphite surface. The sessile drop contact angles estimated with our methodology for the first two systems are shown to be in good agreement with the angles predicted from Young's equation. The interfacial tensions required for this equation are computed by employing the test-area perturbation method for the corresponding planar interfaces. Our findings suggest that the widely adopted spherical-cap approximation should be used with caution, as it could take a long time for a sessile drop to relax to a spherical shape, of the order of 100 ns, especially for water molecules initiated in a lattice configuration on a solid surface. But even though a water drop can take a long time to reach the spherical shape, we find that the contact angle is well established much faster and the drop evolves toward the spherical shape following a constant-contact-angle relaxation dynamics. Making use of this observation, our methodology allows a good estimation of the sessile drop contact angle values even for moderate system sizes (with, e.g., 4000 molecules), without the need for long simulation times to reach the spherical shape.

Atomistic and Coarse-Grained Modeling of the Adsorption of Graphene Nanoflakes at the Oil-Water Interface

The high interfacial tension between two immiscible liquids can provide the necessary driving force for the self-assembly of nanoparticles at the interface. Particularly, the interface between water and oily liquids (hydrocarbon chains) has been exploited to prepare networks of highly interconnected graphene sheets of only a few layers thickness, which are well suited for industrial applications. Studying such complex systems through particle-based simulations could greatly enhance the understanding of the various driving forces in action and could possibly give more control over the self-assembly process. However, the interaction potentials used in particle-based simulations are typically derived by reproducing bulk properties and are therefore not suitable for describing systems dominated by interfaces. To address this issue, we introduce a methodology to derive solid-liquid interaction potentials that yield an accurate representation of the balance between interfacial interactions at atomistic and coarse-grained resolutions. Our approach is validated through its ability to lead to the adsorption of graphene nanoflakes at the interface between water and n-hexane. The development of accurate coarse-grained potentials that our approach enables will allow us to perform large-scale simulations to study the assembly of graphene nanoparticles at the interface between immiscible liquids. Our methodology is illustrated through a simulation of many graphene nanoflakes adsorbing at the interface.

Hydrophilicity of Graphene in Water through Transparency to Polar and Dispersive Interactions

Establishing contact angles on graphene-on-water has been a long-standing challenge as droplet deposition causes free-floating graphene to rupture. The current work presents ice and hydrogels as substrates mimicking water while offering a stable support for graphene. The lowest water contact angles on graphene ever measured, namely on graphene-on-ice and graphene-on-hydrogel, are recorded. The contact angle measurements of liquids with a range of polarities allow the transparency of graphene toward polar and dispersive interactions to be quantified demonstrating that graphene in water is hydrophilic. These findings are anticipated to shed light on the inconsistencies reported so far on the wetting properties of graphene, and most particularly on their implications toward rationalizing how molecules interact with graphene in water.

Graphene as Barrier to Prevent Volume Increment of Air Bubbles over Silicone Polymer in Aqueous Environment

The interaction of air bubbles with surfaces immersed in water is of fundamental importance in many fields of application ranging from energy to biology. However, many aspects of this topic such as the stability of surfaces in contact with bubbles remain unexplored. For this reason, in this work, we investigate the interaction of air bubbles with different kinds of dispersive surfaces immersed in water. The surfaces studied were polydimethylsiloxane (PDMS), graphite, and single layer graphene/PDMS composite. X-ray photoelectron spectroscopy (XPS) analysis allows determining the elemental surface composition, while Raman spectroscopy was used to assess the effectiveness of graphene monolayer transfer on PDMS. Atomic force microscopy (AFM) was used to study the surface modification of samples immersed in water. The surface wettability has been investigated by contact angle measurements, and the stability of the gas bubbles was determined by captive contact angle (CCA) measurements. CCA measurements show that the air bubble on graphite surface exhibits a stable behavior while, surprisingly, the volume of the air bubble on PDMS increases as a function of immersion time (bubble dynamic evolution). Indeed, the air bubble volume on the PDMS rises by increasing immersion time in water. The experimental results indicate that the dynamic evolution of air bubble in contact with PDMS is related to the rearrangement of surface polymer chains via the migration of the polar groups. On the contrary, when a graphene monolayer is present on PDMS, it acts as an absolute barrier suppressing the dynamic evolution of the bubble and preserving the optical transparency of PDMS.

Communication: Is a coarse-grained model for water sufficient to compute Kapitza conductance on non-polar surfaces?

Coarse-grained models have increasingly been used in large-scale particle-based simulations. However, due to their lack of degrees of freedom, it is a priori unlikely that they straightforwardly represent thermal properties with the same accuracy as their atomistic counterparts. We take a first step in addressing the impact of liquid coarse-graining on interfacial heat conduction by showing that an atomistic and a coarse-grained model of water may yield similar values of the Kapitza conductance on few-layer graphene with interactions ranging from hydrophobic to mildly hydrophilic. By design the water models employed yield similar liquid layer structures on the graphene surfaces. Moreover, they share common vibration properties close to the surfaces and thus couple with the vibrations of graphene in a similar way. These common properties explain why they yield similar Kapitza conductance values despite their bulk thermal conductivity differing by more than a factor of two.

Direct Measurement of the Surface Energy of Graphene

Graphene produced by chemical vapor deposition (CVD) is a promising candidate for implementing graphene in a range of technologies. In most device configurations, one side of the graphene is supported by a solid substrate, wheras the other side is in contact with a medium of interest, such as a liquid or other two-dimensional material within a van der Waals stack. In such devices, graphene interacts on both faces via noncovalent interactions and therefore surface energies are key parameters for device fabrication and operation. In this work, we directly measured adhesive forces and surface energies of CVD-grown graphene in dry nitrogen, water, and sodium cholate using a modified surface force balance. For this, we fabricated large (∼1 cm²) and clean graphene-coated surfaces with smooth topography at both macro- and nanoscales. By bringing two such surfaces into contact and measuring the force required to separate them, we measured the surface energy of single-layer graphene in dry nitrogen to be 115 ± 4 mJ/m², which was similar to that of few-layer graphene (119 ± 3 mJ/m²). In water and sodium cholate, we measured interfacial energies of 83 ± 7 and 29 ± 6 mJ/m², respectively. Our work provides the first direct measurement of graphene surface energy and is expected to have an impact both on the development of graphene-based devices and contribute to the fundamental understanding of surface interactions.

Insight on Structure of Water and Ice Next to Graphene Using Surface-Sensitive Spectroscopy

The water/graphene interface has received considerable attention in the past decade due to its relevance in various potential applications including energy storage, sensing, desalination, and catalysis. Most of our knowledge about the interfacial water structure next to graphene stems from simulations, which use experimentally measured water contact angles (WCAs) on graphene (or graphite) to estimate the water-graphene interaction strength. However, the existence of a wide spectrum of reported WCAs on supported graphene and graphitic surfaces makes it difficult to interpret the water-graphene interactions. Here, we have used surface-sensitive infrared-visible sum frequency generation (SFG) spectroscopy to probe the interfacial water structure next to graphene supported on a sapphire substrate. In addition, the ice nucleation properties of graphene have been explored by performing in situ freezing experiments as graphitic surfaces are considered good ice nucleators. For graphene supported on sapphire, we observed a strong SFG peak associated with highly coordinated, ordered water next to graphene. Similar ordering was not detected next to bare sapphire, implying that the observed ordering of water molecules in the former case is a consequence of the presence of graphene. Our analysis indicates that graphene behaves like a hydrophobic (or negatively charged) surface, leading to enhanced ordering of water molecules. Although liquid water orders next to graphene, the ice formed is proton disordered. This research sheds light on water-graphene interactions relevant in optimizing the performance of graphene in various applications.

Influence of O2, H2O and airborne hydrocarbons on the properties of selected 2D materials

Adsorption of molecules from the ambient environment significantly changes the optical, electrical, electrochemical, and tribological properties of 2D materials.