Solid Surface Research Articles

In Situ Visual Observation of Surface Energy-Controlled Heterogeneous Nucleation of Metal Nanocrystals.

Electrochemical growth of metal nanocrystals is pivotal for material synthesis, processing, and resource recovery. Understanding the heterogeneous interface between electrolyte and electrode is crucial for nanocrystal nucleation, but the influence of this interaction is still poorly understood. This study employs advanced in situ measurements to investigate the heterogeneous nucleation of metals on solid surfaces. By observing the copper nanocrystal electrodeposition, an interphase interaction-induced nucleation mechanism highly dependent on substrate surface energy is uncovered. It shows that a high-energy (HE) electrode tended to form a polycrystalline structure, while a low-energy (LE) electrode induced a monocrystalline structure. Raman and electrochemical characterizations confirmed that HE interface enhances the interphase interaction, reducing the nucleation barrier for the sturdy nanostructures. This leads to a 30.92-52.21% reduction in the crystal layer thickness and a 19.18-31.78% increase in the charge transfer capability, promoting the formation of a uniform and compact film. The structural compactness of the early nucleated crystals enhances the deposit stability for long-duration electrodeposition. This research not only inspires comprehension of physicochemical processes correlated with heterogeneous nucleation, but also paves a new avenue for high-quality synthesis and efficient recovery of metallic nanomaterials.

Interaction of Diisopropyl methyl Phosphonate (DIMP) with magnesium oxide at elevated temperatures

Diisopropyl Methyl Phosphonate (DIMP) is a liquid widely considered a surrogate for chemical warfare agents (CWA). Respectively, its interactions with different surfaces must be understood to predict the fate of CWA in various scenarios involving their release. Magnesium oxide (MgO) is a combustion product of metallized energetic materials and can serve as a model material for other common oxides. Thus, its interaction with DIMP is of both fundamental and practical interest. In this work, powders of MgO produced by different methods and having different surface areas and crystallite sizes were exposed to liquid DIMP and heated above the DIMP boiling point in a thermal analyzer. The residue remaining on the surface of different powders was quantified and examined using infrared spectroscopy. It was found that the mass of the residue scaled with the specific surface area determined based on the MgO crystallite sizes. The results suggest that the residue consists of P-CH3 molecular groups with the phosphorus attached to multiple surface oxygen anions. The residue was found to be discontinuous on smaller crystallites, but it could form a continuous monolayer on larger crystallites. The presence of Mg(OH)2 prevented interactions between DIMP and the solid surface.

Water dynamics in calcium silicate hydrates probed by inelastic neutron scattering and molecular dynamics simulations

Calcium-silicate-hydrate (C-S-H) is a disordered, nanocrystalline material, acting as a primary binding phase in Portland cement. C-S-H and C-A-S-H (an Al-bearing substitute present in low-CO2 cement) contain thin films of water on solid surfaces and inside nanopores. Water controls multiple chemical and mechanical properties of C-S-H, including drying shrinkage, ion transport, creep, and thermal behavior. Therefore, obtaining a fundamental understanding of its properties is essential. We applied a combination of inelastic incoherent neutron scattering and molecular dynamics simulations to unravel water dynamics in synthetic C-(A)-S-H conditioned at five hydration states (from drier to more hydrated) and with three Ca/Si ratios (0.9, 1, and 1.3). Our results converge towards a picture where the evolution from thin layers of interfacial water to bulk-like capillary water is dampened by the structure of C-(A)-S-H. In particular, the hydrophilic Ca2+ sites organize the distribution of interfacial C-(A)-S-H water.

Normal impact of a ball rotating around its linear velocity.

We report on an experimental study of the normal impact on a solid surface of a table tennis ball that rotates around its linear velocity vector. We observe that the ratio of the reflected velocity to the incident velocity does not depend on the initial spin. In contrast, the reflected spin depends not only on the incident spin but also on the incident velocity. The experimental results, which reveal the tricky role played by the friction in the region of contact, are accounted for by simple theoretical arguments.

Computer simulation and modeling of glow discharge optical emission coded aperture elemental mapping

BackgroundElemental mapping (EM) yields necessary insights into mechanisms of interest in solid samples across multiple disciplines. There are several EM techniques available but long acquisition time is a common limitation. Glow discharge optical emission spectroscopy (GDOES) allows direct quantitative multi-EM at very high throughput (∼10 s s) when coupled to traditional hyperspectral imaging (HSI) techniques. However, GDOES consumes the sample via sputtering, such that traditional HSI sequential scanning requirements lead to loss of information/resolution, which is compounded for multi-EM and limits nanomaterials analysis. Thus, there is a need for faster HSI to enable GDOES multi-EM of nanoscale materials. ResultsHere, a new technique is described, Glow discharge Optical emission Coded Aperture elemental Mapping (GOCAM), that takes advantage of compressive coded aperture spectral imaging to enable multi-EM in a single camera exposure. In this first phase of development, computer model simulations were implemented to study the effects of coded aperture parameters on data fidelity, which showed the best fidelity is achieved at smaller mask element sizes and transmittance of 60 %. In addition, SeSCIGPU demonstrated the best fidelity performance compared to several compressed sensing reconstruction algorithms, including TwIST, GAP-TV, SeSCICPU, and ADMM-TV, as evaluated by studying the effects of varying the corresponding hyperparameters. SignificanceThis study shows GOCAM's feasibility and provides a starting point for the second phase hardware development currently underway. GOCAM's potential to allow multi-EM from solid surfaces in a fraction of a second will be particularly enabling for nanostructured materials characterization.

Liquid Springs for High‐Speed Contamination‐Free Manipulation of Droplets and Solid Particles

AbstractAccurate and flexible control of droplets is essential in many industrial applications, such as water harvesting, chemical assays, and biological detection. Magnetic force‐based methods have been broadly exploited to fulfill the goals due to the advantage of non‐contact, easy control, and long‐range navigation. Nevertheless, it still suffers from some challenges, such as sample fouling, and the paradox between the droplet efficient motion and the large volume. Here a ferrofluid‐based liquid spring to achieve contamination‐free and fast droplet transportation on non‐wetting solid surfaces is proposed. The liquid spring is based on the actuation of a ferrofluid droplet in an external uniform magnetic field. The actuation enables the liquid spring to propel tiny objects, including the non‐magnetic miscible droplets and water‐repellent solid particles, with adjustable motion velocity. Furthermore, the magnetic force, applied on the ferrofluid via an additional permanent magnet, makes it possible to navigate the liquid spring in a programmable way. With the aid of the liquid spring, the single or multiple droplets/solid particles advancing, on‐demand droplets coalescence, and out‐of‐plane droplet motion are achievable.

Microscopic behavior of heteroatom asphaltene in the multi-media model: The effect on heavy oil properties

The high viscosity and surface adsorption of heavy oil pose significant challenges for its exploration. Asphaltenes, the most polar and heaviest components of crude oil, play a crucial role in these properties. Heteroatom asphaltenes (A-X, S-Y) exhibit stronger interaction energy compared to non-heteroatom asphaltenes A-C. The self-assembly of A-X/S-Y forms ordered "reinforcement-cement" aggregates, whereas A-C aggregates are more chaotic. The presence of heteroatoms, particularly the sulfur atom in the side chains, may significantly influences asphaltene aggregation structures. The diffusion coefficient of heteroatom asphaltene in oil droplet is smaller than that of A-C, and these heteroatom compound can bind other oil components, reducing the overall fluidity. Notably, the adsorption energy of asphaltenes on solid surfaces exceeds their interaction energy. Surface aggregation of A-X/S-Y is more dispersed compared to A-C adsorption system. Additionally, the existence of heteroatom asphaltenes can shorten the distance between oil droplets and the solid surface by 16.20 Å compared with non-heteroatom system. These phenomena, caused by A-X/S-Y, further complicate the detachment of crude oil components from surfaces. This study aims to share some new insights into the micro-mechanisms of heteroatom asphaltenes self-assembly and the behavior in the oil-sludge process, which may contribute to the theoretical foundation for the targeted and efficient exploitation of heavy oil.

Electrochemical Investigation of the Stability of Poly-Phosphocholinated Liposomes.

Poly[2-(methacryloyloxy)ethyl phosphorylcholine] liposomes (pMPC liposomes) gained attention during the last few years because of their potential use in treating osteoarthritis. pMPC liposomes that serve as boundary lubricants are intended to restore the natural lubrication properties of articular cartilage. For this purpose, it is important that the liposomes remain intact and do not fuse and spread as a lipid film on the cartilage surface. Here, we investigate the stability of the liposomes and their interaction with two types of solid surfaces, gold and carbon, by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). With the aid of a hydrophilic species used as an electroactive probe in the solution, the charge transfer characteristics of the electrode surfaces are obtained. Additionally, from EIS, the capacitance characteristics of the surfaces are derived. No decrease of the peak currents and no displacement of the peak potentials to greater overpotentials are observed in the CV experiments. No decrease in the apparent capacitance and increase in the charge transfer resistance is observed in the EIS experiments. On the contrary, all parameters in both CV and EIS do change in the opposite direction. The obtained results confirm that there is only physical adsorption without fusion and spreading of the pMPC liposomes and without the formation of lipid films on the surfaces of both gold and carbon electrodes.

Tannin-on-protein adlayers: Secondary adsorption quantified by quartz crystal microbalance with dissipation monitoring (QCM-D)

The quartz crystal microbalance with dissipation monitoring (QCM-D) technique allows to detect adsorption and mechanical characteristics of protein adsorption layers at solid surfaces, including subsequent secondary adsorption of additional species interacting with the proteins. Here, we study the secondary adsorption of two different classes of tannins - hydrolysable vs. condensed - onto a β-lactoglobulin base layer using QCM-D in a protein/tannin adlayer configuration. Sequential flow experiments of protein solutions, tannin solutions and neat solvent or buffer are used to distinguish reversible from desorption-resistant layer formation, evaluate the viscoelastic characteristics the layers, and map protein/tannin interactions in view of their structural properties and origin. We find that for the majority of tannins, secondary layer structural changes (detected via a change in the dissipation factor) occur before significant mass adsorption (detected via the resonance frequency shift). In contrast, simultaneous onset of structural change and adsorbed mass within an observation window of 100 ms was only found for certain hydrolysable tannins. We use these data to build adsorption maps and group the tannins according to their adlayer properties, providing a physical basis for an improved understanding of the organoleptic perception of tannins and polyphenols, in particular with respect to attributes such as ‘astringency’ and ‘lingering’. Moreover, these maps provide insight into the capacity of different tannins to modify the mechanical properties of protein adsorption layers.

Propulsive performance of a heaving and pitching foil with large amplitudes in unsteady ground effect

To reveal the effect of ground on the force behavior, propulsion performance, and vortex dynamics of a flapping foil, we numerically study a flapping foil actuated by combined heaving and pitching motions with large amplitudes near a solid ground surface. The instantaneous forces in both streamwise and cross-stream directions undergo a sudden drop during upstroke in close ground effect. The time-averaged thrust is enhanced by the ground but varies non-monotonically with ground distance at high foil oscillating frequencies. Three force regimes are identified in lift production, giving rise to two equilibrium altitudes of zero lift, one stable and another unstable. Moreover, we propose a simple scaling law model by a path-length-based Strouhal number and ground effect for thrust and power performance of a large-amplitude flapping foil. The results of the model are consistent with numerical results and previous experimental studies. The vortex dynamics in the wake structure are largely affected by ground proximity; in particular, the formed vortex pairs and their orientations are sensitive to their initial relative positions. Different sources and mechanisms of satellite vortex formation are then discussed. The ground exerts little effect on the phase angles that optimize the propulsive efficiency. This study is expected to provide new insights into the development and maneuvering of flapping foil-based propulsive systems.

Design of electrical energy harvesting devices utilizing air bubbles sliding along a fluoropolymer immersed in water

Recent research has shown that it is possible to utilize contact electrification combined with electrostatic induction to harvest electrical energy from the mechanical motion of air bubbles sliding along a charged solid surface immersed in water. The working principle of these devices is simple, but the design is usually complicated as transduction efficiency depends on a number of interdependent parameters. Here we propose a simple analytical model and demonstrate how it can be used to determine the optimal energy per bubble for a given resistive load. The model allows one to estimate the optimal energy harvested per bubble in terms of polymer thickness, electrode separation and load resistance. It is shown that the model provides a good fit to experimental data. The model may be used as an initial step when designing energy harvesting devices utilizing air bubbles sliding along a solid surface.

Low-energy atomic and molecular hydrogen ion interaction with low-work function electride 12CaO · 7Al2O3

To efficiently generate H− ions from positive atomic or molecular hydrogen beams injected onto a solid surface, it has been suggested to use a material with a low-work function as the target material. However, it is not clear under what conditions the most efficient H− production is realized for incident beam parameters or reflection angles. Therefore, we studied the interaction between low-energy atomic and molecular hydrogen beams (less than 1 keV/nucleon) with a low-work function electride 12CaO⋅7Al2O3 (C12A7). The production ratio of H− to H+ ions from the C12A7 electride was much higher than Mo targets with higher work functions, especially at smaller incident and smaller reflection angles. The H− to H+ production ratio slightly increased as the incident energies were decreased, but there was no significant difference between the electride and Mo targets. These results indicate that smaller incident angles and lower beam energies of the incident hydrogen beam are favorable for the enhancement of the production ratio of H− to H+ ions in C12A7. The higher production ratio appeared at the vertical beam energies less than on the order of 100 eV, where quantum mechanical processes may become important.

Liquefaction efficiency study of heterogeneous condensation of methane-ethane binary gas mixtures with different component contents

Natural gas is a mixture of hydrocarbons. During the liquefaction of natural gas, the mixed gas undergoes heterogeneous condensation on the surface of the heat exchanger. Currently, the heterogeneous condensation mechanism of mixed gases is not fully understood, and the influence of component content on the liquefaction efficiency is not clear. Therefore, the molecular dynamics simulation was employed to investigate the heterogeneous condensation process of methane-ethane binary mixed gas on a solid surface. The characteristics of condensation nucleation cluster growth was explored and the variations in temperature, density, heat flux, and liquefaction rate were analyzed. The impact of ethane on methane condensation was also discussed. The results indicate that increasing wettability enhances surface nucleation quantity and shortens nucleation time, resulting in a 3.6- times increase in heat flux. The addition of ethane gas in the system promotes condensation nucleation, which increases the liquefaction rate by 60 % when x = 20 % compared to the pure methane system. However, for methane gas, ethane only benefits the liquefaction of methane molecules in the initial nucleation stage. As the mixture in the system enters the cluster growth stage, component migration in the mixed gas leads to a lower liquefaction rate compared to pure methane gas.

An analysis of the contact time of nanodroplets impacting superhydrophobic surfaces with square ridges

Reducing the contact time between liquid droplets and solid surfaces is important in numerous engineering applications. Mechanisms for reducing the contact time have been extensively studied. However, there is still a lack of systematic research on the mechanisms governing the changes in the contact time of nanodroplets impacting superhydrophobic surfaces with square ridges. Additionally, there has been relatively little examination of the differences between nanodroplets and macroscopic droplets in this scenario. In this work, we investigate the impact of nanodroplets on both superhydrophobic planes and superhydrophobic surfaces with square ridges. When nanodroplets impact a superhydrophobic plane, we find that the droplet contraction time determines the total contact time. When nanodroplets impact superhydrophobic surfaces with square ridges, we find for the first time that the critical factor determining its contact time is the detachment time of the droplet from the square ridges as it lifts off. Compared to millimeter-sized droplets, nanodroplets exhibit stronger adhesion to solid surfaces. Additionally, we propose for the first time that nanodroplets are more resistant to deformation and splitting. We define the Weber number (We) at which the droplet rebounds as the critical rebound Weber number (We), and use this to evaluate the effect of square ridges on improving droplet rebound. We observe that a moderate increase in the square ridge height is beneficial to reducing the contact time. By altering the square ridge width, we find that a narrower ridge is more favorable for reducing the contact time and for facilitating droplet splitting.

What Drives Deformation of Smart Nanoporous Materials During Adsorption and Electrosorption?

Nanoporous solids have high surface area, so processes at the surface affect the sample as a whole. When guest species adsorb in nanopores, be they molecules adsorbing from the gas phase, or ions adsorbing from solution, they cause material deformation. While often undesired, adsorption- or electrosorption-induced deformation provides a potential for nanoporous materials to be used as actuators. Progress in this direction requires understanding the mechanisms of adsorption- or electrosorption-induced deformation. These two processes are rarely discussed together, and this Perspective aims to fill this gap to some extent, focusing on driving forces for both processes. Typically the main driving force for both is the solvation (disjoining) pressure, acting normally to the pore walls. However, in some cases, solvation pressure is not sufficient to describe the effects even qualitatively. We highlight examples in which the surface stress acting along the solid surface is an additional driving force for deformation.

Tracking the critical roles of Cu+ and Cu0 sites and the optimal Cu+/Cu0 ratio for CH3OH steam reforming (MTSR) to manufacture H2

Understanding the nature of surface-active sites is of critical importance for catalyst design, yet remains great challenges because of the complexity of solid surfaces. To tracking the valence states and roles of various Cu sites played in CH3OH steam reforming (MTSR) on Cu-based catalysts and the factors to determine the reactivity, appropriate Cu/Ce1-xZrxO2 solid solution catalysts have been purposely designed. On a catalyst having a Ce/Zr ratio of 0.7/0.3 (Cu/Ce0.7Zr0.3O2), the optimal Cu+/Cu0 ratio around 1.00 can be obtained to accomplish the best performance, with a high H2 production rate but very low CO selectivity. With the combination of experiments and DFT calculation, it has been discovered that CH3OH molecules mainly be adsorbed/activated on the Cu+ sites, but H2O molecules could mainly be adsorbed/activated on the Cu0 sites. We have substantiated that to fabricate Cu/Ce1-xZrxO2 catalysts with high efficiency, nearly equal amount of surface Cu+ and Cu0 sites should be engineered.

On-Surface Self-Assembly Kinetic Study of Cu-Hexaazatriphenylene 2D Conjugated Metal-Organic Frameworks on Coinage Metals and MoS2 Substrates.

Supramolecular coordination self-assembly on solid surfaces provides an effective route to form two-dimensional (2D) metal-organic frameworks (MOFs). In such processes, surface-adsorbate interaction plays a key role in determining the MOFs' structural and chemical properties. Here, we conduct a systematic study of Cu-HAT (HAT = 1,4,5,8,9,12-hexaazatriphenylene) 2D conjugated MOFs (c-MOFs) self-assembled on Cu(111), Au(111), Ag(111), and MoS2 substrates. Using scanning tunneling microscopy and density functional theory calculations, we found that the as-formed Cu3HAT2 c-MOFs on the four substrates exhibit distinctive structural features including lattice constant and molecular conformation. The structural variations can be attributed to the differentiated substrate effects on the 2D c-MOFs, including adsorption energy, lattice commensurability, and surface reactivity. Specifically, the framework grown on MoS2 is nearly identical to its free-standing counterpart. This suggests that the 2D van der Waals (vdW) materials are good candidate substrates for building intrinsic 2D MOFs, which hold promise for next-generation electronic devices.

Anomalous friction of supercooled glycerol on mica

Although friction of liquids on solid surfaces is traditionally linked to wettability, recent works have unveiled the role of the solid’s internal excitations on interfacial dissipation. In order to directly evidence such couplings, we take advantage of the considerable variation of the molecular timescales of supercooled glycerol under mild change of temperature to explore how friction depends on the liquid’s molecular dynamics. Using a dedicated tuning-fork AFM, we measure the slippage of glycerol on mica. We report a 100 fold increase of slip length upon cooling, while liquid-solid friction exhibits a linear scaling with molecular relaxation rate at high temperature. This scaling can be explained by a contribution of mica’s phonons which resonate with density fluctuations in the liquid, allowing efficient momentum transfer to mica. These results suggest that engineering phononic spectra of materials could enhance flow performance in nanofluidic channels and industrially relevant membranes.

Surface structure-based model to calculate thermal and optical properties in nanoscale and quantum dot semiconductors

Surface combination structure was used to design the construction of nano and quantum films, particles, and dots. The created model was applied to tetrahedral semiconductor nanoparticles and quantum dots with a successful performance. It works better than reported experimental data across the whole size range, from the bulk state to the critical size of quantum dots. The model helps to figure out the melting point that changes with the nanosize scale as applied on Si and Sn, which are simple tetrahedral semiconductors, and for CdS and CdSe, which are among II-VI group compounds. It was also used to calculate melting-related parameters such as cohesive energy, Debye temperature, vibrational entropy, and melting enthalpy. The nanosize dependence of the lattice volume and the atom's ionization energy describes the solid surface structure. From the model, an equation was obtained to calculate the energy gaps in Si, CdS, and CdSe particles that are nano- and quantum-dot-sized, and the results are compared to the experimental data that goes with these particles.

Kinetic evaluation of Azadirichtin and related phytoconstituents extraction from Azadirachtaindica (neem) and Melia azedarach (chinaberry) seeds using ultrasound assisted extraction and maceration

The comparative extraction of total phenolic compounds from A. indica and M. azedarach seed powder using two different methodologies is evaluated. The solvent screening was done using soxhlet extraction for six different solvents and methanol had the maximum extraction capacity for both seed powder. Extraction of total phenolic compounds from A. indica and M. azedarach seed powder was done using maceration and ultrasound. The kinetics and extraction mechanism was evaluated using popular three empirical and five semi-empirical models along with scanning electron micrograph analysis. The rate constants showed an enhancement in extraction kinetics of about 4.5 fold for M. azedarach and 7.8 fold for A. indica seeds using ultrasound. Based on model evaluation, extraction using maceration for both seeds follows inter-particle diffusion as the rate-limiting step with non-swelling behaviour. Whereas, in the case of ultrasound, the dependence of rate is on the diffusion of solute within the solid and the solid surface to the solvent. In addition, HPLC and GC-MS analysis suggested differences in the composition of extracts and some important phytochemicals were extracted in larger quantities using ultrasound as compared to maceration.