Liquid-air Interface Research Articles

Outstanding Electrochemical Performance of Planar On-chip Micro-supercapacitors Based on Nitrogen and Oxygen Co-doped Graphene Quantum Dots at High Scan Rates

In recent years, researchers continue optimizing and innovating in the selection of supercapacitor electrode materials and fabrication processes. In this paper, the fabrication process and electrochemical performance of an all-solid-state planar on-chip micro-supercapacitors (POMSs) with nitrogen and oxygen co-doped graphene quantum dots as electrode material will be demonstrated. Graphene quantum dots (GQDs) electrode films were prepared by a modified liquid-air interface self-assembly method, and patterned by photolithography. The resulting POMSs achieved an area capacitance of 25.53 μFcm−2, and its areal energy density and areal power density are 3.55nWh cm−2 and 8.79 μW cm−2 correspondingly. It is worth noting that the POMSs have excellent rate capability and can maintain outstanding performance at ultrahigh scan rate of 10000 V s−1. The POMSs have a short time constant which is 3.16 μs, and the capacitance characteristics can be maintained over 99.1% in 10000 cycles of charge and discharge at a scan rate of 5000 V s−1 in the cyclic stability test.

Assessment of antibacterial properties and skin irritation potential of anodized aluminum impregnated with various quaternary ammonium

The importance of the inert environment in the transmission of pathogens has been reassessed in recent years. To reduce cross-contamination, new biocidal materials used in high touch surfaces (e.g., stair railings, door handles) have been developed. However, their impact on skin remains poorly described. The present study aimed to evaluate the antibacterial properties and the risk of skin irritation of two materials based on hard-anodized aluminum (AA) impregnated with quaternary ammonium compound solutions (QAC#1 or QAC#2). The QAC#1 or QAC#2 solutions vary in composition, QAC#2 being free of dioctyl dimethyl ammonium chloride (Dio-DAC) and octyl decyl dimethyl ammonium chloride (ODDAC). Unlike AA used as a control, both AA-QAC#1 and AA-QAC#2 had excellent and rapid antibacterial efficacy, killing 99.9 % of Staphylococcus aureus and Escherichia coli bacteria, in 15 s and 1 min, respectively. The impregnation solutions (QAC#1 and QAC#2) did not show any skin sensitizing effect on transformed human keratinocytes. Nevertheless, these solutions as well as the materials (AA-QAC#1, AA-QAC#2), and the liquid extracts derived from them, induced a very rapid cytotoxicity on L929 murine fibroblasts (>70 % after 1 h of contact) as shown by LDH, MTS and neutral red assays. This cytotoxicity can be explained by the fast QACs release occurring when AA-QAC#1 and AA-QAC#2 were immersed in aqueous medium. To overcome the limitation of assays based on liquid condition, an in vitro skin irritation assay on reconstructed human epidermis (RHE) was developed. The effect of the materials upon their direct contact with the epidermis grown at the liquid-air interface was determined by evaluating tissue viability and quantifying interleukin-1 alpha (IL-1α) which is released in skin during injury or infection. AA-QAC#1 induced a significant decrease in RHE viability, close to OECD and ISO 10993-10 acceptability thresholds and enhanced the pro-inflammatory IL-1α secretion compared with AA-QAC#2. Finally, these results were corroborated by in vivo assays on mice using erythema and edema visual scores, histological observations, and epidermal thickness measurement. AA had no effect on the skin, while a stronger irritation was induced by AA-QAC#1 compared with AA-QAC#2. Hence, these materials were classified as moderate and slight irritants, respectively. In summary, this study revealed that AA-QAC#2 without Dio-DAC and ODDAC could be a great candidate for high touch surface applications, showing an extremely effective and rapid bactericidal activity, without inducing adverse effects for skin tissue.

The Influence of Liquid Menisci on Nanoparticle Dosimetry in Submerged Cells.

Understanding the interaction between cells and nanoparticles (NPs) is vital to understand the hazard associated with nanoparticles. This requires quantifying and interpreting dose-response relationships. Experiments with cells cultured in vitro and exposed to particle dispersions mainly rely on mathematical models that estimate the received nanoparticle dose. However, models need to consider that aqueous cell culture media wets the inner surface of hydrophilic open wells, which results in a curved liquid-air interface called the meniscus. Here the impact of the meniscus on nanoparticle dosimetry is addressed in detail. Experiments and build an advanced mathematical model, to demonstrate that the presence of the meniscus may bring about systematic errors that must be considered to advance reproducibility and harmonization is presented. The script of the model is co-published and can be adapted to any experimental setup. Finally, simple and practical solutions to this problem, such as covering the air-liquid interface with a permeable lid or soft rocking of the cell culture well plate is proposed.

Experimental studies of droplet nucleate boiling characteristics on micro-cavities surfaces with different wettability

In this study, micro-cavities surfaces with different wettability are prepared by combining nanosecond laser micro-nano processing technology, chemical oxidation and electrochemical corrosion. The effects of micro-cavities and surface wettability on evaporation time and bubble dynamics are experimentally investigated. The results show that micro-cavities surfaces with smooth structures exhibit excellent evaporation performance as micro-cavity serves as the nucleation site for bubble. The existence of CuO film on micro-cavities surfaces with superhydrophilic structures increases the thermal resistance and hinders the bubble growth, which causes poor droplet evaporation process. On the other hand, the Rayleigh-Taylor instability is easily triggered on micro-cavities surfaces with superhydrophobic structures due to lower surface tension of the solid–liquid interface. This phenomenon causes the strong fluctuation and prolongs the evaporation process. The vapor jet or fluctuating liquid–air interface is observed at the droplet center from the thermal pattern of micro-cavities surfaces with smooth structures. On micro-cavities surfaces with superhydrophilic structures, the fluid at the top of droplet has an obvious temperature gradient due to the existence of vapor bubble. However, strong fluctuation is developed on micro-cavities surfaces with superhydrophobic structures, leading to the fact that the fluid near the liquid–air interface is cooled. Entrapped air inside the micro-cavities serves as the bubble seed for micro-cavities surfaces with smooth structures and micro-cavities surfaces with superhydrophobic structures, thus shortening the time for bubble nucleation.

Excessive Iodine Enabled Ultrathin Inorganic Perovskite Growth at the Liquid-Air Interface.

The liquid-air interface offers a platform for the in-plane growth of free-standing materials. However, it is rarely used for inorganic perovskites and ultrathin non-layered perovskites. Herein the liquid-air interfacial synthesis of inorganic perovskite nanosheets (Cs3 Bi2 I9 , Cs3 Sb2 I9 ) is achieved simply by drop-casting the precursor solution with only the addition of iodine. The products are inaccessible without iodine addition. The thickness and lateral size of these nanosheets can be adjusted through the iodine concentration. The high volatility of the iodine spontaneously drives precursors that normally stay in the liquid to the liquid-air interface. The iodine also repairs in situ iodine vacancies during perovskite growth, giving enhanced optical and optoelectronic properties. The liquid-air interfacial growth of ultrathin perovskites provides multi-degree-of-freedom for constructing perovskite-based heterostructures and devices at atomic scale.

Numerical study on the nonwetting ability of trapezoid topography

Pattern texturing has been proved to effectively improve the water repellency on hydrophobic surfaces. To explore the effect of pattern profile on the dynamic wetting state, here, we conduct numerical study of a water droplet impinging on surfaces with a single trapezoid, rectangular, and V-shaped grooves, respectively. Simulations are performed using a volume-of-fluid method based interFoam solver shipped with OpenFOAM. We begin with a numerical validation by comparing with experiment. Subsequently, the wetting states of the droplet on these three substrates are examined, which demonstrate the capability of trapezoid topography to prevent being wetted. Theoretical model well explains that trapezoid structure can lead to a great increase in energy barrier brought by the penetration of liquid–air interface, which in turn improves its non-wetting ability even under impingement conditions. By masking the surface with mix-hydrophobicity, we also prove that wettability in grooves plays a significant role on the dynamic wetting state. A stable dry surface can be obtained by only masking the groove side-wall by superhydrophobic patterns.

Chemical Feedback in the Self-Assembly and Function of Air-Liquid Interfaces: Insight into the Bottlenecks of CO2 Direct Air Capture.

As fossil fuels remain a major source of energy throughout the world, developing efficient negative emission technologies, such as direct air capture (DAC), which remove carbon dioxide (CO2) from the air, becomes critical for mitigating climate change. Although all DAC processes involve CO2 transport from air into a sorbent/solvent, through an air-solid or air-liquid interface, the fundamental roles the interfaces play in DAC remain poorly understood. Herein, we study the interfacial behavior of amino acid (AA) solvents used in DAC through a combination of vibrational sum frequency generation spectroscopy and molecular dynamics simulations. This study revealed that the absorption of atmospheric CO2 has antagonistic effects on subsequent capture events that are driven by changes in bulk pH and specific ion effects that feedback on surface organization and interactions. Among the three AAs (leucine, valine, and phenylalanine) studied, we identify and separate behaviors from CO2 loading, chemical changes, variations in pH, and specific ion effects that tune structural and chemical degrees of freedom at the air-aqueous interface. The fundamental mechanistic findings described here are anticipated to enable new approaches to DAC based on exploiting interfaces as a tool to address climate change.

The Cultivation Modality and Barrier Maturity Modulate the Toxicity of Industrial Zinc Oxide and Titanium Dioxide Nanoparticles on Nasal, Buccal, Bronchial, and Alveolar Mucosa Cell-Derived Barrier Models.

As common industrial by-products, airborne engineered nanomaterials are considered important environmental toxins to monitor due to their potential health risks to humans and animals. The main uptake routes of airborne nanoparticles are nasal and/or oral inhalation, which are known to enable the transfer of nanomaterials into the bloodstream resulting in the rapid distribution throughout the human body. Consequently, mucosal barriers present in the nose, buccal, and lung have been identified and intensively studied as the key tissue barrier to nanoparticle translocation. Despite decades of research, surprisingly little is known about the differences among various mucosa tissue types to tolerate nanoparticle exposures. One limitation in comparing nanotoxicological data sets can be linked to a lack of harmonization and standardization of cell-based assays, where (a) different cultivation conditions such as an air-liquid interface or submerged cultures, (b) varying barrier maturity, and (c) diverse media substitutes have been used. The current comparative nanotoxicological study, therefore, aims at analyzing the toxic effects of nanomaterials on four human mucosa barrier models including nasal (RPMI2650), buccal (TR146), alveolar (A549), and bronchial (Calu-3) mucosal cell lines to better understand the modulating effects of tissue maturity, cultivation conditions, and tissue type using standard transwell cultivations at liquid-liquid and air-liquid interfaces. Overall, cell size, confluency, tight junction localization, and cell viability as well as barrier formation using 50% and 100% confluency was monitored using trans-epithelial-electrical resistance (TEER) measurements and resazurin-based Presto Blue assays of immature (e.g., 5 days) and mature (e.g., 22 days) cultures in the presence and absence of corticosteroids such as hydrocortisone. Results of our study show that cellular viability in response to increasing nanoparticle exposure scenarios is highly compound and cell-type specific (TR146 6 ± 0.7% at 2 mM ZnO (ZnO) vs. ~90% at 2 mM TiO2 (TiO2) for 24 h; Calu3 93.9 ± 4.21% at 2 mM ZnO vs. ~100% at 2 mM TiO2). Nanoparticle-induced cytotoxic effects under air-liquid cultivation conditions declined in RPMI2650, A549, TR146, and Calu-3 cells (~0.7 to ~0.2-fold), with increasing 50 to 100% barrier maturity under the influence of ZnO (2 mM). Cell viability in early and late mucosa barriers where hardly influenced by TiO2 as well as most cell types did not fall below 77% viability when added to Individual ALI cultures. Fully maturated bronchial mucosal cell barrier models cultivated under ALI conditions showed less tolerance to acute ZnO nanoparticle exposures (~50% remaining viability at 2 mM ZnO for 24 h) than the similarly treated but more robust nasal (~74%), buccal (~73%), and alveolar (~82%) cell-based models.

Time-Resolved Measurements of Interfacial Tension and Flow Speed of the Inclined Water Surface around a Self-propelled Camphor Boat by the Quasi-elastic Laser Scattering Method.

An inclined liquid surface, such as a meniscus, plays an important role in advection and transport phenomena at a liquid's surface. However, there is no time-resolved measurement method for the interfacial tension of an inclined liquid-air interface. Here, a noninvasive method for simultaneous measurements of the interfacial tension and surface flow speed for an inclined water surface is described. This is an upgrade of the quasi-elastic laser scattering method with a closed-loop control system that introduces the dynamically tracked scattered and referential light into the detector. For the evaluation of the tilt compensation by dynamic tracking, the relationship between the apparent interfacial tension and surface inclination was examined for a water meniscus at 0-5° inclinations. It was also demonstrated that simultaneous measurements of the interfacial tension and surface flow speed around a self-propelled camphor boat on a pure water surface inclined by >3° at the back end of the boat are difficult to conduct accurately without dynamic tracking. Both the interfacial tension difference and the backward flow speed increased as the boat speed increased to 0.1 m/s; that had not been evaluated to date because of the high velocity of the boat and the surface inclination of the water around it. The direct experimental evaluation of the interfacial tension and the flow speed supported the model that the driving force of the camphor boat is the interfacial tension difference and the resistance force proportional to the boat velocity reduces its acceleration.

Engulfment of a drop on solids coated by thin and thick fluid films

When an aqueous drop contacts an immiscible oil film, it displays complex interfacial dynamics. When the spreading factor is positive, upon contact, the oil spreads onto the drop's liquid–air interface, first forming a liquid bridge whose curvature drives an apparent drop spreading motion and later engulfs the drop. We study this flow using both three-phase lattice Boltzmann simulations based on the conservative phase field model, and experiments. Inertially and viscously limited dynamics are explored using the Ohnesorge number $Oh$ and the ratio between the film height $H$ and the initial drop radius $R$ . Both regimes show that the radial growth of the liquid bridge $r$ is fairly insensitive to the film height $H$ , and scales with time $T$ as $r\sim T^{1/2}$ for $Oh\ll 1$ , and as $r\sim T^{2/5}$ for $Oh\gg 1$ . For $Oh\gg 1$ , we show experimentally that this immiscible liquid bridge growth is analogous with the miscible drop–film coalescence case. Contrary to the growth of the liquid bridge, however, we find that the late-time engulfment dynamics and final interface profiles are significantly affected by the ratio $H/R$ .

Microphysics of liquid water in sub-10 nm ultrafine aerosol particles

Abstract. Ultrafine aerosol particles with sizes smaller than 50 nm have been shown in recent studies to serve as a large source of cloud condensation nuclei that can promote additional cloud droplet formation under supersaturation conditions. Knowledge of the microphysics of liquid water in these droplets remains limited, particularly in the sub-10 nm particle size range, due to experimental and theoretical challenges associated with the complexity of aerosol components and the small length scales of interest (e.g., difficulty of precisely sampling the liquid–air interface, questionable validity of mean-field theoretical representations). Here, we carried out molecular dynamics (MD) simulations of aerosol particles with diameters between 1 and 10 nm and characterized atomistic-level structure and water dynamics in well-mixed and phase-separated systems with different particle sizes, NaCl salinities, and pimelic acid (PML) organic surface loadings as a function of distance from the time-averaged Gibbs dividing interface or instantaneous water–air interface. We define a sphericity factor (ϕ) that can shed light on the phase-mixing state of nanodroplets, and we reveal an unexpected dependence of mixing state on droplet size. Our results also evidence an ion concentration enhancement in ultrafine aerosols, which should modulate salt nucleation kinetics in sub-10 nm droplets, and provide detailed characterization of the influence of droplet size on surface tension and on water self-diffusivity near the interface. Analysis of water evaporation free energy and water activity demonstrates the validity of the Kelvin equation and Köhler theory at droplet sizes larger than 4 nm under moderate salinities and organic loadings and the need for further extension to account for ion concentration enhancement in sub-10 nm aerosols, droplet-size-dependent phase separation effects, and a sharp decrease in the cohesiveness of liquid water in sub-4 nm droplets. Finally, we show that an idealized fractional surface coating factor (fs) can be used to categorize and reconcile water accommodation coefficients (α*) observed in MD simulations and experimental results in the presence of organic coatings, and we resolve the droplet size dependence of α*.

Durable Biomimetic Two-Tier Structured Superhydrophobic Surface with Ultralow Adhesion and Effective Antipollution Property.

Superhydrophobic surfaces with low adhesion have attracted great attention in recent years owing to their extensive applications. Enlightened by multifunctional rice leaves, a micro/nanobinary structured superhydrophobic surface was successfully fabricated on the Ti6Al4V substrate by photoetching, acid etching, alkaline etching, as well as fluorination treatments. Water droplets exhibited a Cassie impregnating wetting state on this superhydrophobic surface, under which the contact area fraction of the liquid-air interface caused by primary micron-scale stripped bumps (fp) and secondary nanoflower-like structures (fs) were calculated for the first time. The water adhesion force of this nonwetting surface was precisely measured as 7 μN, which was much lower than that (362 μN) of the original flat substrate and the previous reported surfaces. Moreover, this low-adhesive surface displayed good chemical stability after exposing to air, soaking in aqueous solutions (acid, alkaline, and salt), and cyclic icing/melting treatment. It also showed good mechanical durability after a series of abrasion treatments. Besides, this multifunctional superhydrophobic surface exhibited superior antipollution property to different kinds of contaminants. This multifunctional superhydrophobic surface displays a huge potential for industrial droplet transportation and self-cleaning applications.

Janus liquid marbles: Fabrication techniques, recent developments, and applications

AbstractLiquid marbles (LMs) are nonsticky droplets stabilized by hydrophobic solid particles that are adsorbed at the liquid–air interface. LMs are emerging as a potential platform in digital microfluidics, sensing applications, storage unit, biological incubation, and cosmetic applications. Incorporating multifunctional particles to form Janus liquid marble (JLM) could enhance the capabilities of the pristine LM. JLMs are multifunctional next‐generation LMs comprising two hemispherical domains of distinct physicochemical properties such as size, hydrophobicity, electrical conductivity, surface functionalization, stimuli‐responsivity, and so on. The JLMs offer precise control on the manipulability and enhanced performance over pristine LMs. Though the properties, applications, and progress of LMs are detailed in the recent literature, a focused review encompassing the fabrication, recent developments, potential applications of JLMs, and the challenges regarding its reliable fabrication remains a gap in the literature. The review provides insights into the importance of JLMs, systematically discussing the fabrication strategies, applications, challenges, and future directions.

Elucidating the Origins of High Preferential Crystal Orientation in Quasi‐2D Perovskite Solar Cells (Adv. Mater. 5/2023)

Perovskite Solar Cells In article 2208061, Lukas E. Lehner, Martin Kaltenbrunner, and co-workers report the controlled nucleation of quasi-2D perovskites at the liquid–air interface of the precursor solution. Restricting the initial crystal growth to the surface of the liquid results in highly aligned perovskite thin films with improved charge transport, enabling efficient solar cells with excellent stability.

Growth of chemical gardens in gaseous acidic atmospheres.

The generation of chemobrionic architectures through slow injection of aqueous silicate solution in gaseous TiCl4 is demonstrated. The tubes were characterized by XRD, SEM and wet chemistry control experiments, and their mechanism of formation was unraveled. These structures serve as laboratory models for calthemites or soda straws.

Structure Analysis of Adsorption Layers at Air-Liquid, Solid-Liquid, and Liquid-Liquid Interfaces with Neutron Reflectometry

Structure Analysis of Adsorption Layers at Air-Liquid, Solid-Liquid, and Liquid-Liquid Interfaces with Neutron Reflectometry

Advancing in situ single-cell microbiological analysis through a microwell droplet array with a gradual open sidewall.

The utilization of microfluidic analysis technology has resulted in the advancement of fast pathogenic bacteria detection, which can accurately provide information on biochemical reactions in a single cell and enhance detection efficiency. Nevertheless, the achievement of rapid and effective in situ detection of single-bacteria arrays remains a challenge due to the complexity of bacterial populations and low Reynolds coefficient fluid, resulting in insufficient diffusion. We develop microwell droplet array chips from the lateral hydrodynamic wetting approach to address this issue. The sidewall of the microwell gradually opens which aids in advancing the liquid-air interface and facilitates the impregnation of the solid microwells, preserving the Wenzel state and assisting in resisting the liquid force to separation from the drop. The feasibility of preparing cell arrays and identifying them inside the microwells was demonstrated through the simulated streamlined distribution of gradual and traditional microwells with different sizes. The water-based ink diffusion experiment examined the relationship between diffusion efficiency and flow velocity, as well as the position of the microwell relative to the channel. It showed that the smaller gradual microwell still has a good diffusion efficiency rate at a flow velocity of 2.1 μL min-1 and that the infiltration state is easier to adjust. With this platform, we successfully isolated a mixed population containing E. coli and S. aureus, obtained single-bacteria arrays, and performed Gram assays after in situ propagation. After 20 hours of culture, single bacteria reproduced demonstrating the capability of this platform to isolate, cultivate, and detect pathogenic bacteria.

Vertically oriented self-assembly of colloidal CdSe/CdZnS quantum wells controlled via hydrophilicity/lipophilicity balance: optical gain of quantum well stacks for amplified spontaneous emission and random lasing.

We propose and demonstrate vertically oriented self-assembly of colloidal quantum wells (CQWs) that allows for stacking CdSe/CdZnS core/shell CQWs in films for the purposes of amplified spontaneous emission (ASE) and random lasing. Here, a monolayer of such CQW stacks is obtained via liquid-air interface self-assembly (LAISA) in a binary subphase by controlling the hydrophilicity/lipophilicity balance (HLB), a critical factor for maintaining the orientation of CQWs during their self-assembly. Ethylene glycol, as a hydrophilic subphase, orients the coalition of these CQWs into self-assembled multi-layers in the vertical direction. Stacking CQWs into large micron-sized areas as a monolayer is facilitated by adjusting HLB with diethylene glycol addition as a more lyophilic subphase during LAISA. ASE was observed from the resulting multi-layered CQW stacks prepared via sequential deposition onto the substrate by applying the Langmuir-Schaefer transfer method. Random lasing was achieved from a single self-assembled monolayer of the vertically oriented CQWs. Here, highly rough surfaces resulting from the non-close packing nature of the CQW stack films cause strongly thickness-dependent behavior. We observed that in general a higher roughness-to-thickness ratio of the CQW stack films (e.g., thinner films that are intrinsically rough enough) leads to random lasing, while it is possible to observe ASE only in thick enough films even if their roughness is relatively higher. These findings indicate that the proposed bottom-up technique can be used to construct thickness-tunable, three-dimensional CQW superstructures for fast, low-cost, and large-area fabrication.

Sol–gel-derived nanoparticles coated liquid entities: liquid marbles, liquid plasticine, and flat interface

Particle-coated liquid entities are the basis of many useful products, such as foams, emulsions, bijels, and liquid marbles (LMs). Particles stabilized at the liquid–air interface have been widely explored during the past two decades. Such interfaces usually consist of micro- to nano-sized particles that often tend to aggregate and lose transparency as well as the smoothness of the interface. Recently, these obstacles have been overcome by using sol–gel-derived silica nanoparticle coating. Sol–gel-derived nanoparticles offer a monolayer structure at the interface with high transparency and reproducibility. This paper critically reviews three sol–gel-derived liquid entities: LMs, liquid plasticine (LP), and nanoparticle-coated flat interface. LM, a particle-covered droplet, was significantly explored for its application as a container in many biological and chemical processes. Unlike powder-derived LM, sol–gel-derived LM can be formed into any desired shape, commonly known as LP. Monolayer-covered sol–gel entities also offer a simple physical platform for investigating many fundamental properties of particle-coated interfaces. In the present review, starting from its preparation to the application, all critical aspects are summarized. Some unaddressed issues compared to the powder LM are also discussed for future research.

Probing Colloidal Assembly on Non-Axisymmetric Droplet Surfaces via Electrospray.

Microparticles trapped on the surface of a sessile droplet interact via electrostatic and capillary forces. The assembly of colloids at a fluid-fluid interface is governed by particle size, surface chemistry, and contact line roughness. We created nonspherical droplets using surface energy patterning and delivered microparticles to the liquid-air interface with electrospray atomization. Using a water droplet as the target, the particle assembly was observed over time. We found that the underlying surface energy pattern significantly influenced the colloidal assembly and drove particles toward the center of the droplet. The particles were arranged into a single, non-close-packed cluster with local hexagonal ordering but left a clear region with very few particles near the contact line. This depletion region is attributed to long-range electrostatic repulsion from the photoresist used to create the surface energy pattern, which retained electric charge from the electrospray. To understand the effect of electrostatic interactions, we explored target droplets with dissimilar dielectric properties. Using patterned substrates and electrospray for particle deposition, we can harness the assembly of colloids at a fluid interface to build repeatable monolayer patterns.