Solid-water Contact Research Articles

Simultaneous generation of hydroxyl and hydrogen radicals from H+/OH- pairs caused by water-solid contact electrification.

Water-solid contact electrification is a common physical phenomenon involving interfacial electron and ion transfer, recently discovered to trigger unique redox reactions. Here, we demonstrate the generation of both hydroxyl and hydrogen radicals when water contacts SiO2. The coexistence of hydroxyl and hydrogen radicals is confirmed by simultaneous nitrate reduction and nitrite oxidation during the contact. Increased density of hydroxyl groups on the SiO2 surface enhances its surface electronegativity before the contact, as well as boosting charge transfer and radical generation during the contact. We propose that the simultaneous generation of hydroxyl and hydrogen radicals originates from electron gain and loss between hydroxide anions in water and hydrogen cations adsorbed on the solid surface, which are ion pairs separated by the interfacial electric field. This discovery advances our understanding of redox processes induced by contact electrification.

Water-solid contact electrification and catalysis adjusted by surface functional groups

Water-solid contact electrification and catalysis adjusted by surface functional groups

Crystallization-driven evolution of water occurrence states with implications on dewaterability improvement of waste-activated sludge

This study proposed to improve the dewaterability of waste-activated sludge (WAS) through crystallization-driven evolution of water occurrence states. Primarily, the feasibility of clathrate hydrate (i.e., CO2 hydrate) formation in WAS was examined. The thermodynamic analysis indicated that the CO2 hydrate formation with the excessive water in WAS followed pseudo-first-order kinetics, and fit of the data yielded a kobs value of 3.905 × 10−5 L∙mol−1∙s−1 for 274.15 K. With the water conversion efficiency of 100%, the crystallization-dissociation process of CO2 hydrate significantly improved the dewaterability of WAS in term of capillary suction time (CST) decreasing from 251.5 s to 57.4 s. Also, the relief of gas pressure can induce the hydrate dissociation, which creates a novel way to recycle CO2 gas and save the consumption of chemicals required by sludge dewatering process. Regarding the mechanism of hydrates-based sludge dewatering, the evolution of water occurrence state was investigated. The in-situ synchrotron X-ray computed microtomography visually analyzed the micro-scale porosity and interstitial water of WAS flocs. The model of three-dimensional pore structure was established and the porosity parameters of solid aggregates were determined. It was found that the volume of connected pores and the total pore volume fraction of solid compositions increased. But the mean volume and mean area of isolated pores simultaneously decreased by 14.6% and 12.4%, respectively, which meant that the steric hindrance caused by isolated pores was weakened due to the reduced solid-water contact area. In addition, the crystallization of water caused the reformation of conformation arrangement of vicinal water and solid molecules, which highly organized the water molecules into the crystal structure. Accordingly, an estimation method for vicinal water layer thickness was developed based on atom force microscope. The thickness of vicinal water layer was found to be reduced by 77.4% and the hydration repulsion among solid compositions was correspondingly weakened, which facilitated the aggregation of solid compositions, and the relatively separated hydrate phase and solid phase could be formed. All the above results open up a novel strategy for enhanced water-solid separation of WAS through the crystallization-driven evolution of water occurrence states. As distinguished from the conventional approaches, the hydrates-based sludge dewatering enhances the water-solid separation only with regulating the spatial arrangement of water-solid molecules, but without altering the chemical compositions. Thus, more chances can be created to increase the environmentally friendly attributes related to WAS dewatering.

Can fluid-solid contact area quantify wettability during flow? – A parametric study

Wettability characterisation in a porous medium is challenging owing to the heterogeneity and large-scale of the interacting surface. Measuring the liquid-solid contact area can be used as a real-time wettability quantification at the Darcy scale. However, flow, grain size, and saturation path can affect the liquid-solid contact area. In this work, we use the two-tracer experiments to quantify the liquid-solid contact area and relate it with different parameters affecting the liquid-solid contact area. We do experiments at different conditions, i.e. (a) when the organic phase is at residual saturation and (b) when both phases flow. When the organic phase is immobile, increasing the flow rate does not change the residual saturation significantly; however, the water-solid contact area increases because of the increased corner flow. When both organic and aqueous phases flow, the relationship between the water saturation and water-solid contact area is found to be dependent on the grain size.

Detachment of Dodecane from Silica Surfaces with Variable Surface Chemistry Studied Using Molecular Dynamics Simulation.

The adsorption and detachment processes of n-dodecane (C12H26) molecules were studied on silica surfaces with variable surface chemistry (Q2, Q3, Q4 environments), using molecular dynamics simulations. The area density of the silanol groups varied from 9.4 to 0 per nm2. The shrinking of the oil-water-solid contact line was a key step for the oil detachment, due to water diffusion on the three-phase contact line. The simulation results showed that oil detachment was easier and faster on a perfect Q3 silica surface which had (≡Si(OH))-type silanol groups, due to the H-bond formation between the water and silanol groups. When the surfaces contained more Q2 crystalline type which had (≡Si(OH)2)-type silanol groups, less oil detached, due to the formations of H-bonds among the silanol groups. There were no silanol groups on the Si-OH 0 surface. Water cannot diffuse on the water-oil-silica contact line, and oil cannot detach from the Q4 surface. The detachment efficiency of oil from the silica surface not only depended on the area density, but also on the types of silanol groups. The density and type of silanol groups depend on the crystal cleavage plane, particle size, roughness, and humidity.

Water droplet impact on perfluoropolyether-based liquid-like surfaces

Water droplet impact on water-repellent surfaces is of relevance to many important engineering applications. Although the droplet impact behavior on water-repellent surfaces has been extensively investigated, in particular, textured superhydrophobic surfaces, that on water-repellent liquid-like surfaces has been barely reported. In this study, we experimentally investigated the oblique impact of water droplets on a surface with a liquid-like perfluoropolyether coating using a high-speed camera. The direct observation showed that all the impinging water droplets released from various heights exhibited similar outcomes, including an inelastic collision without rebound as well as a continuous sliding over the liquid-like surface. The liquid-like perfluoropolyether molecular chain on the surface was suggested to behave like a high viscosity oil layer that could cause more intense energy dissipation during the impact process. The crucial impact parameters of the maximum tangential droplet extension, the spreading time and the sliding length were further analyzed quantitatively by the methodology widely adopted in the exploration of the impact behavior in previous studies. It was indicated that the process of the impinging droplet spreading on the liquid-like surface was more likely dominated by the competition between the inertial and capillary forces although the increased viscous dissipation arising from the liquid-like layer significantly suppressed the rebound of the impinging droplet. Compared to superhydrophobic surfaces, the liquid-like surface could generate a stronger friction force for the slide motion of the impinging water droplet. The triboelectric effect arising from the water-solid contact was proven to inhibit the retraction of the impinging droplet with relatively higher impact velocities. This study is expected to improve the fundamental understanding of the droplet impact process and facilitate us to make more rational designs of liquid-like surfaces for relevant applications.

Water-solid contact electrification causes hydrogen peroxide production from hydroxyl radical recombination in sprayed microdroplets.

Contact electrification between water and a solid surface is crucial for physicochemical processes at water-solid interfaces. However, the nature of the involved processes remains poorly understood, especially in the initial stage of the interface formation. Here we report that H2O2 is spontaneously produced from the hydroxyl groups on the solid surface when contact occurred. The density of hydroxyl groups affects the H2O2 yield. The participation of hydroxyl groups in H2O2 generation is confirmed by mass spectrometric detection of 18O in the product of the reaction between 4-carboxyphenylboronic acid and 18O-labeled H2O2 resulting from 18O2 plasma treatment of the surface. We propose a model for H2O2 generation based on recombination of the hydroxyl radicals produced from the surface hydroxyl groups in the water-solid contact process. Our observations show that the spontaneous generation of H2O2 is universal on the surfaces of soil and atmospheric fine particles in a humid environment.

Surface wettability effect on heat transfer across solid-water interfaces

• Mechanisms of surface wettability effect on interfacial heat transfer are unraveled. • Hydrophilic surface promotes heat transfer through enhanced hydrogen-bonded network. • Molecular roughness with high biphasic contact area is beneficial to heat transfer. • Multiple and shallow vacancies are better than the deep but fewer ones on substrate. Although it has been found that the heat transfer across solid–liquid interface can be affected by the surface wettability, the underlying mechanism remains ambiguous. Herein non-equilibrium molecular dynamics simulations are conducted to unravel the microscopic mechanisms of heat transfer across solid-water interfaces. Specifically, two aspects of the solid surface wettability are investigated including modifying surface chemistry and introducing molecular roughness. We find that enlarging the solid-water interaction strength facilitates the formation of hydrogen-bonded network of water, hence enhancing the interfacial heat transfer. In addition, introducing molecular roughness can promote heat transfer by increasing the solid-water contact area. When the contact area remains the same, the substrates with multiple and shallow vacancies are more beneficial to heat transfer than those with deep but fewer vacancies. This work sheds insights into the mechanistic understanding of the heat transfer process and provides theoretical guidance for the development of efficient heat transfer devices.

Controllable and Scalable Fabrication of Superhydrophobic Hierarchical Structures for Water Energy Harvesting

We report a controllable and scalable fabrication approach for the superhydrophobic hierarchical structures and demonstrate the excellent ability to harvest water energy when applied to water-solid contact triboelectric nanogenerator (TENG). A strategy combined with multiple photolithography and micromolding process was developed to accurately regulate the diameters and the center distances of the two-level micropillars. A variety of hierarchical structures were successfully fabricated and presented the advantages of structure control, large scale, high accuracy, and high consistency. The hydrophobic property characterizations were conducted, and the results indicated that the hierarchical structures showed a larger contact angle than the single-level structures and achieved superhydrophobicity. Then the hierarchical structures were applied to water-TENGs with flowing water continuously dripping on, and the effect of the structure parameter on the triboelectric output was analyzed. The hierarchical structures exhibited a superior ability to harvest water energy than the flat film and the single-level structures due to the enhanced friction area and superhydrophobic property. At a flowing velocity of 8 mL/s, the hierarchical structure generated the output voltage of approximately 34 V and the short-circuit current of around 5 μA. The water-TENG device exhibited a power density peak of 7.56 μW/cm2 with a resistive load of 16.6 MΩ at a flowing velocity of 10 mL/s. These findings shed light on the potential applications of the hierarchical structures-based water-TENGs to water energy harvesting and self-powered sensor devices.

Electrohydrodynamic jet printed silver-grid electrode for transparent raindrop energy-based triboelectric nanogenerator

A transparent water-solid contact triboelectric nanogenerator (TENG) integrated with a solar cell is expected to be a reliable source of stable energy during both dry and wet seasons. This is particularly significant in countries with tropical climates, where both seasons are pronounced. In this paper, a highly transparent and conductive electrohydrodynamic jet-printed Ag nanoparticle (NP) electrode-based TENG (PA-TENG) was developed and integrated with a solar cell. The fabricated PA-TENG exhibited a maximum power output of 1.17 W/m2, and average and maximum optical transmittance of 91% and 96%, respectively. The conventional indium tin oxide (ITO) electrode was selected to serve as a control group. ITO-TENG exhibited maximum and average optical transmittances of 92% and 88%, respectively. In addition, its power output was approximately three times lower than that of PA-TENG. This performance improvement of PA-TENG is attributed to the effective charge-inducing ability caused by the high contact barrier at the AgNP/polydimethylsiloxane (PDMS) interface as well as the high conductivity of the printed AgNP electrode (sheet resistance: 3.6 Ω/sq). We also find that the PA-TENG exhibits the highest transparency and power output among the reported data of the transparent water-solid contact TENG. Finally, integration of the solar cell within PA-TENG reduced its power density by 3.6% while it reduced that of ITO-TENG by 5.2%. This indicates that PA-TENG is more suitable for integration with a solar cell than ITO-TENG on both rainy and sunny days. By estimating the loss in electricity in the solar cell induced by the opacity of PA-TENG and the electricity generated by it, we concluded that PA-TENG integrated with a solar cell could serve as a potential source of stable eco-friendly energy in regions with a tropical wet-dry climate.

Screw Pump‐Type Water Triboelectric Nanogenerator for Active Water Flow Control

Triboelectric nanogenerators (TENGs) have been spotlighted due to the high efficiency, low cost, and easy accessibility. Despite their advantage, TENGs exhibit certain limitations such as a low lifetime. To overcome the limitation, the use of water–solid contact TENGs has been suggested. Most of the studies about water–solid contact TENG are focused on optimizing the device to adjust to the stagnant water or natural water movement. However, in most actual fluid machinery applications, the flow of water can be changed by mechanical components. The development of the water–solid contact TENG that actively changes water movement can broaden the application of TENGs. Herein, a screw pump TENG (SP–TENG) that can generate electricity by pumping water is developed. By analyzing contact surface area between water and screw in different screw rotational velocity and electrical difference from different electrode area, the device is able to optimize. The developed SP–TENG can light four light‐emitting diodes (LEDs) and activate temperature sensor. In addition, because SP–TENG can function as an active water TENG, it can enable the circulation of water to improve the ecosystem. Thus, SP–TENG can be used as an ecofriendly device that can simultaneously harvest energy and induce self‐purification of water.

Triboelectricity-based self-charging droplet capacitor for harvesting low-level ambient energy

Abstract In this study, we propose a self-charging droplet capacitor for harvesting low-level mechanical energy. The capacitor comprises of a conductive liquid droplet, which is placed on a heterogeneous and hydrophobic surface of dielectric materials coated onto a conductive substrate. The substrate and the droplet, along with the dielectric materials in between, form a parallel-plate type capacitor. The droplet is free to move on the surface, and thus, enables the position-dependent variation of capacitance. The surface consists of two regions, each with a different material and thickness. The different strengths of solid-water contact electrification of the two materials give rise to a self-charging mechanism. The variation in thickness allows for the capacitance change required for energy harvesting. A device is fabricated with two droplet capacitors and one ceramic capacitor. Passive diode switches are used to enable reconfiguration of the connectivity of the capacitors, which leads to a geometrical growth of the energy in the system. With a 450 μL water drop in each capacitor, the device can effectively harvest energy from low ambient vibrations. The energy harvested grows by 100 times within 11 cycles, sufficient to illuminate 30 LEDs connected in series.

Colloid transport through soil and other porous media under transient flow conditions—A review

AbstractUnderstanding colloid transport in porous media under transient‐flow conditions is crucial in understanding contaminant transport in soil or the vadose zone where flow conditions vary constantly. In this article, we provide a review of experimental studies, numerical approaches, and new technologies available to determine the transport of colloids in transient flow. Experiments indicate that soil structure and preferential flow are primary factors. In undisturbed soils with preferential flow pathways, macropores serve as main conduits for colloid transport. In homogeneously packed soil, the soil matrix often serves as filter. At the macroscale, transient flow facilitates colloid transport by frequently disturbing the force balance that retains colloids in the soil as indicated by the offset between colloid breakthrough peaks and discharge peaks. At the pore‐scale and under saturated condition, straining, and attachment at solid–water interfaces are the main mechanisms for colloid retention. Variably saturated conditions add more complexity, such as immobile water zones, film straining, attachment to air–water interfaces, and air–water–solid contact lines. Filter ripening, size exclusion, ionic strength, and hydrophobicity are identified as the most influential factors. Our review indicates that microscale and continuum‐scale models for colloid transport under transient‐flow conditions are rare, compared to the numerous steady‐state models. The few transient flow models that do exist are highly parameterized and suffer from a lack of a priori information of required pore‐scale parameters. However, new techniques are becoming available to measure colloid transport in real‐time and in a nondestructive way that might help to better understand transient flow colloid transport.This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Quality

Recent Progress in Power Generation from Water/Liquid Droplet Interaction with Solid Surfaces

AbstractWater covers ≈70% of the Earth's surface and it contains a tremendous amount of energy that remains unexploited. With the advance of nanotechnology, new strategies toward harnessing water energy based on new mechanisms are proposed. Here, the interaction mechanisms between a water droplet and a solid surface for harvesting energy, including water–solid contact electrification in the four basic working modes of a triboelectric nanogenerator and streaming current, are reviewed. Among them, nanogenerators based on the contact electrification show the highest output. Practical applications are also presented, such as sensing application, wearable power generation, all‐weather power generation, and blue energy solutions. At last, perspectives and opportunities for using water/liquid‐based energy are discussed.

Hybrid nanogenerator and enhancement of water–solid contact electrification using triboelectric charge supplier

Artificially embedded surface charges can dramatically improve the output energy of water–solid contact electrification devices, but the embedded charges easily escape or are cancelled out by adsorption of opponent ions in ambient conditions. Here, we demonstrate a hybrid energy harvesting nanogenerator by combing water–solid and solid–solid contact electrification schemes. It consists of a water–solid contact electrification device and wind-fluttering ribbons that regularly supplies triboelectric surface charges on the solid surface and hence, the high density of surface charges can remain on the surface. This scheme can enhance the harvesting energy by up to a factor of 30 during rain. Moreover, the fluttering ribbons independently collect electric energy from mild winds without rain. This scheme provides a facile, cost-effective, and robust approach to enhance the energy-harvesting efficiency of a water–solid contact electrification device and to combine the energy harvesting from simultaneous rain and wind.

Theoretical study of micro/nano roughness effect on water-solid triboelectrification with experimental approach

The application of micro and nano surface roughness in solid-solid contact electrification based triboelectric nanogenerators (TENGs) has been a widely accepted method for pursuing higher energy output. However, the roughness enhancement needs to be reconsidered in the case of TENGs based on water-solid contact electrification because water-solid contact has clearly different interfacial behavior from solid-solid contact. This study reports a theory for investigating the effect of micro/nano roughness on water-solid contact electrification. The relation between micro/nano roughened surface and water-solid contact electrification is developed and specified by utilizing wetting and rheological properties such as fraction of the area in contact and thickness of the air gap layer. For the fabrication of water-driven TENG (WTENG), aluminum plates are anodized and fluorinated to form the dielectric layer and triboelectric layer, respectively. Three types of surface roughness were fabricated by adjusting the fabrication conditions. The WTENG with the lowest roughness shows approximately 14 times higher current and 9 times higher voltage than the WTENGs with higher roughness. The theoretical equations for water-solid contact electrification correspond well with the experimental results with high accuracy. The theory derived in this work is the first insightful perspective on roughness related liquid-solid electrification.

Physical properties controlling water repellency in synthesized granular solids

SummaryThe wettability of granular solids such as soil is known to depend primarily on two factors: their inherent chemistry and their physical properties, such as their particle size, particle shape and surface roughness. Nevertheless, the distinctive physical properties of such materials have not been fully explored to gauge their wettability. In this study, the difference in wettability between a flat solid (microscope slide) and three granular solids, namely glass beads (GB), Leighton Buzzard Sand (LBS) and crushed Glass (CG), which have different physical properties, were examined. The effect of chemistry was isolated by strongly hydrophobizing the above materials by treatment with dimethyldichlorosilane. Wettability measurements were made by measuring the water–solid contact angle (CA) by the sessile drop method after adhering one layer of uniformly oriented granular solids on to double‐sided adhesive tape initially attached to a microscope slide. Techniques for particle characterization included sieving for particle size, dynamic image analysis for particle shape and confocal laser microscopy to determine surface roughness. Results show that all CAs of the granular solids exceeded that of the hydrophobized microscope slide (103°). The crushed glass had the largest CA (125°). With all three granular solids, there was an increase in CAs as particle size decreased. In addition, as particles became more angular, CAs increased. The influence of shape on wettability became more predominant as particle size decreased. The surface roughness parameter, Ra, was investigated and shown to be sensitive to both the size and shape of the particles. A decrease in Ra from 95.4 to 34.1 μm increased CAs from 107 to 125°. A similar change in CA was shown to correspond to an increase in void fraction from 40.7 to 77.4%. Our results have practical implications for the optimum use of soil by enhancing or suppressing water repellency.Highlights How do the physical properties of granular solids such as soil influence their wettability? Effects of physical properties of particles on CAs were investigated when isolated from effect of chemistry. Particle shape becomes more important in gauging wettability as particle size decreases. Wettability of granular solids may be physically controlled by specific particle characteristics.

Molecular Dynamics Study on the Effect of Surface Hydroxyl Groups on Three-Phase Wettability in Oil-Water-Graphite Systems.

In this paper, a hydroxylated graphite surface is generated as a hydrophilic oleophobic material for the application of oil-water separation, and the effects of hydroxyl density on the three-phase wettability are studied in oil-water-graphite systems. We analyze the adsorption of water molecules on the hydroxylated surfaces and obtain the relationship between water-oil-solid interfacial properties and the hydroxyl density, which results from the synthetic effects of the orientation of molecules and hydrogen bonds. With the increase of hydroxyl density, the water-solid contact angle first decreases rapidly, and then remains constant. The density of the hydrogen bond formed between hydroxyls and water molecules in the adsorption layer can explain the regularity of the three-phase wettability. The orientation of the water molecules in the adsorption layer shows insignificant variation, owing to the hydrogen bond network formed between the water molecules; thus, little change is observed in the hydrogen bond density in the adsorption layer.

Deciphering buried air phases on natural and bioinspired superhydrophobic surfaces using synchrotron radiation-based X-ray phase-contrast imaging

Superhydrophobicity is an important phenomenon in nature that inspires the design of numerous biomimetic functional materials. A superhydrophobic surface is expected to have a three-phase solid–liquid-vapor interface. To directly image the buried air phase in wetted superhydrophobic surfaces, we employed synchrotron radiation-based X-ray phase-contrast imaging to non-invasively probe the surfaces of natural lotus leaves and artificial carbon nanotube films in three dimensions. Reconstructed images of the three-phase distribution surrounding the superhydrophobic surfaces were presented with high resolution and contrast. Further extraction of the phases enabled direct analysis of the relationship between the surface morphology and its wetting property. We found that the protruding micro/nano-structures trapped a layer of air up to 14 μm thick, which played an important role in the observed superhydrophobicity. Direct evidence of the presence of a buffer layer air cushion deepens our understanding of hydrophobicity and opens new opportunities for designing novel bioinspired materials. Synchrotron X-ray imaging has given researchers the clearest view yet of how trapped air pockets can make surfaces extremely water repellent. Electron microscopy has helped researchers determine that rough, micro and nanoscale textures on natural surfaces such as lotus leaves cause water droplets to bead up. But below the droplets are hidden air cushions that elude direct observation. Now, Chunhai Fan and colleagues in Shanghai, China, have used non-invasive synchrotron radiation to clarify the role of the air phase in superhydrophobic lotus leaves and carbon nanotube films. High-colour and high-resolution images of interfacial electron density revealed that individual air pockets prevent water from infiltrating the surfaces grooves and can connect into large cushions that dramatically reduce the solid-water contact area. Any protruding structures on the superhydrophobic surfaces serve mainly as droplet anchor points. Synchrotron radiation-based X-ray phase-contrast imaging was employed to observe the solid–liquid-vapor interface on superhydrophobic surfaces. The reconstructed 3D images revealed that the water exists in the Cassie-to-Wenzel transit state. The micro/nano-structures trapped a large amount air on the supherhydrophobic surface to repel water.

3-Dimensional broadband energy harvester based on internal hydrodynamic oscillation with a package structure

Energy harvesting techniques which convert ambient waste energy into usable electrical power have emerged with the increased energy demand and rapid growth of self-powered systems. Among them, the triboelectric nanogenerator (TENG), which utilizes contact-electrification, has attracted a great deal of attention owing to its high output energy, good cost-effectiveness, and simple fabrication process. Here, the TENG harvesting ambient mechanical energy based on internal hydrodynamic oscillation (Hy-TENG), is demonstrated. The Hy-TENG shows an output voltage and current of 22V and 1.45µA, respectively, under vibration of 5Hz. The maximum instantaneous power density is 26.5mW/m2. By utilizing water in the mechanical energy harvesting process, several powerful advantages are also guaranteed due to the shape adaptability of water. First, both a wide bandwidth and a low resonance frequency are verified, which are advantageous for applications based on human motion. These frequency response characteristics indicate the strong potential of the Hy-TENG in actual environments. Furthermore, the Hy-TENG is highly scalable, as water can easily be restored to its initial state without a mechanical conversion apparatus. Finally, Hy-TENG harvesting 3-dimensional random motion (3-D Hy-TENG) is developed with a simple design based on the easily shape-transformable characteristic of water. Additionally, enhanced endurance against environmental factors and mechanical damage can be expected based on the packaged structure and on the non-destructive water–solid contact.