Spreading dynamics of a droplet upon impact with a liquid film containing solid particles.

This study examines how a deionized water droplet behaves when it centrally collides with a liquid film containing TiO2 nanoparticles at low impact velocities, aiming to understand how nanoparticles affect droplet spreading, in particular its maximum spreading diameter. Typically, we found that both the spreading velocity and dynamic contact angle of the droplet would be similarly affected by increasing TiO2 nanoparticle concentration. During retraction, the droplet's dimensionless spreading diameter oscillates, with more pronounced oscillations at higher nanoparticle concentrations. Moreover, both the droplet's maximum dimensionless rebound height and dynamic contact angle show similar trends with increasing TiO2 nanoparticle concentration. Interestingly, we proved that the influence of the solid-liquid interaction (Stokes force) on the fluid during the spreading process accounts for less than 2% of the surface energy when the droplet reaches its maximum spreading diameter, indicating a negligible effect on droplet spreading. We hypothesize that the droplet's initial energy is fully converted into surface energy and viscous dissipation at maximum spreading diameter, which involves viscous dissipation both between the fluid and the solid wall surface and the fluid and solid particle surface. Based on this, we developed a model for predicting the droplet's maximum spreading diameter that includes parameters associated with the solid particles. Compared to models in the literature that do not consider the effect of solid particles, our model aligns more closely with experimental data. The results indicate that adding solid particles leads to increased viscous dissipation, which in turn reduces the droplet's maximum spreading diameter.

Density Analysis of Adsorption Phase in the Thermodynamic Study of Shale Gas Adsorption.

Understanding the adsorption mechanism and precisely predicting the thermodynamic adsorption properties of methane at high pressure are crucial while very challenging for shale gas development. In this study, we demonstrated that the Langmuir adsorption model combining with different empirical methods to determine the adsorption phase density makes the calculated isothermal adsorption heat violate Henry's law at low pressure. For instance, the isothermal adsorption heat calculated by the Langmuir-Freundlich model contradicts Henry's law when the absolute adsorption quantity is zero. Given the current challenge in accurately calculating the adsorption phase density, it is necessary to impose constraints on the parameters of the adsorption model by adhering to Henry's law to maintain thermodynamic consistency. We found that the adsorption phase volume of methane molecules lies between the micropore volume and the total pore volume when shale adsorption reaches saturation. The adsorption mechanism involves not only filling micropores but also monolayer adsorption in meso-macro pores. The high-energy adsorption sites for methane are primarily concentrated in organic matter, while within these methane adsorption areas in shale, the high-energy adsorption sites for water are mainly located in kaolinite within clay minerals. The zero-pressure heat of adsorption is a temperature-independent thermodynamic index, yet it is influenced by the water content. It can therefore be selected as a quantitative measure to evaluate the impact of methane adsorption on water.

Integrated Janus Evaporator with an Enhanced Donnan Effect and Thermal Localization for Salt-Tolerant Solar Desalination and Thermal-to-Electricity Generation.

Solar-driven interfacial evaporation (SIE) technology has great advantages in seawater desalination. However, during the long-term operation of a solar evaporator, salts can be deposited on the solar absorbing surface, which, in turn, hinders the evaporation process. Therefore, there is an urgent need to propose new antisalt strategies to solve this problem. Here, we present a novel cogeneration system leveraging a salt-tolerant, heterogeneous Janus-structured evaporator (FHJE) for simultaneous solar desalination and thermoelectric generation. The top evaporation layer is composed of a graphene-based photothermal membrane pre-embedded with Fe3+ cations, which enhanced solar absorption and energy conversion abilities. Meanwhile, the Fe3+ cations further contribute to the Donnan effect, effectively repelling salt ions in saltwater. The bottom layer comprises a hydrogel composed of hydrophilic phytic acid (PA) and poly(vinyl alcohol) (PVA), fostering facilitation of water transport. The FHJE was demonstrated to exhibit evaporation rate and efficiency as high as 3.655 kg m-2 h-1 and 94.7% in 10 wt% saltwater, respectively, and superior salt resistance ability without salt accumulation after 8 h of continuous evaporation (15 wt%). Furthermore, a hydropower cogeneration evaporator device was constructed, and it possesses an open-circuit voltage (VOC) and a maximum output power density of up to 143 mV and 1.33 W m-2 under 1 sun, respectively. This study is expected to provide new ideas for comprehensive utilization of solar energy.

Droplet Rolling Transport on Hydrophobic Surfaces Under Rotating Electric Fields: A Molecular Dynamics Study.

Driving droplets by electric fields is usually achieved by controlling their wettability, and realizing a flexible operation requires complex electrode designs. Here, we show by molecular dynamics methods the droplet transport on hydrophobic surfaces in a rolling manner under a rotating electric field, which provides a simpler and promising way to manipulate droplets. The droplet internal velocity field shows the rolling mode. When the contact angle on the solid surface is 144.4°, the droplet can be transported steadily at a high velocity under the rotating electric field (E = 0.5 V nm-1, ω = π/20 ps-1). The droplet center-of-mass velocities and trajectories, deformation degrees, dynamic contact angles, and surface energies were analyzed regarding the electric field strength and rotational angular frequency. Droplet transport with a complex trajectory on a two-dimensional surface is achieved by setting the electric field, which reflects the programmability of the driving method. Nonuniform wettability stripes can assist in controlling droplet trajectories. The droplet transport on the three-dimensional surface is studied, and the critical conditions for the droplet passing through the surface corners and the motion law on the curved surface are obtained. Droplet coalescence has been achieved by surface designs.

Transforming Droplets into Liquid Films in Nanochannels under Rotating Electric Fields Studied by Molecular Dynamics.

It is first proposed to use a rotating electric field to stretch a droplet into a liquid film pinned to the insulated channel inner wall as a new type of active liquid valve. The molecular dynamics (MD) simulations are performed to prove that droplets in nanochannels can be stretched and expanded into closed liquid films under the action of rotating electric fields. The variations of the liquid cross-sectional area and droplet surface energy with time are calculated. The liquid film formation occurs mainly through two modes: gradual expansion and liquid column rotation. In most cases, increasing the electric field strength and angular frequency favors liquid film closing. At higher angular frequencies, decreasing the angular interval favors liquid film closing. The opposite is true at lower angular frequencies. The process of closing the hole-containing liquid film, which has formed a dynamic equilibrium, is a surface energy increase process, which requires greater electric field strength and angular frequency.

Solar Hydrocarbons: Single-Step, Atmospheric-Pressure Synthesis of C2 -C4 Alkanes and Alkenes from CO2.

Cobalt ferrite (CoFe2 O4 ) spinel has been found to produce C2 -C4 hydrocarbons in a single-step, ambient-pressure, photocatalytic hydrogenation of CO2 with a rate of 1.1 mmol g-1 h-1 , selectivity of 29.8 % and conversion yield of 12.9 %. On stream the CoFe2 O4 reconstructs to a CoFe-CoFe2 O4 alloy-spinel nanocomposite which facilitates the light-assisted transformation of CO2 to CO and hydrogenation of the CO to C2 -C4 hydrocarbons. Promising results obtained from a laboratory demonstrator bode well for the development of a solar hydrocarbon pilot refinery.

The enhanced photocatalytic and photothermal effects of Ti3C2 Mxene quantum dot/macroscopic porous graphitic carbon nitride heterojunction for Hydrogen Production.

A new heterostructure between Ti3C2 MXene quantum dot and 3D macroscopic porous graphitic carbon nitride (PGCN) was successfully obtained by integrating Ti3C2 quantum dots onto porous graphitized carbon nitride (Ti3C2QDs/PGCN) using in situ electrostatic self-assembly techniques. The photocatalytic H2 evolution rate of optimized 5.5 wt% Ti3C2 QD/PGCN composites is nearly 15.24 and 3.53 times higher than pristine CN, and PGCN, respectively. Ti3C2 quantum dots can significantly enhance the hydrogen production activity of PGCN. In addition, their good photothermal conversion ability accelerates the overall reaction process and enhances the light absorption and carrier density. Furthermore, to elucidate the photocatalytic mechanism, a series of tests involving electron spin resonance (ESR) and density functional theory (DFT) calculations were performed. The results confirmed that the Schottky barrier between PGCN and Ti3C2 QD can effectively promote spatial charge separation and significantly improve the photocatalytic performance. This work provides a new approach for the construction of photocatalytic systems and the application of MXene QD.

Enhanced photocatalytic activity by regulating charge transferring: Unveiling the decisive role of cerium oxide crystal-facet engineering over heterojunction.

Heterojunctions have been verified to be effective for separation of photogenerated electrons and holes, therefore improving the photocatalytic efficiency. Meanwhile, cerium oxide (CeO2) is an ideal semiconductor for studying the influence of different exposed crystal facets on regulation of electron transport pathways over heterojunctions. Herein, various kinds of crystal facet-dependent CeO2/g-C3N4 (graphitic carbon nitride) heterojunctions have been successfully engineered as representative model catalysts, and their critical role in regulating charge transfer pathways has been confirmed by systemic characterizations. It was found that facet-dependent heterojunctions followed different charge transport pathways, leading to different H2 evolution activities. In detail, heterojunctions with (100) and (110) exposed surfaces followed the Z-scheme transport pathways, while heterojunction with (111) exposed surface followed the type-II pathway. The H2 evolution rates via these three kinds of heterojunctions were determined to be 3.084, 1.925, and 1.128 mmol·g-1·h-1, respectively, which were 13.3, 7.9, 4.2 times that of bare g-C3N4. It's revealed that the different exposed crystal facets of CeO2 with different Fermi levels determine the transport pathways of photogenerated carriers. This work shows an example of controlling photocatalytic activity by facet-dependent heterojunctions and reveals the importance role of crystal-facet engineering toward heterojunction construction, which is expected to provide an important guidance for the design of new photocatalytic systems.

Optimal electrical conductivity and interfacial polarization induced by loaded nanoparticles on carbon nanotubes for excellent electromagnetic wave absorption performance.

Carbon materials have aroused wide attention in the field of electromagnetic wave absorption because of their advantages of good electrical conductivity, low density and adjustable structure. To obtain enhanced performance of electromagnetic wave absorption, the elaborate design of structures for carbon materials has become essential. In this work, the nitrogen-doped and large diameter carbon nanotubes modified with cobalt nanoparticles (M-Co/C-CNTs) composites were successfully prepared by adsorption and carbonization of the gases generated by organic matter pyrolysis. It is concluded that the enhanced conduction loss and polarization loss are attributed to the interfacial electronic engineering induced by the sensibly loaded cobalt nanoparticles and nitrogen doping. As a result, the samples achieved a broad effective absorption bandwidth of 4.56 GHz, and a strong reflection loss of -52.2 dB with a thin thickness of 2.1 mm. This work proposes a tailored way to fabricate the large diameter carbon nanotube composites and enhance electromagnetic wave absorption through novel structural modulation.

Efficient photothermal-assisted photocatalytic hydrogen production over a plasmonic CuNi bimetal cocatalyst.

It is challenging to maximize the utilization of solar energy using photocatalysis or photothermal catalysis alone. Herein, we report a full spectrum solar energy driven photothermal-assisted photocatalytic hydrogen production over CuNi bimetallic nanoparticles co-loaded with graphitized carbon nitride nanosheet layers (CuxNiy/CN) which are prepared by a facile in-situ reduction method. Cu5Ni5/CN shows a high hydrogen production rate of 267.8 µmol g-1 h-1 at room temperature, which is 70.5 and 1.34 times of that for pure CN (3.8 µmol g-1 h-1) and 0.5 wt% Pt/CN (216 µmol g-1 h-1), respectively. The photothermal catalytic hydrogen activity can be further increased by 3.7 times when reaction solution is external heated to 100 °C. For the photothermal catalytic system, the local surface plasmon resonance (LSPR) effect over active Cu nanoparticles can absorb near-infrared light to generate hot electrons, which are partially quenched to generate heat for heating of the reaction system and partially transported to the active sites, where the Ni nanoparticles as another functional component couple the electrons and heat to finally promote the photothermal catalytic activity. Our result suggests that a rational design of the catalyst with bifunctional atomic components can photothermocatalysis-assisted photocatalysis to maximize utilization solar energy for efficient full spectrum conversion.

Polymer Photoelectrodes for Solar Fuel Production: Progress and Challenges.

Converting solar energy to fuels has attracted substantial interest over the past decades because it has the potential to sustainably meet the increasing global energy demand. However, achieving this potential requires significant technological advances. Polymer photoelectrodes are composed of earth-abundant elements, e.g. carbon, nitrogen, oxygen, hydrogen, which promise to be more economically sustainable than their inorganic counterparts. Furthermore, the electronic structure of polymer photoelectrodes can be more easily tuned to fit the solar spectrum than inorganic counterparts, promising a feasible practical application. As a fast-moving area, in particular, over the past ten years, we have witnessed an explosion of reports on polymer materials, including photoelectrodes, cocatalysts, device architectures, and fundamental understanding experimentally and theoretically, all of which have been detailed in this review. Furthermore, the prospects of this field are discussed to highlight the future development of polymer photoelectrodes.

Enhanced charge separation in La2NiO4 nanoplates by coupled piezocatalysis and photocatalysis for efficient H2 evolution.

Photocatalytic hydrogen evolution is one promising method for solar energy conversion, but the rapid charge recombination limits its efficiency. To this end, in this work, grain size, and hence the charge carrier migration path, is reduced by lowering the synthesis temperature of two-dimensional visible light-responsive La2NiO4 perovskite. Interestingly, the hydrogen yield for the piezoelectric response of La2NiO4 under only 40 kHz ultrasonic vibration is as high as 680 µmol h-1 g-1, which is 80 times that under only 600 mW cm-2 visible light irradiation. More surprisingly, the hydrogen production rate under both light illumination and ultrasonic vibration is 129 times higher than under visible light irradiation alone. Clearly, a synergistic effect exists between piezocatalysis and photocatalysis. The hydrogen production activity of the samples with water splitting can reach 1097 µmol h-1 g-1 without any sacrificial reagent or co-catalyst, when the light intensity reaches about 1000 mW cm-2, which is a much higher hydrogen evolution rate by piezo-photocatalysis than is achieved by either piezocatalysis or photocatalysis individually. Further analysis indicates that the internal electric field generated by deformation of the La2NiO4 edge under piezoelectric action facilitates the directional separation and migration of photogenerated charges, which in turn significantly enhances the efficiency of use of photogenerated charges for hydrogen production. The investigation here provides a novel approach to design a new reaction system for hydrogen production by coupling multiple external physical fields.

Significantly enhanced photothermal catalytic hydrogen evolution over Cu2O-rGO/TiO2 composite with full spectrum solar light.

Reduced graphene oxide (rGO) has conspicuous photothermal characteristics in photothermal applications. Thus in our previous work, we used reduced graphene oxide (rGO) supported titanium dioxide (TiO2) nanocomposite (rGO/TiO2) to absorb the ultraviolet and infrared light in the photothermal hydrogen evolution process. In order to make use of the full spectrum solar energy into other clear energy, the visible light should be also considered in following research. Herein, we report a cuprous oxide (Cu2O) decorated reduced graphene oxide (rGO) supported titanium dioxide (TiO2) (Cu2O-rGO/TiO2) catalysts, which can absorb full spectrum solar light in an innovative way. The Cu2O-rGO/TiO2 catalyst is synthesized through a one-step hydrothermal method. The rates of hydrogen evolution are 17800 µmol·g-1h-1 under photothermal condition (90°C), 3800 µmol·g-1h-1 under photocatalysis condition only (25°C) and 0 µmol·g-1h-1 under thermal catalysis condition only. The result of photothermal catalytic hydrogen evolution rate is about 4.7 times that of the sum of the photocatalytic and thermal reactions. The photothermal synergetic effect promotes the photo-generated electron-holes separation through the rGO due to the temperature rising, and accelerates the reaction rates on the catalyst surface in hydrogen evolution process simultaneously. This work could provide us a new promising way for the conversion of full spectrum solar energy to hydrogen energy.

Generating Shear Flows without Moving Parts by Thermo-osmosis in Heterogeneous Nanochannels.

Shear flows play critical roles in biological systems and technological applications and are achieved experimentally using moving parts. However, when the system size is reduced to micro- and nanoscale, fabrication of moving parts becomes exceedingly challenging. We demonstrate that a heterogeneous nanochannel composed of two parallel walls with different wetting behaviors can generate shear flow without moving parts. Molecular dynamics simulations show that shear flows can be formed inside such a nanochannel under a temperature gradient. The physical origin is that thermo-osmosis velocities with different rates and directions can be tuned by wetting behaviors. Our analysis reveals that thermo-osmosis is governed by surface excess enthalpy and nanoscale interfacial hydrodynamics. This finding provides an efficient method of generating controllable shear flows at micro- and nanoscale confinement. It also demonstrates the feasibility of using fluids to drive micromechanical elements via shear torques generated by harvesting energy from temperature differences.

Hollow Carbon Sphere-Modified Graphitic Carbon Nitride for Efficient Photocatalytic H2 Production.

A novel hybrid photocatalyst composed of hollow carbon nanospheres (NCS) and graphitic carbon nitride (CN) curly nanosheets has been prepared by the calcination of a NCS precursor and freeze-dried urea. The optimized photocatalyst exhibits an efficient photocatalytic performance under visible light irradiation with a highest H2 generation rate of 3612.3 µmol g-1 h-1 , leading to an apparent quantum yield of 10.04 % at 420 nm, five times higher than the widely reported benchmark photocatalyst CN (2.01 % AQY). The materials characterization shows that NCS-modified CN curly nanosheets can promote photoelectron transfer and suppress charge recombination through their special coupling interface and NCS as an electron acceptor, which significantly improves the photocatalytic efficiency. Thus, this study provides an efficient strategy for the design of highly efficient photocatalyst, particularly suitable for a totally metal-free photocatalytic system.

Investigation of operation parameters on the removal efficiency of methyl orange pollutant by cellulose/zinc oxide hybrid aerogel.

In this study, Cellulose/Zinc Oxide-x (CA/ZnO-x) hybrid aerogel with various concentrations of Zn2+ ion is synthesized through the hydrothermal method and used for organic pollutant removal. The CA/ZnO-x hybrid aerogel was characterized by XRD, UV-Visible, DRS, FTIR, FESEM, EDS, BET, LC-MS-MS analyses. In the present study, CA/ZnO-x hybrid aerogel as photocatalyst was used to investigate the influence of the photocatalytic degradation of methyl orange (MO) from an aqueous solution under UV irradiation. The morphology of CA/ZnO-x hybrid aerogels exhibited that the ZnO rice grain-like shape were successfully synthesized on the cellulose aerogel matrix. Also, the influence of various factors such as photocatalyst dosage, pH, initial concentration of MO, and irradiation intensity was investigated on the photocatalytic degradation rate of MO. The results revealed that the highest degradation efficiency and mineralization rate of MO were about 99.02% and 51.68%, respectively, after 90 min under UV irradiation using CA/ZnO-0.4 photocatalyst when pH = 3, C0 = 10 ppm, photocatalyst dose = 9 g L-1 under the 300 W UV light irradiation. Furthermore, the synthesized photocatalyst exhibited a considerable firmness and easily separated from an aqueous solution for reuse.

Prediction and New Insight for the Drag Reduction of Turbulent Flow with Polymers and Its Degradation Mechanism.

A physicochemical understanding of the mechanism of turbulent flow drag reduction with polymer and its degradation is of great interest from both science and industry perspectives. Although the correlation based on the Fourier series has been proposed to predict the drag reduction and its degradation, its physical meaning was not clear until now. This letter aims to clarify this issue. We develop a comprehensive model to predict the drag reduction and degradation of polymers in turbulent flow from a chemical thermodynamics and kinetics viewpoint. We demonstrate that the Fourier series employed to predict the drag reduction and its degradation is due to the viscoelastic property of drag-reducing polymer solution, and the phase angle in the model, in physical nature, represents the hysteresis of the polymer in turbulent flow. Besides, our new insight of drag reduction with flexible polymers can also explain why a maximum drag reduction in rotational flow appears before degradation happens.

Whole Contact Line Pinning for Droplets Impacting on a Hydrophobic Surface Due to Hydrophilic TiO2 Nanoparticle Addition.

Controlling droplet deposition on a hydrophobic surface has received much attention due to its wide applications. Addition of certain elements into a working droplet is a feasible way to improve drop deposition, which, however, often leads to a significant change in droplet spreading properties. In this work, we show that adding a small amount of hydrophilic TiO2 nanoparticles without any surfactant can significantly suppress the droplet rebound and even generate a whole contact line pinning on the hydrophobic surface. The whole contact line pinning is positively related to the Weber number (i.e., impact velocity) and suspension concentration. Specifically, when the suspension concentration exceeds a critical value, the pinning and droplet deposition occur in the same We range. A mechanism is proposed to explain the observed unique pinning and depinning behaviors, according to which the agglomerated TiO2 particles depositing at the triple line can change the wettability of the local surface, which leads to pinning, while the disturbance of capillary oscillation leads to depinning. Interestingly, a long-time whole contact line pinning for more than a second was observed under certain conditions. This work can be of value for many practical applications such as pesticide deposition and spray cooling.

Net Unidirectional Fluid Transport in Locally Heated Nanochannel by Thermo-osmosis.

Thermo-osmosis driven by temperature gradients generally requires two liquid reservoirs at different temperatures connected by porous bodies or capillaries. We demonstrate, by molecular dynamics simulation, a new phenomenon toward nanoscale thermo-osmosis. Upon heating at a certain region of a nanochannel, multiple nanoscale convective layers are formed and can be manipulated to generate a net fluid transport from one reservoir to another, even without a temperature difference between them. A net unidirectional fluid transport with different rates can be achieved by precisely controlling location of the heated region. The net fluid transport can be enhanced further by tuning liquid-wall interactions. The demonstrated phenomenon provides a strategy for enhancing fluid mixing, which is often inefficient in nanoscale flows. Our finding is promising for chip-level cooling. The heat generated by chips can be employed to produce asymmetric temperature gradients in channels through proper configuration. Coolant liquids can thus be circulated without extra pumps.

Asymmetric Two-Layer Porous Membrane for Gas Separation.

We present that the porous two-layer membranes of graphene and hexagonal boron nitride (h-BN) are promising for gas mixture separation. For the two-layer membranes, the mechanisms of the gas separation are (i) the different adsorption properties of gases on two membranes inducing a permeation flux difference from one side to the other and (ii) the asymmetric potential energy curves (potential energy of a gas molecule vs distance between the pore center and a gas molecule) of a two-layer membrane leading to a potential energy difference, which can affect gas permeation through the pore. As a concrete example, we explore the gas separation of CO2 and CH4 by the two-layer membrane using molecular dynamics simulations. Finally, on the basis of the distinctive permeation rates in the two directions, a gas separation system with two back-to-back arrayed graphene/h-BN membranes with big pores is designed to realize gas separation.