Liquid-solid Contact Research Articles

Liquid-solid contact electrification through the lens of surface and interface science

Triboelectric nanogenerators (TENGs) are cutting-edge technology that converts mechanical energy into electrical energy through contact electrification (CE). The CE process involves the chemical, physical, and mechanical properties of material surfaces and interfaces. Studying CE from the perspective of interfacial science can provide a comprehensive understanding of the charging mechanism and improve the performance of TENGs through structural design and technical optimization. Therefore, this paper reviews the research progress in CE from the perspective of surface and interface science. Specifically, we introduce the static and dynamic behavior of the liquid-solid (L-S) interaction during CE separately. Then, the interface phenomenon involved in gas-liquid-solid CE, such as surface wettability and ionic effect on liquid film drainage between the bubble and solid surface in water is discussed. This new interdisciplinary area of surface science of TENGs provides many research opportunities, while it is essential to explain critical phenomena in the CE process.

Wetting and interfacial reactivity of graphite by liquid TiCuNi

Understanding the wetting and interfacial reactivity of graphite in contact with molten alloys is important to optimize brazing or to prepare carbon-based composites and coatings.In this work, the isothermal wetting of graphite substrates by a molten Ti-based alloy, namely TiCuNi (Ti: 70 wt.%), was tested at 1000 °C at different holding times by the sessile drop method with the aim to understand the interfacial reactivity. A final contact angle of 15° was achieved after 60 min of liquid-solid contact as a result of spreading kinetics. A thorough microstructural characterization by SEM and TEM was performed and evaluated with the help of the relevant phase diagrams: a limited interfacial reactivity to form TiC particles at the metal/graphite interface occurred. The interfacial compounds grew with time and determined the spreading of the liquid on the graphite substrate which resulted to be reaction-limited.

Construction of efficient silicon solar cells through polymetallic oxidation-reduction triggered by thermite reaction

Optimal metal-semiconductor contact interface is crucial for semiconductor devices, particularly for silicon solar cells aluminum-silicon contact is a key to improve the efficiency of the cell. In this work, a new type of Sb2O3-MoO3-B2O3 glass frits that can trigger a thermite reaction was utilized to improve the Al-Si interface. The low melting temperatures of these glass frits enhance the effect of thermite reaction by providing solid-liquid contact, which enables gradient distribution of the elements and leads to differences in the valence states of the Al-Si layer at the inner and surface regions. The valence states of Al and Sb at the inner layer are higher than those at the surface, while Mo is the opposite. Metallic Mo with a matched work function can lower the potential barrier, thereby improving hole selectivity and enhancing the potential skip-step. As a result, the fabricated solar cell achieved a high open-circuit voltage, a high short-circuit current, and a low series resistance. Consequently, a power conversion efficiency of 19.94% was obtained for a monocrystalline silicon solar cell with full Al-BSF. This work not only presents a new hole-selective contact for silicon solar cells, but also introduces a new approach for regulating the distribution and valence states of interface elements for enhanced efficiency.

Self-powered siphon rain gauge based on triboelectric nanogenerators

Rainfall measurement is of great significance to agriculture, weather forecasting and water resource management, while rain energy harvesting is a desirable solution to energy demand of wireless sensor networks of rainfall information. In this work, a novel self-powered siphon rain gauge (SR) integrating with a siphon unit, a sensing unit and an energy harvesting unit is proposed, which can simultaneously measure rainfall information and capture rain energy. In the siphon unit, a siphon structure is used to periodically automatically trigger and stop siphon events. In the sensing unit, a liquid–solid multi-tube triboelectric nanogenerator (TENG) is proposed for the first time to detect rainfall information by a multi-tube synergistic sensing strategy. In the energy harvesting unit, a rotary TENG excited by the siphon emptying event is utilized to generate electricity by a rainwater potential energy collection-based generation strategy. After optimization design, the water level resolution of the multi-tube TENG is around 2.5 times higher than that of traditional single-tube TENG, resulting in a rainfall resolution of 20.45 μm of SR. Besides, the rotary TENG successfully converts random, disordered, high-entropy raindrops energy into regular, ordered, low-entropy fluid mechanical energy. It can finally help the SR achieve a power density of 97.2 µW/m2, which is nearly 2.5 times higher than that of liquid–solid contact TENG using the traditional instantaneous impact-based generation strategy. The proposed self-powered SR not only obtains an excellent real-time rainfall sensing capability, but also has remarkable rain energy harvesting ability. Therefore, this SR has great potential for extensive applications in the hydrology and meteorology fields.

Dual-sensory fusion self-powered triboelectric taste-sensing system towards effective and low-cost liquid identification.

Infusing human taste perception into smart sensing devices to mimic the processing ability of gustatory organs to perceive liquid substances remains challenging. Here we developed a self-powered droplet-tasting sensor system based on the dynamic morphological changes of droplets and liquid-solid contact electrification. The sensor system has achieved accuracies of liquid recognition higher than 90% in five different applications by combining triboelectric fingerprint signals and deep learning. Furthermore, an image sensor is integrated to extract the visual features of liquids, and the recognition capability of the liquid-sensing system is improved to up to 96.0%. The design of this dual-sensory fusion self-powered liquid-sensing system, along with the droplet-tasting sensor that can autonomously generate triboelectric signals, provides a promising technological approach for the development of effective and low-cost liquid sensing for liquid food safety identification and management.

Stable and impalement-resistant superamphiphobic coatings enabled by phase-separated adhesive for anti-adhesion of viscous liquids with low surface tension

Superamphiphobic coatings have great potential in various fields but surfer from weak mechanical stability, low impalement resistance and poor repellency to viscous liquids with low surface tension. Here, we report stable and impalement-resistant superamphiphobic coatings enabled by phase-separated adhesive for anti-adhesion of viscous liquids with low surface tension. By fluorination of silica (F-silica) nanoparticles and solvent induced phase separation of an adhesive, a homogeneous suspension was obtained, which could be readily used for preparing superamphiphobic coatings via spray-coating. The effects of phase separation degree and adhesive/F-silica mass ratio on superamphiphobicity, impalement resistance and mechanical stability of the coatings were studied. The coatings have a hierachical micro-/nanostructure and numerous perfluorodecyl groups on the surface. Thus, the coatings show excellent static and dynamic superamphiphobicity, e.g., high contact angles, low sliding angles, high maximum release height, short solid-liquid contact time and large bouncing height for various liquids with high viscosity and low surface tension. The coatings also show excellent mechanical stability, good chemical and temperature stability. Furthermore, the coatings are applicable onto diverse substrates and could be easily scaled-up.

The thermodynamic stability of the Cassie–Baxter regime determined by the geometric parameters of hierarchical superhydrophobic surfaces

Understanding the geometric parameters of hierarchical superhydrophobic surfaces and their impact on the thermodynamic stability of the Cassie-Baxter regime are invaluable for surface wettability–related applications. Herein, we fabricated hierarchical micro-microstructured silicone rubber surfaces having superhydrophobic properties via an industrially applicable direct replication method. The mold inserts were fabricated using photochemical milling, laser ablation, and wet chemical etching to create different hierarchical levels. We calculated surface roughness ratios and equilibrium contact angles and considered the contribution of sub-microstructures in the wettability properties via physical and statistical analyses. Comparing the calculated theoretical wettability properties and experimental measurements revealed a good agreement among all samples because of the accurate predictions of the governing wetting regime. It was worthwhile insights into the design and fabrication of superhydrophobic structured surfaces. The presence of superimposed sub-microstructures produced desirable water-repellency properties because of the reduced solid–liquid contact area as low as 0.086. Given the primary importance of the Cassie–Wenzel wetting transition on the design and fabrication of superhydrophobic surfaces, we evaluated the thermodynamic persistence of the Cassie–Baxter regime by analyzing the energy barrier to be overcome by droplets with various volumes. Finally, we discuss the contribution of the dimensional parameters of microstructures on the stability of the Cassie–Baxter regime.

Effect of Microstructure on Coalescence-Induced Droplet Jumping Behavior of a Superhydrophobic Surface and Its Application for Marine Atmospheric Corrosion Protection

Coalescence-induced droplet jumping behavior (CIDJB) refers to the spontaneous jumping of droplets on a specific superhydrophobic surface (SS) without any external energy, which offers a new approach to the field of marine atmospheric corrosion protection by isolating corrosive media. In this study, a flower-like micro–nanocomposite structure SS (F-SS) and a sheet-like nanostructure SS (S-SS) were prepared on copper sheets by ammonia immersion and chemical vapor deposition. Firstly, we observed the microstructure characteristics of the samples and secondly analyzed its chemical composition and wettability. Moreover, the CIDJB was studied by simulated condensation experiments, and the influence of the microstructure on CIDJB was revealed. Meanwhile, the atmospheric corrosion resistance of samples was analyzed by electrochemical impedance spectroscopy (EIS) measurements, and the protection mechanism of SS through CIDJB was proposed. The results showed that the S-SS had a smaller solid–liquid contact area and lower interfacial adhesion, which is more conducive to CIDJB. Since a larger solid–liquid contact area requires greater interface adhesion energy for the droplets to overcome, droplet jumping behavior was not observed on the F-SS. Compared with the F-SS, the S-SS exhibited outstanding corrosion resistance due to the wettability transition of droplets by CIDJB, which facilitated the restoration of the air film to insulate the corrosive medium. The present study provides a reference for a marine atmospheric corrosion resistance technique through CIDJB on an SS.

Suppressing the pancake bouncing induced secondary contact on superhydrophobic surfaces via jet splash

Pancake bouncing of water droplets on superhydrophobic surfaces has been extensively studied, because the reduction in solid–liquid contact time shows great potential for self-cleaning, anti-icing, etc. However, the behavior of a pancake-bouncing droplet in the ambient air and its subsequent interaction with the underlying surface remain unrevealed, which is actually crucial for practical applications. In this Letter, we comprehensively investigate the overall dynamics of droplets on post-array superhydrophobic surfaces by extending the range of impact velocities. An unreported phenomenon was observed, whereby the retracting droplet undergoes vertical elongation and re-contacts the underlying surface following the pancake bouncing event. As the impact velocity increases, the submillimeter-scale posts notably influence the droplet splash, where tiny droplets are ejected from the edges of lateral liquid jets through the posts. Experimental results and scale analysis show that the critical Weber number for this jet splash phenomenon decreases with the post spacing and the post edge length over a certain range. The violent jet splash occurring at higher Weber numbers reduces the mass of the remaining droplet and, consequently, diminishes the diameter prior to retraction, thereby suppressing the secondary contact with the surface. Our findings are believed to provide valuable insight for the understanding and the application of the pancake bouncing effect.

Preparation of PVDF-PDMS-SiO2 multi-stage rough superhydrophobic coating with excellent anti-corrosion and drag reduction performance via one-step cold spraying

Suitable superhydrophobic surface micromorphology, which can capture air underwater to form an air film, was used to decrease the solid-liquid contact area between the substrate and water, and thus reduce the intrusion of corrosive media and the resistance of movement. In this research, PVDF-PDMS-PF@μ/nSiO2 (PPPSS) multi-stage rough superhydrophobic coating was prepared on aluminum alloy by one-step cold spraying. Perfluorodecyltrimethoxysilane (PFDT) modified dual-scale SiO2 microparticles were used to provide the desired roughness. Low surface energy polymers polyvinylidene fluoride (PVDF) and polydimethylsiloxane (PDMS) were introduced as the polymer skeleton. The water contact angle of the PPPSS coating was 162.6°, with good superhydrophobicity. The coating exhibited excellent anti-corrosion that |Z|0.01 Hz was still three orders of magnitude higher than the bare substrate after 10 days of immersion. The flow experiment results showed that a maximum of 35 % drag reduction was achieved in Reynolds from 6 × 103 to 4 × 104. The equivalent circuit and drag reduction mechanism of the coatings were further investigated by combining surface morphology and material composition. Besides, the PPPSS coating demonstrated good stability and self-cleaning performance. The large-area superhydrophobic surface made by simple and convenient one-step cold spraying, is expected to have practical applications in marine transportation, underwater mapping, and pipeline transportation.

Liquid flow in heap leaching using the discrete liquid flow model and graph-based void search algorithm

The liquid flow in many heap leaching processes is discontinuous in nature as it occurs either as droplets or rivulets or in combination. Therefore, it cannot be represented correctly using continuum-based models at very low liquid flow rates, such as in heap leaching, or in non-wetting conditions, where liquid flow is discrete in nature. Many researchers have modelled the liquid flow through the heap packing using continuum models. However, this directly contradicts the experimental observations made by various researchers. In the current study, liquid flow is considered discrete in nature as rivulets and droplets with their rupture and merging phenomena. A single vertical layer of particles is considered for the study. A novel void search algorithm is devised to detect the position, shape, and size of the voids in a heap. The liquid flow behaviour is studied in terms of particle size, tortuosity, liquid distribution, breakthrough time, and contact angle. The study shows that heap leaching processes can be modelled more accurately using Discrete Liquid Flow (DLF) theory by avoiding uncertain experimental parameters such as bed permeability. It is found that particle size in the heap structure has significant effect on the liquid-solid contact area, which is an important parameter to determine the efficiency of the leaching process at an industrial scale.

Liquid–Solid Triboelectric Probes for Real‐Time Monitoring of Sucrose Fluid Status

AbstractTriboelectric probes have rapidly developed as an efficient tool for understanding contact electrification at liquid–solid interfaces. However, the liquid–solid electrification process is susceptible to interference from chemical components in mixed solutions, severely limiting the potential applications of triboelectric probes in various liquid environments. This study for the first time reports a triboelectric probe capable of sucrose solution concentration sensing, finding that the dissolution of sucrose destroys the hydrogen bond network between water molecules and forms sucrose–water hydrogen bonds, which alters the fluid mechanics characteristics of the solution and enhances its conductivity, thereby reducing the droplet size and causing an ion charge shielding effect that significantly affects the electron transfer in liquid–solid contact electrification. Owing to the feedback of the triboelectric probe on the sucrose concentration gradient‐type sensing electrical signals, efficient sensing of sucrose solution was achieved (sensitivity of −0.0038%−1, response time of 90 ms). The triboelectric probe is also used as a wireless smart terminal to enable real‐time detection of sucrose solution. This work contributes to the understanding of the structure–function relationship between micro hydrogen bonding and macro performance, and provides a promising solution for building sustainable intelligent sensors.

Molecular dynamics simulation of the evaporation of thin liquid sodium film on the conical nanostructure surface

Deeply understanding the evaporation of the nanoscale thin liquid film on the nanostructure substrates is significant for further constructing the novel wick structure with nanostructure surfaces, which would be used in the optimal design of the sodium heat pipe. For this purpose, this paper investigates the evaporation of thin liquid film on conical nanostructured surfaces by the molecular dynamics method. A cuboid evaporation system is constructed. It consists of the bottom solid substrate and thin liquid sodium film. A flat solid wall is set as the reference. By changing the nanoscale cone's number and height, five conical nanostructure surfaces are built. The number of gas atoms and the total energy of fluid atoms are observed respectively. The evaporation rate and the average heat flux (q) of six cases are compared. The conical nanostructure suppresses evaporation. With the roughness of the conical nanostructures, the evaporation rate decrement maximum can reach 83% and the q decrement maximum can reach 58%. Based on the thermal resistances in the solid-liquid contact region, the D (Self-diffusion coefficient), and the peak values of RDF (Radial distribution function), the mechanisms of the conical nanostructure surfaces are analyzed. The suppressing effects of the conical nanostructure surfaces are achieved by weakening the heat conduction in the solid-liquid contact region and mitigating the collision heat transfer of liquid atoms.

Study on Dynamic Contact Behavior of Multi-Component Droplet and Dust Surface

The dynamic contact behavior between multi-component droplets and the surface of iron ore dust was taken as the research object, analysis of the maximum spreading coefficient, maximum acting diameter, maximum acting area, and maximum bouncing height of solid-liquid contact, from a microscopic perspective, using high-speed photography and image analysis and processing technology. The experimental results indicate that (1) with the particle size of dust particles decreases, the solid-liquid contact behavior sequentially manifests as spread immediately after broken, retraction, negative bounce, primary bounce, and secondary bounce. (2) When the surface tension of the droplets decreases from 55.5 to 34.8 mN/m, the maximum spreading diameter of the droplet has increased by 30% to 40%, the maximum bounce heights (coefficients) decreased by 100%, 57.14%, and 53.57%, respectively, the maximum spreading coefficient of the droplet exhibits no obvious pattern. (3) With decreasing droplet surface tension, the unidirectional acting diameter and the maximum acting area increase when the dust surface size is over 100 μm. When the surface particle size is less than 100 μm, there is no significant change in the unidirectional acting diameter and maximum acting area despite decreasing surface tension. Thus, droplet diffusion is mainly influenced by particle size. These findings contribute to enhancing the theory of water mist dust removal and improving dust removal efficiency.

Геометрические характеристики нанокапли. Ранжирование энергии капли

The equilibrium condition of a hanging axially symmetric droplet is derived for the Deryagin model, which takes into account the thickness of the wedging/molecular layer of the droplet surface. For an axisymmetric drop, the forming line of which depends on a natural parameter, a differential equation is found, the solution of which is the integrand function of the functional of the wedging energy. The solution of this differential equation is found. The functional of the wedging energy of the drop is constructed. The functional of the total energy of an axisymmetric hanging drop is refined, taking into account the second-order component of the smallness of the energy of intermolecular interaction. As a result of solving the variational problem, the equilibrium condition of a liquid droplet is obtained for the Deryagin model, which takes into account the thickness of the surface layer of the droplet. Due to the fact that the first variation of the functional of the total energy of a drop for an equilibrium drop is zero, a more accurate form of the dependence of the liquid-solid contact angle on the radius and height of the drop is obtained. The decomposition of the potential energy of the intermolecular interaction by degrees of the potential well is obtained for the Deryagin model, which takes into account the thickness of the wedging/molecular layer of the droplet surface, which is why a second-order element of smallness appeared in the decomposition by degrees of the potential well. A comparison with other models specifying the relationship of the wetting angle depending on the height of the drop is carried out.

The Effect of a Magnetic Field on Solid–Liquid Contact Electrification for Streaming Flow Energy Harvesting

In recent years, the triboelectric nanogenerator (TENG) has been recognized as a promising method for energy harvesting and self-powered devices. However, in order to improve the output efficiency of the TENG, it is necessary to change the types of dielectric materials, which requires advanced technology and a high cost to implement. To address this issue, we developed a parallel electrode magnetic-TENG (Mag-TENG) based on contact electrification of a liquid–solid interface under the effect of the magnetic field, which enhances the output performance of the TENG without having to develop the dielectric material. Our experimental results achieved a higher output of the TENG under the influence of a magnetic field when an increase of the magnetic field strength went from 0 to 360 mT, and the flow rate of unsteady seawater was variable from 390 to 690 mL/min. Specifically, compared to the without-magnetic field case, the output current increased by approximately 6.5 times and the output voltage by 2.7 times. These findings suggested that using a magnetic field to enhance the TENG’s efficiency has significant potential for energy harvesting from seawater and self-powered flow sensors.

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.

Bioinspired Superhydrophobic Fibrous Materials.

Natural fibers with robust water repellency play an important role in adapting organisms to various environments, which has inspired the development of artificial superhydrophobic fibrous materials with applications in self-cleaning, antifogging, water harvesting, heat exchanging, catalytic reactions, and microrobots. However, these highly textured surfaces (micro/nanotextured) suffer from frequent liquid penetration in high humidity and abrasion-induced destruction of the local environment. Herein, bioinspired superhydrophobic fibrous materials are reviewed from the perspective of the dimension scale of fibers. First, the fibrous dimension characteristics of several representative natural superhydrophobic fibrous systems are summarized, along with the mechanisms involved. Then, artificial superhydrophobic fibers are summarized, along with their various applications. Nanometer-scale fibers enable superhydrophobicity by minimizing the liquid-solid contact area. Micrometer-scale fibers are advantageous for enhancing the mechanical stability of superhydrophobicity. Micrometer-scale conical fibrous structures endow a Laplace force with a particular magnitude for self-removing condensed tiny dewdrops in highly humid air and stably trapping large air pockets underwater. Furthermore, several representative surface modification strategies for constructing superhydrophobic fibers are presented. In addition, several conventional applications of superhydrophobic systems are presented. It is anticipated that the review will inspire the design and fabrication of superhydrophobic fibrous systems.

On the lack of influence of contact area on the solid–liquid lateral retention force

We experimentally show that, unlike the solid–solid frictional force, the solid–liquid retention force is determined by interactions at the triple line rather than over the solid–liquid contact area, as predicted by theory. We have prepared drops whose triple line enclosed a uniform surface, a hydrophobic island, and topographical inhomogeneities, and measured the same retention force for all three cases. We have also studied the retention force on drops whose initial triple line was non-circumferential and measured different retention forces for different shapes, also as predicted by theory. The experiments with non-circumferential drops provide (1) another way to show that the retention force is not proportional to the contact area, and (2) a manner to falsify a recent theory on drop depinning.

Mechanical criteria and analytical modeling of mixed-state wetting on multiscale surfaces: Of the importance of nanoscale topography for water-repellency

Multiple natural surfaces exhibit excellent water-repellent properties. Such surfaces systematically demonstrate multiscale surface topographies. However, current mechanical criteria for composite-wetting equilibrium can only be applied on model micrometric pillars. This paper suggests a theoretical basis on the foundation of which we introduce multiscale mechanical criteria for mixed-state equilibrium on real surfaces. A method for the analytical modeling of mixed-state wetting on real textured surfaces and the retrieval of critical wetting parameters such as the total solid-liquid contact area and the total triple line perimeter at the effective anchorage depth is introduced. This method is applied to the case of the sacred Lotus leaf on which we predict the effective anchorage depth of the triple lines. Results show that the nanoscale topography of these leaves leads to the minimization of the effective anchorage depth of the droplet on the sides of the micrometric asperities, a determining parameter with regards to its effect on the total solid-liquid contact area and the total triple line perimeter, unveiling a little more the role of nanoscale wax crystals in nature’s strategies for water-repellency.