Search

The self-driving of water droplets on textured substrate is momentous for understanding the directional transport of water on biological surface. In this paper, a novel design of monolayer graphene-covered wedge-shaped copper substrate (GWCS) is put forward to realize the directional and ultrafast spontaneous driving of water droplets from the tip of the wedge-shaped substrate to the wide end. The self-driving behaviors of water droplets on GWCS are studied by classical molecular dynamics. The results show that the maximum spontaneous driving velocity of water droplet driven by surface wetting gradient and Young-Laplace pressure gradient can reach 73.8 m/s. The law of energy variation during the whole droplet self-driving on GWCS indicates that there is a competitive relationship between the potential energy of water droplet and the interaction energy between the droplet and GWCS, i.e. the interaction energy between water droplet and GWCS is partially converted into the potential energy of water droplet. The relationship of the maximum displacement of water droplet with the wedge angle, wettability of GWCS and the droplet surface tension is proposed in theory, and the influence of the discontinuous linear increase of the wedge-shaped substrate width on a nanoscale on the self-driving is analyzed and used to explain the little difference between the theoretical and simulation results. Furthermore, a smaller droplet is easier to obtain larger spontaneous driving velocity, and the influence of long-distance decelerating motion of high-speed small droplet on the non-wetting gradient substrate on the droplet displacement law is clarified. Finally, the mechanism of graphene suspended on both sides of the wedge-shaped copper structure to enhance the droplet transport efficiency is determined. The results will have theoretical significance in designing the functional texture surface covered by monolayer graphene to realize droplet self-driving.

Because of the high water flux and excellent ion rejection, the pores graphene is considered as a promising candidate material for fabricating the membranes in reverse osmosis (RO) process. Unfortunately, water molecules cannot pass through the perfect graphene, and how to effectively create a large number of nanopores with controllable size remains a challenge, which seriously prevents the practical application and development of graphene membrane for desalination. Recently, the emergence of pillared graphene (PGN) might open a new way for designing the graphene-based membranes, which can compensate for the deficiency of porous graphene membrane. The PGN has been extensively studied in gas storage and separation, and its RO characteristics and mechanism still remain unclear because the limitation of large area preparation in desalination. In this paper, the RO process of seawater through PGN membranes is investigated by molecular dynamics simulations, and the influences of the pressure within feed solution, temperature and the shearing of membrane on the desalination properties are considered. It is found that the water flux increases linearly with the pressure within feed solution increasing, and the PGN membrane with nanopore diameter of 0.8 nm can conduct water molecules but completely rejects high-concentration ions. As the diameter of nanopores increases to 1.2 nm, the rise of temperature can increase the permeability of water molecules, whereas the salt rejection is not appreciably sensitive to the temperature. Particularly, the shearing membrane can improve the salt rejection and hinder the water molecules from permeating at the same time. The designed PGN membrane exhibits excellent performance of water purification, and the ultrahigh water flux obtained in this work reaches 56.15 L·cm–2·day–1·MPa–1 with a salt rejection of 88.9%. Subsequently, the hydrogen bond dynamics is calculated in order to explain the variation of water permeability under different conditions. The result shows that the rise of temperature reduces the stability of hydrogen bonds and leads the water flux to increase, while the increase of shearing speed will enhance the stability of hydrogen bonds and inhibit water seepage. Furthermore, the analysis results of hydrogen bond and ionic hydration structure show that the shear motion on RO membrane will improve the stability of ionic hydration shell, which makes it more difficult for the ions to pass through nanopores by weakening the hydration shell. On the contrary, rising temperature will impair the strength of ionic hydration shell, leading more ions to pass through the RO membrane. The simulation results can provide an in-depth understanding of the desalination performance of PGN membrane under different key conditions, and further demonstrate the promising applications of graphene-based membrane in water desalination.

Triboelectric nanogenerator (TENG) and its self-powered sensor based on the principles of contact electricity generation and electrostatic induction have important application prospects in the fields of new energy and internet of things (IoT). In the contact separation process of polymer materials with different electronegativity values, due to the transfer of electrons, a changing electrostatic field will be generated in the space around the polymer. In the existing TENG research, the field strength perpendicular to the plane of the friction layer and the electrode layer is mainly used to generate electrostatic induction, and the electric field effect around the polymer is ignored. According to the principle of electrostatic induction, the internal charge of the conductor in the electric field will be redistributed, which provides a way for the conductor to generate an induced electrical signal on the surface of the conductor without contacting the friction material. In this paper, we design a non-contact triboelectric nanogenerator (NC-TENG) based on changing electrostatic field. The influence of the distance between the conductor and the friction material, the induction area of the conductor and the position of the conductor relative to the friction material on the induced electrical output performance are studied when silicone rubber and nitrile rubber are used as a friction material. The results show that the NC-TENG can produce a stable electrical signal output when the conductor is completely separated from the friction material. The induced voltage of NC-TENG decreases with the increase of the distance between the conductor and the friction material, and gradually increases with the increase of the conductor's induction area. For the friction material with a size of 30 mm × 30 mm, the electrical output of NC-TENG tends to be stable when its conductor area is 60 mm × 45 mm. In addition, the different orientation of the conductor relative to the friction material also has a significant effect on the induced electrical output. The NC-TENG designed in this paper provides a novel electrical output generation mode, which provides a higher possibility for the subsequent research on TENG and the application of self-powered sensors.

Graphene-based materials have aroused great interest for their potential applications in water desalination and purification membranes attributed to their ultrathin thickness, high mechanical strength, and anti-foiling properties. Reverse osmosis (RO) technology is currently the most progressive, energy-saving and efficient separation technology by membranes, therefore the new materials with high strength, strong pollution resistance and excellent performance are urgently needed. The ability of porous graphene to serve as a kind of novel advanced RO membrane is due to two major potential strengths of this atomically thin two-dimensional material, i.e., ultrahigh permeability and super selectivity. Thus, the reverse osmotic properties of the porous graphene membranes should be further investigated theoretically. In this paper, classical molecular dynamics method is used to investigate the reverse osmosis characteristics of brine in hydrogenated porous graphene reverse osmosis membrane. The results show that the water permeation rate increases with the driving force, pore size and temperature increasing, for the pore diameter larger than the hydration radius. The ion rejection rate decreases with the driving force and temperature increasing. Interestingly, as the porous graphene moves in the tangential direction to perform a shearing process, the interception rate of the salt ions can be effectively improved and the concentration difference polarization phenomenon can be reduced with the tangential velocity increasing, although the water flux decreases slightly. The influence mechanism of each parameter on permeability and on water flux are explored by analyzing the hydrogen bond distribution, the ionic hydration in feed solution, and the energy barrier of the water molecules in penetrating process. In order to further evaluate the effects of various parameter changes on the benefits of reverse osmosis membranes, both the selectivity and permeability are calculated to evaluate the tradeoff between permeability and selectivity, indicating that the increase of the pore diameter can obtain both high permeability and selectivity under the shearing circumstance of the membrane. The research results in this paper will provide a theoretical understanding of porous graphene-based desalination membrane and also may be helpful in designing the shearing graphene-based water filtration devices.

With the advent of global warming and energy crisis, the search for renewable energy to reduce carbon emissions has become one of the most urgent challenges. Ithas become a research hotspot to collect or harvest various mechanical energy in nature and convert it into electric energy. Vibration is a common form of mechanical movement in our daily life. It is visible both on most working machines and in nature and is a type of potential energy. There are several methods that can convert such mechanical energy into electric energy. Triboelectric nanogenerator (TENG) based on the principle of contact electrification and electrostatic induction which first appeared in 2012 by Zhonglin Wang provides a feasible method of efficiently collecting the vibrational energy with different vibrating frequencies. In this paper, a contact-separation mode of TENG is designed and implemented. The voltage- quantity of charge- distance(V-Q-x)relation of TENG is calculated. During the experiment, the factors such as load resistance, vibration frequency, etc. which affect the output performance, are considered and analyzed. An electrically driven crank-connecting rod mechanism is employed to provide the vibration source with adjustable frequency in a range of 1-6 Hz. The result shows that the amount of charge transfer in each working cycle remains almost unchanged, while the voltage and current increase with frequency increasing. When the frequency is 5 Hz, the best power matching resistance of the TENG is about 33 MΩ and the maximum output power reaches 0.5 mW. For a further study, a COMSOL software is used to simulate the distribution rule and variation rule of the electric potential in the contact-separation process, then the theoretical charge density and the experimental charge density on the polymer surface are compared and analyzed in order to provide theoretical and practical support for the design of TENG with collected vibration energy and self-powered vibration sensor. The result shows that the electric potential is proportional to the distance between two friction layers. While as the distance between two friction layers increases, the electric potential and the charge density both show a tendency to concentrate in the middle of the friction layer. The huge difference between experimental result and the simulation predicts thatmuch work should be done continually to improve the output of the TENG. Finally, the obtained results conduce to understanding the contact electrification and electrostatic induction mechanism and also provide a new method of harvesting the vibration energy.

The two-dimensional material graphene is usually required to be transferred on the target substrate for some special applications, thus it is important to understand the adsorption properties in the graphene transferring and stripping processes. In this paper, the adsorption properties of a single-layered graphene on the grooved copper substrate are investigated using molecular dynamics simulations. The influence of geometric characteristic size of the groove on the adsorption force of the graphene deriving from the substrate is explored. For the fixed boundary conditions of the graphene, the adsorption force increases up to maximum and then decreases with reducing the distance between the graphene and substrate in the adsorbing process. The maximum adsorption force increases with groove depth increasing, with the groove width kept constant. Nevertheless, as the groove depth increases continuously, the adsorption force decreases greatly until the graphene cannot be adsorbed into the groove. In the graphene stripping process, the critical force that can strip the graphene completely from the substrate increases first and then decreases with the increase of the groove depth, which is also dependent on the steady adsorbing configuration of the system before stripping. With the groove depth kept constant, the magnitude of the adsorption force between the graphene and substrate is determined by the steady adsorbing configuration of the graphene in the groove region. The adsorption force versus the distance between the graphene and the grooved substrate can be divided into two groups according to whether the graphene can be adsorbed into the groove. In both adsorbing and stripping processes, the adsorption force for the graphene adsorbed into the groove is obviously larger than that for the graphene covered on the groove. Moreover, the influence of the boundary condition of the graphene on the adsorption properties in the groove region on the substrate is considered preliminarily. It indicates that the tensile plane stress within the graphene sheet induced by the fixed boundaries can hinder the graphene from being adsorbed into the groove. The findings may be helpful for the graphene-based fabrication of nano-apparatus and functionalized surface modification.

In this paper, the classical molecular dynamics method is used to investigate the permeability of pressure-driven water fluid in the hybrid structure of graphene-carbon nanotube (CNT). The results indicate that the permeability of water molecules for the hybrid structure of graphene-CNT is obviously higher than that for the assembled structure of graphene-CNT. The combination between the graphene sheet and CNT in the hybrid structure is found to be a key point to improve the permeability of water molecules. Subsequently, the potential of mean force (PMF) is calculated in order to explain the influences of the combined structure on the permeabilities for the water fluid passing through both the hybrid and assembled graphene-CNT structures. The result shows that the PMF for the water molecules penetrating through the assembled structure is larger than that for the hybrid structure appreciably. It implies that the structure of the combined chemical bonds in the hybrid structure can efficiently improve the permeability of water molecules. As for the water penetrating through the hybrid structured graphene-CNT, the permeability of water increases with water pressure rising, and decreases with the electric field intensity increasing. The water molecules cannot pass through the proposed hybrid structure below a pressure threshold of 100 MPa. The permeability of water in the hybrid structure decreases with the increasing charge quantity on CNT below a threshold of 0.8e. The PMF for water penetrating through the hybrid structure decreases with charge quantity decreasing. The results suggest that the water permeability can be controlled by regulating the water pressure and the electric field intensity. Furthermore, the influences of the temperature and the axis spacing of two CNTs in the hybrid structure on the water permeability are considered. The permeability of water in the hybrid structure increases with the increasing temperature above a threshold of 200 K. The PMF for water penetrating through the hybrid structure increases with the decreasing temperature. Interestingly, the water permeability decreases with the increasing axis spacing. As the axial spacing increases, the water permeability decreases gradually and even approaches to two times of the permeability in the case of the hybrid structure with a single CNT channel. The findings can provide a theoretical basis for designing nanopumps or osmotic membranes based on the graphene-CNT hybrid structures.

A theory which was proposed by Scheid et al. in 2010 (Scheid B, van Nierop E A, Stone H A 2010 Appl. Phys. Lett. 97 171906) suggests that very thin ribbons of molten material can be drawn out of a melt by adequately tuning the temperature gradient along the dynamic meniscus that connects the static meniscus at the melting bath to the region of the drawn flat film. Based on this theory, one-step manufacturing ultra-thin silicon wafer by pulling out from a molten silicon bath has attracted considerable attention in recent year due to its many attractive performances such as low cost, simple process, etc. By using this method, solar cell can have intensive applications due to its low cost and stable output efficiency. The results show that the thermal capillarity effect plays a great role in preparing the ultra-thin silicon. The thickness of the silicon wafer is sensitive to the capillary length and the strength of the surface tension variation as well. In order to reveal the mechanism for the effect of thermal capillary on the fabrication of ultra-thin silicon wafer, a thermal capillary finite element model is developed for the horizontal ribbon growth system to study the wetting behaviors of molten silicon on graphite. The mathematical model is established and simulated by using the commercial software; several parameters such as mass, viscous stress and capillary force are calculated. The wetting processes are tested by changing surface roughness (Ra=0.721 m and Ra=0.134 m), system temperatures (17371744 K), and durations (1030 s) at constant temperature on a high-temperature, high-vacuum contact angle measurement instrument. It is found that the wetting angle of silicon droplet on graphite decreases with surface roughness and temperature increasing; the wetting angle comes down with time going by (lasting 30 s) at constant temperature, which is consistent with the theoretical result of Wenzel. The influence of surface tension on wetting process is studied by analyzing the distributions of pressure and velocity field. It is shown that the differential pressure at the solid-liquid interfaces, induced by thermal capillary effect, decreases in the wetting process and reaches a balance which prevents the droplet from being wetted. At T=1700 K, the wetting angle and the shape of droplet change quickly within 0.4 ms and eventually become stable after 5 ms as shown in the simulation. The spreading length L and droplet height h at the steady-state are calculated with considering the influence of droplet radius on the wetting process. The results show that both L and h are directly related to the steady-state of wetting angle. The surface tension dominates the wetting process for droplet radius R0 5mm; while for R0 5 mm, the wetting process is dominated by gravity.

Contact electrification between insulators, manifesting as static or triboelectricity is a well-known effect. The triboelectric nanogenerator (TENG) which is based on the contact triboelectricification and electrostatic induction provides a promising route for harvesting ambient mechanical energy and converting it into electric energy. The TENG which is due to its unique properties such as simple structures, low cost, high electric density etc., can offset or even replace the traditional power source for small portable electronics, sensors and so on. So far, the influence of factors on the output performance of TENG is still trapped in unsettled questions and under debate. In this paper, we prepare several textured polydimethylsiloxane (PDMS) films with micro rod array by model method and fabricate a TENG with a size of 2222 mm. The electric generation can be achieved with a cycled process of contact and separation between a polymer and metal electrode (PDMS and aluminum respectively in this study). Several influences as the surface structure and external load on the electrical output of the TENG are systematically studied by integrating use of experimenal tests and ANSYS simulation. Results show that the existence of micro rod array on the PDMS films effectively enlarges the contact area and provides more surfaces for charge storage and hence improve the output performance of TENG. When keeping the external load constant, the output increases with decreasing distance between micro rods. When the external load is 5 N and the distance is 15 m, the average output voltage and current as high as 88 V and 15 A can be achieved respectively, which is 1.5 times higher than the output generated when the distance is 50 m. The electrical output increases quasilinearly with the increase of the external load. Simulation results show that the micro rods of PDMS films are mainly compressed by normal load, which results in a bigger diameter of micro rods. The deformations of PDMS substrate leads to the lateral friction between the micro rods and the upper electrode, which produces more charges because of the friction. For 5 N normal load, the deformations of PDMS substrate and micro rods contribute to the sum of displacement vector and the deformations along Z-axis are 32.7 m and 21.3 m respectively, and are 4.96 and 5.04 times higher than the deformation at the load of 1 N. All the results in an enlarging surface area and the larger output correspondingly. Not only does this work present a new type of generator with micro rods on the PDMS surface, which can be an effective method to improve the electrical output of TENG, but also offers a unique point of view for further understanding of the working principle of TENG.

A capillary model is developed for calculating the wetting angle of molten silicon on different walls by using the microfluidic two-phase flow level set method and studying the characteristics of the rising process. A mathematical model formulation rigorously accounts for the mass and momentum conservation by using the improved Navier-Stokes equation and considering the Marangoni effect. Compared with the experimental data, the change of the wetting angle on the chemical vapor deposition (CVD) diamond wall indicates the grids independence and the validity of the numerical algorithm. We also discuss the influence of surface tension, and Marangoni stress induced by the gradient of surface tension coefficient, and wall adhesion to the change of wetting angle for three different walls, which include SiC wall, graphite wall, and CVD diamond wall, at different temperatures (1683-1873 K). Result shows that at the same temperature, the thermal-capillary effects that induce the molten silicon to undulation are raised. The wetting angle is reduced after first being increased and finally stabilized. At the initial stage, the fluctuation of the liquid-air interface is volatile due to the large changes of the liquid-air and the wall-air surface tensions, and subsequently, the fluctuation tends to be stable while the wetting angle is close to a fixed value. It is also found that with the graphite wall, these changes are more likely to be stable. This research provides a theoretical guide to obtain a stable growth environment for silicon belt fabricated from the molten silicon.