Solid-liquid Interfacial Area Research Articles

The growth of condensed nanodroplets in electric fields: A molecular dynamics study

Electric fields (E-Fields) can promote droplet detachment and improve dropwise condensation. Since droplet growth is dominant in the dropwise condensation cycle, the influence of the applied electric field on the growth of condensed droplets is worth exploring. In this work, the effect of molecule orientation polarization in the electric field on the growth of condensed nanodroplets is studied via molecular dynamics simulation. The results show that when the size of a growing cluster reaches a critical size Rc, the orientation polarization of water molecules within the cluster significantly increases. The orientation polarization is shown to reduce the interaction between water molecules, and the condensed molecules are easier to escape. Thus, the reduced interaction between water molecules hindered the growth of condensed clusters. Especially when the cluster size exceeds Rc, the increased orientation polarization makes this hindrance more pronounced. However, the morphology of the cluster simultaneously transforms from ellipsoidal to needle-like at the critical size, and the shape deformation of the cluster can also affect the growth rate. When the cluster size is larger than Rc, a vertical E-Field stretches the condensed cluster perpendicular to the substrate, which decreases the solid-liquid interfacial area of the cluster. The reduction of solid-liquid interfacial area and the reduced interaction between water molecules jointly hinder the growth of condensed clusters. On the contrary, needle-shaped deformation induced by parallel electric fields can increase the contact area between the cluster and the substrate. Therefore, the increased solid-liquid interface area can compensate for the negative effect of orientation polarization on the growth. Besides, when the cluster size is smaller than Rc, the growth rate is mainly related to the polarization-induced weakened interactions between molecules. Thus, in stage R < Rc, both vertical and parallel electric fields hinder the growth.

Experiment of contact angle hysteresis of a droplet on hydrophobic fibrous membrane

Fibrous membranes are widely used in liquid-gas filtration and separation, porous structures formed by randomly cross-layered fibers have a significant influence on interfacial free energies, showing complex changes in contact angle. The present paper aims to measure the pore structures of a hydrophobic ((C2F4)n, PTFE) fibrous membrane by Confocal Laser Scanning Microscope (CLSM), and analyze the droplet contact angle hysteresis along its circumference. The capillary force of the solid-liquid interface is calculated by the surface integral of Laplace pressure on half droplet surface, and the solid-liquid and liquid-vapor interface area ratios are obtained according to the Cassie-Baxter model. The liquid penetration depth in pore structures is calculated by the quasi-equilibrium of the interface free energy. The experimental result shows that for the PTFE fibrous membrane, the liquid-vapor interface area ratio (f2The experimental result shows that for the PTFE fibrous membrane, the liquid-vapor interface area ratio (f2

High-performance pneumatic solid–liquid triboelectric nanogenerator

In the field of blue energy harvesting, solid-liquid triboelectric nanogenerators (SL-TENG) show significant potential. However, their low output performance in energy applications is limited due to the relatively small contact area and slow separation speed during the friction process. To overcome these limitations, we develop a pneumatic solid-liquid triboelectric nanogenerator (PSL-TENG) with ultrahigh output performance based on pneumatic solid-liquid triboelectrification and electrostatic breakdown. The pneumatic design effectively increases the solid-liquid interface area and the solid-liquid contact and separation rate, which can enhance solid-liquid triboelectrification effect. The synergistic design of the horn and the bent pipe can converge positively charged mist into large droplets, which can enhance electrostatic breakdown effect. This innovative design approach has achieved outstanding output performance, with a peak instantaneous voltage of 6436 V and a peak instantaneous current of 3.05 mA. This exceptional output performance is sufficient to directly power 3000 LED lights or a 55 W commercial lamp. These research achievements provide new insights for SL-TENG in energy harvesting, as significant output can be generated by collecting the inherent charges of liquids themselves.

Experiment of droplet contact angle hysteresis on PTFE fibrous membrane in pore-scale

The present paper aims to study the influence of pore-scale fiber structure on the Cassie-Baxter contact angle of droplets on PTFE fibrous hydrophobic membranes. The fiber structure is obtained by the Confocal Laser Scanning Microscope (CLSM), and an optical test bench is built to measure the Cassie-Baxter contact angles of a droplet on a PTFE membrane. The free energy of the solid-liquid interface is measured by the surface tension integral on the droplet profile. An iterative algorithm for liquid-vapor interface shrinking is brought forward to analyze the liquid-vapor interface area between the droplet and the membrane, and the interface parameters of the Cassie-Baxter model are recognized. The experiment shows that the Cassie-Baxter contact angles and the contact line radii along the droplet circumference are in a normal distribution due to random fiber structures, and the variance of the normal distribution increases with the membrane pore diameter. The normal distribution of Cassie-Baxter’s contact angle indicates the constraint of the solid-liquid interface to the liquid-vapor interface. On the same normal contact pressure condition, the liquid-vapor area ratio of the droplet and membrane interface changes little, and Cassie-Baxter’s contact angle increases with the solid-liquid interface area ratio.

Heat transfer characteristics of solid-liquid interface on nanostructure surface under external electric field

With the size of high-performance electronic device decreasing (down to nanoscale), and the accompanying heat dissipation becomes a big problem due to its extremely high heat generation density. To tackle the ever-demanding heat dissipation requirement, intensive work has been done to develop techniques for chip-level cooling. Among the techniques reported in the literature, liquid cooling appears to be a good candidate for cooling high-performance electronic devices. However, when the device size is reduced to the sub-micro or nanometer level, the thermal resistance on the solid-liquid interface cannot be ignored in the heat transfer process. Usually, the interfacial thermal transport can be enhanced by using nanostructures on the solid surface because of the confinement effect of the fluid molecules filling up the nano-grooves and the increase of the solid-liquid interfacial contact area. However, in the case of weak interfacial couplings, the fluid molecules cannot enter into the nano-grooves and the interfacial thermal transport is suppressed. In the present work, the heat transfer system between two parallel metal plates filled with deionized water is investigated by molecular dynamics simulation. Electronic charges are applied to the upper plate and lower plate to create a uniform electric field that is perpendicular to the surface, and three types of nanostructures with varying size are arranged on the lower plate. It is found that the wetting state at the solid-liquid interface can change from Cassie state into Wenzel state with strength of the electric field increasing. Owing to the transition from the dewetting state to wetting state (from Wenzel to Cassie wetting state), the Kapitza length can be degraded and the solid-liquid interfacial heat transfer can be enhanced. The mechanism of the enhancing hart transfer is discussed based on the calculation of the number density distribution of the water molecules between the two plates. When the charge is further increased, electrofreezing appears, and a solid hydrogen bonding network is formed in the system, resulting in the thermal conductivity increasing to 1.2 W/(m·K) while the thermal conductivity remains almost constant when the electric charge continues to increase.

Molecular dynamics study of bubble nucleation on trigonometric nanostructured surfaces

Nanostructures have a crucial impact on bubble nucleation. In this paper, different nano-grooved surfaces are constructed using complex trigonometric functions, and the nucleation process of liquid argon on the surfaces is investigated using molecular dynamics simulations. The nanostructures affect the bubble nucleation location, and the results show that the bubble nucleation always occurs in the groove region during the boiling process. In contrast, the bubbles on the flat surface appear randomly. In addition, bubble growth is also affected by the nanostructure. Increasing nanostructure size can reduce the bubble nucleation time and enhance the bubble growth rate. The nanostructure increases the solid-liquid interface area, which enhances the heat transfer between solid and liquid. Finally, the reasons for bubble nucleation on different nanostructured surfaces are analyzed. The heat transfer between solid and liquid is the leading factor for bubble nucleation when the difference in liquid atoms’ potential energy is slight. As the nanostructure size increases, the substrate attracts more argon atoms to form a non-evaporating liquid layer, improving heat transfer and promoting bubble nucleation.

Superhydrophobic Fe-based amorphous alloy coatings with excellent anti-fouling and anti-corrosion properties by picosecond laser texturing

Producing superhydrophobic surfaces on the amorphous alloy coatings with excellent anti-corrosion and anti-fouling properties offers a promising strategy to broaden the applications of amorphous alloys in marine fields. In this work, the superhydrophobic surfaces on the Fe-based amorphous alloy coatings with micro-nano hierarchical structures were fabricated using picosecond pulsed laser texturing (LT) and surface chemical modification. The fabricated superhydrophobic coating surfaces have achieved a maximum contact angle of 166.5°and a minimum sliding angle of 3.6°. Additionally, these produced coatings also demonstrate outstanding self-cleaning and anti-fouling properties for preventing surface contamination. The superhydrophobicity originated from the formation of micro-nano hierarchical structures and the grafting of organic compounds with low surface free energy on the LT fabricated surfaces. The changes in wettability were attributed to the differences in the solid–liquid interfacial area fractions. The electrochemistry tests revealed that the corrosion resistance of the produced coatings with superhydrophobic surfaces had improved considerably compared with the as-sprayed amorphous alloy coatings, which could broaden the functional applications of the Fe-based amorphous alloy coatings.

Reconstruction of dendritic growth by fast tomography and phase field filtering

Three dimensional models of dendritic structures during solidification are valuable for building physical models, validating simulated results, estimating some properties such as permeability in the mushy, simulating semisolid deformation and so on. Thus, it is of interest to observe microstructure evolution in situ. Time-resolved tomography combined with X-ray diffraction has allowed us to observe the evolution of dendritic structures and to measure crystallographic orientation in situ. Reconstruction still proves to be difficult for some alloy systems because of the tradeoff between time and spatial resolution. This paper demonstrates the reconstruction of dendritic structures for three different alloy systems (Al-10mass%Cu alloy with a diameter of 4 mm, CrMnFeCoNi alloy with 1 mm, and Zn-4mass%Al alloy with 0.7 mm). The observations were performed in a synchrotron radiation facility SPring-8. A filter using a phase field model was introduced to reconstruct the three-dimensional images. Parameters used in the filtering were consistently determined based on the raw reconstruction images. Evaluation of solid-liquid interface area and curvature was significantly improved by the filter. For the Al-Cu alloy, a three-dimensional model containing approximately 300 million voxels was obtained. For the CrMnFeCoNi alloys, the preferred growth direction <100> was confirmed by tomography and X-ray diffraction. For the Zn-Al alloy, the observed 14 growth directions were not simply defined by the crystallographic orientations, although the directions were consistent with the hexagonal symmetry. This study verifies that time resolved tomography, X-ray diffraction and the filter using a phase field model provide three dimensional models for light metal alloys with rather large diameters and 3d transition-metal alloys with rather large X-ray absorption coefficients. The models are expected to be used for further studies.

Mesoporous CuFe2 O4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties.

Metal oxide-based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid-liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe2 O4 (CFO) thin film photoanodes were prepared by dip-coating and soft-templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127), polyisobutylene-block-poly(ethylene oxide) (PIB-PEO) and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquid™-PEO, KLE). The non-ordered CFO showed the highest photocurrent density of 0.2 mA/cm2 at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. This interpretation is confirmed by intensity-modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS). The lowest surface recombination rate was observed for the ordered KLE-based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order.

Dynamic Electrolyte Spreading during Meniscus-Confined Electrodeposition and Electrodissolution of Copper for Surface Patterning.

Meniscus-confined electrodeposition and electrodissolution are a facile maskless approach to generate controlled surface patterns and 3D microstructures. In these processes, the solid-liquid interfacial area confined by the meniscus dictates the zone on which the electrodeposition or the electrodissolution occurs. In this work, we show that the process of electrodeposition or electrodissolution in a meniscus-confined droplet system can lead to dynamic spreading of the meniscus, thereby changing the solid-liquid interfacial area confined by the meniscus. Our results show that the wetting dynamics depends on the applied voltage and the type of interface underneath the droplet, specifically a smooth surface with a homogeneous solid-liquid interface or a superhydrophobic surface with a heterogeneous solid-liquid and liquid-vapor interface. It is found that both electrodissolution and electrodeposition processes induced droplet spreading in the case of a smooth surface with a homogeneous interface. However, a superhydrophobic surface with a heterogeneous interface under the droplet produced nonlinear spreading during electrodissolution and spreading inhibition during electrodeposition. The underlying mechanisms resulting in the observed behavior have been explicated. The dynamic droplet spreading could modify the dimensions of the patterns formed and hence is of immense importance to the meniscus-confined electrochemical micromachining. The findings also provide fundamental insights into the spreading behavior and wetting transitions induced by electrochemical reactions.

A novel approach for wettability estimation in geological systems by fluid–solid interfacial area measurement using tracers

Wettability plays a vital role in many applications of flow in porous media and affects Darcy scale flow parameters by influencing the fluid–solid interfacial area. Therefore, quantifying the fluid–solid interfacial area can provide a way to measure wettability at the Darcy scale. Here, we experimentally explore a dual-tracer method, which can also be scaled to large geological reservoirs to quantify the fluid–solid interfacial area during the multiphase flow through a porous medium for different wetting conditions. Using our experiments, we demonstrate the influence of different saturations, wettability and flow conditions on the solid–liquid interfacial area. When oil is in the residual phase, we observe that the solid–water interfacial area increases with the increase in water saturation for the water-wet and mixed-wet cases. However, the water–solid interfacial area decreases with an increase in water saturation for the oil-wet case. We increase the water saturation by increasing the water flow rate; therefore, the anomalous behaviour seen in the oil-wet case can be attributed to the rearrangement of oil and water at higher water flow rates. When both oil and water are flowing, the solid–water interfacial area increases with water saturation for all the wettability cases and increases in water wettability as anticipated.Synopsis: Wettability measurements at Darcy-scale give a broad idea of overall subsurface wetting conditions for application in CO2 sequestration, ground-water remediation or oil recovery.

Superhydrophobic brass surfaces with tunable water adhesion fabricated by laser texturing followed by heat treatment and their anti-corrosion ability

Superhydrophobic metallic surfaces with tunable water adhesion have gained increasing interest due to their significance in both fundamental researches and practical applications. However, the relationship between the water adhesion and the anti-corrosion ability of the superhydrophobic metallic surfaces has been seldom studied. In this work, superhydrophobic brass surfaces with tunable water adhesion were fabricated by laser texturing followed by heat treatment, and the anti-corrosion ability of these surfaces was studied by performing electrochemistry tests. The analyses of surface structures and compositions revealed that the observed superhydrophobicity results from the micro/nanostructures produced by the laser texturing and the adsorption of organic matter from ambient air during the heat treatment; the different water adhesions of these superhydrophobic surfaces are attributed to their different solid–liquid interfacial area fractions. The results of the electrochemistry tests revealed that the anti-corrosion abilities of the investigated surfaces are the following order: the superhydrophobic surface with low water adhesion > the superhydrophobic surface with high water adhesion > the bare brass surface, which stems from the different solid-electrolyte interfacial area fractions of these surfaces.

Impact Dynamics and Freezing Behavior of Surfactant-Laden Droplets on Non-Wettable Coatings at Subzero Temperatures.

The present study investigates the impact and freezing behavior of the droplets of surfactant solutions on non-wettable coatings at very low temperatures of -10 to -30 °C. Our goal is to elucidate the critical role of concentration, molecular weight, and ionic nature of surfactants on these phenomena. To achieve this goal, we used sodium dodecyl sulfate (anionic), hexadecyltrimethylammonium bromide (cationic), and n-decanoyl-n-methylglucamine (nonionic) at four concentrations ranging from 0 to 2 × CMC (critical micelle concentration). We captured the impact-freezing of the droplets on superhydrophobic alkyl ketene dimer coatings using a high-speed camera at 5000 frames per second. The results show that the ability of the droplets to spread and retract on the coatings is a function of concentration, ionic nature, and molecular weight of the surfactants, as well as the temperature-dependent viscosity of the solutions. Additionally, surfactant-laden droplets generally demonstrated an accelerated freezing compared to pure water. This might be due to the fact that the presence of surfactants can promote both heterogeneous ice nucleation from within the liquid and a larger solid-liquid interfacial area by filling the air pockets of the surface, leading to enhanced heat transfer. The behavior of the cationic surfactant at certain concentrations was, however, an exception leading to a freezing delay, for which a mechanism will be proposed.

A highly-efficient and durable Pt-based electrocatalyst decorated by Co2C-Mo2C@CS composite for methanol oxidation reaction

Designing highly efficient electrocatalysts for the methanol electro-oxidation is an ongoing challenge. Herein, we report that molybdenum carbide (Mo2C) and cobalt carbide (Co2C) are grown in situ on the surface of carbon spheres to form Co2C-Mo2C@CS carrier material by the impregnation-reduction-annealing process, and then Pt nanoparticles are deposited on the Co2C-Mo2C@CS surface through sodium borohydride reduction method. In-depth material characterizations and electrochemical analyses demonstrate that the introduction of small-sized Mo2C-Co2C composites can facilitate mass transfer and increase the solid-liquid interface area. As expected, the resulting Pt/Co2C-Mo2C@CS catalyst displays excellent methanol oxidation activity (1504.5 mA mgPt−1), enhanced CO tolerance and stability comparing with other catalysts, which mainly derived from the metal-support interaction between Co2C-Mo2C and small-sized Pt NPs. The rational design of Pt/Co2C-Mo2C@CS catalyst provides a feasible strategy for boosting the overall performance of Pt-based catalysts towards methanol oxidation, and broadens the potential applications of metal carbides.

Solid-fluid interfacial area measurement for wettability quantification in multiphase flow through porous media

Quantification of wettability inside a porous medium is challenging. Most of the currently available methods use either a proxy surface of the porous solid or a small sample of the porous medium for wettability quantification. Recognizing that more wetting fluid has more interfacial area with porous solid, we develop a tracer method to quantify the solid-liquid interfacial area for a porous medium. We use two tracers for a fluid-phase, an ideal tracer, and an adsorbing tracer. The ideal tracer follows the flow path, and the adsorbing tracer dynamically adsorbs on the porous surface. We use the tracers’ mean residence time to quantify the solid-liquid interfacial area during multiphase flow in a porous medium. We conduct the tracer flow experiments at various saturations for a given wettability condition. We observe a monotonic behaviour of the wetted area with the fluid saturations. At the same residual saturation, oil has 10% less interfacial area with the porous solid in comparison to water. This difference in contact area can be used for quantifying the wettability using our methodology.

Molecular dynamics study of the nanoscale boiling heat transfer process on nanostructured surfaces

In this study, the effects of nanostructures on nanoscale water boiling heat transfer are investigated using nonequilibrium molecular dynamics simulations. The nanostructured substrates show great superiority over the smooth surface. The bubble nucleation time is advanced and the heat transfer efficiency is enhanced on structured surfaces. Besides, the heat transfer enhancement differs on surfaces with different nanostructures with the same nanostructure volume. Through calculation, it is found that the solid-liquid interfacial area varies with the surface structure configuration and the square nanogroove case which achieves the best heat transfer performance among the nanostructured systems has the largest solid-liquid contact area. To prove the hypothesis that the heat transfer efficiency increases with the interfacial surface area, a solid-liquid-solid system is developed to measure the interfacial thermal resistance between the different solid surfaces and water molecules. The interaction energy per unit area is also calculated to explain the heat transfer variations. Results show that with the increment of surface area which adsorbs more water molecules at the interface could increase the interaction energy per unit area, thereby decreasing the interfacial thermal resistance. The results in this study could provide some insight into the nanostructure design to enhance the heat transfer involving boiling.

Effect of Nanobubbles on the Slime Coating of Kaolinite in Coal Flotation

Understanding the coating behavior of fine gangue slimes in the presence of nanobubbles (NBs) is important for the application of NB technology in flotation. In this study, slime coating of kaolinite in the flotation of a low-ash coal using deionized (DI) water and NB water was investigated. Kaolinite was found to depress coal flotation by the formation of coating on coal surfaces, but its deleterious effect was less pronounced in the NB water with mitigated slime coating. Setting tests, Brunauer–Emmett–Teller surface area measurements, and dynamic light scattering were conducted to understand the underpinning mechanism. In comparison with DI water, the degree of kaolinite aggregation was enhanced in the NB water. The intensified self-aggregation of kaolinite platelets which appears to be induced by the presence of NBs reduces the solid–liquid interfacial area as well as the number of free kaolinite particles in the suspension, mitigating the coating of kaolinite on coal surfaces in NB water flotation.

Interfacial Strategies for Smart Slippery Surfaces

The problem of contact line pinning on surfaces is pervasive and contributes to problems from ring stains to ice formation. Here we provide a single conceptual framework for interfacial strategies encompassing five strategies for modifying the solid-liquid interface to remove pinning and increase droplet mobility. Three biomimetic strategies are included, (i) reducing the liquid-solid interfacial area inspired by the Lotus effect, (ii) converting the liquid-solid contact to a solid-solid contact by the formation of a liquid marble inspired by how galling aphids remove honeydew, and (iii) converting the liquid-solid interface to a liquid-lubricant contact by the use of a lubricant impregnated surface inspired by the Nepenthes Pitcher plant. Two further strategies are, (iv) converting the liquid-solid contact to a liquid-vapor contact by using the Leidenfrost effect, and (v) converting the contact to a liquid-liquid-like contact using slippery omniphobic covalent attachment of a liquid-like coating (SOCAL). Using these approaches, we explain how surfaces can be designed to have smart functionality whilst retaining the mobility of contact lines and droplets. Furthermore, we show how droplets can evaporate at constant contact angle, be positioned using a Cheerios effect, transported by boundary reconfiguration in an energy invariant manner, and drive the rotation of solid components in a Leidenfrost heat engine. Our conceptual framework enables the rationale design of surfaces which are slippery to liquids and is relevant to a diverse range of applications.

When condensed matter physics meets biology: Does superhydrophobicity benefiting the cryopreservation of human spermatozoa?

With the increasing demand in regenerative and reproductive medicine for successful conservation of living matter, the need of reliable platform in cell banking seems inevitable. Whilst the cells storage at cryogenic temperatures is a well-developed method, far less is known about the efficiency of nanotechnology in cryogenics. The primary objective of this study is to represent the first of its kind experimental results related to cryopreservation of human spermatozoa by means of superhydrophobic carbon soot coatings. The inclusion of soot-based water repellent interface during the freezing and thawing of human semen minimizes the solid-liquid interfacial area, retards the heat transfer rate and promotes the recovery of up to 80% of initial motility of post-thaw sperm cells. Our discoveries reveal a fundamentally new and exciting direction of development of cryopreservation technologies in the battle against painful biopsies and repetitive surgeries.

Assessment and Interpretation of Surface Wettability Based on Sessile Droplet Contact Angle Measurement: Challenges and Opportunities

AbstractSurface wettability, a property that governs the interaction between the solid and liquid phases, is central in various biological systems and technological applications. A typical approach to assess the wetting interaction is based on the sessile droplet contact angle. Despite its experimental simplicity, caution is required in the measurement and interpretation of the sessile contact angle as a wettability metric. In this work, the major challenges related to the use of the sessile droplet contact angle for characterizing surface wetting behavior are reviewed. The complexity of interpreting surface wettability using the contact angle value arising from the following factors is discussed: the role of solid–liquid interfacial area and three‐phase contact line, the existence of multiple metastable states with a range of contact angle values, and the droplet size dependence of contact angles. An overview of the emerging alternative wetting evaluation techniques and parameters that can complement the sessile contact angle measurement is outlined.