Solid-liquid Interactions Research Articles

Molecular dynamic simulation on the density of titanium dioxide and silver water-based nanofluids using ternary mixture model

This study utilizes molecular dynamics (MD) simulations to scrutinize the influence of type and volume fraction of nanoparticle on the density of nanofluids, i.e. silver (as a hydrophilic case) and titanium dioxide (as a hydrophobic case) water-based nanofluids. Ionic structure of TiO2 distinguishes the solid–liquid interactions with those in silver-water nanofluid. The thickness of nanolayer is estimated from density profile formed around the nanoparticle and is included in the density calculation. It is found that the type of interatomic potential between the nanoparticle and surrounding water can highly affect the thickness of nanolayer. The hydrophobicity/hydrophilicity of nanoparticle is of great importance in nanolayer density and its thickness. It is observed that the difference between the traditional binary mixture and the ternary mixture formula for calculating the density of nanofluid increases by increasing the volume fraction of nanoparticle. This indicates that the effect of nanolayer gets more considerable at high volume fractions.

Dynamic observation of dislocation evolution and interaction with twin boundaries in silicon crystal growth using in – situ synchrotron X-ray diffraction imaging

The grown-in dislocation dynamics and interaction mechanisms with growth twins are investigated in-situ during the directional solidification of silicon crystal. The melting, solidification and cooling down process is performed in a dedicated installation at the European synchrotron radiation facility and is followed by X-ray Bragg diffraction imaging techniques (X-ray topography) at the mesoscale in real-time. Existing dislocations in the seed are observed to propagate in the up-grown crystal via replicas. They expand vertically with the moving solid-liquid interface being always aligned perpendicular to the growth front. During the solidification process when they meet a growth twin lamella (Σ3{111}), they neither pile-up nor transmit through the boundary. They are blocked by the twin, but they continue to move laterally behind the growth front due to the thermomechanical stresses in the system. The existence of dislocations at the solid-liquid interface, their evolution and interaction with twin boundaries is discussed, as growth proceeds, based on a detailed crystallographic analysis of the system.

Enhancing methanogenic fermentation of waste activated sludge via isoelectric-point pretreatment: Insights from interfacial thermodynamics, electron transfer and microbial community

The usefulness of waste activated sludge (WAS) as an energy source is limited by the poor generation efficiency of methane from WAS, which is mainly due to the complex nature of sludge and low abundance of functional microbes. In this study, the interfacial thermodynamics, electron transfer and microbial community of sludge were investigated to reveal the enhancing effects of isoelectric-point (pI) pretreatment on the efficiency of methane generation from WAS. Experimentally, after pI pretreatment, the methane production potential, maximum methane production rate and maximum methane proportion in the biogas increased by 122.2%, 154.4% and 17.4%, respectively, indicating that pI pretreatment enhanced the generation efficiency of methane. Analyses of changes in the solid-liquid interfacial non-covalent interaction energy, electron transfer capacity (ETC) and reductive peak potential values of sludge samples with and without pI pretreatment during a 170-day methanogenic fermentation period revealed that pI pretreatment enhanced the self-driven solid-liquid interfacial hydrophobic attractions of sludge, increased the abiotic driving forces of interfacial enzymatic reactions, promoted the electron transfer efficiency and lowered the barrier of the reduction reaction. It was thus hypothesised that these changes would be responsible for increasing methane production, which was confirmed by the correlation analyses between the interfacial free energy (IFE) and ETC versus daily methane production. Moreover, statistical analyses of the differences between the microbial communities of sludge samples with and without pI pretreatment during fermentation demonstrated that pI pretreatment significantly (P < 0.05) improved the relative abundances of the main functional microbes with respect to hydrolysis, acidification and methanation. A further investigation of the relationships of IFE and ETC with the relative abundances of the main genera of methanogens indicated that the hydrophobic attraction of sludge surface and a high ETC are conducive to the enrichment of hydrogenotrophic methanogens (+29.9%). These findings are expected to provide a conceptual framework for developing second-generation pretreatment methods and provide a methodological reference for revealing the details of the ‘black-box’ anaerobic digestion process.

Solid-liquid interaction caused by minor wetting in gravel-ice mixtures: A key factor for the mobility of rock-ice avalanches

Rock-ice avalanches pose serious risks to humans and infrastructures due to their extremely large volumes and high mobility in glacierized mountain regions. The mobility of rock-ice avalanches is supposed to be influenced by water, while the solid-liquid interactions induced by minor wetting remain an open issue. A series of flume tests was performed to investigate the influence of a small amount of water (no more than 4% of the gravel weight) on the behavior and mobility of gravel-ice mixtures. The test results show that intergranular refreezing bonds were created with a preliminary addition of water. The breakup of some weak refreezing bonds during particle rearrangement likely dissipated some kinetic energy, and thus, the mixture mobility was reduced. As the water content increased, the thermal conductivity could likely not remain efficient enough to refreeze all the trapped water and thus allowed unfrozen water to lubricate particle corners, which enhanced mixture mobility. With a further increase in the water content, suction-induced cohesion cemented particles as clusters and reduced mixture mobility. However, a high ice content resulted in mobility enhancement with increasing water content because more gravel was refrozen into the ice matrix, and thus, the basal friction was lowered by the occupation of ice cubes at the bottom. The redistribution pattern of trapped water during mass propagation changed at a characteristic water content. When the water content was lower than the characteristic content, superficially cohered water detached from the surface asperity of one particle and transferred to another during particle rearrangement, i.e., local redistribution of trapped water. When the water content was higher than the characteristic content, the downward transfer of a considerable amount of trapped water was triggered by particle rearrangement under the influence of gravity, i.e., bulk redistribution of trapped water. The water experiencing bulk redistribution influenced the mixture mobility by exerting intergranular drag force via the build-up and break-up of liquid bridges, while locally redistributed water influenced mixture mobility by altering the surface properties of particles. This study is expected to be helpful in better understanding the propagation mechanisms of water-poor rock-ice avalanches and in improving relevant hazard assessments. • Water influence on rock-ice avalanche mobility was studied via a small flume test. • Lubrication of unfrozen water enhanced mixture mobility. • Suction-induced cohesion reduced mixture mobility. • The refreezing of minor trapped water might reduce mixture mobility. • Water redistribution pattern changed at a typical water content of 2.2%.

Surface wettability of silicon nanopillar array structures fabricated by biotemplate ultimate top-down processes

We fabricated high aspect ratio 10-nm Si nanopillar (NP) array structures with a few-tenths-nm-gap arranged by fusing biotemplate and neutral beam etching processes to investigate the wettability [e.g., contact angle (CA)] with and without surface silicon oxide film. The NP array with silicon native oxides in all gaps exhibited super-hydrophilicity due to the chemical liquid-solid interface interaction and larger surface area than the Si flat surface thanks to the NP structure. These phenomena can be explained by using the Wenzel model. In contrast, when we selectively removed the native oxide on Si NP surface with our radical treatment, a gap variation from 11 to 43 nm stably resulted in a CA of more than 96° (hydrophobicity) with a maximum of 115°. This can be explained by using the Cassie–Baxter model with a filling factor. Our findings demonstrate that controlled surface wettability can be achieved by combining our controllable gap silicon NP array structures and the surface with or without silicon native oxides. The gap of a Si NP fills with water due to the capillarity on a silicon native oxide, but on a pure stable silicon one with a defect-free surface, does not completely fill. We found that Si NP structures with controllable gaps exhibit a surface wettability ranging from super-hydrophilicity to high-hydrophobicity.

A perspective on intelligent design of engineered materials and structures by interface mechanics

Engineering design of materials and structures has evolved radically from earliest employment of single phase materials with a focus on optimization of geometric shapes to recent multiphase materials with tactful integration and assembly in both geometric shapes and material domains. In these recent design concepts and strategies, the interfaces are crucial and leveraged to achieve properties and functionalities surpassing those of individual material/phase components and/or their simple superpositions. This perspective focuses on the underpinned interface mechanics view in which insights are essential to understand fundamentals of integration, tailoring and assembly of multiphase materials with diverse geometric layouts. The view also provides insights to enable creations of intelligent new design concepts and approaches of materials and structures capable of being readily manufactured with current technologies. Selected examples of engineered materials and structures enabled by either solid-solid or solid-liquid interactions are provided to highlight the driving role of interface mechanics in designs and to outlook the associated future opportunities and challenges.

Effect of Asphaltene on Threshold Pressure Gradient of Heavy Oil in Porous Media

Many studies have shown that heavy oil with high asphaltene content has a yield stress. Coupled with the solid-liquid interaction between porous media and heavy oil, there is a threshold pressure gradient when heavy oil flows in porous media. Meanwhile, some previous research has indicated that the high viscosity of heavy oil is the decisive factor for its threshold pressure gradient. Hence, this concept needs more clarification, especially because its accuracy is questionable. In this research, different oil samples with the same viscosity and also different asphaltene contents heavy oil samples were prepared. The viscosity of the different heavy oil samples was measured. Threshold pressure gradient experiments under different permeabilities and temperatures were also conducted on heavy oils. The results proved that the viscosity was not directly related to threshold pressure gradient of heavy oil. They also suggested that the heavy oil viscosity increased with the increase of asphaltene content. Moreover, the formula of the factors affecting threshold pressure gradient was regressed, and also its applicability was verified. As the temperature and core permeability increase, the threshold pressure gradient was also proven to decrease significantly. Furthermore, it was found that the threshold pressure gradient increased significantly with the increase of asphaltene content. Therefore, the heavy oil threshold pressure gradient could be characterized as a function of temperature, permeability, and asphaltene content. This study provided some theoretical support for the research attempts on the reduction of threshold pressure gradient and also on the effective development of heavy oil reservoirs.

Directional Droplet Transport Mediated by Circular Groove Arrays. Part II: Theory of Effect.

In the first part of this research, we reported the experimental study of the drop impact on the superhydrophobic circular groove arrays, which resulted in a directional droplet transport. In the second part, we further explored the influence of the Weber number (We), ridge height (H), and the deviation distance (r) between the impacting point and the center of curvature on the lateral offset distance (ΔL) of bouncing drops. The suggested theoretical analysis is in reasonable agreement with the experimental observations. We demonstrate that a Cassie-Wenzel wetting transition occurred within the microstructures of the relief under the threshold Weber number, for example, We ≅ 19-25, which switched the nature of drop bouncing. The dynamic pressure plays a decisive role in the directional droplet transport. The reported investigation may shed light on the solid-liquid interactions occurring on the patterned hierarchical surfaces and open up new opportunities for directional droplet transportation.

Universal and tunable liquid\u2013liquid separation by nanoparticle-embedded gating membranes based on a self-defined interfacial parameter

Superwetting porous membranes with tunable liquid repellency are highly desirable in broad domains including scientific research, chemical industry, and environmental protection. Such membranes should allow for controllable droplet bouncing or spreading, which is difficult to achieve for low surface energy organic liquids (OLs). Here we develop an interfacial physical parameter to regulate the OL wettability of nanoparticle-embedded membranes by structuring synergistic layers with reconfigurable surface energy components. Under the tunable solid-liquid interaction in the aggregation-induced process, the membranes demonstrate positive/negative liquid gating regularity for polar protic liquids, polar aprotic liquids, and nonpolar liquids. Such a membrane can be employed as self-adaptive gating for various immiscible liquid mixtures with superior separation efficiency and permeation flux, even afford successive achievement of high-performance in situ extraction-back extraction coupling. This study should provide distinctive insights into intrinsic wetting behaviors and have pioneered a rational strategy to design high-performance separation materials for diverse applications.

Wetting dynamics of nanoliter water droplets in nanoporous media

HypothesisThe Lucas-Washburn (L-W) equation is the classical theory to describe the dynamics of spontaneous imbibition in single micro-channels and micro-scale porous media. However, for nanoliter droplets imbibition in nanoporous media, the L-W equation may not be suitable, due to the nanoscale liquid-solid interactions, e.g., contact line pinning and capillary condensation. In addition, for an intrinsically hydrophobic nanoporous substrate, spontaneous imbibition of a nanoliter droplet is hypothesized to occur if capillary condensation had occurred internally already. ExperimentsA nanoporous carbon scaffold was synthesized and used as a model nanoporous medium. A recently-developed micro-injection technique was used to generate a series of nanoliter water droplets (2.8–34 nL); the entire wetting dynamics (i.e., apparent contact angle and droplet volume as a function of time) were observed inside an environmental scanning electron microscope. FindingsThe L-W equation does not describe the wetting dynamics of nanoliter water droplets in nanoporous media. A new theoretical model is developed to characterize the corresponding dynamics. It is demonstrated that, even for an intrinsically hydrophobic nanoporous substrate, spontaneous imbibition of a nanoliter droplet can occur if capillary condensation had occurred internally already.

Numerical and experimental investigations of interdigital transducer configurations for efficient droplet streaming and jetting induced by surface acoustic waves

Surface acoustic wave (SAW) based technologies have recently been explored for various sensing and microfluidic applications, and numerous experimental studies and numerical modelling of SAW streaming and liquid-solid interactions have been performed. However, the large deformation of droplet interface actuated by SAWs has not been widely explored, mainly due to the complex physics of SAW-droplet interactions and interfacial phenomena. In this paper, a computational interface tracking method is developed based on the couple level set the volume of fluid (CLSVOF) approach to simulate the interactions between liquid and acoustic waves and deformation of the liquid-air surface. A dynamic contact angle boundary condition is developed and validated by experimental results to simulate the three-phase contact line dynamics. The modified CLSVOF method is then used to study the droplet jetting and internal streaming behaviours by analyzing the energy terms within the liquid medium. Furthermore, by applying the numerical model, effects of configurations and positions of two interdigital transducers (IDTs) on droplet actuation have been investigated to achieve efficient mixing, separation, and jetting. Results show that two perfectly aligned IDTs are optimal for mixing applications. In contrast, two offset IDTs are optimal for concentration and separation applications. The maximum jetting velocity and minimum jetting time are achieved by using a pair of aligned IDTs, whereas by using the two offset IDTs, effective liquid mixing and jetting are observed which can be used in bioprinting applications.

The influence of wall properties on convective heat transfer in isothermal nanochannel

The effect of solid wall properties on the convective heat transfer characteristic in a three-dimensional nanochannel is probed with molecular dynamics simulation. The interfacial velocity slip and temperature jump are treated as the function of the interatomic interaction, the atomic mass and thickness, respectively. The result shows that the wall properties, including interatomic interaction among wall atoms, atomic mass, as well as coating thickness based on certain interatomic interaction and atomic mass, will lead to significant changes in the temperature jump. These effects are caused by the variations of coupling degree of solid-liquid vibration frequency induced by the mismatch between solid wall and fluid atoms. Furthermore, temperature jump is also related to the solid-liquid interaction while the velocity slip is only related to the solid-liquid interaction. It's found that both the decrease of interatomic interaction and the increase of atomic mass can further enhance the heat transfer. That is, the interatomic interaction among wall atoms is closer to that among fluid atoms, the atomic mass of wall atoms is much greater than that of the fluid atoms, and the convective heat transfer in the channel is better. However, the variation of Nusselt number with coating thickness is notably, which exhibits a peak value in the hydrophilic and hydrophobic cases. It's indicated that vibrational density of states (VDOS) corresponding to a selected coating wall properties (including interatomic interaction, atomic mass and thickness) lies in between the VDOS of the channel wall atoms and that of fluid atoms, can play a role of thermal bridge between wall and fluid atoms and lead to a remarkable increase of heat transfer. Based on the selected solid wall properties, a reasonable selection of wall-fluid interaction can effectively increase velocity slip and achieve drag reduction.

Promoting rebound of impinging viscoelastic droplets on heated superhydrophobic surfaces

The rebound of impinging droplets is a defining characteristic of superhydrophobic surfaces; yet, such an intriguing interfacial phenomenon can be effectively suppressed by adding a tiny amount of flexible polymers to induce non-Newtonian viscoelastic properties. In this work, however, we demonstrate the promoting effects of surface heating on the rebound of impinging viscoelastic droplets on superhydrophobic surfaces. The underlying mechanism for the promotion is that the local heat transfer at the liquid–solid interface causes the fast evaporation of the liquid and thus the breakup of the formed viscoelastic filaments, which hinder droplet recoiling. Therefore, the lower threshold velocity for rebound increases while the upper threshold velocity for rebound suppression decreases with increasing surface temperature, resulting in a wider regime for droplet rebound in the impact phase diagram. The surface heating effect on liquid–solid interactions also leads to a nontrivial dependence of the contact time on the impact velocity and a linear decrease of the restitution coefficient with the Weber number for diverse bouncing viscoelastic droplets, which can be rationalized by coupling the interfacial force and energy analyses. We envision that these findings would be useful in technological processes requiring control the retention of viscoelastic liquids on solid surfaces.

A generalized examination of capillary force balance at contact line: On rough surfaces or in two-liquid systems

We investigate the capillary force balance at the contact line on rough solid surfaces and in two-liquid systems. Our results confirm that solid-liquid interactions perpendicular to the interface have a significant influence on the lateral component of the capillary force exerted on the contact line. Surface roughness of the solid substrate reduces the mobility of liquid and alters how the perpendicular solid-liquid interactions transfer into a force acting parallel to the interface. A quantitative relation between surface roughness and the transfer strategy is proposed. Moreover, when a liquid is in coexistence with another immiscible liquid on a solid, the capillary forces exerted on liquids of both sides are involved in our theoretical model. The contact angle can be predicted by calculating three interfacial tensions. These arguments are then verified by molecular dynamics simulations. Our findings set up the generalized theoretical framework for the capillary force balance at the contact line and broaden its application in more realistic scenarios.

Molecular Investigation of Contact Line Movement in Electrowetted Nanodroplets.

Molecular dynamics (MD) simulation of an electrowetted nanodroplet is performed to understand the fundamental origin of the involved parameters resulted from the molecular movement in the vicinity of the three-phase contact line (TPCL). During the spreading of the droplet, contact line friction (CLF) force is found to be the controlling one among all other resistive forces. Being molecular in nature, MD study is required to unveil the CLF, which is manifested by the TPCL friction coefficient ζ. The combined effect of temperature, electric field, and surface wettability, manifested by the solid-liquid Lennard-Jones interaction parameter, is studied to explore the droplet spreading. The entire droplet wetting dynamics is divided into two different regimes, namely, spreading regime and equilibrium regime. The molecular frequency during the TPCL movement in the equilibrium regime is affected by the presence of any external perturbation and results in an alteration of ζ. The predetermined knowledge of the alteration of CLF due to the coupling effect of electric field and temperature will have a potential application towards designing electric field-inspired droplet movement devices.

Effect of Solid–Liquid Interactions on Substrate Wettability and Dynamic Spreading of Nanodroplets: A Molecular Dynamics Study

Solid–liquid interaction plays a key role in substrate wettability and spreading dynamics of liquid droplets. Yet, how the solid–liquid interaction controls wettability and the spreading process is...

Spontaneous outflow efficiency of confined liquid in hydrophobic nanopores

The suspension of nanoporous particles in a nonwetting liquid provides a unique solution to the crux of superfluid, sensing, and energy conversion, yet is challenged by the incomplete outflow of intruded liquid out of nanopores for the system reusability. We report that a continuous and spontaneous liquid outflow from hydrophobic nanopores with high and stable efficiency can be achieved by regulating the confinement of solid-liquid interactions with functionalized nanopores or/and liquids. Full-scale molecular-dynamics simulations reveal that the grafted silyl chains on nanopore wall surfaces will promote the hydrophobic confinement of liquid molecules and facilitate the molecular outflow; by contrast, the introduction of ions in the liquid weakens the hydrophobic confinement and congests the molecular outflow. Both one-step and multistep well-designed quasistatic compression experiments on a series of nanopores/nonwetting liquid material systems have been performed, and the results confirm the outflow mechanism in remarkable agreement with simulations. This study offers a fundamental understanding of the outflow of confined liquid from hydrophobic nanopores, potentially useful for devising emerging nanoporous-liquid functional systems with reliable and robust reusability.

Viscosity measurement and simulation of microbubble wetting on flat surfaces with many-body dissipative particle dynamics model

One of the most important flow properties of liquid is its viscosity. In the present paper, two non-equilibrium methods, Poiseuille flow method and Couette flow method, are employed to measure the viscosity of many-body dissipative particle dynamics (MDPD) model. The results obtained by these two non-equilibrium methods are consistent with each other. Based on Poiseuille flow method, we obtain a map for the viscosity of MDPD model with different numerical parameters. Furthermore, the static wetting of microbubbles is investigated by a selected system from the map. The relationship between the contact angle of bubbles and the solid-liquid interaction is obtained, which can be helpful for further microfluidic research and applications based on microbubbles.

Influence of Turbulence, Density, Phase Change, and Phase Interfaces Models on the Performance of the Numerical Simulation of a Two-Phase Closed Thermosyphon

A heat pipe can be considered a highly effective thermal conduction device, which is especially desirable in heat transfer operations in order to ensure high energy efficiency. However, the operation of heat pipes comprises different heat and mass transfer phenomena, such phase change, heat conduction and convection, solid-liquid and vapor-liquid surface interactions, surface vaporization, and nucleate boiling. Therefore, modelling heat pipes is a highly complex task that demands detailed knowledge of the physical phenomena involved and choosing suitable theoretical models to obtain a good representation of the real nature of the heat and mass transfer processes. In this study, some models and parameters available in the commercial CFD software ANSYS Fluent for turbulence, density, phase change, and phase interfaces were examined to determine their influence on the prediction of the heat and mass transfer in a two-phased closed thermosyphon (TPCT). The numerical results show that using a turbulence viscous model is not necessary and that a variable density model improves the temperature distribution inside the TPCT. Furthermore, using high mass and energy transfer coefficients during condensation makes the vapor remain close to the saturation temperature. Finally, a sharp interphase model is strongly recommended for this type of process.

Effect of twist blade distributor on velocity distribution in a swirling fluidized bed

Swirling Fluidized Bed (SFB) is one of the liquid-solid interaction in fluidization improved from conventional systems. This system is usually viewable in the power generation, chemical industry, material production and drying processes. Inside the SFB, there is annular blade distributor which will cause the air to pass through and create a swirl motion on the bed. The energy consumption of a fluidized bed system depends on the distributor’s design. The current study has purpose a new design that applies to existing designs of blade inclination angle. The simulation study was conducted using Computational Fluid Dynamics (CFD) to obtain the result of velocity distribution and pressure drops on various blade distributor designs. This study uses two (2) twist angle (80° and 100°) via number of blade distributor (40, 50 and 60). In this study, tangential velocity is the main velocity component by reason of the velocity represents the rotating air velocity in fluidization system. Overall, the design of the 100° twist angle and 40 blades distributor are the best distributors of blades compared to others.