Oil spills and industrial organic pollutants have induced severe water pollution and threatened every species in the ecological system. To deal with oily water, special wettability stimulated materials have been developed over the past decade to separate oil-and-water mixtures. Basically, synergy between the surface chemical composition and surface topography are commonly known as the key factors to realize the opposite wettability to oils and water and dominate the selective wetting or absorption of oils/water. In this review, we mainly focus on the development of materials with either super-lyophobicity or super-lyophilicity properties in oil/water separation applications where they can be classified into four kinds as follows (in terms of the surface wettability of water and oils): (i) superhydrophobic and superoleophilic materials, (ii) superhydrophilic and under water superoleophobic materials, (iii) superhydrophilic and superoleophobic materials, and (iv) smart oil/water separation materials with switchable wettability. These materials have already been applied to the separation of oil-and-water mixtures: from simple oil/water layered mixtures to oil/water emulsions (including oil-in-water emulsions and water-in-oil emulsions), and from non-intelligent materials to intelligent materials. Moreover, they also exhibit high absorption capacity or separation efficiency and selectivity, simple and fast separation/absorption ability, excellent recyclability, economical efficiency and outstanding durability under harsh conditions. Then, related theories are proposed to understand the physical mechanisms that occur during the oil/water separation process. Finally, some challenges and promising breakthroughs in this field are also discussed. It is expected that special wettability stimulated oil/water separation materials can achieve industrial scale production and be put into use for oil spills and industrial oily wastewater treatment in the near future.

Biomimetic super-lyophobic and super-lyophilic materials applied for oil/water separation: a new strategy beyond nature

Advanced Mathematical Methods for Scientists and Engineers

Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Advanced Thermoelectric Design: From Materials and Structures to Devices

Three-dimensional nanofibrous aerogels (NFAs) that are both highly compressible and resilient would have broad technological implications for areas ranging from electrical devices and bioengineering to damping materials; however, creating such NFAs has proven extremely challenging. Here we report a novel strategy to create fibrous, isotropically bonded elastic reconstructed (FIBER) NFAs with a hierarchical cellular structure and superelasticity by combining electrospun nanofibres and the fibrous freeze-shaping technique. Our approach causes the intrinsically lamellar deposited electrospun nanofibres to assemble into elastic bulk aerogels with tunable densities and desirable shapes on a large scale. The resulting FIBER NFAs exhibit densities of >0.12 mg cm(-3), rapid recovery from deformation, efficient energy absorption and multifunctionality in terms of the combination of thermal insulation, sound absorption, emulsion separation and elasticity-responsive electric conduction. The successful synthesis of such fascinating materials may provide new insights into the design and development of multifunctional NFAs for various applications.

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Ultralight nanofibre-assembled cellular aerogels with superelasticity and multifunctionality

We numerically study the dynamics of buoyancy-driven bubbles in a constricted vertical capillary in which a throat with an arc shape is present. To investigate at what conditions and how the bubble would be entrapped at the capillary throat, a diffuse-interface immersed-boundary method is used in numerical simulations. Axisymmetric simulations are performed for various bubble and throat sizes, represented by the diameter ratio of the throat to the bubble, η ([Formula: see text]), the Bond number ([Formula: see text]), and the Reynolds number ([Formula: see text]). We find that small bubbles have insignificant deformation and, thus, cannot pass through a throat with [Formula: see text], while relatively large bubbles encounter noticeable interface oscillations at their lower part when approaching the throat. In particular, the interface oscillations are composed of a standing wave arising from buoyancy and a capillary wave propagating radially. A phase diagram is presented regarding the eventual bubble morphology: pass-through and entrapment. For the critical diameter ratio ηc at the onset of bubble entrapment, we proposed two scaling laws based on the analysis of the deformability and oscillation of the bubble, i.e., [Formula: see text] for Bo < 1 and [Formula: see text] for Bo > 1. These theoretical predictions are in good agreement with our numerical results.

Buoyancy-driven bubbles in a constricted vertical capillary

Abstract Natural oscillations of sessile drops with a free or pinned contact line in different gravity environments are studied based on a linear inviscid irrotational theory. The inviscid Navier–Stokes equations and boundary conditions are reduced to a functional eigenvalue problem by the normal-mode decomposition. We develop a boundary element method model to numerically solve the eigenvalue problem for predicting the natural frequencies. Emphasis is placed on the frequency shifts of modes due to gravity for a wide range of contact angles $\alpha$ and Bond numbers $Bo$. Three types of $\alpha$–$Bo$ diagrams reflecting how gravity shifts the frequency are identified. Specifically, the frequency of zonal modes shifts downwards (upwards) when $\alpha$ is smaller (larger) than a critical value, while the frequencies of most sectoral modes are shifted downwards regardless of $\alpha$. As a result, gravity can transform the lowest mode from a zonal mode to a sectoral mode. The spectral degeneracy of hemispherical drops inherited from the Rayleigh–Lamb spectrum is also broken by gravity. However, we discover that gravity has no effect on the mode associated with the horizontal motion of the centre of mass, whose frequency is always zero regardless of $\alpha$ and $Bo$. This implies that the ‘walking’ drop instability reported in previous literature does not exist.

Effects of gravity on natural oscillations of sessile drops

In this paper, the impact of a viscoelastic drop onto a hydrophobic solid substrate is numerically investigated at low Weber number and Reynolds number. Previous experiments showed that viscoelastic fluids could make a big difference to drop spreading and retraction after impact, compared to Newtonian fluids. To quantitatively study the effect of viscoelasticity, Oldroyd-B model and diffuse interface methods are adopted in the direct numerical simulation of viscoelastic drops impacting onto a hydrophobic solid surface. The viscoelasticity is represented by the Deborah number, which is the ratio of the relaxation time of the viscoelastic fluid to the flow characteristic time. Based on the numerical results, we find that the spreading process is hardly affected by the viscoelasticity, and follows the scaling law: t m ∼ W e R m 3 / 2 , where R m is the maximum radius of the wetted area and t m is the corresponding time. During the drop retraction, the Newtonian drop would bounce up from the substrate. By contrast, the retraction velocity of the viscoelastic drops gradually slow down with the increase of Deborah number, thereby eventually suppressing the drop bouncing. The mechanism of anti-rebound effect due to the viscoelasticity can be interpreted by the occurrence of the downward elastic stress near the interface, and the bigger the Deborah number is, the more likely the bouncing is inhibited.

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Dynamics of viscoelastic drops impacting onto a hydrophobic solid substrate

The accurate sedimentation of metal droplets is of great importance in metal droplet-based three-dimensional (3D) printing. Detailed investigations of the process of metal droplet collision in a liquid-liquid system are still lacking, relative to studies on the atmospheric environment. In this study, the dynamics of the sedimentation behavior of metal droplets in a liquid-liquid system are investigated experimentally using a high-speed imaging system. The experimental results showed that with increased impact velocity, metal droplets successively appear after the collision as coalescence, coalescence accompanied by rebound, and rebound. There is a critical impact velocity between the rebound and coalescence, which is related to the surface tension and droplet size of the metal droplets. Analysis of the mechanism of coalescence showed that mechanical oscillations occur during coalescence, which leads to variation in the inherent surface tension. The greater the impact velocity, the greater the variation. In addition, a semi-empirical prediction formula for the Weber number and maximum spreading factor in the coalescence of metal droplets is developed. This work provides an improved theoretical understanding and superior practical printing efficiency and quality.

Experimental investigation of the sedimentation behavior of metal droplets in liquid-liquid systems

Abstract To improve the efficiency of the discrete unified gas kinetic scheme (DUGKS) in capturing cross-scale flow physics, an adaptive partitioning-based discrete unified gas kinetic scheme (ADUGKS) is developed in this work. The ADUGKS is designed from the discrete characteristic solution to the Boltzmann-BGK equation, which contains the initial distribution function and the local equilibrium state. The initial distribution function contributes to the calculation of free streaming fluxes and the local equilibrium state contributes to the calculation of equilibrium fluxes. When the contribution of the initial distribution function is negative, the local flow field can be regarded as the continuous flow and the Navier–Stokes (N-S) equations can be used to obtain the solution directly. Otherwise, the discrete distribution functions should be updated by the Boltzmann equation to capture the rarefaction effect. Given this, in the ADUGKS, the computational domain is divided into the DUGKS cell and the N-S cell based on the contribution of the initial distribution function to the calculation of free streaming fluxes. In the N-S cell, the local flow field is evolved by solving the N-S equations, while in the DUGKS cell, both the discrete velocity Boltzmann equation and the corresponding macroscopic governing equations are solved by a modified DUGKS. Since more and more cells turn into the N-S cell with the decrease of the Knudsen number, a significant acceleration can be achieved for the ADUGKS in the continuum flow regime as compared with the DUGKS. 

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Hang Ding

Papers

Buoyancy-driven bubbles in a constricted vertical capillary

Effects of gravity on natural oscillations of sessile drops

Dynamics of viscoelastic drops impacting onto a hydrophobic solid substrate

Experimental investigation of the sedimentation behavior of metal droplets in liquid-liquid systems

Adaptive partitioning-based discrete unified gas kinetic scheme for flows in all flow regimes