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A conservative sharp interface method is proposed in this work to simulate two-dimensional/axisymmetric incompressible two-phase flows with phase change. In this method, we use the cut cell method to generate unstructured meshes near the interface, of which the cell edges overlap with the interface at each time step. On such mesh, the mass and heat transfer during phase change and all the jump conditions can be incorporated into the calculation of fluxes at the cell edges, to ensure that they are strictly satisfied at the interface in a sharp manner. The governing equations, including the incompressible Navier–Stokes equations, heat equation, and vapor mass fraction equation, are discretized by a second-order finite volume method in the arbitrary Lagrangian–Eulerian framework. To well couple the mass, heat, momentum, and interface evolution, the solution procedure is carefully designed and performed with several techniques. In such a way, the sharp discontinuity of the velocity, stress, temperature gradient, and vapor fraction, caused by the mass/heat transfer during phase change, can be simulated accurately and robustly. The performance of this method is systematically examined by cases of phase change at or below the saturated temperature, including vapor bubble in superheated liquid, film boiling, droplet evaporation at different relative humidity conditions, droplet evaporation under gravity, and droplet evaporation under forced convection. The applicability of the present method for incompressible two-phase flows with phase change is well demonstrated by comparing the numerical results with the benchmark, theoretical or experimental ones. [ABSTRACT FROM AUTHOR]

Droplet collisions on superhydrophobic cylindrical surfaces are widely seen in industrial applications. To investigate their dynamic behavior, numerical simulations of droplets impacting eccentrically on the surface of a small superhydrophobic cylinder are performed in this work. The eccentricity e ranges from 0 to 1.2 mm, and the impact velocity ranges from 0.5 to 2 m/s. The effects of the impact velocity and eccentricity are studied in detail. The results show that increasing the eccentricity e reduces the maximum spreading factor and exacerbates the asymmetry of droplets in the azimuthal direction. When the droplets impact on the small cylindrical surface, two collision modes are observed: an asymmetric stretching regime and a stretched rebound regime. The formulation (W e c r / D ∗ = 230 ε + 31) is employed as a criterion to distinguish between the two modes. With increasing eccentricity e, an asymmetrical flow of droplets from the non-impact side to the impact side occurs, accompanied by a transition in the dynamic behavior of the droplets from stretching to bouncing. The asymmetrical stretching and stretched rebound can effectively decrease the contact time between the droplet and the cylindrical surface, resulting in a reduction of up to 32% during eccentric impact. [ABSTRACT FROM AUTHOR]

The accurate sedimentation of metal droplets is of great importance in metal droplet-based three-dimensional 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 experimentally investigated using a high-speed imaging system. The experimental results showed that with an 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 a 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. [ABSTRACT FROM AUTHOR]

We numerically study the impact of a liquid drop onto no-slip rigid substrates with different wettabilities using a diffuse interface method, aiming to obtain a universal model for the maximal spreading of the impacting drop at moderate Weber numbers. We find that the wettability plays an important role in the maximal spreading and that the ratio of the surface energy to the initial kinetic energy of the drop at the maximal spreading, η, follows η ∼ W e − 1 / 2 at high fixed Reynolds numbers, where We is the Weber number. Taking account of the wettability effect, we obtain a scaling law at high Reynolds numbers from an analysis of energy transformation. This scaling law is compatible with the one derived from the momentum balance at the high impact velocity by Clanet et al. ["Maximal deformation of an impacting drop," J. Fluid Mech. 517, 199–208 (2004)]. Moreover, we attribute it to the presence of a viscous–capillary regime, in which the viscous dissipation of the kinetic energy from the substrate is as significant as the kinetic energy transformed into the surface energy. Accordingly, we identify a new impact parameter, which makes all the numerical results of maximum drop deformation (from the viscous regime to the viscous–capillary regime with Reynolds number up to 104) collapse onto a single curve. Finally, we propose a universal model, the predictions of which are shown to agree well with numerical results for a wide range of Weber and Reynolds numbers. [ABSTRACT FROM AUTHOR]

We experimentally investigate the impact of a millimetric superhydrophobic sphere on a water pool covered by a thin oil layer, with the aim of seeking the critical conditions for sphere entrapment at the interfaces. The interfacial tension and viscosity of the thin oil layer are found to have a significant effect on the fate of the impacting spheres that are denser than the liquids: sinking or floating. For the oil layer of low viscosity, the impact dynamic is dominated by the capillary force, and the sphere experiences more or less uniform acceleration after the impact, which is similar to a sphere impacting onto a pure water pool. For the oil layer of relatively high viscosity, the viscous dissipation inside the thin oil layer greatly hinders the descending of the sphere, and thus, it is the viscosity of the oil layer that dictates the acceleration process of the spheres at the early stage of impact. At the late stage, the sphere moves very slowly under water (particularly at the onset of sinking), and the competition between the oil–water interfacial tension and buoyancy determines whether the sphere would eventually sink or float. We then conduct the theoretical analysis of the dynamic processes of the impacting sphere and give the theoretical predictions of the respective critical conditions, which agree well with the experimental observations. [ABSTRACT FROM AUTHOR]

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 (0.55 ≤ η ≤ 1.35), the Bond number (0.1 ≤ B o ≤ 15), and the Reynolds number (78.5 ≤ R e ≤ 3367). We find that small bubbles have insignificant deformation and, thus, cannot pass through a throat with η < 1 , 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., η c ≈ 1.1 for Bo < 1 and η c ∼ B o − 1 / 4 for Bo > 1. These theoretical predictions are in good agreement with our numerical results. [ABSTRACT FROM AUTHOR]

Contactless droplet transport has gained intensive attention in recent years. In this study, the motion of a sessile binary droplet or a pure water droplet under the contactless Marangoni effect has been investigated experimentally and theoretically. The concrete form of the driving force originates from the evaporation of a pure liquid "source" droplet was achieved and verified by our experimental data. For a "target" droplet consisting of pure water, we discovered a non-negligible increase in its spreading radius R during motion. Based on the experimental results, we proposed a linear approximation between R and the droplet spacing and successfully introduced its influence on the driving force calculation through the perturbation theory. [ABSTRACT FROM AUTHOR]

When two sessile droplets of miscible fluids come into contact, the coalescence process can be significantly delayed owing to the competition between the capillary and Marangoni effects. It is important to reveal the mechanism of the deformation and displacement of the sessile droplets at the early stage of the delayed coalescence, which determines the self-stabilized shape and joint motion of the two droplets later on. In this work, we numerically investigate the early-stage dynamics of the delayed coalescence between two sessile droplets of equal size and laden with aqueous mixtures of different solvent mass fractions. A three-dimensional numerical model is adopted based on lubrication theory and is validated by comparison against previous experimental results. Through simulation, we first showed how the concentration transport is coupled with droplet deformation. Then, we explained the underlying mechanism of delayed coalescence by analyzing the liquid bridge numerically and theoretically. A scaling law for the duration of liquid bridge growth is given and agrees well with the numerical results. Finally, the effects of the solubility on the dynamics are investigated. Our study reveals how the capillary and Marangoni effects dominate the flow during the early stage of the delayed coalesce and thus determine its following behavior. [ABSTRACT FROM AUTHOR]

The breakup dynamics of coaxial liquid interfaces into compound droplets in a co-flow focusing process is studied systematically. In experiments, the jetting and dripping modes downstream the focusing orifice are identified within the parametric regime where a coaxial liquid cone can be established steadily, and the phase diagram is plotted under different flow rates of inner, outer, and driving liquids. The force balance for the jet interface is analyzed numerically to explore the critical conditions for the jetting-dripping transition. It is found that the instability of the inner interface is much easier to trigger the modes transition, and the transition criterion is decided by the balance of inertia force, shear stress, and interfacial tension at the local inner jet. The linear spatiotemporal instability analysis is further carried out to study the convective and absolute instabilities of the coaxial jets. The effects of main process parameters on jet instability are accessed, and the boundary between the absolute/convective instabilities is further compared with the experimental and numerical results, which achieves good agreement. Finally, the energy budget analysis of the instability of coaxial liquid jets is performed to provide more understanding of physical mechanisms for the mode transition. [ABSTRACT FROM AUTHOR]

We report a systematic study on the structural, electronic, magnetic, and ferroelectric properties of [111]-oriented BiFeO3/BiAlO3 (BFO/BAO) superlattice using density-functional calculations. It is found that the Fe-O-Fe superexchange interactions in BFO/BAO superlattice are greatly suppressed by the inserted BAO layers, with the antiferromagnetic-ferromagnetic transition energy decreasing from around 280 meV per BFO formula unit (five atoms) to 11.6 meV per BFO/BAO formula unit (ten atoms). The tensile strain can further decrease this energy, making the magnetic transition more plausible. In addition, we find that BFO/BAO superlattice preserves the large ferroelectric polarization as well as energy gap of bulk BFO. Therefore, BAO may be a good candidate for constructing the BFO-based superlattice with improved multiferroicity. [ABSTRACT FROM AUTHOR]

We numerically study the head-on collisions of two immiscible droplets of different components and focus on the effects of droplet inertia and interfaces, which are expected to play a crucial role in the interaction between the two droplets. A ternary-fluid diffuse-interface method is used here after being validated by comparing against experiments of the collision between an aqueous droplet and a silicone oil droplet. In order to figure out how the droplet inertia and interfaces affect the dynamic behavior after the collision, axisymmetric simulations are performed with various Weber number We and surface tension ratio λ, i.e., the ratio of the surface tension coefficient of the liquid–liquid to the liquid–gas interfaces. Their effects on the film thickness, maximal deformation of the colliding droplets, and the corresponding contact time are investigated. To describe the collision dynamics, we propose an equivalent surface tension σ* based on the analysis of the energy conservation and morphology of the colliding droplets. Using the equivalent surface tension σ*, we theoretically predict the film thickness, maximal spreading time, and deformation of the colliding droplets. The theoretical predictions are in good agreement with the numerical results. [ABSTRACT FROM AUTHOR]

The rupture of coaxial liquid jets in co-flow focusing is studied experimentally and numerically, aiming to deepen the understanding of interface coupling and its effect on the morphology of the generated compound droplets. The weak, transitional, and strong coupling regimes of interface instabilities are identified in experiments by varying the flow rate ratio of the inner jet to the coaxial jets (denoted by rQ). The dynamics of coaxial liquid jets are further resolved by direct numerical simulations after being validated against experiments. The flow structures and pressure contours for the coaxial jets can be given under different coupling conditions. Moreover, scaling laws are proposed to correlate the breakup length of coaxial jets and the sizes of compound droplets and encapsulated cores with rQ, and a good agreement is obtained. It can be found that the transition from weak, transitional and strong coupling occurs approximately at rQ = 0.15 and rQ = 0.4, respectively. This work provides underlying insight into the interface coupling of coaxial liquid jets, which would provide theoretical guidance for microcapsule generation in various potential applications. [ABSTRACT FROM AUTHOR]

The tensile strain and oxygen vacancy effects on the electronic and magnetic properties of multiferroic material BiCoO3 have been studied by density functional theory + U calculations. It is found that the oxygen vacancy at the top of the pyramid structure along the <001> direction (denoted as O1) can be stabilized at 1+ and 2+ charge states, which significantly changes the local magnetic moment of Co ions and possibly provides net magnetic moments to the BiCoO3 system. While the tensile strain degrades the polarization of BiCoO3 about 20 μC/cm2, the formation of oxygen vacancies becomes easier as the strain increases up to 5%. It indicates that the polarization and magnetic property of BiCoO3 could be tuned by the strain and oxygen vacancies, though the polarization of BiCoO3 is slightly degraded by the oxygen vacancies. Meanwhile, the change of electronic structure and magnetic property introduced by oxygen vacancy is illustrated according to the crystal field theory. [ABSTRACT FROM AUTHOR]

In the pure axisymmetric flow focusing (AFF), it is difficult to generate uniform droplets due to the random breakup of bulk flows. Therefore, applying external perturbations is a desirable approach to control the monodispersity of the droplets and makes it possible to produce uniform droplets at relatively high flow rates. In the present study, the effects of different external perturbations (waveform shape, frequency f and amplitude A) on the droplet generation are numerically investigated. When the focused phase is singly actuated, the size of the uniform droplets can be obtained and manipulated by adjusting f and A. In particular, the jet breakup has the same frequency as the external perturbation in the generation of uniform droplets. There exists a cutoff frequency beyond which the perturbation cannot control the jet breakup, even with large A. This is found to be associated with the critical condition for the onset of the Rayleigh-Plateau instability. In addition, the reservoir effect of the cone in the AFF effectively reduces the influence of the perturbation on the liquid supply to the liquid jet, accounting for the presence of jetting at low f and large A. Moreover, we apply the perturbations either singly to the focusing phase or simultaneously to the focused and focusing phases and assess their effects on the jet breakup. Finally, the square waveform perturbations acting on the droplet generation are discussed. The present work provides a guidance of the actuation-aided AFF for practical applications of on-demand droplet generation. [ABSTRACT FROM AUTHOR]

We consider the growth rate of the height of the connecting bridge in rapid surface-tension-driven coalescence of two identical droplets attached on a partially wetting substrate. For a wide range of contact angle values, the height of the bridge grows with time following a power law with a universal exponent of 2/3, up to a threshold time, beyond which a 1/2 exponent results, that is known for coalescence of freely-suspended droplets. In a narrow range of contact angle values close to 90°, this threshold time rapidly vanishes and a 1/2 exponent results for a 90° contact angle. The argument is confirmed by three-dimensional numerical simulations based on a diffuse interface method with adaptive mesh refinement and a volume-of-fluid method. [ABSTRACT FROM AUTHOR]