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Using experiments and numerical simulations, we investigate the spontaneous spreading of droplets of aqueous glycerol (Newtonian) and aqueous polymer (shear-thinning) solutions on smooth surfaces. We find that in the first millisecond the spreading of the shear-thinning solutions is identical to the spreading of water, regardless of the polymer concentration. In contrast, aqueous glycerol solutions show a different behavior, namely, significantly slower spreading rate than water. In the initial rapid spreading phase, the dominating forces that can resist the wetting are inertial forces and contact-line friction. For the glycerol solutions, an increase in glycerol concentration effectively increases the contact-line friction, resulting in increased resistance to wetting. For the polymeric solutions, however, an increase in polymer concentration does not modify contact-line friction. As a consequence, the energy dissipation at the contact line can not be controlled by varying the amount of additives for shear-thinning fluids. The reduction of the spreading rate of shear-thinning fluids on smooth surfaces in the rapid wetting regime can only be achieved by increasing solvent viscosity. Our results have implications for phase-change applications where the control of the rapid spreading rate is central, such as anti-icing and soldering

Droplet spreading and transport phenomenon is ubiquitous and has been studied by engineered surfaces with a variety of topographic features. To obtain a directional bias in dynamic wetting, hydrophobic surfaces with a geometrical asymmetry are generally used, attributing the directionality to one-sided pinning. Although the pinning may be useful for directional wetting, it usually limits the droplet mobility, especially for small volumes and over wettable surfaces. Here, we demonstrate a pinning-less approach to rapidly transport millimeter sized droplets on a partially wetting surface. Placing droplets on an asymmetrically structured surfaces with micron-scale roughness and applying symmetric horizontal vibration, they travel rapidly in one direction without pinning. The key, here, is to generate capillary-driven rapid contact-line motion within the time-scale of period of vibration. At the right regime where a friction factor local at the contact line dominates the rapid capillary motion, the asymmetric surface geometry can induce smooth and continuous contact-line movement back and forth at different speed, realizing directional motion of droplets even with small volumes over the wettable surface. We found that the translational speed is selective and strongly dependent on the droplet volume, oscillation frequency, and surface pattern properties, and thus droplets with a specific volume can be efficiently sorted out.

The movement of the triple contact line plays a crucial role in many applications such as ink-jet printing, liquid coating and drainage (imbibition) in porous media. To design accurate computational tools for these applications, predictive models of the moving contact line are needed. However, the basic mechanisms responsible for movement of the triple contact line are not well understood but still debated. We investigate the movement of the contact line between water, vapour and a silica-like solid surface under steady conditions in low capillary number regime. We use molecular dynamics (MD) with an atomistic water model to simulate a nanoscopic drop between two moving plates. We include hydrogen bonding between the water molecules and the solid substrate, which leads to a sub-molecular slip length. We benchmark two continuum methods, the Cahn–Hilliard phase-field (PF) model and a volume-of-fluid (VOF) model, against MD results. We show that both continuum models reproduce the statistical measures obtained from MD reasonably well, with a trade-off in accuracy. We demonstrate the importance of the phase-field mobility parameter and the local slip length in accurately modelling the moving contact line.

Textured hydrophobic surfaces that repel liquid droplets unidirectionally are found in nature such as butterfly wings and ryegrass leaves and are also essential in technological processes such as self-cleaning and anti-icing. However, droplet impact on such surfaces is not fully understood. Here, we study, using a high-speed camera, droplet impact on surfaces with inclined micropillars. We observed directional rebound at high impact speeds on surfaces with dense arrays of pillars. We attribute this asymmetry to the difference in wetting behavior of the structure sidewalls, causing slower retraction of the contact line in the direction against the inclination compared to with the inclination. The experimental observations are complemented with numerical simulations to elucidate the detailed movement of the drops over the pillars. These insights improve our understanding of droplet impact on hydrophobic microstructures and may be a useful for designing structured surfaces for controlling droplet mobility., Comment: 24 pages, 9 figures

The motion of the three-phase contact line between two immiscible fluids and a solid surface arises in a variety of wetting phenomena and technological applications. One challenge in continuum theory is the effective representation of molecular phenomena close to the contact line. Here, we characterize the molecular processes of the moving contact line to assess the accuracy of two different continuum two-phase models. Specifically, molecular dynamics (MD) simulations of a two-dimensional droplet between two moving plates are used to create reference data for different capillary numbers and contact angles. We use a simple-point-charge/extended (SPC/E) water model with particle-mesh Ewald electrostatics treatment. This model provides a very small slip and a more realistic representation of the molecular physics than Lennards-Jones models. The Cahn-Hilliard phase-field model and the Volume-of-Fluid model are calibrated against the drop displacement from MD reference data. It is demonstrated that the calibrated continuum models can accurately capture droplet displacement and droplet breakup for different capillary numbers and contact angles. However, we also observe differences between continuum and atomistic simulations in describing the transient and unsteady droplet behavior, in particular, close to dynamical wetting transitions. The molecular dynamics of the sheared droplet provide insight of the line friction experienced by the advancing and receding contact lines and evidence of large-scale temporal "stick-slip" like oscillations. The presented results will serve as a stepping stone towards developing accurate continuum models for nanoscale hydrodynamics., 45 pages, 23 figures, 4 tables, 1 supplementary PDF, 2 supplementary movies, 1 supplementary archive, accepted for publication in "Journal of Fluid Mechanics"

A Volume of Fluid (VOF) method is applied to study the interaction between two liquid drops with the same initial diameter in uniform flow. Various arrangements of the drops are studied, based on t ...

The impact of liquid drops on a rigid surface is central in cleaning, cooling and coating processes in both nature and industrial applications. However, it is not clear how details of pores, roughness and texture on the solid surface influence the initial stages of the impact dynamics. Here, we experimentally study drop impacting at low velocities onto surfaces textured with asymmetric (tilted) ridges. We define the line-friction capillary number $Ca_f={\mu_f V_0}/{\sigma}$ (where $\mu_f$, $V_0$ and $\sigma$ are the line friction, impact velocity and surface tension, respectively) as a measure of the importance of the topology of surface textures for the dynamics of droplet impact. We show that when $Ca_f \ll 1$, the contact line speed in the direction against the inclination of the ridges is set by line-friction, whereas in the direction with inclination the contact line is pinned at acute corners of the ridge. When $Ca_f \sim 1$, the pinning is only temporary until the liquid-vapor interface reaches to the next ridge where a new contact line is formed. Finally, when $Ca_f\gg 1$, the geometric details of non-smooth surfaces play little role., Comment: 12 pages, 7 figures, under consideration for Physical Review Fluids. Revised February 04 2021

In this paper, we numerically study pool boiling of a binary (water and nitrogen) mixture on a surface endowed with a combination of hydrophobicity and hydrophilicity (i.e., the so called biphilic surface). Here we adopt a numerical approach based on the phase field theory, where the vapor-liquid interface is assumed to be of a finite thickness (hence diffusive in nature) and requires no explicit tracking schemes. The theoretical modeling of two-phase heat and mass transfer in water diluted with nitrogen demonstrates the significant impact of impurities on bubble dynamics. The simulations show that locally high concentrations of nitrogen gas within the vapor bubble is essential to weakening the condensation effect, which results in sustained bubble growth and ultimately (partial) departure from the surface under the artificially enlarged gravity. Simply increasing the solubility of nitrogen in water, however, turns out to be counterproductive because possible re-dissolution of the aggregated nitrogen by the bulk water could deprive the bubble of vital gas contents, leading instead to continuous bubble shrinkage and collapse. Additionally, it is found that with the significant accumulation of nitrogen, the bubble interface is increasingly dominated by a strong interfacial thermocapillary flow due to the Marangoni effect.

The heat and mass transfer of deformable droplets in turbulent flows is crucial to a wide range of applications, such as cloud dynamics and internal combustion engines. This study investigates a single droplet undergoing phase change in isotropic turbulence using numerical simulations with a hybrid lattice Boltzmann scheme. Phase separation is controlled by a non-ideal equation of state and density contrast is taken into consideration. Droplet deformation is caused by pressure and shear stress at the droplet interface. The statistics of thermodynamic variables are quantified and averaged over both the liquid and vapour phases. The occurrence of evaporation and condensation is correlated to temperature fluctuations, surface tension variation and turbulence intensity. The temporal spectra of droplet deformations are analysed and related to the droplet surface area. Different modes of oscillation are clearly identified from the deformation power spectrum for low Taylor Reynolds number $Re_{\unicode[STIX]{x1D706}}$, whereas nonlinearities are produced with the increase of $Re_{\unicode[STIX]{x1D706}}$, as intermediate frequencies are seen to overlap. As an outcome, a continuous spectrum is observed, which shows a decrease in the power spectrum that scales as ${\sim}f^{-3}$. Correlations between the droplet Weber number, deformation parameter, fluctuations of the droplet volume and thermodynamic variables are also developed.

We conduct numerical experiments on viscoelastic droplets hitting a flat solid surface. The results present time-resolved non-Newtonian stresses acting in the droplet. Comparing with the simulation of the impact of a Newtonian droplet, the effects of viscoelasticity on droplet behaviors such as splashing, the maximum spreading diameter and deformation are analyzed. With detailed information on the contact line region, we demonstrate how the contact line behaves according to the transition of the fluid property from elasticity dominated to shear-thinning dominated when a droplet expands and contracts on the substrate. The propose of this work is to discuss whether and how the elasticity in an impinging droplet takes effect.

For phase-change cooling schemes for electronics, quick activation of nucleate boiling helps safeguard the electronics components from thermal shocks associated with undesired surface superheating at boiling incipience, which is of great importance to the long-term system stability and reliability. Previous experimental studies show that bubble nucleation can occur surprisingly early on mixed-wettability surfaces. In this paper, we report unambiguous evidence that such unusual bubble generation at extremely low temperatures—even below the boiling point—is induced by a significant presence of incondensable gas retained by the hydrophobic surface, which exhibits exceptional stability even surviving extensive boiling deaeration. By means of high-speed imaging, it is revealed that the consequently gassy boiling leads to unique bubble behaviour that stands in sharp contrast with that of pure vapour bubbles. Such findings agree qualitatively well with numerical simulations based on a diffuse-interface method. Moreover, the simulations further demonstrate strong thermocapillary flows accompanying growing bubbles with considerable gas contents, which is associated with heat transfer enhancement on the biphilic surface in the low-superheat region.

The phenomenon of droplet impact on a smooth, flat, partially wetted surface is studied by phase field simulation. A map of the different impact regimes is constructed for Reynolds numbers ranging ...

Microstructured surfaces that control the direction of liquid transport are not only ubiquitous in nature, but they are also central to technological processes such as fog/water harvesting, oil-water separation, and surface lubrication. However, a fundamental understanding of the initial wetting dynamics of liquids spreading on such surfaces is lacking. Here, we show that three regimes govern microstructured surface wetting on short time scales: spread, stick, and contact line leaping. The latter involves establishing a new contact line downstream of the wetting front as the liquid leaps over specific sections of the solid surface. Experimental and numerical investigations reveal how different regimes emerge in different flow directions during wetting of periodic asymmetrically microstructured surfaces. These insights improve our understanding of rapid wetting in droplet impact, splashing, and wetting of vibrating surfaces and may contribute to advances in designing structured surfaces for the mentioned applications., 9 pages, 5 figures, under consideration for Soft Matter

Wetting phenomena, i.e. the spreading of a liquid over a dry solid surface, are important for understanding how plants and insects imbibe water and moisture and for miniaturization in chemistry and biotechnology, among other examples. They pose fundamental challenges and possibilities, especially in dynamic situations. The surface chemistry and micro-scale roughness may determine the macroscopic spreading flow. The question here is how dynamic wetting depends on the topography of the substrate, i.e. the actual geometry of the roughness elements. To this end, we have formulated a toy model that accounts for the roughness shape, which is tested against a series of spreading experiments made on asymmetric sawtooth surface structures. The spreading speed in different directions relative to the surface pattern is found to be well described by the toy model. The toy model also shows the mechanism by which the shape of the roughness together with the line friction determines the observed slowing down of the spreading.

The effect of gasification on the dynamics and kinematics of immersed spherical and non-spherical solid particles have been investigated using the three-dimensional lattice Boltzmann method. The ga ...

We investigate the Marangoni effects in a hexane droplet under evaporation and close to its critical point. A lattice Boltzmann model is used to perform 3D numerical simulations. In a first case, the droplet is placed in its own vapor and a temperature gradient is imposed. The droplet locomotion through the domain is observed, where the temperature differences across the surface is proportional to the droplet velocity and the Marangoni effect is confirmed. The droplet is then set under a forced convection condition. The results show that the Marangoni stresses play a major role in maintaining the internal circulation when the superheated vapor temperature is increased. Surprisingly, surface tension variations along the interface due to temperature change may affect heat transfer and internal circulation even for low Weber number. Other results and considerations regarding the droplet surface are also discussed.

A Volume of Fluid (VOF) method is applied to study the deformation and breakup of a single liquid drop in shear flows superimposed on uniform flow. The effect of shearing on the breakup mechanism i ...

Amongst an extensive collection of surface characteristics that could affect boiling performance, surface wettability (as measured by the contact angle with water) proves to play a unique role in p ...

Hindrance of dynamic wetting due to surface microstructures can be deactivated by applying electric fields., Dynamic wetting problems are fundamental to understanding the interaction between liquids and solids. Even in a superficially simple experimental situation, such as a droplet spreading over a dry surface, the result may depend not only on the liquid properties but also strongly on the substrate-surface properties; even for macroscopically smooth surfaces, the microscopic geometrical roughness can be important. In addition, because surfaces may often be naturally charged or electric fields are used to manipulate fluids, electric effects are crucial components that influence wetting phenomena. We investigate the interplay between electric forces and surface structures in dynamic wetting. Although surface microstructures can significantly hinder spreading, we find that electrostatics can “cloak” the microstructures, that is, deactivate the hindering. We identify the physics in terms of reduction in contact-line friction, which makes the dynamic wetting inertial force dominant and insensitive to the substrate properties.

The addition of alloying elements in low alloyed PM steels in the form of a master alloy gives the advantage of introducing oxidation sensitive but less expensive elements and also allows manipulation in composition adjustment to achieve desired properties. In this work, interrupted sintering trials of the Fe–2MA–0.5C (%) (MA = Cu based master alloy) are performed. The behaviour of the liquid forming master alloy, for instance in terms of liquid phase formation, alloying element redistribution and effect on the dimensional changes, is investigated. The results show that master alloy particles melt over a range of temperature, which is also supported by the thermodynamic calculations. The low swelling in the master alloy system, compared to a reference system of Fe–2Cu–0.5C, is attributed to the progressive melting of the master alloy. The mean diffusion distance of Cu in Fe at the interparticle boundaries is 5.8 μm after 34 min of isothermal holding.

A Volume of Fluid (VOF) method is applied to investigate the deformation and breakup of an initially spherical drop in the bag- and shear breakup regimes, induced by steady disturbances. The onset ...

In this paper, we numerically study particle formation in the rapid expansion of supercritical solution (RESS) process in a two dimensional, axisymmetric geometry, for a benzoic acid + CO2 system. ...

Axisymmetric rapid expansion of supercritical carbon dioxide is investigated in this article. The extended generalized Bender equation of state is used to give a good description of the fluids over ...

The use of supercritical carbon dioxide (scCO2) as an apolar solvent has been known for decades. It offers a greener approach than, e.g., hexane or chloroform, when such solvents are needed. The use of scCO2 in microsystems, however, has only recently started to attract attention. In microfluidics, the flow characteristics need to be known to be able to successfully design such components and systems. As supercritical fluids exhibit the exciting combination of low viscosity, high density, and high diffusion rates, the fluidic behavior is not directly transferrable from aqueous systems. In this paper, three flow regimes in the scCO2–liquid water two-phase microfluidic system have been mapped. The effect of both total flow rate and relative flow rate on the flow regime is evaluated. Furthermore, the droplet dynamics at the bifurcating exit channel are analyzed at different flow rates. Due to the low viscosity of scCO2, segmented flows were observed even at fairly high flow rates. Furthermore, the carbon dioxide droplet behavior exhibited a clear dependence on both flow rate and droplet length.

The effective use of alloying elements in powder metallurgical steels requires a deep understanding of their redistribution kinetics during sintering. In this work, interrupted sintering trials of Fe–2Cu and Fe–2Cu–0·5C compacts were performed. Moreover, diffusion simulations of Cu in γ-Fe using Dictra were performed. It is found that transient liquid phase penetrates the Fe interparticle and grain boundaries in less than 3 min of holding time. However, C addition limits the penetration of liquid Cu, particularly into grain boundaries of large Fe particles. The results also show that the mean diffusion distance of Cu in γ-Fe in the C added system is slightly lower than that in the C-free system at 3 min of holding time; however, after 33 min, the mean diffusion distance is similar in both systems. The diffusion distances of Cu in γ-Fe, predicted by Dictra, are in good agreement with the measured values.

In this work, the effect of external loading on the martensitic transformation is analyzed using an elasto-plastic phase field model. The phase field microelasticity theory, incorporating a non-lin ...

The initial rapid wetting of a solid surface by a liquid phase is an important step in many industrial processes. Liquid-phase sintering of powder metallurgical steels is one such industrial process, where metallic powders of micrometer size are used. Investigating the dynamic wetting of a high-temperature metallic drop of micrometer size experimentally is very challenging. Here, a phase-field-based numerical model is first implemented and verified by accurately capturing the initial dynamic wetting of millimeter-sized metal drops and then the model is extended to predict the dynamic wetting of a micrometer-sized metal drop. We found, in accordance with recent observations, that contact line friction is required for accurate simulation of dynamic wetting. Our results predict the wetting time for a micrometer-sized metal drop and also indicate that the dynamic wetting patterns at the micro- and millimeter length scales are qualitatively similar. We also found that the wetting process is much faster for a micrometer-sized metal drop compared to a millimeter-sized metal drop.

Presented in this paper is a finite-element-method-based model for phase change material actuators, modeling the active material as a fluid as opposed to a solid. This enables the model to better conform to localized loads and offering the opportunity to follow material movement in enclosed volumes. Modeling, simulation, and analysis of an electrothermally activated paraffin microactuator have been conducted. The paraffin microactuator used for the analysis in this study exploits the large volumetric expansion of paraffin upon melting, which, combined with its low compressibility in the liquid state, allows for high hydraulic pressures to be generated. The purpose of this study is to supply a geometry-independent model of such a microactuator through the implementation of a fluid model rather than a solid one, which has been utilized in previous studies. Numerical simulations are conducted at different frequencies of the heating source and for different geometries of the microactuator. The results are compared with the empirical data obtained on a close to identical paraffin microactuator, which clearly show the advantages of a fluid model instead of a solid-state approximation.

In this paper we consider a thermal multiphase lattice Boltzmann method (LBM) to investigate the heating and vaporization of a suspended droplet. An important benefit from the LBM is that phase sep ...

A droplet that impacts onto a solid substrate deforms in a complex dynamics. To extract the principal mechanisms that dominate this dynamics we deploy numerical simulations based on the phase field method. Direct comparison with experiments suggests that a dissipation local to the contact line limits the droplet spreading dynamics and its scaled maximum spreading radius $\beta_\mathrm{max}$. By assuming linear response through a drag force at the contact line, our simulations rationalize experimental observations for droplet impact on both smooth and rough substrates, measured through a single contact line friction parameter $\mu_f$. Moreover, our analysis shows that at low and intermediate impact speeds dissipation at the contact line limits the dynamics and we describe $\beta_\mathrm{max}$ by the scaling law $\beta_\mathrm{max} \sim (Re \mu_\mathrm{l}/\mu_f)^{1/2}$ that is a function of the droplet viscosity ($\mu_\mathrm{l}$) and its Reynolds number ($Re$).

The Phase Field Microelasticity model proposed by Khachaturyan is used to perform 3D simulation of Martensitic Transformation in polycrystalline materials using finite element method. The effect of ...

An Eulerian model was developed for turbulent gas-particle flow that takes into account the influence of particles on the gas-phase turbulence. For the description of the particle-phase stress the kinetic theory of granular flow and the simpler Hinze model were adopted. A K- ω model was used as the gas phase turbulence model. The difference between one- and two-way coupling was investigated for different particle volume fractions and particle diameters. It was found that particles with a much higher density than the fluid substantially affect the gas-phase in turbulent channel flow for particle volume fractions as low as 10 − 4 . The models with the particle-phase stress described by the kinetic theory of granular flow and the simpler Hinze model produce similar results for particles with small response times but deviate for larger response times. The study shows that two-way coupling and the turbophoretic effect must be taken into account in models even at relatively low particle volume fractions.

In the present work a 3-D elastoplastic phase-field (PF) model is developed, based on the PF microelasticity theory proposed by A.G.Khachaturyan and by including plastic deformation as well as anis ...

In this article, we present a modelling approach for rapid dynamic wetting based on the phase field theory. We show that in order to model this accurately, it is important to allow for a non-equilibrium wetting boundary condition. Using a condition of this type, we obtain a direct match with experimental results reported in the literature for rapid spreading of liquid droplets on dry surfaces. By extracting the dissipation of energy and the rate of change of kinetic energy in the flow simulation, we identify a new wetting regime during the rapid phase of spreading. This is characterized by the main dissipation to be due to a re-organization of molecules at the contact line, in a diffusive or active process. This regime serves as an addition to the other wetting regimes that have previously been reported in the literature.

The correlation correlation between the fluctuating particle and gas velocity in isotropic turbulence is studied with a set of stochastic differential equations taking into account both particle-pa ...

An Eulerian turbulent two phase flow model using kinetic theory of granular flows for the particle phase was developed in order to study evolving upward turbulent gas particle flows in a pipe. The model takes the feedback of the particles into account and its results agree well with experiments. Simulations show that the pipe length required for particle laden turbulent flow to become fully developed is up to five times longer than an unladen flow. To increase the understanding of the dependence of the development length on particle diameter a simple model for the expected development length was derived. It shows that the development length becomes shorter for increasing particle diameters, which agrees with simulations up to a particle diameter of 100 μm. Thereafter the development length becomes longer again for increasing particle diameters because larger particles need a longer time to adjust to the velocity of the carrier phase.

In the present paper we present a phenomenological description of droplet dynamics in a bifurcating channel that is based on three-dimensional numerical experiments using the Phase Field theory. Droplet dynamics is investigated in a junction, which has symmetric outflow conditions in its daughter branches. We identify two different flow regimes as the droplets interact with the tip of the bifurcation, splitting and non-splitting. A distinct criterion for the flow regime transition is found based on the initial droplet volume and the Capillary (Ca) number. The Rayleigh–Plateau instability is identified as a driving mechanism for the droplet breakup close to the threshold between the splitting and non-splitting regime.

This paper presents a model, using a phase-field method, that is able to simulate the motion of a solid sphere impacting on a liquid surface, including the effects of capillary and hydrodynamic forces. The basic phenomena that were the subject of our research effort are the small scale mechanism such as the wetting property of the solid surface which control the large scale phenomena of the interaction. The coupled problem during the impact will be formulated by the inclusion of the surface energies of the solid surface in the formulation, which gives a reliable prediction of the motion of solid objects in/on/out of a liquid surface and the hydrodynamic behaviours at small scales when the inertia of fluid is less important than its surface tension. Numerical results at different surface wettabilities and impact conditions will be presented and compared with the experiments of Duez el al. [C. Duez, C. Ybert, C. Clanet, L. Bocquet, Nat. Phys. 3 (2007) 180-183] and Lee and Kim [D. Lee, H. Kim, Langmuir 24 (1) (2008) 142].

This paper presents a technology for dispensing droplets through thin liquid layers. The system consists of a free liquid film, which is suspended in a frame and positioned in front of a piezoelect ...

Dissolutive wetting is investigated numerically using a diffuse-interface model that incorporates fluid flow, solute diffusion and phase change. A range of materials parameters are investigated: (1 ...

Numerical simulation of liquid-phase sintering using a multicomponent and multiphase model is presented. The model consists of convective concentration and phase-field equations coupled with the Na ...

PurposeIn welding there is an intricate coupling between the composition of the material and the shape and depth of the weld pool. In certain materials, the weld pool may not penetrate the material easily, so that it is difficult or impossible to weld, while other seemingly quite similar materials may be well suited for welding. This is due to the convective heat transfer in the melt, where the flow is driven primarily by surface tension gradients. This paper aims to study how surface active agents affect the flow and thus the welding properties by surveying some recent 3D simulations of weld pools.Design/methodology/approachSome basic concepts in the modelling of flow in a weld pool are reviewed. The mathematical models for a convecting melt, with a detailed model for the surface tension and the Marangoni stress in the presence of surfactants, are presented. The effect of the sign of the Marangoni coefficient on the flow pattern, and thus, via melting and freezing, on the shape of the weld pool, is discussed.FindingsIt is seen that it is beneficial to have surfactants present at the pool surface, in order to have good penetration. Results from a refined surface tension model that accounts for non‐equilibrium redistribution of surfactants are presented. It is seen that the surfactant concentration is significantly modified by the fluid flow. Thereby, the effective surface tension and the Marangoni stresses are altered, and the redistribution of surfactants will affect the penetration depth of the weld pool.Originality/valueThe importance of surfactants for weld pool shapes, and in particular the convective redistribution of surfactants, is clarified.

In this paper we simulate the evolution and free particle motion of an individual nucleus that grows into a dendritic crystal. The melt flow and the convective heat transfer around the crystal are simulated as they settle due to gravity. There is an intricate coupling between the settling and the evolution of the crystal. The relative flow induced by the settling enhances the growth at the downward facing parts, which in its turn affects the subsequent settling motion.Simulations have been done in two dimensions using a semi-sharp phase-field model. The flow was constrained to a rigid body motion by using Lagrange multipliers inside the solidified part. The model was formulated using two different meshes. One is a fixed background mesh, which covers the whole domain. The other is an adaptive mesh, where the node points are also translated and rotated with the movement of the solid particle. In the latter, the dendritic growth is simulated by the semi-sharp phase-field method.

We numerically study the thermohydrodynamics of boiling for a CO(2) + ethanol mixture on lyophilic and lyophobic surfaces in both closed and open systems, based on a diffuse interface model for a two-component system. The corresponding wetting boundary conditions for an isothermal system are proposed and verified in this paper. New phenomena due to the addition of another component, mainly the preferential evaporation of the more volatile component, are observed. In the open system and the closed system, the physical process shows very different characteristics. In the open system, except for the movement of the contact line, the qualitative features are rather similar for lyophobic and lyophilic surfaces. In the closed system, the vortices that are observed on a lyophobic surface are not seen on a lyophilic surface. More sophisticated wetting boundary conditions for nonisothermal, two-component systems might need to be further developed, taking into account the variations of density, temperature, and surface tension near the wall, while numerical results show that the boundary conditions proposed here also work well even in boiling, where the temperature is nonuniform.

We conduct numerical experiments on spreading of viscoelastic droplets on a flat surface. Our work considers a Giesekus fluid characterized by a shear-thinning viscosity and an Oldroyd-B fluid, which is close to a Boger fluid with constant viscosity. Our results qualitatively agree with experimental observations in that both shear thinning and elasticity enhances contact line motion, and that the contact line motion of the Boger fluid obeys the Tanner-Voinov-Hoffman relation. Excluding inertia, the spreading speed shows strong dependence on rheological properties, such as the viscosity ratio between the solvent and the polymer suspension, and the polymeric relaxation time. We also discuss how elasticity can affect contact line motion. The molecular migration theory proposed in the literature is not able to explain the agreement between our simulations and experimental results.