Precursor film controlled wetting of Pb on Cu

Engulfment and Pushing of Cylindrical Liquid Nano-Inclusion by Advancing Crystal/Melt Interface: An Atomistic Simulation Study

We reported a molecular dynamics (MD) simulation study of an advancing pure Al(100)/melt interface that encounters a foreign immiscible liquid Pb cylindrical nano-inclusion. When the advancing interface approaches the inclusion, the interface may engulf, push to an extent and then engulf or push the nano-inclusion away from the solidifying phase depending on the velocity of the interface. Here, we investigated cylindrical liquid Pb nano-inclusion pushing or engulfment by a growing crystal Al that strongly depends on the velocity of the crystal/melt interface, and a critical velocity (vc) is deduced. If the velocity of the interface is less than vc, then the inclusion is pushed and engulfed otherwise. The relationship between vc and the radius of the nano-inclusion is expressed using a power function that agrees well with the previous studies. For velocity above the vc, the crystal/melt interface plays a vital role; it hinders the matrix atoms from setting below the cylindrical nano-inclusion due to insufficient mass transfer below the inclusion, resulting in the engulfment.

Line tension of sessile droplets: Thermodynamic considerations

We deduce a thermodynamically consistent diffuse interface model to study the line tension phenomenon of sessile droplets. By extending the standard Cahn-Hilliard model via modifying the free energy functional due to the spatial reflection asymmetry at the substrate, we provide an alternative interpretation for the wall energy. In particular, we find the connection of the line tension effect with the droplet-matrix-substrate triple interactions. This finding reveals that the apparent contact angle deviating from Young's law is contributed by the wall energy reduction as well as the line energy minimization. Besides, the intrinsic negative line tension resulting from the curvature effect is observed in our simulations and shows good accordance with recent experiments [Tan et al. Phys. Rev. Lett. 130, 064003 (2023)0031-900710.1103/PhysRevLett.130.064003]. Moreover, our model sheds light upon the understanding of the wetting edge formation which results from the vying effect of wall energy and line tension.

High-Temperature Wetting and Dewetting Dynamics of Silver Droplets on Molybdenum Surfaces

The wetting and dewetting behaviors of Ag droplets on Mo(100), Mo(110), and Mo(111) surfaces were investigated over 1200-2000 K via molecular dynamics simulations. We used the diffusion energy barriers of Ag droplets on the three surfaces to analyze the phenomenon of different precursor films and adsorption layers on the different surfaces. Alloying enabled the Mo(111) surface better wettability in both Mo(110) and Mo(111) surfaces, where there were significant precursor films. We observed that the dewetting rate was the fastest on the surface with the densest adsorption layer. Simulations proved that the same molecular kinetic theory model was applicable to not only the wetting process but also the dewetting process on the same surface. We also provided evidence to support the fact that an increased temperature could reduce the time to reach equilibrium for the wetting and dewetting processes.

Molecular Dynamics Study on the Wettability of the Lithium Droplet and Tungsten Surface

In this paper, molecular dynamics (MD) simulation was used to study the wettability of lithium and tungsten. The surface energy barrier and evaporation control the static contact angle with increasing temperature. The effects of 4 different sizes of droplets and 10 different tungsten sections were evaluated. Moreover, it was found that the different arrangements of atoms on the solid surface will affect the wettability, but the size of the droplet has little effect. In addition, the situation of the droplets driven by six different external forces was evaluated. When the force increases, the two states of the droplet and stream will have different properties. Finally, we studied the phase behavior between lithium and tungsten. For example, lithium overflows from the tungsten plate. The tungsten phase is separated in the lithium plate. Lithium is faster than tungsten when it aggregates in the gas phase, and wettability will drive the effects of engulfing and spitting.

Non-locality of the contact line in dynamic wetting phenomena

HYPOTHESIS

The notion of the contact line is fundamental to capillary science, where in a large category of wetting phenomena, it was always regarded as a one-dimensional object involving only microscopic length scales. This prevailing opinion had a strong impact and repercussions on the developing theories and methodologies used to interpret experimental data. It is hypothesised that this is not the case under certain conditions leading to non-local effects and requiring the development of a modified force balance at the contact line.

THEORY AND SIMULATIONS

Using the first principles of molecular dynamic simulations and a unique combination of steady state conditions and observables, the microscopic structure of the contact region and its connections with macroscopic quantities of capillary flows was revealed for the first time.

FINDINGS

The contact line is shown to become a non-local, macroscopic object involving rather complex interplay between microscopic distributions of density, velocity and friction force. It was established that the non-locality effects, which cannot be in principle captured by localised methodologies, kick off at a universal tipping point and lead to a modified force balance. The developed framework is applicable to a wide range of capillary flows to identify and analyse this regime in applications.

Molecular insights on NaCl crystal formation approaching PVDF membranes functionalized with graphene

Atomistic simulations of graphene–PVDF membranes speeding up NaCl crystal nucleation and growth in comparison to the pristine PVDF membranes.

Orientation dependence of heterogeneous nucleation at the Cu-Pb solid-liquid interface

In this work, we examine the effect of surface structure on the heterogeneous nucleation of Pb crystals from the melt at a Cu substrate using molecular-dynamics (MD) simulation. In a previous work [Palafox-Hernandez et al., Acta Mater. 59, 3137 (2011)] studying the Cu/Pb solid-liquid interface with MD simulation, we observed that the structure of the Cu(111) and Cu(100) interfaces was significantly different at 625 K, just above the Pb melting temperature (618 K for the model). The Cu(100) interface exhibited significant surface alloying in the crystal plane in contact with the melt. In contrast, no surface alloying was seen at the Cu(111) interface; however, a prefreezing layer of crystalline Pb, 2-3 atomic planes thick and slightly compressed relative to bulk Pb crystal, was observed to form at the interface. We observe that at the Cu(111) interface the prefreezing layer is no longer present at 750 K, but surface alloying in the Cu(100) interface persists. In a series of undercooling MD simulations, heterogeneous nucleation of fcc Pb is observed at the Cu(111) interface within the simulation time (5 ns) at 592 K-a 26 K undercooling. Nucleation and growth at Cu(111) proceeded layerwise with a nearly planar critical nucleus. Quantitative analysis yielded heterogeneous nucleation barriers that are more than two orders of magnitude smaller than the predicted homogeneous nucleation barriers from classical nucleation theory. Nucleation was considerably more difficult on the Cu(100) surface-alloyed substrate. An undercooling of approximately 170 K was necessary to observe nucleation at this interface within the simulation time. From qualitative observation, the critical nucleus showed a contact angle with the Cu(100) surface of over 90°, indicating poor wetting of the Cu(100) surface by the nucleating phase, which according to classical heterogeneous nucleation theory provides an explanation of the large undercooling necessary to nucleate on the Cu(100) surface, relative to Cu(111), whose surface is more similar to the nucleating phase due to the presence of the prefreezing layer.

Dynamics of Dissolutive Wetting: A Molecular Dynamics Study

Dissolutive wetting, i.e., dynamic wetting of a liquid droplet on dissolvable substrates, has been studied by molecular dynamics simulations. In dissolutive wetting, the geometry and properties of the solid-liquid interface evolve with the solid dissolving into the droplet; meanwhile, the droplet spreads on the receding solid surfaces. The droplets advance on the dissolvable substrate following different dynamic laws, compared with spreading on nondissolutive substrate. On the basis of molecular kinetic theory, we develop a theoretical model to reveal physical mechanisms behind the dissolutive wetting phenomena. We also find that solid particles are pulled by their hydration shells to dissolve into liquid, changing the flow field, the atomic structure, and the hydrogen bond network in the droplet. Our findings may help to comprehend the dynamics of dissolutive wetting and assist future design in practical applications.

Self-pinning of a nanosuspension droplet: Molecular dynamics simulations

Results are presented from molecular dynamics simulations of Pb(l) nanodroplets containing dispersed Cu nanoparticles (NPs) and spreading on solid surfaces. Three-dimensional simulations are employed throughout, but droplet spreading and pinning are reduced to two-dimensional processes by modeling cylindrical NPs in cylindrical droplets; NPs have radius R_{NP}≅3nm while droplets have initial R_{0}≅42nm. At low particle loading explored here, NPs in sufficient proximity to the initial solid-droplet interface are drawn into advancing contact lines; entrained NPs eventually bind with the underlying substrate. For relatively low advancing contact angle θ_{adv}, self-pinning on entrained NPs occurs; for higher θ_{adv}, depinning is observed. Self-pinning and depinning cases are compared and forces on NPs at the contact line are computed during a depinning event. Though significant flow in the droplet occurs in close proximity to the particle during depinning, resultant forces are relatively low. Instead, forces due to liquid atoms confined between the particles and substrate dominate the forces on NPs; that is, for the NP size studied here, forces are interface dominated. For pinning cases, a precursor wetting film advances ahead of the pinned contact line but at a significantly slower rate than for a pure droplet. This is because the precursor film is a bilayer of liquid atoms on the substrate surface but it is instead a monolayer film as it crosses over pinning particles; thus, mass delivery to the bilayer structure is impeded.

Classification of precursors in nanoscale droplets

Molecular precursors, ultrathin films that precede spreading droplets, are still far from being understood, despite intensive study. The inherent microscopic length scales make small-scale experimental techniques and molecular simulation ideal methods to study this phenomenon. Previous work on molecular precursors using nanoscale droplets, however, consistently suffers from incorrect measurement of the dimensions of the precursor film. An alternative method to accurately characterize the precursor film is presented here. In contrast to previous measures, this method (i) allows for easy detection and characterization of precursors and (ii) yields wetting dynamics that agree with experimental observations. Finally, we briefly comment on previous studies whose conclusions may merit reconsideration in light of the present work.

Requirements for the Formation and Shape of Microscopic Precursors in Droplet Spreading

While the mass transport mechanisms and dynamics of molecular precursors, ultrathin films that precede spreading droplets, are well understood, the requirements for their formation and the reasons for the occurrence of different precursor shapes remain unclear. In this work, we study these requirements using molecular dynamics simulations of spreading droplets and extensive free energy computations. For a simple simulation model, we demonstrate that with growing droplet-substrate attraction, spreading passes succesively through regimes with no precursor, a monolayer precursor, and a continuously growing precursor. We show that the onset of layer formation and the changes in the precursor shape correlate with the free energy of layer formation. On the basis of these findings, we show that a positive spreading coefficient is sufficient but not necessary for precursor formation.

Capillary wave propagation during the delamination of graphene by the precursor films in electro-elasto-capillarity

Molecular dynamics simulations were carried out to explore the capillary wave propagation induced by the competition between one upper precursor film (PF) on the graphene and one lower PF on the substrate in electro-elasto-capillarity (EEC). During the wave propagation, the graphene was gradually delaminated from the substrate by the lower PF. The physics of the capillary wave was explored by the molecular kinetic theory. Besides, the dispersion relation of the wave was obtained theoretically. The theory showed that the wave was controlled by the driving work difference of the two PFs. Simulating the EEC process under different electric field intensities (E), the wave velocity was found insensitive to E. We hope this research could expand our knowledge on the wetting, electrowetting and EEC. As a potential application, the electrowetting of the PF between the graphene and the substrate is a promising candidate for delaminating graphene from substrate.

Heterogeneous crystal nucleation: the effect of lattice mismatch

A simple dynamical density functional theory is used to investigate freezing of an undercooled liquid in the presence of a crystalline substrate. We find that the adsorption of the crystalline phase on the substrate, the contact angle, and the height of the nucleation barrier are nonmonotonic functions of the lattice constant of the substrate. We show that the free-growth-limited model of particle-induced freezing by Greer et al. [Acta Mater. 48, 2823 (2000)] is valid for larger nanoparticles and a small anisotropy of the interface free energy. Faceting due to the small size of the foreign particle or a high anisotropy decouples free growth from the critical size of homogeneous nuclei.

Controlling the velocity of jumping nanodroplets via their initial shape and temperature

Controlling the movement of nanoscale objects is a significant goal of nanotechnology. Dewetting-induced ejection of nanodroplets could provide another means of achieving that goal. Molecular dynamics simulations were used to investigate the dewetting-induced ejection of nanoscale liquid copper nanostructures that were deposited on a graphitic substrate. Nanostructures in the shape of a circle, square, equilateral, and isosceles triangle dewet and form nanodroplets that are ejected from the substrate with a velocity that depends on the initial shape and temperature. The dependence of the ejected velocity on shape is ascribed to the temporal asymmetry of the mass coalescence during the droplet formation; the dependence on temperature is ascribed to changes in the density and viscosity. The results suggest that dewetting induced by nanosecond laser pulses could be used to control the velocity of ejected nanodroplets.

Topology-dominated dynamic wetting of the precursor chain in a hydrophilic interior corner

The topology-dominated dynamic wetting of a droplet in a hydrophilic interior corner was explored using molecular dynamics simulations and molecular kinetic theory. A wetting transition in the interior corner of a single-file water-molecule precursor chain (PC), which eliminated the stress singularity and advanced much faster than the precursor film, was controlled by the interior angle. Owing to the confinement in the interior corner, the potential surface is lower and smoother. The one-dimensional hydrogen-bond chain transferred the disjoining pressure to drive the PC to slip-like ice. As an example, a stable and long metallic monatomic chain was formed using the unique transport properties of the PC for the first time. Our results may help in understanding the topology-dominated dynamic wetting in a hydrophilic interior corner, expand ‘Taylor conjecture’ to nanoscale and develop new applications at nanoscale.

Molecular dynamics study of the dewetting of copper on graphite and graphene: implications for nanoscale self-assembly

Thin-film dewetting can be exploited to self-assemble and organize nanoparticles. Crucial to this effort is the understanding of the nanoscale liquid phase dynamics, and molecular dynamics simulations (MD) provide a powerful tool in this respect. In this paper we demonstrate that MD simulations utilizing a Lennard-Jones (LJ) interface potential can be effectively used to study various wetting regimes of nanoscale Cu disks on graphite. It was found that both the dewetting velocity and the equilibrium contact angle increase with a decrease in the Cu-C potential, and that the retraction velocities obtained are characteristic of dewetting phenomena governed by inertial and capillary forces. This phenomena leads to a change in morphology, from disks to nanodroplets, which, in turn, when using the most accurate LJ potential, jump off the graphitic substrate with a velocity on the order of 140 m/s. This ejection velocity is consistent with the previous experimental observation that nanoscale Au triangles deposited on graphite or glass jump when exposed to a pulsed laser above the melting threshold. Interestingly, the Cu ejection velocity decreases when the liquid Cu disks are deposited on a suspended graphene membrane. Finally, a Rayleigh-Plateau-like instability, which leads to the breakup of a pseudo-one-dimensional liquid Cu nanowire in nanodroplets, is revealed when the MD simulations are performed using different LJ interface potentials.

Polymorphism, crystal nucleation and growth in the phase-field crystal model in 2D and 3D

We apply a simple dynamical density functional theory, the phase-field crystal (PFC) model of overdamped conservative dynamics, to address polymorphism, crystal nucleation, and crystal growth in the diffusion-controlled limit. We refine the phase diagram for 3D, and determine the line free energy in 2D and the height of the nucleation barrier in 2D and 3D for homogeneous and heterogeneous nucleation by solving the respective Euler-Lagrange (EL) equations. We demonstrate that, in the PFC model, the body-centered cubic (bcc), the face-centered cubic (fcc), and the hexagonal close-packed structures (hcp) compete, while the simple cubic structure is unstable, and that phase preference can be tuned by changing the model parameters: close to the critical point the bcc structure is stable, while far from the critical point the fcc prevails, with an hcp stability domain in between. We note that with increasing distance from the critical point the equilibrium shapes vary from the sphere to specific faceted shapes: rhombic dodecahedron (bcc), truncated octahedron (fcc), and hexagonal prism (hcp). Solving the equation of motion of the PFC model supplied with conserved noise, solidification starts with the nucleation of an amorphous precursor phase, into which the stable crystalline phase nucleates. The growth rate is found to be time dependent and anisotropic; this anisotropy depends on the driving force. We show that due to the diffusion-controlled growth mechanism, which is especially relevant for crystal aggregation in colloidal systems, dendritic growth structures evolve in large-scale isothermal single-component PFC simulations. An oscillatory effective pair potential resembling those for model glass formers has been evaluated from structural data of the amorphous phase obtained by instantaneous quenching. Finally, we present results for eutectic solidification in a binary PFC model.

Droplet spreading driven by van der Waals force: a molecular dynamics study

The dynamics of droplet spreading is investigated by molecular dynamics simulations for two immiscible fluids of equal density and viscosity. All the molecular interactions are modeled by truncated Lennard-Jones potentials and a long-range van der Waals force is introduced to act on the wetting fluid. By gradually increasing the coupling constant in the attractive van der Waals interaction between the wetting fluid and the substrate, we observe a transition in the initial stage of spreading. There exists a critical value of the coupling constant, above which the spreading is pioneered by a precursor film. In particular, the dynamically determined critical value quantitatively agrees with that determined by the energy criterion that the spreading coefficient equals zero. The latter separates partial wetting from complete wetting. In the regime of complete wetting, the radius of the spreading droplet varies with time as [Formula: see text], a behavior also found in molecular dynamics simulations where the wetting dynamics is driven by the short-range Lennard-Jones interaction between liquid and solid.

Spreading with evaporation and condensation in one-component fluids

We investigate the dynamics of spreading of a small liquid droplet in gas in a one-component simple fluid, where the temperature is inhomogeneous around 0.9T{c} and latent heat is released or generated at the interface upon evaporation or condensation (with T{c} being the critical temperature). In the scheme of the dynamic van der Waals theory, the hydrodynamic equations containing the gradient stress are solved in the axisymmetric geometry. We assume that the substrate has a finite thickness and its temperature obeys the thermal diffusion equation. A precursor film then spreads ahead of the bulk droplet itself in the complete wetting condition. Cooling the substrate enhances condensation of gas onto the advancing film, which mostly takes place near the film edge and can be the dominant mechanism of the film growth in a late stage. The generated latent heat produces a temperature peak or a hot spot in the gas region near the film edge. On the other hand, heating the substrate induces evaporation all over the interface. For weak heating, a steady-state circular thin film can be formed on the substrate. For stronger heating, evaporation dominates over condensation, leading to eventual disappearance of the liquid region.

Post-Tanner stages of droplet spreading: the energy balance approach revisited

The spreading of a circular liquid drop on a solid substrate can be described in terms of the time evolution of its base radius R(t). In complete wetting, the quasistationary regime (far away from initial and final transients) typically obeys the so-called Tanner law, with R∼t(α(T)), α(T) = 1/10. Late-time spreading may differ significantly from the Tanner law: in some cases the drop does not thin down to a molecular film and instead reaches an equilibrium pancake-like shape; in other situations, as revealed by recent experiments with spontaneously spreading nematic crystals, the growth of the base radius accelerates after the Tanner stage. Here we demonstrate that these two seemingly conflicting trends can be reconciled within a suitably revisited energy balance approach, by taking into account the line tension contribution to the driving force of spreading: a positive line tension is responsible for the formation of pancake-like structures, whereas a negative line tension tends to lengthen the contact line and induces an accelerated spreading (a transition to a faster power law for R(t) than in the Tanner stage).

Nonreactive spreading at high-temperature revisited for metal systems via molecular dynamics

The spreading for Cu and Ag droplets on top of a rigid solid surface modeling Mo is herewith considered via molecular dynamics. The dynamics of the base radius and the contact angle are recorded and fitted using the molecular-kinetic theory. A method is described to determine for liquid metals at the microscopic level the parameters appearing in this theory. These microscopic parameters are calculated directly in the simulations and compared to the fitted values. The agreement between the fitted values and the calculated ones shows that the dissipation of energy within the drop is caused primarily by the friction of liquid atoms over the substrate. This validation supports the understanding of the mechanisms controlling the spreading of liquid metals which, up to now, were based on experimental data and fitting procedures.

Nanoscale wetting on groove-patterned surfaces

In this paper, nanoscale wetting on groove-patterned surfaces is thoroughly studied using molecular dynamics simulations. The results are compared with Wenzel's and Cassie's predictions to determine whether these continuum theories are still valid at the nanoscale for both hydrophobic and hydrophilic types of surfaces when the droplet size is comparable to the groove size. A system with a liquid mercury droplet and grooved copper substrate is simulated. The wetting properties are determined by measuring contact angles of the liquid droplet at equilibrium states. Correlations are established between the contact angle, roughness factor r, and surface fraction f. The results show that, for hydrophobic surfaces, the contact angle as a function of roughness factor and surface fraction on nanogrooved surfaces obeys the predictions from Wenzel's theory for wetted contacts and Cassie's theory for composite contacts. However, slight deviations occur in composite contacts when a small amount of liquid penetration is observed. The contact angle of this partial wetting cannot be accurately predicted using either Cassie's or Wenzel's theories. For hydrophilic surfaces, only wetted contacts are observed. In most cases, the resulting contact angles are found to be higher than Wenzel's predictions. At the nanoscale, high surface edge density plays an important role, which results in contact line pinning near plateau edges. For both hydrophobic and hydrophilic surfaces, substantial amount of anistropic spreading is found in the direction that is parallel to the grooves, especially at wetted or partially wetted contacts.

Nonreactive spreading at high temperature: molten metals and oxides on molybdenum

The spontaneous spreading of small liquid metal (Cu, Ag, Au) and oxide drops on Mo substrates has been studied using a drop transfer setup combined with high-speed video. Under the experimental conditions used in this work, spreading occurs in the absence of interfacial reactions or ridging. The analysis of the spreading data indicates that dissipation at the triple junction (that can be described in terms of a triple-line friction) is playing a dominant role in the movement of the liquid front. This is due, in part, to the much stronger atomic interactions in high-temperature systems when compared to organic liquids. As a result of this analysis, a comprehensive view of spreading emerges in which the strength of the atomic interactions (solid-liquid, liquid-liquid) determines the relative roles of viscous impedance and dissipation at the triple junction in spreading kinetics.

Phase field theory of heterogeneous crystal nucleation

The phase field approach is used to model heterogeneous crystal nucleation in an undercooled pure liquid in contact with a foreign wall. We discuss various choices for the boundary condition at the wall and determine the properties of critical nuclei, including their free energy of formation and the contact angle as a function of undercooling. For particular choices of boundary conditions, we may realize either an analog of the classical spherical cap model or decidedly nonclassical behavior, where the contact angle decreases from its value taken at the melting point towards complete wetting at a critical undercooling, an analogue of the surface spinodal of liquid-wall interfaces.

Surface wetting of liquid nanodroplets: droplet-size effects

The spreading of liquid nanodroplets of different initial radii R0 is studied using molecular dynamics simulation. Results for two distinct systems, Pb on Cu(111), which is nonwetting, and a coarse-grained polymer model, which wets the surface, are presented for Pb droplets ranging in size from approximately 55,000 to 220,000 atoms and polymer droplets ranging in size from approximately 200,000 to 780 000 monomers. In both cases, a precursor foot precedes the spreading of the main droplet. This precursor foot spreads as r(2)(f)(t) = 2D(eff)t with an effective diffusion constant that exhibits a droplet-size dependence D(eff) approximately R(1/2)(0). The radius of the main droplet r(b)(t) approximately R(4/5)(0) is in agreement with kinetic models for the cylindrical geometry studied.