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The present study investigates impact of nanodroplets on liquid films with varied film thicknesses over hydrophilic to hydrophobic surfaces via molecular dynamics simulations. A new crown pattern, hollow crown, is observed on medium liquid films for nanodroplets, whereas it is not reported for millimeter-sized droplets. Holes (or hollows) are attributed to capillary vibration of a nanometer-thick liquid film. The occurrence of holes is inhibited effectively for deep liquid films because the cavity thickness is much larger than the amplitude of capillary vibration. Surface wettability has a significant influence on thin and medium liquid films for nanodroplets, whereas it only affects thin liquid films for macrodroplets because the range of solid–liquid intermolecular forces is comparable to the size of droplets when the size of droplets reduces to the nanoscale. Subsequently, the accuracies of the existing macroscale models for predicting the time-dependent crown radius of nanodroplets are tested by simulated results. The results show that the macroscale models cannot capture the time-dependent crown radius of nanodroplets. On the basis of energetics analysis, we prove that the failure of macroscale models is attributed to the distinct difference in viscous dissipation mechanisms between macroscale and nanoscale droplets. [ABSTRACT FROM AUTHOR]

In this work, the effects of in-plane shear stress on the thermal conductivities of two important kinds of graphene configurations (rectangular graphene and annular graphene) are investigated using molecular dynamics simulations. The various strain angles are set as θ = 0°, 3°, 5°, 10°, and 15° for rectangular graphene and θ = 0°, 3°, 5°, and 8° for annular graphene. A spectral energy density approach is applied to carry out phononic analyses of the two configurations under in-plane shear stress for the first time. It is found that for rectangular graphene, the thermal conductivity shows a significant decreasing trend from 239.6 to 79.1 W m−1 K−1 with the θ increasing from 0° to 15°. Besides, the application of shear stress can significantly widen the energy bands of branches, which indicates shorter phonon lifetimes. Further phononic analysis shows that most of the phonon scattering caused by in-plane shear stress emanates from the in-plane mode, which is an updated conclusion in this field. On the other hand, the thermal conductivity of annular graphene also shows a decreasing evolution with the increasing strain angles, but phonon transport in rectangular graphene is more susceptible to shear stress than in annular graphene due to a stronger phonon-boundary scattering in the latter configuration. In this work, the monotonically decreasing trends of the thermal conductivities of both graphene configurations with the increasing strain angle are different from the V-shape trend reported in the literature. Based on phononic analyses, it can be concluded that an abnormal increase of the thermal conductivities at a low shear stress can be attributed to the application of the AIREBO potential model in the literature. [ABSTRACT FROM AUTHOR]

Rising nanoscale technologies arouse interest in investigating the impact dynamics of nanodroplets. In this work, the impact of nanodroplets on solid spheres is investigated by the molecular dynamics simulation method, to comprehensively report outcome regimes and reveal the curvature effect, in wide ranges of Weber numbers (We) from 1.5 to 235.8, diameter ratios (λ) of nanodroplets to solid spheres from 0.3 to 5, and contact angles (θ) from 105° to 135°. Five outcomes are identified, including deposition, bouncing, splash, covering, and dripping. The former three outcomes are found in the high diameter ratio range (λ > 1), showing similar dynamic behaviors with impacts on flat surfaces, whereas in the low diameter ratio range (λ ≤)1, splash disappears, and covering and dripping take place additionally. At each contact angle, the outcomes are recorded in λ-We phase diagrams. It is found that the bouncing, splash, covering, and dripping are all promoted by decreasing diameter ratios; in addition, the critical Weber numbers for trigging bouncing and splash increase with decreasing θ. However, the critical We of the boundary between the bouncing to other regimes in the low diameter ratio range is not sensitive to wettability owing to the relatively small diameter of solid spheres. For quantitatively describing the curvature effect, the boundaries between the deposition and bouncing regimes in the high diameter ratio range and between the bouncing and other regimes in the low diameter ratio range are established. Both the established models show satisfactory agreement with the boundaries in the phase diagrams. [ABSTRACT FROM AUTHOR]

On nanopillar-arrayed superhydrophobic surfaces, the contact time of oblique nanodroplet impacts is for the first time investigated via molecular dynamics simulations. Here, oblique nanodroplet impacts are triggered by nanodroplets impacting superhydrophobic surfaces under various impact angles, α. The simulation results show that owing to the non-axisymmetry of spreading factors on nanopillar-arrayed superhydrophobic surfaces, the contact time of oblique nanodroplet impacts is always less on nanopillar-arrayed rather than smooth superhydrophobic surfaces under same impact angles. As the impact angle is increased from 5° to 65°, that is, under different impact conditions, the non-axisymmetry is more remarkable as α > 35° instead of α < 35° at the low, medium, and high normal Weber numbers, Wen. Hence, the contact time is sharp as α > 35° and then slowly reduced as α < 35° at the low, medium, and high Wen, at which the drastically increased sliding length as α > 35° further promotes the rapid reduction in contact time. As the impact angle is constant at 35°, the non-axisymmetry is more remarkable as h/w < 1 instead of h/w > 1 as the aspect ratio of nanopillars, h/w, is increased, that is, under different surface conditions. Hence, the contact time is sharp as h/w < 1 and then slowly reduced as h/w > 1 at the low, medium, and high Wen, at which the drastically reduced sliding length as h/w > 1 further hinders the rapid reduction in contact time. [ABSTRACT FROM AUTHOR]

Nanodroplet impacts have attracted significant attention, while the effect of surface wettability on contact time is evaluated poorly. Utilizing molecular dynamics simulations, the current work with a special focus on the contact time studies nanodroplets impacting solid surfaces in a wide range of static contact angles (θ0 = 105°–175°) and the Weber number (We = 0.1–200). The complete trends in contact time and restitution coefficient with surface wettability are analyzed and reported for the first time. For surfaces with θ0 > 160°, four different regimes are identified for the contact time and restitution coefficient as a function of the Weber number. For surfaces with 110° < θ0 < 160°, the fourth regime is not observed. The restitution coefficient is employed to analyze the contact time of distinct rebound patterns in the individual wettability range. Intriguingly, surface wettability has a remarkable influence on the contact time of nanodroplets even for superhydrophobic surfaces. The main reason for the difference between the macroscale and nanoscale is attributed to the significantly enhanced viscous effect and interfacial effect of the nanoscale impact. Considering the different effects of surface wettability on spreading and retraction dynamics, the theoretical models for the maximum spreading factor, spreading velocity, and retraction velocity are established. Finally, scaling laws of the spreading time τspr ∼ (R0/Vi)We2/3Re−1/3 and retraction time τret ∼ (R0/Vi)We2/3Re−1/3(1 − cos θ0)−1/2 are proposed. An excellent agreement with both the current data sources and the results in the literature verifies the universality of the current scaling law from hydrophobic to superhydrophobic surfaces. [ABSTRACT FROM AUTHOR]

Under parallel electric fields and free evaporation conditions, the statics and dynamics of spreading–evaporating nanodroplets are investigated on an isothermally heated surface via molecular dynamics (MD) simulations. The simulation results show that at the substrate temperature of Ts = 320 K, the static and dynamic contact angles on the left and right edges are initially asymmetric and then symmetric with increasing field strengths of E = 0.00–0.06 V Å−1, resulting in the asymmetric-to-symmetric spreading transition of spreading–evaporating nanodroplets. Under weak evaporation condition, that is, at Ts = 320 K, the asymmetric-to-symmetric spreading transition is triggered by enhancing the intrinsic surface wettability θ0 = 49°–80° at a constant field strength of E = 0.03 V Å−1. However, at the substrate temperature of Ts = 350 K, the symmetric-to-asymmetric spreading transition first appears for the static and dynamic contact angles on the left and right edges, and then the asymmetric-to-symmetric spreading transition appears with increasing field strength. Under strong evaporation condition, that is, at Ts = 350 K, as the field strength is constant at E = 0.03 V Å−1, the asymmetric-to-symmetric spreading transition also appears with increasing surface wettability. [ABSTRACT FROM AUTHOR]

Efficient droplet shedding from surfaces is fundamentally interesting and important due to its promising potential in numerous applications, such as anti-erosion, anti-icing, and self-cleaning. In this work, the bouncing dynamics of the head-on impact of two nanodroplets on superhydrophobic surfaces are investigated through molecular dynamics simulations. Three bouncing patterns, including regular-coalescence-bouncing, coalescence-hole-bouncing, and ring-bouncing, are identified at a wide range of impacting Weber numbers. For three bouncing patterns, the time evolutions of the spreading factors and the vertical velocity components are employed to analyze the particular dynamic behaviors and elucidate the underlying physics. As a counter-intuitive bouncing pattern, the ring-bouncing that two impact nanodroplets coalesce, spread, and then leave the surface in a ring shape without retracting exhibits a remarkable reduction in contact time by up to 60%. Considering four typical states for the ring-bouncing pattern, the comparison of the velocity distribution within the droplet clearly reveals that the ring-shaped droplet reshapes interfaces, which leads to a special hydrodynamics distribution. As a result, the internal flows at the inner and outer edges along the opposite direction collide with each other, leading to a sudden increase in the upward velocity. Combining the largely decreased contact area between solid and liquid with the small surface adhesion, the ring-shaped droplet rapidly bounces off the surface at the maximum spreading state. Finally, it is significantly highlighted that the ring-bouncing pattern offers a new avenue to break the contact time limit for efficient droplet shedding. [ABSTRACT FROM AUTHOR]

This work investigates the retraction and bouncing dynamics of an impacting low-viscosity nanodroplet on superhydrophobic surfaces via molecular dynamics simulations, aiming to reveal the scaling laws of retraction and bouncing velocities and to establish the relationship between them. The retraction velocity, Vre, is found to scale as Vre ∼ Dmax/τc,n, where Dmax is the maximum spreading diameter, τc,n = (D0/V0)We1/2Oh1/3 is the inertial-capillary-viscous time, and We and Oh are the Weber number and Ohnesorge number, respectively. The bouncing stems from the collision of the retracting rim at the center of the nanodroplet, leading to the bouncing velocity scaling as the retraction velocity. Combining the relationship of Vre ∼ Dmax/τc,n with the scaling law of Dmax ∼ We1/2Oh1/3D0 yields both the retraction and bouncing velocities scaling as the impact velocity, indicating that both the retraction and bouncing velocities of low-viscosity nanodroplets on a superhydrophobic surface depend merely on the impact velocity. An energy analysis shows that the proportion of the surface energy at the maximum spreading state (Es,max) to the initial kinetic energy (Ek,ini) follows Es,max/Ek,ini ∼ Oh2/3, whereas the proportion of the bouncing kinetic energy (Ek,b) to the surface energy at the maximum spreading state follows Ek,b/Es,max ∼ Oh−2/3, leading to constant Ek,b/Ek,ini and also constant restitution coefficient for low-viscosity nanodroplets impacting superhydrophobic surfaces. [ABSTRACT FROM AUTHOR]

The impact dynamics of water nanodroplets on flat solid surfaces was studied by molecular dynamics simulations over a wide range of Weber numbers (We) and surface wettability (θ0), where θ0 is the Young contact angle. A phase diagram in the parameter space of We vs θ0 was established accommodating eight impact outcomes noted in the final stage of impact, with three of them, holes rebound, partial-rebound splash, and rebound splash, for the first time being identified and reported. The eight impact outcomes were classified into three categories, i.e., non-bouncing, bouncing, and splash. The results show that the splash is triggered only when Wecr > 140. The boundaries separating bouncing from non-bouncing were determined based on the phase diagram. When θ0 > 160°, the boundary is described as Wecr = a ≪ 1; when 110° < θ0 < 160°, the boundary depends on both We and θ0, with a larger We required to trigger bouncing on a less hydrophobic surface, expressed as Wecr = b + ccosθ0; when θ0 < 110°, bouncing never takes place, and hence, the boundary is determined only by the critical contact angle, expressed as θ0,cr = 110°. Here, a, b, and c are constants. [ABSTRACT FROM AUTHOR]

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Rapid boiling of liquid film appears on solid surfaces with ultrahigh temperatures or heat fluxes, where a vapor layer is rapidly formed between the liquid film and the surface, leading to the film detachment from the surface and hence significantly reducing the heat transfer rate. In this work, a new idea, solid surface is charged with surface charges, is proposed to suppress the formation of vapor layer and prevent the liquid film detachment. To examine the effectiveness of the idea, the boiling behaviors of water films on gold (1 0 0) surfaces with various wettability conditions as well as with or without surface charges are investigated by molecular dynamics simulations. The results show that the electric field induced by the surface charges leads to an increased attractive interaction between the water film and the gold surface. Meanwhile, it is very interesting that a cone-shaped liquid column moves upward from the free surface of the water film due to the directional arrangement of water molecules along the electric field direction, which helps the nucleation bubble to break through the water film. Thus, the formation of vapor layer is suppressed and the water film detachment is never observed during the whole phase-change process of water film. The results also reveal that the electric field significantly increases the collision rate between water molecules and gold atoms, and hence enhances heat transfer from the gold surface to the water film. [ABSTRACT FROM AUTHOR]

Reactive and non-reactive wettings of pure Al and Al–Ni alloy droplets on an NiAl(100) substrate were numerically investigated. The embedded atoms method potentials were applied to account for interatomic interactions for the Al–Ni systems. The results show that since the Al–Ni alloy droplets has lower equilibrium contact angle, their spreading on the substrate will be slightly faster than the pure Al droplet. However, owing to atom interdiffusion effects on the relaxed substrate, enhanced dissolution of Ni atoms from the substrate to the droplet would remarkably accelerate the spreading using rigid substrate as control. Thus, the pure Al droplet yields the fastest spreading due to its corresponding farthest-from-equilibrium concentration, which is followed by 90% Al-10% Ni droplet and 80% Al-20% Ni droplet. The enhanced spreading kinetics is attributed to the decreased bulk viscous dissipation and local dissipation in the vicinity of the contact line caused by the dissolution. [ABSTRACT FROM AUTHOR]

This work employs non-equilibrium molecular dynamics (NEMD) simulations to examine the applicability of four kinds of interatomic potential models: the Tersoff, the REBO, the opt-Tersoff and the AIREBO, which are widely used to model the thermal transport in single- and multi-layer graphene, as well as graphite crystallites. Thermal conductivities of ∼17 × 5 nm 2 and ∼50 × 5 nm 2 graphene are calculated in the temperature range of 200∼500 K with the four potentials and quantum correction is applied due to an extremely high Debye temperature of about 2100 K for graphene. The predicted thermal conductivities are compared with experimental data and phonon spectrum functions are calculated to quantify the degree of phonon scattering. The results show that two original potentials, the Tersoff and the REBO, as well as the AIREBO significantly underestimate thermal conductivities of single-layer graphene but they can qualitatively describe the trend of thermal conductivities with temperature. The opt-Tersoff is found to be the most suitable potential for modeling the thermal conductivity of both single- and multi-layer graphene because it predicts a larger frequency range and a larger frequency value for the high frequency peak, while appropriately capturing phonon scattering in thicker multi-layer graphene when Lennard-Jones term is added into the opt-Tersoff to describe interlayer atomic interactions. [ABSTRACT FROM AUTHOR]

In the presence of electric field induced by either constant or pulsed direct current (DC), a critical electric strength ( E c ) exists for the transition from full coalescence to partial coalescence of adjacent charged droplets. In this study, the electro-coalescence behaviors of two charged droplets under constant and pulsed DC electric fields are studied via molecular dynamics simulations. The results show that the two droplets experience an attractive force when the constant DC electric field is applied, which causes an accelerated approach from one another before contact. However, under the pulsed DC electric field, the approach velocities of the two droplets accelerate within the pulse duration, while remain approximately constant during the pulse interval. Thus, the value of E c for full coalescence and partial coalescence of the two droplets under the pulsed DC electric field becomes larger as compared with the constant DC electric field, which significantly reduces the time required for bringing the two droplets into contact. Hence, higher electro-coalescence efficiency is achieved. Furthermore, for a given pulse period, E c is inversely proportional to the pulse duration; for a given duty ratio, E c reduces with the increase in the pulse period. [ABSTRACT FROM AUTHOR]

Graphene has received great attention due to its fascinating thermal properties. The inevitable defects in graphene, such as single vacancy, doping, and functional group, greatly affect the thermal conductivity. The sole effect of these defects on the thermal conductivity has been widely studied, while the mechanisms of the coupling effects are still open. We studied the combined effect of defects with N-doping, the -CH3 group, and single vacancy on the thermal conductivity of multi-layer graphene at various temperatures using equilibrium molecular dynamics with the Green-Kubo theory. The Taguchi orthogonal algorithm is used to evaluate the sensitivity of N-doping, the -CH3 group, and single vacancy. Sole factor analysis shows that the effect of single vacancy on thermal conductivity is always the strongest at 300 K, 700 K, and 1500 K. However, for the graphene with three defects, the single vacancy defect only plays a significant role in the thermal conductivity modification at 300 K and 700 K, while the -CH3 group dominates the thermal conductivity reduction at 1500 K. The phonon dispersion is calculated using a spectral energy density approach to explain such a temperature dependence. The combined effect of the three defects further decreases the thermal conductivity compared to any sole defect at both 300 K and 700 K. The weaker single vacancy effect is due to the stronger Umklapp scattering at 1500 K, at which the combined effect seriously covers almost all the energy gaps in the phonon dispersion relation, significantly reducing the phonon lifetimes. Therefore, the temperature dependence only appears on the multi-layer graphene with combined defects. [ABSTRACT FROM AUTHOR]

The present work employed a molecular dynamics method to investigate the effect of NaCl concentration on the deformation, breakup, and evaporation characteristics of bio-nanodroplets under a strong electric field. It aims to reveal the size control mechanism for a nanoparticle produced by electrospray deposition. The results show that when the droplet dissolves NaCl, it is elongated to be a longer “spindle” by the electric field force, compared to the droplet without NaCl, and several small clusters are ejected from two tips of the spindle due to the hydration effect of Na+and Cl−. In addition, the formation of ion pairs is observed when the droplet dissolves NaCl. The NaCl concentration affects the hydration degree and the formation of ion pairs significantly, which leads to different spindle lengths and number of clusters. The longer spindle and the larger cluster number could enlarge the free surface area and remarkably accelerate evaporation. Fabrication of smaller bio-nanoparticle requires both a faster evaporation rate and larger cluster number, which can be achieved by selecting an appropriate NaCl concentration. [ABSTRACT FROM AUTHOR]

Abstract: The study on evaporation of water droplet with dissolved salt is important for the understanding of electrospinning and electrospray mass spectrometry experiments. In this work the dynamics of evaporation of water droplet and aqueous droplet with dissolved either LiCl, NaCl or KCl salt in the absence and presence of a homogeneous electric field are comparatively studied by molecular dynamics simulations. The results show that (1) the evaporations of water and aqueous droplets are all accelerated by applying an electric field; (2) the aqueous droplet evaporates slower than the water droplet at low electric field of 0.4Vnm−1, however, its evaporation rate is enhanced significantly and exceeds the water droplet at high electric fields of 0.6 and 0.8Vnm−1; (3) aqueous droplet with dissolved KCl molecules evaporates the fastest, and then followed by dissolved NaCl and LiCl molecules; (4) an optimal salt concentration is observed for the aqueous droplet, above which the evaporation rate is reduced significantly and may cause a electrospinning failure. The present results are qualitatively agree with the observations in the electrospinning experiment and are attributed to the fact that the electric field applied causes the droplet deformation, directional arrangement of water molecules along the electric field direction, acceleration of water molecules due to ions acceleration, as well as hydration effect. [Copyright &y& Elsevier]

Highlights: [•] Nanodroplet evaporation in a nanochannel is studied using molecular dynamics method. [•] The effect of wall adsorption on the droplet evaporation is analyzed. [•] The evaporation in nanochannel differs from that in infinite space significantly. [ABSTRACT FROM AUTHOR]

Interfacial nanolayer acting as a bridge linking a bulk solid phase and a bulk liquid phase has a crucial impact on the heat transfer from the solid to the liquid. In this work, we proposed a promising method to enhance the heat transfer between a Cu surface and liquid water by applying surface charges to the surface, which was expected to change the structure of the interfacial nanolayer. We employed molecular dynamics simulations to verify the effectiveness of the method. Our simulations showed that applying surface charges led to an orientational alignment of water molecules, reducing the spacing between the solid surface and the interfacial nanolayer and increasing the number of water molecules located at the interface nanolayer. Meanwhile, the presence of surface charges also improves the mismatch of vibrational density of states between solid atoms and water molecules. As a results, the heat transfer was enhanced as compared with that without surface charges and the enhancement becomes more remarkeble when applying more surface charges to the Cu surface. [ABSTRACT FROM AUTHOR]

• Condensed cluster growth under an electric field is studied. • The orientation polarization reduces the interaction between water molecules. • A critical cluster size partitions the orientation polarization into two stages. • The cluster growth rate is independent of theF E-Field direction in the first stage. • In stage 2, the growth is affected by the contact area and molecule interaction. Electric fields (E-Fields) can promote droplet detachment and improve dropwise condensation. Since droplet growth is dominant in the dropwise condensation cycle, the influence of the applied electric field on the growth of condensed droplets is worth exploring. In this work, the effect of molecule orientation polarization in the electric field on the growth of condensed nanodroplets is studied via molecular dynamics simulation. The results show that when the size of a growing cluster reaches a critical size R c , the orientation polarization of water molecules within the cluster significantly increases. The orientation polarization is shown to reduce the interaction between water molecules, and the condensed molecules are easier to escape. Thus, the reduced interaction between water molecules hindered the growth of condensed clusters. Especially when the cluster size exceeds R c , the increased orientation polarization makes this hindrance more pronounced. However, the morphology of the cluster simultaneously transforms from ellipsoidal to needle-like at the critical size, and the shape deformation of the cluster can also affect the growth rate. When the cluster size is larger than R c , a vertical E-Field stretches the condensed cluster perpendicular to the substrate, which decreases the solid-liquid interfacial area of the cluster. The reduction of solid-liquid interfacial area and the reduced interaction between water molecules jointly hinder the growth of condensed clusters. On the contrary, needle-shaped deformation induced by parallel electric fields can increase the contact area between the cluster and the substrate. Therefore, the increased solid-liquid interface area can compensate for the negative effect of orientation polarization on the growth. Besides, when the cluster size is smaller than R c , the growth rate is mainly related to the polarization-induced weakened interactions between molecules. Thus, in stage R < R c , both vertical and parallel electric fields hinder the growth. [ABSTRACT FROM AUTHOR]

Reducing hydrophilic spot size on hybrid wetting substrates can decrease droplet pinning. When spot size is reduced to that of nucleation, whether condensation is still controllable remains to be explored. This work studies condensation patterns on substrates with nano-spots using molecule dynamics. Density fluctuation results in the uneven distribution of vapor, leading to vapor nucleation at partial nano-spots first. These nucleated clusters compete with bare spots to attract vapor, resulting in different condensation patterns. If clusters dominate in attracting vapor, nucleation on bare spots is hindered, and condensation patterns are disordered. When the nano-spot wettability exceeds critical θ cr , vapor preferentially nucleation at bare spots and condensation patterns become uniform. This preference is attributed to the strong attraction of solids to vapor and more hydrogen bonds formed between adsorbed water molecules. The growth of individual clusters on substrates with different condensation patterns is evaluated by the power law of temporal-evolved cluster size (N c = C 0 t b ). The competition between formed clusters to attract vapor molecules hinders the cluster growth, and two hindrance effects are distinguished: (I) weak competition only reduces the coefficient C 0 , whereas power b maintains 3/2, and (II) severe competition also reduces b , which finally shrinks to 1. • Condensation pattern on surfaces with nano hydrophilic spots is studied. • Competition between spots to attract molecules results in a disordered pattern. • Molecules prefer aggregate at exposed solid areas of wettability above critical. • More hydrogen bonds formed at the S-L than at the L-V interface when θ < θ cr. • The relation between hindered growth of clusters and competition is illustrated. [ABSTRACT FROM AUTHOR]

Abstract: The macroscopic flow geometry has long been assumed to have little impact on dynamic wetting behavior of liquids on solid surfaces. This study experimentally studied both spontaneous spreading and forced wetting of several kinds of Newtonian and non-Newtonian fluids to study the effect of the macroscopic flow geometry on dynamic wetting. The relationship between the dynamic contact angle, θD , and the velocity of the moving contact line, U, indicates that the macroscopic flow geometry does not influence the advancing dynamic wetting behavior of Newtonian fluids, but does influence the advancing dynamic wetting behavior of non-Newtonian fluids, which had not been discovered before. [Copyright &y& Elsevier]

[Display omitted] • The influence of the surface structure features on the bouncing dynamics is revealed. • The phase diagrams of the bouncing patterns are constructed. • Theoretical models are developed to unravel the mechanisms of pancake bouncing of impacting nanodroplets on nanopillared surfaces. • The critical Weber number for triggering the pancake bouncing is proposed. Superhydrophobic surfaces that promote rapid droplet detachment in a pattern of pancake bouncing are fundamentally interesting and important in substantial applications, such as self-cleaning, anti-corrosion, and anti-icing. However, the accurate modeling of the trigger conditions and the underlying dynamic mechanism for the pancake bouncing have yet to be completed. Here, using molecular dynamics simulations, the bouncing dynamics of impacting water nanodroplets on superhydrophobic Pt surfaces with nanopillar arrays are comprehensively studied considering a wide range in the impact Weber number, solid fraction (the pillar width w is kept constant), and pillar height. Correspondingly, the phase diagrams of the bouncing patterns are constructed in the parameter space of the Weber number against the solid fraction. It has been found that there are critical values of the solid fraction and pillar height to induce the pancake bouncing at the moderate Weber number. Furthermore, a theoretical model is developed and quantitatively proclaims the dependence of nanostructure features and the critical Weber number on the pancake bouncing. This work provides rational design principles for attaining a controllable pancake bouncing on structured superhydrophobic surfaces for a wide range of applications. [ABSTRACT FROM AUTHOR]

• Aqueous nano-film evaporation is studied by molecular dynamics simulation. • The external electric field on the evaporation of the aqueous nano-film is focused in this work. • Aqueous film evaporation was enhanced using a high electric field parallel to the surface of aqueous film. In this work, molecular dynamics simulation has been applied to investigate the influence of external electric field on the evaporation of the aqueous nano-film. The evaporation of the aqueous nano-film with 2240 water molecules and 50 NaCl on a gold (1 0 0) surface is analyzed at the electric fields with various intensities (0, 0.05, 0.1, 0.2 and 0.3 V nm−1) and directions. The predictions show that the evaporation of aqueous film is remarkably enhanced when the electric field E x = 0.2 or 0.3 V nm−1 is parallel to the aqueous film surface. It is also noted that free ions in the aqueous film are accelerated under the action of the higher E x and water molecules in the hydration shell move together with the ions due to the hydration effect. As a result, the interaction between water molecules decreases, which is responsible for increasing the evaporation of the aqueous film under the action of the higher E x. While applying the electric field E y = ±0.3 V nm−1 perpendicular to the aqueous film, ions cannot be in accelerated motion due to the existence of a solid-liquid interface and a liquid-gas surface in y -direction. Therefore, the evaporation enhancement is much lower than that of the aqueous film under the action of the E x. [ABSTRACT FROM AUTHOR]

Highlights • Anisotropic properties at black phosphorus (BP)-water interface are first identified. • Rippled BP surface leads to the anisotropic boundary slip and interfacial friction. • Boundary slip decreases and interfacial friction increases with the chiral angle. • BP-water interfacial friction and slip can be adjusted by changing the chiral angle. Abstract Black phosphorus (BP) has become a novel focus of the 2D material due to its natural direct band gap and high carrier mobility, but researchers seldom refer to the water-BP interfacial properties while the natural rippled surface of the BP raises great hope to develop the controllable fluid transmission devices. In this paper, the interfacial slip and friction between monolayered BP and water are first investigated using classical molecular dynamics simulations based on the BP sheet-water Couette model. The results show that the rippled structure of the BP surface leads to the anisotropic boundary slip and interfacial friction between water and BP sheets. The maximum boundary slip and the minimum interfacial friction are obtained for the water flow direction parallel to the ripple direction of BP monolayer. With increasing the intersection angle between water flow direction and BP ripple direction, the boundary slip decreases and the interfacial friction increases continuously. The mechanism of the anisotropic interfacial properties is clarified from the configuration of the water molecules adjacent to the BP surfaces. This finding will generate excitement in designing the BP-based fluid transmission device. [ABSTRACT FROM AUTHOR]

Highlights • Phase change behaviors of nanoscale liquid films are studied via MD simulations. • A critical thickness is proposed to distinguish the macro-micro boiling transition. • The critical thickness is found to be 10.34 nm for water. Abstract Two main reasons including practical and scientific significance continuously motivate the studies of explosive boiling of nanoscale liquid films. Whether the nanoscale explosive boiling agrees with classical nucleation theory is still an open issue. In this work, we study the effects of surface wettability on the explosive boiling of nanoscale liquid films using molecular dynamics simulations. A critical film thickness is proposed to address the debate of whether the classical nucleation theory fails in nanoscale boiling. The explosive boiling modes and dynamics are summarized with the phase diagram based on various simulation cases, considering the effects of surface wettability, film thickness, and surface superheat. When the films thickness exceeds the critical thickness, hydrophobic surfaces are more favorable for explosive boiling, which still agrees with the classical nucleation theory. However, a much higher superheat is required to trigger the explosive boiling of a thin liquid film when its thickness is less than the critical thickness on hydrophobic surfaces, which consists with previous molecular dynamics simulations. Furthermore, we also find that the explosive boiling is trigged faster for films on hydrophilic surfaces than those on hydrophobic surfaces with the same superheat owing to the lower Kapitza thermal resistance. With such heat transfer superiority, hydrophilic surfaces can heat liquid films faster to explosion. The deliveries of this work and the concept of the critical thickness might help to understand the still fledgling field of nanoscale boiling phase change and its relevant mechanisms. [ABSTRACT FROM AUTHOR]

Because of scaling effect, phase change behaviors of nano-liquid films on solid surfaces display distinct features from macroscale pool boiling. In this work, evaporation and explosive boiling of nano-liquid argon films with various film thicknesses on hydrophilic or hydrophobic platinum surfaces are investigated via molecular dynamics simulations. The simulations show that the onset of explosive boiling closely depends on the film initial thickness on both the hydrophilic and hydrophobic surfaces, and the onset temperature for explosive boiling significantly reduces with the increase of the film thickness. In addition, the simulations also demonstrate that, with the same film thickness and the same wall superheat, explosive boiling is triggered on the hydrophilic surface, while only evaporation is observed on the hydrophobic surface, indicating that a high surface wettability reduces the onset temperature, which is opposite to the prediction by classical nucleation theory and the observation in pool boiling experiments. For a thicker film and a higher surface wettability, it is found that a larger temperature gradient is generated along the film thickness direction due to the heat accumulation effect, which leads to a very high temperature occurring at the film bottom and thereby is responsible for the lower onset temperature for explosive boiling on these circumstances. [ABSTRACT FROM AUTHOR]

The high-temperature wetting of metal droplets on alloy metal substrates is of practical interest owing to its wide range of applications in brazing, welding and soldering. The surface and bulk of the alloy metals contain very many grain boundaries, whose effects on high-temperature wetting and the associated mechanisms have not been satisfactorily studied. In this work, two spreading scenarios are considered using molecular dynamics simulations to investigate the effects of grain boundaries on the wetting kinetics of the spreading of Al droplets on substrates with (polycrystalline NiAl) or without (monocrystalline NiAl) dissolutive reactions and at various rates of solution (monocrystalline and polycrystalline Ni 3 Al). Dissolutive reactions occur only when an Al droplet spreads over a polycrystalline NiAl substrate, and they significantly accelerate the wetting kinetics. Irregular atomic alignment and associated with grain boundaries, atomic migration or dissolution reactions occur more easily on grain boundaries than on ideal crystal substrates. Dissolutive reactions occur when Al droplets spread on either monocrystalline or polycrystalline Ni 3 Al substrates; however, grain boundaries influence the dissolution rates in various spreading stages. The dissolution rate of the Al(l)/polycrystalline Ni 3 Al(s) system with grain boundaries exceeds that of the Al(l)/monocrystalline Ni 3 Al(s) system in the initial stage of spreading, but is less than that of the Al(l)/monocrystalline Ni 3 Al(s) system in the final stage of spreading. Correspondingly, the spreading of the Al(l)/polycrystalline NiAl(s) system is faster than that of Al(l)/monocrystalline NiAl(s) system in the first stage but slower in the last stage. This result confirms that dissolutive wetting is dominated by dissolutive rate rather than by the total dissolutive quantity, and a high dissolution rate accelerates wetting. [ABSTRACT FROM AUTHOR]

Dewetting kinetics of Al and NiAl metallic liquid films on NiAl (1 0 0) substrates was studied using molecular dynamics simulations. A new dewetting-spreading transitional behavior was observed for high temperature dewetting. The dewetting-spreading transition comes from the competition between unbalanced Young’s force and dissolutive reaction. Without dissolutive reaction, liquid films keep dewetting, but immediately turn into spreading when the dissolutive reaction involved. The dissolutive reaction depends on the initial Ni atom contents rather than the contact areas of dewetting films. The far-away-from saturated Ni content is the main mechanism which accelerates the wetting and reverses the dewetting process at high temperatures. [ABSTRACT FROM AUTHOR]

• Vapor condensation on nanoscale hydrophilic spots is studied. • The power of the growth curve is close to the theoretical value. • The interaction between nucleated clusters reduces condensation performance. • Increased spots number facilitate condensation but the marginal utility diminished. • The edge effect disables small hydrophilic spots to attract water molecules. Because of preferable nucleation and departure characteristics, hybrid wetting substrates have been widely used to enhance condensation. Nanoscale hydrophilic spots are expected to reduce the adhesive force between the droplet and substrate, thereby promoting droplet departure; however, whether such hydrophilic spots can enhance nucleation and cluster growth remains poorly understood. Using molecular dynamics simulations, this work studies vapor condensation on a hydrophobic background substrate decorated with nanoscale hydrophilic spots. The results show that on the substrate with larger isolated hydrophilic spots, a higher probability of vapor molecules colliding with hydrophilic spots leads to faster nucleation. In the growth stage, the time-evolved size and surface area of the cluster follow N ∼ t 3/2 and A ∼ t , respectively. With a fixed size of hydrophilic spots, increasing the hydrophilic spot density is found to enhance nucleation probability and increase the number of formed clusters; however, the competition between nucleation sites also suppresses the growth of individual clusters. Besides, an interesting phenomenon is observed that there is a critical size of hydrophilic spots below which vapor condensation cannot take place on the spots. This phenomenon can be explained by the fact that the hydrophobic atoms surrounding a hydrophilic spot weaken the affinity of the hydrophilic atoms that are located at the boundary of the hydrophilic spot, and thus reduce the interaction between the hydrophilic spot and vapor molecules. This edge effect will become extremely prominent if increasing the nucleation site number with a fixed hydrophilic atoms ratio. As a result, an appropriate nucleation site number should be designed to obtain the best condensation performance. [ABSTRACT FROM AUTHOR]

When nanodroplets rebound from nanopillar-arrayed superhydrophobic surfaces, the coupling influence of wetting transitions and scale effects on the bouncing dynamics is for the first time investigated via molecular dynamics (MD) simulations. The simulation results show that two bouncing patterns, the Cassie state bouncing and the Wenzel state bouncing, are always observed on nanopillar-arrayed superhydrophobic surfaces in a wide range of We = 1.5–95 as the coupling influence is changed by various aspect ratios and surface wettability. At same Weber number, the contact time of the Cassie state bouncing is constant, but that of the Wenzel state bouncing being increased with the increased aspect ratio. However, the contact time is always increased with the increased surface wettability regardless of the bouncing pattern. Based on that, scaling laws of contact time are eventually established for these two bouncing patterns, respectively. [Display omitted] [ABSTRACT FROM AUTHOR]

• Retraction of nanodroplets on hydrophilic to hydrophobic surfaces is studied. • Inertial and capillary regimes are distinguished. • Models of retraction velocities in the two regimes are developed. Because of scale effects, the impact of nanodroplets shows different spreading and retraction dynamics from millimeter-sized droplets, which remains poorly understood. This work investigates the retraction dynamics of low-viscosity water nanodroplets on hydrophobic to hydrophilic surfaces via molecular dynamics (MD) simulations. Two retraction regimes, inertial and capillary regimes, are defined based on whether an obvious rim can be observed in retraction processes. The two retraction regimes successively take place on hydrophobic surfaces, whereas only the capillary regime is observed on hydrophilic surfaces. In the inertial regime, the inertial force in the rim and the viscous forces near the rim entrance and the central film both dominate the retraction. The former makes the retraction velocity scale as We −1/2, whereas the latter causes the scaling of Oh −1/3. Moreover, the spreading factor in the inertial regime decreases from its maximum value to equilibrium value in the same interval of time. Based on these dynamics, an expression for retraction velocity in the inertial regime is proposed. In the capillary regime, the retraction slows down because inertial force weakens significantly. The retraction velocity depends only on the impact velocity and surface wettability. The proposed two expressions of retraction velocities are validated by MD simulations for the impact of nanodroplets on hydrophobic to hydrophilic surfaces at various impact velocities, showing good agreement between them. Furthermore, using the present retraction dynamics, a model of contact time for nanodroplets impacting hydrophobic surfaces is also developed. [ABSTRACT FROM AUTHOR]

• Dynamic electrocoalescence of charged droplets with deflected angles is studied. • Effects of field strengths, deflected angles, and ions on electrocoalescence dynamics are focused. • The critical electric field influences the electrocoalescence dynamics. • The dissolved ions are responsible for the appearance of secondary droplets. The electrocoalescence dynamics of two charged droplets with different deflected angles are studied via molecular dynamics simulations. The simulated results show that under a weak electric field, these adjacent droplets can realize the complete coalescence at the deflected angles of 0° and 50°, but cannot realize that at the deflected angle of 90°. As the electric field strength increases to a critical value, owing to escaped ions, a secondary droplet is ejected from coalescing droplet, namely the partial coalescence. However, as the field strength far exceeds the critical field strength, charge transfer appears between these adjacent droplets by the liquid bridge. These two droplets with enough opposite charges move towards the opposite directions, respectively, such that the formed liquid bridge breaks at the interface again, leading to the appearance of non-coalescence. As a result, the complete-to-non coalescence transition is triggered by the strong electric field. [ABSTRACT FROM AUTHOR]

• Wettability of the binary mixed substrate is studied. • A surface potential energy analysis method is used to verify nucleation sites. • The binary mixed substrate can effectively enhance the nucleation process. • No sacrifice on the departure performance of the binary mixed substrate. • A moderated hydrophilic ratio is needed to enhance condensation performance. Sophisticated and expensive surface texture design and manufacturing are commonly needed to achieve efficient surfaces with enhanced condensation performances. The high nucleation rate and high droplet removal capacity of a substrate surface acquire trade-off, whose mechanisms have not explicitly been comprehended. Based on the assumption that hydrophilic spots would improve nucleation while a hydrophobic surface can facilitate droplet removal rate, this work studied the nucleation and departure processes on a binary mixed substrate with randomly distributed hydrophilic atoms, with homogeneous substrates being as comparison references. The molecular dynamics simulation results showed that the wettability of the binary mixed substrate could be estimated by the form of the Cassie equation. Unlike the randomly nucleated clusters on a homogeneous substrate, the surface energy analysis verifies that the clusters tend to form on the areas with enriched hydrophilic atoms on a binary mixed substrate. Conversely, the droplets also tend to stick on the hydrophilic areas to deteriorate droplet removal efficiency. Combined with a probability analysis by the Monte Carlo method and MD simulation, a moderated ratio(r m = 25% in this work) was shown to furnish better nucleation properties with minimal deterioration in departure properties than the mono-wettability substrate with the same contact angle. [ABSTRACT FROM AUTHOR]

• Electrowetting transition on pillar-arrayed surfaces is studied. • MD simulations provide accurate wetting pathway. • Energy-minimization method is used to calculate the energy pathway. • Multiple energy barriers are found for the wetting transition. • Spontaneous dewetting transition occurs for a globally stable Cassie droplet. Two extreme wetting states, a highly non-wetting Cassie state and a wetting Wenzel state, can coexist or even mutually convert on patterned surfaces. Such a conversion process may be spontaneous or induced by external stimuli. This work studies the wetting transition of a nanodroplet on pillar-arrayed surfaces induced by an external electric field via an energy-minimization method in conjunction with molecular dynamics (MD) simulations. The simulation results reveal that driven by the electric field, the initial Cassie state could go through a partial wetting state, and eventually converts to the Wenzel state. The free-energy landscape reveals that there are multiple local energy minima, corresponding to multiple metastable states. For the metastable Cassie state, the wetting transition is irreversible, i.e., the droplet would remain in the Wenzel state when the electric field is removed. Conversely, the spontaneous dewetting transition from the Wenzel to the Cassie state can occur, if only the Cassie droplet is in a global energy minimum configuration. Thus, the stable Cassie wetting configuration is essential for triggering the spontaneous dewetting transition. [ABSTRACT FROM AUTHOR]

The bouncing dynamics of a nanodroplet impacting a superhydrophobic surface under a perpendicular electric field is studied through molecular dynamics (MD) simulations. Using the electric field strength as a parameter, two bouncing regimes are identified: an inertial force rebounding (IFR) regime (E < 0.08 V Å−1) and an electric field force rebounding (EFFR) regime (E > 0.08 V Å−1). In the IFR regime, the restitution coefficient, ε b , is the same as but the contact time, τ c , is shorter than that without an electric field. In the EFFR regime, ε b is proportional to the electric field strength, whereas τ c decreases with an increase in the electric field strength. On the boundary separating the two regimes (around 0.08 V Å−1), both ε b and τ c increase sharply due to the droplet deformation. The droplet bounces off the surface in the shape of a sphere in the IFR regime and as a long strip in the EFFR regime. A new criterion for the bouncing of nanodroplets subjected to a perpendicular electric field is proposed based on the restitution coefficient, Weber number, characteristic length, and factor Φ. The criterion demonstrates that imposing an electric field can help the bouncing of nanodroplets. [Display omitted] • The bouncing of nanodroplets subjected to perpendicular electric fields is studied. • Two bouncing regimes are discovered based on the field strength. • Restitution coefficient and contact time are measured in the two regimes. • A criterion is developed to predict bouncing in the presence of the electric field. [ABSTRACT FROM AUTHOR]

Because electric fields can significantly modify the morphology of impacting droplets, the impact dynamics of droplets subjected to external electric fields have attracted extensive attention in recent years. Owing to the enhanced viscous effect and the altered viscous dissipation mechanisms, nanodroplets show distinctly different impact behaviors from macroscale droplets. However, it is not clear how electric fields affect the impact dynamics of nanodroplets, especially when the field direction is changed. In this study, molecular dynamics (MD) simulations were performed to reveal the bouncing dynamics of a nanodroplet impacting a hydrophobic surface under electric fields with various field strengths and directions. As compared with the case without an electric field, the bouncing dynamics of the nanodroplet were significantly modified in the presence of the electric fields with tilt angles of α = 0°, 30°, 45°, 60°, and 90°, especially when the field strength was higher than 0.08 V Å−1. The restitution coefficient, ε b , was enhanced by the electric fields at a non-zero α with strengths larger than 0.08 V Å−1. Applying an electric field with α = 60° and ≥ 0.08 V Å−1 would lead to the maximum bouncing velocity. The contact time was stretched by a perpendicular electric field when E ≥ 0.08 V Å−1. When the impact velocity was not sufficient to make a droplet bounce off, an electric field with all directions was capable to cause the opposite. [Display omitted] • Bouncing dynamics of nanodroplets subjected to an electric field are studied. • Applying an electric field promotes bouncing. • The electric field with a tilt angle of 60° yields the best bouncing. • A vertical electric field increases the contact time. [ABSTRACT FROM AUTHOR]

Coalescence-induced droplet jumping on pillar-arrayed surfaces has received extensive attention. The pillar size and distribution affect the coalescence and jumping of droplets with unclear mechanisms. For the first time, this study performs molecular dynamics (MD) simulations to reveal the effects of initial wetting states on jumping and associated condensation processes at various intrinsic contact angles (θ Y), solid fractions (f), and roughness factors (r). The results show that although jumping and its velocity strongly depend on θ Y , f , and r , as reported in previous studies, the underlying mechanism to determine whether jumping occurs is the initial wetting state of coalescing droplets on textured surfaces. With the same θ Y and f , when the droplets are initially in the Cassie state, jumping can take place, whereas it may be hindered when the droplets are in the Wenzel state. The jumping velocity shows a decreasing trend for Cassie droplets when increasing f , but the reverse is true for Wenzel droplets. Additionally, it is found that the wetting state of a condensed droplet would transition from the Wenzel state to the Cassie state when the solid fraction increases but the pillar height and intrinsic wettability remain unchanged. [Display omitted] [ABSTRACT FROM AUTHOR]

Impact behaviors of a water nanodroplet on a hydrophobic surface in the presence of an external perpendicular electric field are investigated via classical molecular dynamics (MD) simulations. The results show that the maximum spreading factor, β max , is enhanced by the electric field, especially at field strengths larger than 0.1 V Å-1, whereas it is independent of the field direction when the field strength remains constant. Because the electric field alters the surface tension of the liquid, a factor Φ, which takes into account the effects of dielectric constant and conductivity of the droplet and continuous phase, initial droplet shape, and field strength, is proposed to modify the surface tension. An analytical model is developed to predict the maximum spreading factor for impacting nanodroplets subjected to a perpendicular electric field. The model shows good agreement with MD simulations in a low Reynolds number range. [Display omitted] • Spreading dynamics of impacting nanodroplets are studied under electric fields. • Electric fields enhance spreading and increase the maximum spreading factor. • A factor is proposed to modify the effect of surface tension. • A model of the maximum spreading factor under electric fields is developed. [ABSTRACT FROM AUTHOR]

• Dynamic spreading of nanodroplets on nanostructured surfaces in electric fields is studied. • Effects of field strength, field direction, and intrinsic wettability are focused. • The pinning effect influences the electro-wettability and electro-stretching effects. • The contact angle in the upward electric field is larger than that in the downward electric field. Molecular dynamics simulations are implemented to investigate the statics and dynamics of wetting for a water nanodroplet on a nanostructured surface in the presence of a vertical electric field. The results show that the electric field induces electro-stretching, electro-wettability, modified solid-liquid interfacial tension, and pinning at the triple line, and they jointly affect the spreading exponent and static contact angle of the nanodroplet. Under an upward electric field, the spreading is always hindered, and the static contact angle monotonously increases with the field strength. Interestingly, under a downward electric field, the spreading is first hindered and then is promoted as increasing the field strength, leading to a first increased and then decreased static contact angle. The increased solid-liquid interfacial tension and the enhanced pinning are found to be two main mechanisms for the slowing down of spreading and the increase in static contact angles for both electric fields at low field strengths. The same trends are observed at high field strengths under the upward electric field; however, exactly the reverse trends occur under the downward electric field, leading to the acceleration of spreading and the decrease in static contact angle at high field strengths. Moreover, it is found that the enhancement in intrinsic wettability of the surface can suppress the breaking of hydrogen bonds, thereby reducing the solid-liquid interfacial tension and accelerating the spreading. The enhancement in intrinsic wettability can also reduce the pinning at the triple line as well as the energy barrier of the Cassie-Wenzel transition, which contributes to the reduced static contact angle. [ABSTRACT FROM AUTHOR]

Owing to scale effects, spreading of nanodroplets over heated textured surfaces displays distinct features from that of millimetric-sized droplets. In this work, spreading behaviors of a water nanodroplet on isothermally heated smooth and nanostructured surfaces are investigated for various surface temperatures, wettability, roughness via molecular dynamics (MD) simulations. The simulations show that, in the early spreading stage, the spreading radius of the nanodroplet on the textured surface is smaller than that on the smooth surface, whereas a reverse result was reported for millimetric-sized droplets. Because the nanodroplet size is comparable to the feature size of nanostructures, the pinning effect becomes prominent, leading to a large local dissipation in the vicinity of the contact line and hence hindering the nanodroplet spreading over the nanostructured surface. However, more drastic evaporation on the nanostructured surface in the later spreading stage facilitates the adsorption and desorption of water molecules, weakening the pinning effect. As a result, the difference in the spreading radius over the two surfaces becomes less obvious in the later spreading stage. The simulations also demonstrate that a higher surface temperature increases the spreading coefficient and promotes the evaporation, thereby accelerating the nanodroplet spreading; the nanodroplet spreading is significantly suppressed when the intrinsic wettability of the nanostructured surface changes from hydrophilic to moderate wettability; because of stronger pinning effect, the nanostructured surface with a larger aspect ratio exhibits a slower spreading. Moreover, owing to the influence of disjoining pressure, an abnormal evaporation phenomenon is observed for the intrinsically moderate wettability surface, on which the nanodroplet shows a much faster evaporation rate in the later spreading stage than that on intrinsically hydrophilic surfaces. • Spreading of nanodroplets on isothermally heated textured surfaces is studied. • Effects of surface temperature, intrinsic wettability, and roughness are focused. • Spreading on textured surfaces is slower than on smooth ones due to pining effect. • Evaporation weakens the pinning effect and hence promotes the spreading. • An abnormal evaporation phenomenon is observed in the later spreading stage. [ABSTRACT FROM AUTHOR]

Electrocoalescence has been extensively used to remove water droplets in water-in-oil emulsions. Pulsed direct current (DC) electric fields were found to be more effective than DC electric fields. However, as an essential process in electro-dehydration, coalescence dynamics of two charged droplets and corresponding coalescence mechanisms under pulsed DC electric fields have remained poorly understood. In this work, such coalescence dynamics behaviors are investigated via molecular dynamics simulations. The contact time of droplets, deformation ratio of coalescing droplets, and critical field strength are compared among square, sinusoidal, and triangular waves. The results show that, with the same pulse amplitude, the contact time is the shortest for the square wave, the following are sinusoidal and triangular waves. Sinusoidal and triangular waves have similar waveforms and they significantly differ from the square wave. However, it is astonishing that the contact times are very close for the square and sinusoidal waves, which are far less than that for the triangular wave. This seemingly abnormal result is reasonably explained by the equivalent field strength or RMS (root mean square) field strength. The square and sinusoidal waves have the same RMS field strength, whereas the triangular wave has a smaller RMS field strength. After two droplets coalesce into a large droplet, the coalescing droplet undergoes periodic deformation. The deformation ratio significantly depends on waveforms. As compared with sinusoidal and triangular waves, the deformation is much drastic for the square wave. The critical field strengths are also evaluated for the three waveforms. The result shows that the square and sinusoidal waves have almost the same critical field strength, whereas the triangular wave has a much higher critical field strength. Further analysis demonstrates that the RMS critical electric fields are actually the same for the three waveforms. • Electro-coalescence behaviors under pulsed direct electric fields are studied. • Three typical waveforms square, sinusoidal, and triangular waves are chosen. • Coalescence dynamics are compared for various waveforms. • Contact time, deformation ratio, and critical filed strength are evaluated. • The RMS electric field is used to explain the difference of coalescence dynamics. [ABSTRACT FROM AUTHOR]

• High-temperature spreading of Al-Cu(l)/Ni(s) and Al-Ni(l)/Ni(s) was studied. • Significant dissolution reaction occurs between drops and solid substrate. • Precursor film formation is suppressed by the dissolution reaction. • Dissolution of solid atoms into the droplet correspond to spreading enhancement. • Spreading rate is controlled by the lattice constants. The high-temperature spreading dynamics of Al-Cu(l)/Ni(s) and Al-Ni(l)/Ni(s) wetting systems was studied using molecular dynamics simulations. Accompanied with spreading, a significant dissolution reaction was noted to take place for the two studied systems. The dissolution reaction involves both diffusion of solid atoms into the droplet and incorporation of liquid atoms into the substrate. No precursor film is generated during spreading, since its formation is suppressed by the dissolution reaction. The dissolution reaction induces an enhanced spreading rate, and spreading becomes faster when droplets contain less Cu or Ni atoms. The dissolution of solid atoms into the droplet is faster than that of liquid atoms into the substrate, so that the dissolution of the former is the dominant driving force for the spreading enhancement. The spreading of Al-Cu droplets correlates with that of Al-Ni droplets so long as the Cu concentration in Al-Cu droplets is equal to the Ni concentration in Al-Ni droplets, since both Cu and Ni atoms have the identical lattice constants. [ABSTRACT FROM AUTHOR]