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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]

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]

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]

• 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]

• 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]

• 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]

[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]

• 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]