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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]

The explosive boiling of a Cassie-Baxter (CB) state argon film suspended on nanopillar-arrayed surfaces is for the first time systematically investigated via molecular dynamics (MD) simulations. As the CB state liquid films with various thicknesses explode on nanopillar-arrayed surfaces, owing to the delayed local accumulation of energy on the thicker liquid film, the onset time of explosive boiling is increased with increased thickness of suspended liquid film, resulting from increased hydraulic diameter. Although the hydraulic diameter is decreased with increased nanopillar height, the onset time remains unchanged for a suspended liquid film over a nanopillar-arrayed surface with varying nanopillar heights. The invariant onset time with decreased hydraulic diameter is attributable to the incapability of liquid argon atoms to infiltrate into nanogrooves between nanopillars, producing a constant area for heat transfer between liquid films and solid surfaces, even the nanopillar height is increased. The surface potential energy has been significantly increased with increased inherent wettability of surfaces, leading to a stronger interaction between liquid and solid atoms. Thus, more argon atoms adsorb heat to rapidly trigger the separation of suspended liquid films from the nanopillar-arrayed surface. With enhanced surface wettability, the onset time would be decreased with decreased intrinsic contact angle of the nanopillar-arrayed surface. • Explosive boiling of liquid argon films in the Cassie-Baxter state is studied. • Effects of film thickness, nanopillar height, and inherent wettability are focused. • Effects of hydraulic diameter and surface potential energy are analyzed. • The mismatch of vibrational density of states between liquid and solid atoms is revealed. [ABSTRACT FROM AUTHOR]

• Spreading of nanodroplets on isothermally heated textured surfaces is studied. • A vertical electric field is imposed on droplets during spreading. • Field strength and direction and surface wettability and temperature are focused. • Two special effects induced by high substrate temperatures are identified. The statics and dynamics of electrowetting of a water nanodroplet on an isothermally heated nanostructured surface subjected to a vertical electric field are investigated via molecular dynamics (MD) simulations. The simulation results reveal that when imposing a constant electric field on the water nanodroplet, the static and dynamic contact angles of the water nanodroplet are always decreased with increasing the substrate temperature. At the same time, the spreading becomes faster at higher substrate temperatures. In the no-temperature-difference scenario, the statics and dynamics of electrowetting are found to strongly depend on the field strength and direction owing to small droplet sizes. Intriguingly, this dependence weakens with increasing the substrate temperature even completely disappears at high enough substrate temperatures. The result in the finite-temperature-difference scenario can be attributed to the weakened pining effect caused by vigorous evaporation in the vicinity of the contact line as well as the decreased liquid–vapor interfacial tension (or increased spreading coefficient) arising from the elevated interface temperature. [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]

• 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]