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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 Cassie–Wenzel (C–W) wetting transition has been extensively investigated; however, the wetting transition of water films on textured surfaces with different topologies, together with underlining mechanisms, is unsatisfactorily explored. In this study, the C–W wetting transition of water films on pillar-arrayed and striped surfaces is studied. The results show that, on pillar-arrayed surfaces, the free energy variation during the C–W wetting transition follows the classical wetting pathway. The free energy first increases with the intrusion of water into the asperities and then decreases after a water film touches the basal surface. However, on striped surfaces, there exist multiple partial wetting states with each one occupying a local energy-minimization configuration. Accordingly, the water film needs to overcome multiple energy barriers to realize the C–W wetting transition. Moreover, the effects of aspect ratio and intrinsic wettability of the two textured surfaces on the C–W wetting transition are discussed. [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]

In this work, phase change behaviors of liquid argon nanofilms in the Wenzel or Cassie state on gold nanopillar-arrayed surfaces are investigated via molecular dynamics (MD) simulations. The results show that the argon films with 4.0 and 7.0 nm thickness have lower onset temperatures of explosive boiling than that with 1.5 nm thickness. At the same wall temperature of 190 K and film thickness of 4.0 nm, evaporation only is observed for the films in the Cassie state, whereas explosive boiling is noted for the films in the Wenzel state, indicating that the Wenzel state has a lower onset temperature. The nanopillar height has significant effects on the occurrence of explosive boiling. On the nanopillar-arrayed surfaces with an intrinsic contact angle of 35°, liquid films are always in the Wenzel state, and explosive boiling occurs at a prolonged time for the surface with a larger nanopillar height of 2.040 nm, attributing to the larger energy barrier for the Wenzel-to-Cassie wetting transition. On the nanopillar-arrayed surfaces with an intrinsic contact angle of 101°, films are initially in the Wenzel state for the smaller nanopillar height of 1.224 nm; however, they transition to the Cassie state for the larger nanopillar height of 2.040 nm, leading to the prolonged explosive boiling. [Display omitted] • Explosive boiling of argon nanofilms on nanopillar-arrayed surfaces is studied. • Effects of film thickness, intrinsic wettability, and nanopillar size are focused. • Onset temperature of explosive boiling is lower for thicker collapsed films. • Wetting transition significantly affects onset temperature of explosive boiling. [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]

• 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]

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]

• 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]