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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]

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]

On a single-ridge superhydrophobic surface, the rebound dynamics of double droplet impacts is investigated via lattice Boltzmann method (LBM) simulations, where one droplet is on the left of macro-ridges, whereas the other droplet being on the top of macro-ridges, leading to the asymmetrical impact of double droplets. The simulation results show that under different impact conditions of various Weber numbers, that is, at low, moderate and high Weber numbers, the rebound regime of double droplet impacts always consists of the complete-coalescence-rebound (CCR), partial-coalescence-rebound (PCR) and coalescence-split-rebound (CSR) regimes. Under these three rebound regimes, the contact time of double droplet impacts is always reduced by the increased We , as the center-to-center distances in the horizontal and vertical directions, L x and L z , are constant. At constant L x and We , the contact time of double droplet impacts on single-ridge superhydrophobic surfaces is increased by the increased L z because of the earlier formation of liquid bridges at larger L z. But under different impact conditions of various center-to-center distances in the horizontal direction, that is, at the increased L x , the rebound regime of double droplet impacts is not always composed of the CCR, PCR and CSR regimes at low, moderate and high Weber numbers. At constant We and L z , both coalescence strength and rebound regime are significantly affected by the increased L x for double droplet impacts. So that the contact time of double droplet impacts on singe-ridge superhydrophobic surfaces depends on the center-to-center distance in the horizontal direction, that is, in the x - direction. [Display omitted] • The coupling effect of droplet coalescences and droplet impacts is studied. • The influence of coupling effects on the bouncing dynamics of double droplet impacts is focused. • The influence of Weber numbers and center-center distances on the coupling effect is analyzed. • The correlations between rebound regimes and contact time is revealed. [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]