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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]

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]

When multiple droplets impact a superhydrophobic surface, coalescence between the droplets may lead to an increased viscous dissipation rate and thus an increased contact time. In this study, the impact of double droplets on a superhydrophobic surface is studied via a lattice Boltzmann model. The morphology and contact time of the rebounding droplet are obtained for various droplet distances and Weber numbers. The simulations show that there are three kinds of rebound patterns, complete-coalescence rebound (CCR), partial-coalescence rebound (PCR), and no-coalescence rebound (NCR); and the contact time is the shortest in the PCR regime. An energy analysis is implemented to reveal the energy conversion mechanism. It is found that viscous dissipation strongly depends on the coalescence strength, and it increases monotonously from the CCR regime to the NCR regime. This result implies that the shortest contact time in the PCR regime does not arise from the reduced viscous dissipation but is attributed to the morphology of the rebounding droplet. Moreover, the simulations also show that the total kinetic energy at the rebound moment is the highest in the PCR regime; however, the restitution coefficient or the rebound velocity is lowest in this regime because a larger proportion of the total kinetic energy occurs in the transverse direction. Therefore, a shorter contact time does not imply a higher rebound velocity. [ABSTRACT FROM AUTHOR]

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In this letter, we investigate the rebound dynamics of two equally sized droplets simultaneously impacting a superhydrophobic surface via lattice Boltzmann method (LBM) simulations. We discover three rebound regimes depending on the center‐to‐center distance between the two droplets: a complete‐coalescence‐rebound (CCR) regime, a partial‐coalescence‐rebound (PCR) regime, and a no‐coalescence‐rebound (NCR) regime. We demonstrate that all the rebound regimes are closely associated with dynamic behaviors of the formed liquid ridge or bridge between the two droplets. We also present the contact time in the three regimes. Intriguingly, although partial coalescence takes place, the contact time is still dramatically shortened in the PCR regime, which is even smaller than that of single‐droplet impact. These findings provide new insights into the contact time of multiple‐droplet impact and thereby offering useful guidance for some applications such as anti‐icing, self‐cleaning, and so forth. [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]

[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]

• The multiple droplets impact is compared under symmetrical and asymmetrical impact condition. • Distinct rebound modes are found under symmetrical and asymmetrical impact condition. • The contact time of multiple droplets impact is analyzed under various rebound modes. • The effect of angular momentums on the contact time is revealed for multiple droplets impacts. The rebound dynamics of multiple impinging droplets is investigated and compared under the symmetrical and asymmetrical impact conditions. The result shows that at various relative velocities, when multiple droplets symmetrically impinge macrotextured superhydrophobic surfaces, the rebound modes consist of the left–right non-simultaneous rebound (LRNSR), the coalescence-split simultaneous rebound (CSSR) and the right-left non-simultaneous rebound (RLNSR). With the transition of rebound modes, the contact time is first reduced under the LRNSR mode and then constant under the CSSR and RLNSR modes as the relative velocity is increased. Whereas when multiple droplets asymmetrically impinge macrotextured superhydrophobic surfaces, the rebound modes consist of the left-ridge-right rebound (LRIRR), the left–right-ridge rebound (LRRIR) and LRNSR. When the relative velocity is increased, the contact time is first reduced under the LRIRR mode and then constant under the LRRIR mode. But under the LRNSR mode, the first reduced and then constant contact time appears with increased relative-velocities. [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]