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Bouncing droplets on superhydrophobic surfaces is one of the potential methods used for anti-icing. The surface supercooling is a significant parameter influencing the bouncing dynamic. A droplet impacting cold superhydrophobic surfaces is investigated via experimental methods. The influence of the surface supercooling and the Weber number on the impact dynamic is elucidated. Intriguingly, the surface supercooling shows no influence on the spreading time, and the initial retraction time as the heat exchange can be ignored in these processes, while it shows a strong influence on the late retraction time as it can lead to the wetting transformation. To further quantitatively describe the influence of surface supercooling, the relationships of the retraction rate in the late retraction are developed, considering the changes in the receding contact angle caused by the supercooling degree. Finally, the relationship of the contact time is established over a range of Weber numbers (We = 49.37–70.53), surface supercooling (ΔT = 15–24 °C), and droplet sizes (D0 = 2.2–3.2 mm). This work is the first to establish the relationship of the droplet contact time on cold superhydrophobic surfaces, which can provide a quantitative method to calculate the contact time for anti-icing. [ABSTRACT FROM AUTHOR]

Rising nanoscale technologies arouse interest in investigating the impact dynamics of nanodroplets. In this work, the impact of nanodroplets on solid spheres is investigated by the molecular dynamics simulation method, to comprehensively report outcome regimes and reveal the curvature effect, in wide ranges of Weber numbers (We) from 1.5 to 235.8, diameter ratios (λ) of nanodroplets to solid spheres from 0.3 to 5, and contact angles (θ) from 105° to 135°. Five outcomes are identified, including deposition, bouncing, splash, covering, and dripping. The former three outcomes are found in the high diameter ratio range (λ > 1), showing similar dynamic behaviors with impacts on flat surfaces, whereas in the low diameter ratio range (λ ≤)1, splash disappears, and covering and dripping take place additionally. At each contact angle, the outcomes are recorded in λ-We phase diagrams. It is found that the bouncing, splash, covering, and dripping are all promoted by decreasing diameter ratios; in addition, the critical Weber numbers for trigging bouncing and splash increase with decreasing θ. However, the critical We of the boundary between the bouncing to other regimes in the low diameter ratio range is not sensitive to wettability owing to the relatively small diameter of solid spheres. For quantitatively describing the curvature effect, the boundaries between the deposition and bouncing regimes in the high diameter ratio range and between the bouncing and other regimes in the low diameter ratio range are established. Both the established models show satisfactory agreement with the boundaries in the phase diagrams. [ABSTRACT FROM AUTHOR]

Bouncing droplets on solid surfaces is of great significance in diversified applications such as anti-icing and self-cleaning. It is important to establish a unified model to predict whether an impacting droplet can rebound from a surface or not. This work focuses on the rebound dynamic of a droplet impacting a hydrophobic surface via theoretical methods. Based on energy conservation, a new theoretical model to predict the rebound behavior of an impacting droplet is established. For an ideal surface, the contact angle hysteresis Δθ can be ignored and the rebound condition is θ ≥ θc,i, where θ is the equilibrium contact angle and θc,i is the critical rebounding contact angle (CRCA) of an ideal surface. For a real surface, Δθ is considered and the rebound condition is θr ≥ θc,r, where θr is the receding contact angle and θc,r is CRCA of a real surface. Especially, when Δθ is not large enough, the rebound condition for a real surface can be expressed as θr ≥ θc,i. This work is the first to establish the theoretical model considering both the energy dissipation throughout the impact process and the contact angle hysteresis, which shows a higher consistency with the previous works. [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]

This work presents a study of a droplet impacting an inclined hydrophobic surface using lattice Boltzmann method (LBM) simulation. The influence of surface inclination, surface wettability, and the Weber number on the dynamic of spreading and receding is elucidated. Intriguingly, the contact time is independent of the surface inclination. The surface wettability and Weber number do not affect the spreading time, while they significantly influence the receding dynamic. To further quantitatively describe the influence of surface wettability and Weber number, scaling laws of the receding rate are established as Vret ∼ (1 − cos θ)−0.25 and Vret ∼ Wen0.19, and, thus, scaling laws of the receding time are established as tr ∼ (1 − cos θ)−0.5 and tr ∼ Wen0.1, respectively, where Wen is the normal Weber numbers and θ is the contact angle. Based on this, a relationship of the contact time for a droplet impacting an inclined hydrophobic surface is ultimately established as tc = 3.1(ρR03/σ)1/2 (1 − cos θ)−1/2Wen0.1, where ρ, R0, and σ denote the droplet density, radius, and surface tension, respectively. This study provides a quantitative relationship to calculate the contact time of a droplet impacting an inclined hydrophobic surface, which can simultaneously efficiently evaluate the anti-freezing, anti-icing, and self-cleaning performance of hydrophobic surfaces employed in practical applications. [ABSTRACT FROM AUTHOR]

The impact dynamics of water nanodroplets on flat solid surfaces was studied by molecular dynamics simulations over a wide range of Weber numbers (We) and surface wettability (θ0), where θ0 is the Young contact angle. A phase diagram in the parameter space of We vs θ0 was established accommodating eight impact outcomes noted in the final stage of impact, with three of them, holes rebound, partial-rebound splash, and rebound splash, for the first time being identified and reported. The eight impact outcomes were classified into three categories, i.e., non-bouncing, bouncing, and splash. The results show that the splash is triggered only when Wecr > 140. The boundaries separating bouncing from non-bouncing were determined based on the phase diagram. When θ0 > 160°, the boundary is described as Wecr = a ≪ 1; when 110° < θ0 < 160°, the boundary depends on both We and θ0, with a larger We required to trigger bouncing on a less hydrophobic surface, expressed as Wecr = b + ccosθ0; when θ0 < 110°, bouncing never takes place, and hence, the boundary is determined only by the critical contact angle, expressed as θ0,cr = 110°. Here, a, b, and c are constants. [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]

• 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]