Impact Dynamics of Nanodroplets on V-Shaped Substrates: Asymmetrical Behavior and Fast-Rebound Dynamics

Vibration-Induced Pancake Bouncing of Impacting Droplets on Hydrophobic Surfaces

Rapid detachment of impacting droplets from solid surfaces is fundamentally interesting and important in many practical applications, including self-cleaning, anti-icing, and energy harvesting. The droplet pancake bouncing is strongly preferred for the reduced contact time and high bouncing velocity. However, the trigger conditions for pancake bouncing are rigorous. Driven by this, this work circumvents the limitations via solid surface vibration. Our results show that the impacting droplet patterns are highly sensitive to the surface vibration parameters (i.e., the vibration amplitude and frequency), and only a reasonable design of vibration parameters enables the impacting droplets to bounce in a pancake shape without the contraction stage. Intriguingly, the pancake bouncing induced by surface vibration exhibits a significant reduction of solid-liquid contact time (up to ≈80%) compared to the traditional bouncing pattern, and the bouncing velocity has been dramatically enhanced. Furthermore, the critical conditions for the pancake bouncing on the hydrophobic surface and the underlying dynamic mechanism are also identified. This work provides an effective method to achieve well-controlled pancake bouncing of impacting droplets for extensive applications.

Impact of Droplets on Surfaces Designed with Wettability-Gradient Properties: Directional Migration, Oblique Rebound, and Reduced Contact Time

Efficiently regulating the rebound behavior of droplets post-impact is crucial for various fields, mainly including the development of self-cleaning applications, the design of surface functional materials, and the advancement of industrial techniques. By performing molecular dynamics simulations, we investigated the impact and jumping behavior of droplets on heterogeneous substrates with different wetting regions. We found that, during the impacting evolution process, the retracted droplets would move toward regions with stronger wettability due to the unbalanced force caused by the wettability difference, revealing the directional migration ability. The values of the wettability difference strongly affect the degree of oblique rebound and contact time when droplets can jump off the substrate. We then designed the surfaces with a wettability gradient and found that the oblique rebound angle could be well controlled and the contact time further reduced. Our findings may provide valuable insight into the relationship between the wettability gradient and the behavior of liquid droplets on surfaces, with broad implications for various fields such as surface engineering, materials science, microfluidics, etc.

Coalescence-Induced Jumping for Removing the Deposited Heterogeneous Droplets: A Molecular Dynamics Simulation Study

The removal of the deposited droplets on a solid surface is crucial to considerable practical applications that require self-cleaning properties. In this work, a strategy of cleaning a deposited droplet ("D-droplet") by coalescing with a heterogeneous and easily jumping droplet ("J-droplet") is proposed. Molecular dynamics simulation studies have shown that the coalescence of these two kinds of droplets would not guarantee the removal of D-droplet, unless the lifting ability of J-droplet is enhanced through the reduction of the solid-liquid interaction. However, this is a bad scenario with low efficiency. Further investigation suggests that by introducing two J-droplets to produce triple-coalescence dynamics, the D-droplet could be successfully jumping from the substrates due to the coalescence-induced effect, which is also verified by the free energy calculation. Moreover, the effects of the size of the droplets and the arrangement mode of these three droplets on the jumping dynamics are both considered. The studies not only help advance our understanding of coalescence-induced jumping of heterogeneous droplets, but also open up new ways to remove the deposited impure droplets, which is expected to guide the fields of self-cleaning.

Molecular Dynamics Simulation of Nanodroplets Impacting Stripe-Textured Surfaces

The dynamic behavior of droplets impacting on textured surfaces has an important influence on many engineering applications, such as anti-icing and self-cleaning. However, the mechanism and law of the effect of textured surfaces on the impact behavior of nanodroplets has not been fully revealed yet. In this paper, the molecular dynamics (MD) method is used to model the dynamic behavior of nanodroplets after impacting the solid surface with a striped texture. The influences of texture gap and texture angle on the real contact area, spreading factor, contact time, and bounce velocity of the droplet after impact are also quantitatively analyzed. It is shown that the striped texture produces significant anisotropy in the spreading and contraction behavior of nanodroplets after impact, and the anisotropy is more pronounced on the ridged texture surface than on the grooved texture surface. In addition, we find that the texture gap has little effect on the dynamic behavior of nanodroplets impacting the textured surface. However, as the bottom angle of the texture increases, the real contact area and bounce velocity of the nanodroplet increase significantly, while the contact time and spreading factor decrease. This work further elucidates the characteristics and mechanisms of nanodroplets impacting on stripe-textured surfaces and provides a theoretical basis for the design of nanostructured surfaces in relevant applications.