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Droplet impact on cylindrical surfaces is a widespread phenomenon observed in both nature and industrial applications. The wettability and size of the cylinders, and the initial impact velocity, are expected to influence the impact dynamics, such as generating different droplet morphology, spreading/retracting processes, and contact time, which could finally determine the anti-icing property of power transmission lines. A particle-based mesh-free numerical approach, many-body dissipative particle dynamics, is employed to study the impact dynamics. The cylinders with three different sizes and three types of wettability are modelled, and different initial impact velocities are set. The simulations show the two liquid films (or lamellae) formed after impact can coalesce at the bottom site of a hydrophilic cylinder while they keep separate on hydrophobic and superhydrophobic cylinders. The maximum spreading diameter in the axial direction increases with the increase of impact velocity and cylinder size. Besides, the distribution of droplet detachment mode shows a strong dependence on initial impact velocity and cylinder size. In bouncing mode, the contact time increases with the increase of impact velocity while in dripping/bouncing mode, it has an opposite trend. The splashing angle is also observed to be dependent on the cylinder size. [ABSTRACT FROM AUTHOR]

The droplet anisotropic wetting on chemically heterogeneous stripe-patterned substrates after impact is studied by using many-body dissipative particle dynamics in this work. After a low-velocity impact on substrates with different length ratio (ratio of stripe width to the initial droplet size) and Cassie area fraction, the droplet can show various shapes and contact angles in parallel and orthogonal directions to the stripes. The elongation of the droplet can be increasingly evident when increasing the length ratio. Also, the contact angles at both directions follow the Cassie-Baxter predicted values well at low length ratio while deviate from them at high length ratio. Both the capillary and kinetic effects have a significant influence on the anisotropic wetting. When impact with higher velocity, the droplet leaves some residues on the hydrophilic stripes of the substrates. Surprisingly, the residues for substrates with certain length ratio (≤0.316) are distributed in circular regions with almost identical radii, indicating the spreading stages of them are independent on the surface properties and dominated by the kinetic effect. When the droplet reaches a stable state, the main body shows anisotropic wetting behavior, indicating the capillary effect dominates the wetting instead of the kinetic effect at the retraction stage. Unlabelled Image • Combined effect of length ratio, Cassie area fraction and initial kinetic energy is considered in spreading on substrates. • Critical length ratio is identified to separate the anisotropy and isotropy of wetting. • The droplet shows no anisotropic effect in spreading stage when given a high initial impact velocity. • A simple algorithm is proposed to calculate the contact angle of the droplet with distorted contact line. [ABSTRACT FROM AUTHOR]

The lateral motion of a droplet after its impact on groove-patterned superhydrophobic surfaces is numerically studied in the present work. Different from previous studies, the wettability gradient is not required to produce lateral motions. Instead, a substrate with two halves decorated with same Cassie area fractions but different length ratios is used and four different motions can be observed by varying the impact velocity. The asymmetrical penetration into the grooves of the substrate is found to be the main reason for triggering the different motions. The penetrated liquid could bring upward momentum to lift the droplet from one side of the substrate, or it could block the retracting contact line from one side. As a result, different oblique motion components are generated and added to the vertical rebound motion, finally leading to the different lateral motions. Our simulation findings provide a fresh idea for the control of droplet motion by coating the solid surfaces. [ABSTRACT FROM AUTHOR]

Superhydrophobic surfaces have attracted much attention due to their excellent water-repellent property. In the present study, droplets in the ideal Cassie state were focused on, and a particle-based numerical method, many-body dissipative particle dynamics, was employed to explore the mechanism of droplets impact on textured surfaces. A solid–fluid interaction with three linear weight functions was used to generate different wettability and a simple but efficient method was introduced to compute the contact angle. The simulated results show that the static contact angle is in good agreement with the Cassie–Baxter formula for smaller ∅ S and F a , but more deviation will be produced for larger ∅ S and F a , and it is related to the fact that the Cassie–Baxter theory does not consider the contact angle hysteresis effect in their formula. Furthermore, high impact velocity can induce large contact angle hysteresis on textured surfaces with larger ∅ S and F a . The typical time-based evolutions of the spreading diameter were simulated, and they were analyzed from an energy transformation viewpoint. These results also show that the dynamical properties of droplet, such as rebounding or pinning, contact time and maximum spreading diameters, largely depend on the comprehensive effects of the material wettability, fraction of the pillars and impact velocities of the droplets. [ABSTRACT FROM AUTHOR]

The present work numerically studies the dynamics of a droplet impact on solid spheres with different sizes and surface wettability. Different initial impact velocities are also set to the droplet to consider the velocity effect. A particle-based numerical method, many-body dissipative particle dynamics (MDPD), is employed to perform the simulations. The simulation results show that the post-impact droplet morphology highly depends on the impact velocity, size and surface wettability of solid spheres. At lower impact velocities, the droplet can sit on the top of the solid surfaces and keep a semi-sphere shape; while at higher impact velocities, for smaller solid spheres, the droplet can fully wrap the spheres and even drip off. For impact on larger spheres, on the hydrophilic surfaces, the final stable liquid film thickness is highly velocity-dependent, while on the hydrophobic surfaces, the droplet always sits on top of the sphere with the same liquid thickness. [ABSTRACT FROM AUTHOR]

Droplet impact on non-flat surfaces is a common phenomenon, further understandings of this phenomenon can guide a rational design of relevant applications in daily life. The present work numerically studies droplet impact on superhydrophobic solid spheres of different sizes by using many-body dissipative particle dynamics. The simulation results indicate that the time-dependent wrapping angle highly relies on the initial impact velocity and solid sphere size. Four spreading types, bouncing, wrapping, dripping, and bouncing & dripping, can be identified under different impact conditions. It is found that bouncing type prefers to happen at low impact velocity while dripping type is more likely to happen at high impact velocity. Further, on a solid sphere with small size, the dripping type happens at a large range of impact velocity while bouncing type occupies the large range on sphere surface with large size. The contact time is found much shorter for dripping cases than bouncing cases for all sphere sizes. [Display omitted] [ABSTRACT FROM AUTHOR]

Overhead transmission lines are the most key component in power transmission systems, the bare transmission lines with stranded conductors are the main type for long-range power transmission. However, the long range makes the transmission lines easily be affected by rain icing problem in winter or cold regions, therefore the understandings of how the rain droplet interacting with surfaces of this kind of transmission lines are significant. By using many-body dissipative particle dynamics, the present work simulates the droplet impact on the stranded conductors with varying surface properties, such as surface wettability, number of strands, and twist pitch. The simulation results show that hydrophilic surfaces can retain most of the liquid while the superhydrophobic ones can repel most liquid after impact. A large number of strands, which means a large size of grooves, will force the liquid to flow along the grooves after impact. In this circumstance, grooves on surfaces with short twist pitch can guide the liquid to flow to a more downward direction, while grooves on surfaces with long twist pitch can guide the liquid flow closer to the axial direction of the transmission line and retain more liquid on the surface after impact. Besides, for bouncing cases, longer twist pitch can cause longer contact time. [ABSTRACT FROM AUTHOR]