This paper discusses the dynamic behavior of water drops impacting on inclined superhydrophobic surfaces. For a normal impact on a smooth hydrophobic surface, the spreading (or expansion) and retraction dynamics of an impacting drop varies from complete rebound to splashing depending on its Weber number, (We(d)), calculated using the impact speed and diameter d of the drop. For a slanted impact, on the other hand, the impact dynamics depends on two distinct Weber numbers, based on the velocity components normal, (We(nd)), and tangential, (We(td)), to the surface. Impact on superhydrophobic surfaces is even more complicated as the surfaces are covered with micro- to nano-scale texture. Therefore, we develop an expression for an additional set of two Weber numbers, (We(na), We(ta)), which are counterparts to the first set but use the gap distance a between asperities on the textured surface as the characteristic length. We correlate the derived Weber numbers with the impact dynamics on tilted surfaces covered with three different types of texture: (i) posts, (ii) ridges aligned with and (iii) ridges perpendicular to the impact direction. Results suggest that the first two Weber numbers, (We(nd), We(td)), affect the impact dynamics of a drop such as the degree of drop deformation as long as the superhydrophobicity remains intact. On the other hand, the Weber number We(na) determines the transition from the superhydrophobic Cassie-Baxter regime to the fully-wetted Wenzel regime. Accuracy of our model becomes lower at a high tilting angle (75°), due to the change in the transition mechanism.

http://absimage.aps.org/image/MAR16/MWS_MAR16-2015-007553.pdf

Drop impact on inclined superhydrophobic surfaces

Fabricating aluminium surfaces with superhydrophobic and ice-repellent properties present nowadays a challenging task. In this work, multifunctional structures are manufactured by direct laser writing and direct laser interference patterning methods using pulsed infrared laser radiation (1064 nm). Different periodic patterns with feature sizes ranging from 7.0 to 50.0 µm are produced. In addition, hierarchical textures are produced combining both mentioned laser based methods. Water contact angle tests at room temperature showed that all produced patterns reached the superhydrophobic state after 13 to 16 days. In addition, these experiments were repeated at substrate temperatures from −30 °C to 80 °C allowing to determine three wettability behaviours as a function of the temperature. The patterned surfaces also showed ice-repellent properties characterized by a near three-fold increase in the droplets freezing times compared to the untreated samples. Using finite element simulations, it was found that the main reason behind the ice-prevention is the change in the droplet geometrical shape due to the hydrophobic nature of the treated surfaces. Finally, dynamic tests of droplets imping the treated aluminium surfaces cooled down to −20 °C revealed that only on the hierarchically patterned surface, the droplets were able to bounce off the substrate.

Fabrication of superhydrophobic and ice-repellent surfaces on pure aluminium using single and multiscaled periodic textures.

Impacting-freezing dynamics of a supercooled water droplet on a cold surface: Rebound and adhesion

Experimental and numerical study on the impact and freezing process of a water droplet on a cold surface

Simulation and experiment on supercooled sessile water droplet freezing with special attention to supercooling and volume expansion effects

Modelling of sessile water droplet shape evolution during freezing with consideration of supercooling effect

Spreading of droplets impacting different wettable surfaces at a Weber number close to zero

Experimental observations, numerical simulations, and theoretical analysis are conducted to investigate the impacting dynamics of water droplets on spherical surfaces. A volume of fluid numerical model coupled with a dynamic contact angle model with consideration of the gravity effect is established and validated by comparing the evolutions of droplet profiles and spreading factors obtained from the simulations and the experiments in both the present work and literature. The effects of the Weber number, contact angle, and sphere-to-droplet diameter ratio (D*) on the droplet impacting on a spherical surface are further studied by numerically calculating the spreading factor and the spreading arc angle corresponding to the two-dimensional wetting arc at the maximum spreading state. The results indicate that both the maximum spreading factor and arc angle increase with increasing Weber number and reducing contact angle. When the sphere-to-droplet diameter ratio is reduced, the maximum spreading factor remains almost unchanged for D∗≳10 but it shows a significant increase for D∗<10. The maximum spreading arc angle keeps going up with reducing diameter ratio under all conditions even for D∗≳10. As the Weber number increases and the contact angle decreases, the effect of the diameter ratio on the maximum spreading becomes more conspicuous. Based on the energy conservation, a theoretical model considering the gravity effect is developed to describe the maximum spreading factor of an impacting droplet on a spherical surface. The maximum spreading factors obtained from the theoretical model yield a deviation of ±15% as compared with those from the experiments and simulations.

Xin Liu

Papers

Impacting-freezing dynamics of a supercooled water droplet on a cold surface: Rebound and adhesion

Simulation and experiment on supercooled sessile water droplet freezing with special attention to supercooling and volume expansion effects

Modelling of sessile water droplet shape evolution during freezing with consideration of supercooling effect

Spreading of droplets impacting different wettable surfaces at a Weber number close to zero

Maximum spreading of droplets impacting spherical surfaces