Droplet Impact on Asymmetric Hydrophobic Microstructures

Abstract

Textured hydrophobic surfaces that repel liquid droplets unidirectionally are found in nature such as butterfly wings and ryegrass leaves and are also essential in technological processes such as self-cleaning and anti-icing. In many occasions, surface textures are oriented to direct rebounding droplets. Surface macrostructures (>100 μm) have often been explored to induce directional rebound. However, the influence of impact speed and detailed surface geometry on rebound is vaguely understood, particularly for small microstructures. Here, we study, using a high-speed camera, droplet impact on surfaces with inclined micropillars. We observed directional rebound at high impact speeds on surfaces with dense arrays of pillars. We attribute this asymmetry to the difference in wetting behavior of the structure sidewalls, causing slower retraction of the contact line in the direction against the inclination compared to with the inclination. The experimental observations are complemented with numerical simulations to elucidate the detailed movement of the drops over the pillars. These insights improve our understanding of droplet impact on hydrophobic microstructures and may be useful for designing structured surfaces for controlling droplet mobility.

Spreading, Breakup, and Rebound Behaviors of Compound Droplets Impacting on Microstructured Substrates

“…In Figure , we show that rescaling β max (i.e., normalR normale * ( β max 2 − β 0 2 ) 1 / 2 ), collapses our data for a wide range of α (0.3≤ α ≤ 0.9) values and various values of pillar densities (0.3≤ ρ p ≤ 0.9), which fairly agrees with the universal scaling shown in for smooth substrates. It is very interesting to see that numerical results of rescaled β max for pillar density ρ p = 0.9 are in great agreement with the theoretical model shown in eq for flat surfaces, which has also been confirmed in previous studies for the impact of pure droplets . Although correction reduces the gap between different curves, it does not completely collapse the data for smaller pillar densities, suggesting that effects such as pillar density need to be captured and are therefore subjected to future study.…”

“…It is very interesting to see that numerical results of rescaled β max for pillar density ρ p = 0.9 are in great agreement with the theoretical model shown in eq 6 for flat surfaces, which has also been confirmed in previous studies for the impact of pure droplets. 66 Although correction reduces the gap between different curves, it does not completely collapse the data for smaller pillar densities, suggesting that effects such as pillar density need to be captured and are therefore subjected to future study.…”

Dynamics Behavior of Droplet Impact on a Controllable Curved Micropillar Array Surface Induced by a Magnetic Field

“…Therefore, the accurate prediction and control of the impact behavior of the droplet are of great significance and have attracted much interest. A number of the factors affecting the impact behaviors, such as impact velocity, impact angle, droplet size, surface structure, − surface properties, , and liquid properties, have been investigated. Some models were proposed to predict the change in the impact morphologies of impact products. ,− However, most of the prediction models were for a droplet impact on the superhydrophobic surfaces with symmetric structures.…”

Directional Rebound of Microdroplets on a Magneto-Responsive Micropillar Array Surface

“…The contact time of the droplet and the lateral displacement mechanism were also investigated. Yada et al prepared a superhydrophobic surface with the inclined micropillar array and realized the directional rebound of droplets with a diameter of 2.28 mm. The mechanism of the asymmetric structure on the droplet during directional rebound was studied.…”