Influence of superhydrophobic area occupancy and impact angle on the water entry dynamics of spheres

Abstract

In this work we reported a kind of deflecting air cavity generation strategy through controlling the superhydrophobic (SHB) area occupancy and the impact angle of water entry. The influence of SHB area occupancy and impact angle on the water entry dynamics and drag reduction characteristics of spheres are also explored through both experimental and theoretical analysis. For a hemispherically-coated sphere, it is found that the formed air cavity would reach a maximum deflecting angle when the impact angle of water entry is 90º. With the increasing impact angle, the deflection displacement of hemispherically-coated sphere in the horizontal direction first increases and then decreases. When the impact angle is 90{degree sign}, the deflection displacement reaches the maximum. The deflection displacement of SHB region-modulated sphere in the horizontal direction has the same variation trend. Moreover, the SHB region-modulated sphere exhibits different air cavity morphologies (no cavity, transition state seal, deep seal, surface seal) at different impact velocities for impact angles of 0{degree sign} and 180{degree sign}. The air cavity pinch-off depth and pinch-off time first increase and then stabilize as the SHB area occupancy increases, regardless of whether the impact angle is 0{degree sign} or 180{degree sign}, and the value of (Zpinch-Zp)/Zpinch is not affected by the SHB area occupancy (α ~ 0{degree sign}). Finally, we demonstrate that SHB region-modulated sphere all move faster than the original spheres, and the sphere with a SHB area occupancy of 0.25 (α ~ 180{degree sign}) is able to reduce the drag reduction coefficient to 0.055.

Suppressed Droplet Splashing on Positively Skewed Surfaces for High‐Efficiency Evaporation Cooling

“…It has been shown that corona splash depends on the dynamics of the escaping air beneath the extended sheet. [ 35,36,50 ] The interactions of compressed gas between the liquid sheet and microstructured surfaces are a key determinant of droplet instability in prompt splashing and corona splashing. The discrete micropillars are sufficient to wettability the liquid sheet into the spaces between the solid structures for positively skewed surfaces and it flows along the channel consisting of discrete micropillars.…”

Functional Microtextured Superhydrophobic Surface with Excellent Anti-Wear Resistance and Friction Reduction Properties

“…Superhydrophobic (SHB) surfaces with excellent water repellency and self-cleaning ability are widely attractive in drag reduction, − energy saving, stain resistance for wearable clothing, − microfluidics, and biomedicine fields. , However, it is still a challenge to achieve continuous and durable anti-liquid ability on the SHB surfaces because of their poor mechanical properties and resistance to wear . Under the external pressure and contact load, the prepared multi-scale micro/nano-structures would be easily removed during the friction process, − and the SHB coating would also be destroyed because of low adhesion to the base .…”