Water entry dynamics of rough microstructured spheres

Abstract

In this work we proposed a facile underwater air cavity generation strategy based on rough microstructured spheres and explored its water entry dynamics and drag reduction characteristics. Under the assistance of microstructures, the three-phase contact line is pinned near the sphere equator and inhibits the wetting of liquid film along the sphere surface so that leading the formation of air cavity. The water entry process is mainly divided into four stages: flow formation, cavity opening and stretching, cavity closure and entrapment, and cavity collapse. With the Froude number Fr, the pinch-off depth of air cavity obviously increases and the pinch-off time is also delayed, which contributes to the formation of a longer bottom air cavity. In addition, the spheres with a larger impact velocity would fall faster in water during initial falling period, while the terminal velocities are nearly the same for all the spheres when they are in a stable falling period. It is worth noting that for a same sphere the larger impact velocity could not only contribute to the formation of a longer air cavity, but also makes the generated air cavity keep in a stable and streamlined shape at different underwater depth, which is vitally important for achieving continuous drag reduction. Finally, we demonstrated numerically that the stable streamlined sphere-in-cavity structure could reduce the hydrodynamic resistance levels up to 91.3% at Re ~ 3.12×104, which is related to the boundary slip caused by an air layer trapped in the microstructures.

Armored Superhydrophobic Surfaces with Excellent Drag Reduction in Complex Environmental Conditions

“…To further demonstrate that ASSPs can still achieve drag reduction properties after 24 h of immersion in organic solvents and corrosive solutions, we introduced dimensionless drag coefficients C D to characterize the spheres' kinematic properties (Supporting Information S2). The equation for the drag coefficient of the sphere-in-cavity structure is 33,43…”

“…To further demonstrate that ASSPs can still achieve drag reduction properties after 24 h of immersion in organic solvents and corrosive solutions, we introduced dimensionless drag coefficients C D to characterize the spheres’ kinematic properties (Supporting Information S2). The equation for the drag coefficient of the sphere-in-cavity structure is , C D = 8 g ( m − ρ V normalc ) ρπ D 2 U normalt 2 where m is the mass of the sphere, D is the maximum diameter of the sphere-in-cavity structure, ρ is the density of water, U t is the terminal velocity of the sphere, and V c is the total volume of the sphere-in-cavity structure (including the volume of the sphere). The volume of the sphere-in-cavity structure of the original sphere is 1 because there is no air-cavity entrapment when it enters the water, while the volume of the sphere-in-cavity structure remains in a stable state overall for ASSPs immersed in corrosive solutions (Figure e), although the volume of the sphere-in-cavity structure changes, indicating that the armored superhydrophobic surface has excellent stability.…”

“…Extensive research work has focused on drag reduction and various strategies have been developed to construct an air layer on a structural body’s surface so as to produce significant slip in contact with fluid flow, reducing the shear forces at the boundary and producing drag reduction properties. − Vakarelski et al used the Leidenfrost effect to completely encase a metal sphere falling into a liquid in a stable streamlined air cavity, which eliminates contact between the surface of the sphere and liquid, and the drag coefficient of the streamlined sphere-in-cavity structure is approximately 0.02–0.03. − Liu et al conducted an experimental study on the drag reduction performance of mixed solutions of polymers and surfactants, and the experimental results showed that the mixed solutions could inhibit the generation of vortices more effectively and improve the drag reduction efficiency . Wang et al investigated the mechanism of microstructured spheres to reduce the critical velocity of air cavity entrainment, analyzed the effect of surface morphology effect on hydrodynamic properties, and further revealed the mechanism of surface morphology effect-induced drag reduction in underwater air cavities by numerical simulation. − Liao et al used femtosecond lasers to prepare superhydrophobic glass microchannels with drag-reducing properties, which were able to significantly reduce the flow drag of microfluidics . Zhang et al were inspired by the fine hairs and cilia of living organisms to prepare a hair-like superhydrophobic surface by electrostatic value linting and subsequent surface modification, which has excellent mechanical stability and enables an effective drag reduction effect .…”

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 .…”