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.

Regulation over the generation of the Leidenfrost phenomenon in liquids is vitally important in a cutting fluid/tool system, with benefits ranging from optimizing the heat transfer efficiency to improving the machining performance. However, realizing the influence mechanism of liquid boiling at various temperatures still faces enormous challenges. Herein, we report a kind of microgrooved tool surface by laser ablation, which could obviously increase both the static and dynamic Leidenfrost point of cutting fluid by adjusting the surface roughness (Sa). The physical mechanism that delays the Leidenfrost effect is primarily due to the ability of the designed microgroove surface to store and release vapor during droplet boiling so that the heated surface requires higher temperatures to generate sufficient vapor to suspend the droplet. We also find six typical impact regimes of cutting fluid under various contact temperatures; it is worth noting that Sa has a great influence on the transform threshold among six impact regimes, and the likelihood that a droplet will enter the Leidenfrost regime decreases with increasing Sa. In addition, the synergistic effect of Sa and tool temperature on the droplet kinetics of cutting droplets is investigated, and the relationship between the maximum rebound height and the dynamic Leidenfrost point is correlated for the first time. Significantly, cooling experiments on the heated microgrooved surface are performed and demonstrate that it is effective to improve the heat dissipation ability of cutting fluid by delaying the Leidenfrost effect on the microgrooved heated surface.

In this study, we fabricated two types of functional surfaces with the same roughness (Sa) but entirely opposite surface morphological features, which are defined as the positively skewed surface filled with protruding cylinder array (Ssk > 0) and the negatively skewed surface filled with circular pit array (Ssk < 0). The effect of surface morphology peak-valley features on droplet splash is analyzed, and the formation mechanism of the prompt splash and corona splash is also indicated based on the Kelvin–Helmholtz instability. Our results demonstrate that, under the same roughness conditions of Sa, the interaction between the liquid lamellae and the thin air layer is much stronger on the negatively skewed surface, which would inhibit droplet spreading and promote the generation of droplet splash. Increasing the depth of microstructures, resulting in more pronounced peak-valley features, has been found to facilitate both prompt and corona splash phenomena to some extent. Additionally, it is found that the ease of splash formation on each surface is related to the initial spreading speed variation, with the degree of reduction in the initial spreading speed indirectly reflecting the instability of the liquid lamellae. The findings from our study contribute to the development of advanced surface engineering strategies for controlling droplet splash and enhancing the performance of various industrial applications.

Reducing fluid frictional drag at the solid–liquid interface is a promising strategy for improving the hydrodynamic properties of the structure in water, though so far it has remained unattainable without robust air cavities. Herein, we report a durable generation strategy of robust air cavity on the rough microstructured surface, which could achieve stable drag reduction even after 2000th water entry test. It is worth noting that the generation strategy is almost independent of the wear of surface microstructure, as the worn microstructures still keep a rough morphology and would alter the capillary driving force and prevent the spreading of the liquid film along the structure body. Therefore, the triple contact line is pinned at the solid–liquid interface and induces the generation of a complete air cavity. Comprehensive evaluation, including the mechanical and chemical stability tests, confirm that the microstructured spheres could produce robust cavities even after harsh destruction, and they also reduce the hydrodynamic drag by more than 70.8% at a higher Reynolds number of ∼4.9 × 104. Finally, the boundary slip at the solid–liquid interface of the microstructured surface is simulated, which concludes that the decrease in the contact angle at air–liquid interface and fraction of solid–liquid contact area on the wall would enhance the slip length of fluid, thus resulting in an obvious decreasing of frictional resistance at the solid–liquid interface. We believe that the present work provides a perspective on the sustainable construction of the robust cavity which may have important potential application value in the field of drag reduction.

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.