Superhydrophobic Spheres Research Articles

Visualization study on the dynamic behavior of shear thinning droplets impacting superhydrophobic spheres

The surface and droplet characteristics significantly affect the dynamic of droplet impact on solid surfaces. In present work, a platform is utilized to experimentally study the dynamic behavior of shear thinning fluid droplets impacting superhydrophobic spheres, where the effects of different concentrations, Weber numbers (We), particle size ratios on droplets impact are studied. The results showed that on the superhydrophobic sphere, as the shear thinning droplet concentration increases, the diffusion diameter increases, and the contact time decreases. For the same curvature, the contact time between the droplet and the sphere decreases with the increase of the We. When the We reaches 75 or above, the contact time remains constant with an increase in the We. For various curvature conditions, the contact time increases as the curvature increasing. When the diameter ratio of the sphere to the droplet is D∗≥6, the maximum diffusion coefficient remains almost constant. However, when D∗<6, the maximum diffusion dimensionless diameter significantly increases as the particle size ratio decreases. More importantly, a theoretical model considering gravity effect is presented to predict the maximum dimensionless diameter of shear thinning droplets on superhydrophobic spheres according to energy conservation. The predicted results are rationally agreement with experiments with the deviation within ±10 %.

Resurrection of a superhydrophobic cylinder impacting onto liquid bath

An interesting resurrection phenomenon (including the initial complete submersion, subsequent resurfacing and final rebounding) of a superhydrophobic sphere impacting onto a liquid bath was observed in experiments and direct numerical simulations by Galeano-Rios et al. (J. Fluid Mech., vol. 912, 2021, A17). We investigate the mechanisms of the liquid entry for a superhydrophobic cylinder in this paper. The superhydrophobic cylinder, commonly employed as supporting legs for insects and robots at the liquid surface, can exhibit liquid-entry mechanisms different from those observed with the sphere. The direct numerical simulation method is applied to the impact of a two-dimensional (2-D) superhydrophobic cylinder (modelled as a pseudo-solid) onto a liquid bath. We find that for the impacting cylinder the resurrection phenomenon can also exist, and the cylinder can either rebound (get detached from the liquid surface) or stay afloat after resurfacing. The cylinder impact behaviour is classified into four regimes, i.e. floating, bouncing, resurrecting (resurrecting-floating and resurrecting-bouncing) and sinking, dependent on the Weber number and the density ratio of the cylinder to the liquid. For the regimes of floating and bouncing, the force analysis indicates that the form drag dominates the motion of the cylinder in the very beginning of the impact, while subsequently the surface tension force also plays a role with the contact line pinning on the horizontal midline of the cylinder. For the critical states of the highlighted resurrecting regime, our numerical results show that the rising height for the completely submerged cylinder of different density ratios remains nearly unchanged. Accordingly, a relation between the maximum ascending velocity and the density ratio is derived to predict whether the completely submerged cylinder can resurface.

Phase diagram for nanodroplet impact on solid spheres: From hydrophilic to superhydrophobic surfaces

The impact of droplets on solid surfaces is a crucial fluid phenomenon in the additive industry, biotechnology, and chemistry, where controlling impact dynamics and duration is essential. While extensive research has focused on flat substrates, our understanding of impact dynamics on curved surfaces remains limited. This study seeks to establish phase diagrams for the process of droplet impact on solid spheres and further quantitatively describe the effect of curvature through theoretical analysis. It aims to determine the critical conditions between different impact outcomes and also establish a scaling relationship for the contact time. Here, the post-impact outcome regimes occurring for a wide range of Weber numbers (We) from 1.2 to 173.8, diameter ratio (λ) of solid spheres to nanodroplets from 0.25 to 2, and surface wettability (θ) from 21° to 160°, through the molecular dynamics simulation method (MD) and theoretical analysis. The MD simulations reveal that the phase diagrams of droplet impacts on hydrophilic, hydrophobic, and superhydrophobic spheres differ, with specific distinctions focusing on rebound and three different forms of dripping. Furthermore, a theoretical model based on the principle of energy conservation during impact on superhydrophobic surfaces has been developed to predict the critical conditions between rebound and dripping states, showing good agreement with simulation results. Additionally, a new scaling relationship of contact time for droplet impact on superhydrophobic spherical surfaces has also been established by extending and modifying the existing models, which also agrees well with the simulated results. These insights provide a foundational understanding for designing surface structures.

Study on the construction of durable superhydrophobic coatings using simple spray coating method and their underwater drag reduction performance

Superhydrophobic surfaces have significant applications in industrial and chemical fields. However, their current mechanical stability and underwater durability are insufficient, limiting their widespread application. This study successfully developed a durable superhydrophobic coating suitable for various substrates using a simple one-step spraying technique. By thoroughly investigating the effects of SiO2 concentration and curing temperature on the coating's wettability, underwater durability and mechanical stability, the preparation process was optimized. The prepared superhydrophobic coating achieved a contact angle (CA) of 159.6° and a sliding angle (SA) of less than 1°. In tests of underwater immersion, sandpaper abrasion, repeated water impact, corrosion resistance and ultraviolet (UV) resistance, the coating demonstrated excellent underwater durability, mechanical stability, chemical stability and UV stability. Furthermore, when applied to the steel sphere, the superhydrophobic coating exhibited significant drag reduction effects underwater, reducing the drag coefficient by approximately an order of magnitude and achieving a drag reduction efficiency of 79.1 %. Numerical simulations using STAR-CCM+ showed that the streamlined cavity formed by the superhydrophobic coating effectively suppressed fluid separation, reduced pressure-induced drag, and allowed the superhydrophobic steel sphere to attain a higher terminal velocity. Therefore, the superhydrophobic coating developed in this study has tremendous application potential in underwater drag reduction and is expected to provide effective solutions for energy-saving and drag reduction in fields such as maritime navigation.

A Nusselt number correlation for a superhydrophobic solid sphere encapsulated in a perfect plastron

A Nusselt number correlation for a superhydrophobic solid sphere encapsulated in a perfect plastron

Characteristics of entrapped jet and cavity instability following the water entry of a superhydrophobic sphere

Following the impact of a superhydrophobic sphere on water, the formation of a stable-streamlined gas cavity undergoes several stages characterized by impact load slamming, cavity flow forming, cavity collapse, and unstable cavity. The former three stages have been extensively investigated. However, limited attention has been given to the post-collapse instability of the gas cavity. In the present study, a systematic experiment is conducted to investigate the evolution of the cavity subsequent to the water entry of a superhydrophobic sphere. The pattern of cavity collapse significantly influences the dynamic characteristics of the downward liquid jets entering the cavity. The liquid jets entrapped in the cavity behave as liquid films composed of tiny individual cavities generated with cavity collapse. Deep sealing with a relatively low Weber number consistently promotes the onset of jet penetration, whereas surface sealing results in droplet penetration. The repulsion of entrapped liquid films from the inner rim of the sphere plays a pivotal role in stimulating the periodic instability experienced by the descending gas cavity. The obtained conclusions shed light on the dynamics of the liquid films entrapped in a gas cavity and the cavity stability.

Synthesis of superhydrophobic carbon sphere by pyrolysis of heavy oil fraction of coal tar/ferrocene mixture and their magnetic nanostructure

This present work describes the ferrocene-catalyzed conversion of heavy oil fraction of coal tar to carbon sphere aggregate films in the form of necklace-like carbon spheres, carbon sphere‑carbon nanotube composite, and pomegranate-like carbon spheres by chemical vapor deposition (CVD). Necklace-like carbon spheres exhibit hydrophobic surfaces, whereas pomegranate-like carbon spheres-CNT composites and pomegranate-like carbon spheres display superhydrophobic surfaces. The as-prepared hydrophobic and superhydrophobic surfaces were carefully characterized by Scanning Electron Microscopy (SEM), High-Resolution Transmission Electron Microscopy (HRTEM), Atomic Force Microscopy (AFM), and Magnetic Force Microscopy (MFM) to confirm the effect of geometric carbon aggregates with thermally decomposed ferrocene (FeCp2) catalysts on their magnetic nanostructure. The surface chemistry as well as oxidation resistance was studied by Fourier Transform Infrared spectroscopy (FTIR), Raman spectroscopy, and Thermal Gravimetric Analysis (TGA). FeCp2 catalyst concentration in the heavy oil fraction of coal tar was observed to have a direct correlation with morphology, disordered level of graphitic structure, magnetic domain pattern arrangement, and thermal oxidation resistance of as-produced carbon sphere aggregate.

Construction of a robust MOF-based superhydrophobic composite coating with the excellent performance in antifouling, drag reduction, and organic photodegradation

The superhydrophobic coating blocks a certain air layer between the solid surface and the liquid, thereby changing the interaction between the surface and water, and has potential applications in enhancing buoyancy and drag reduction. Consequently, a superhydrophobic coating designed to reduce drag (the contact angle was up to 168°) was prepared in this study. The coating was applied to copper mesh by spraying a mixed solution comprising ZIF-8 particles, waterborne polyurethane, and a silane coupling agent, resulting in a significant enhancement of load capacity. The superhydrophobic coating can change the contact mode between the copper mesh and water, leading to a remarkable sevenfold increase in the maximum load capacity when compared with the uncoated sample, primarily due to its' increased buoyancy. In addition, the self-made superhydrophobic model ship exhibited an impressive drag reduction efficiency of up to 36 %, indicating that it has significant drag reduction capability. It's worth noting that once water enters, the superhydrophobic spheres will form a gas cavity, which is critical for the coating's drag reduction capability. The self-cleaning property of this superhydrophobic composite coating is highly promising. Moreover, it can also purify seawater by utilizing ZIF-8's photocatalytic activity to degrade organic contaminant. Therefore, in the context of actual ships and underwater vehicles, this superhydrophobic coating holds potential as a relevant strategy for enhancing buoyancy and reducing drag, thus offering commercial value.

From waste plastic to artificial lotus leaf: Upcycling waste polypropylene to superhydrophobic spheres with hierarchical micro/nanostructure

Polypropylene (PP) is the second most commonly used plastic in the industry and a key component in materials packaging and medical consumables. Its accumulation has increased due to the shift from reusable to single-use containers and the significant amounts of biomedical waste generated. We are working towards developing a cost-efficient methodology to convert polypropylene (PP) waste into superhydrophobic materials. Our approach involves the chemical treatment of waste PP to create micron-sized spheres (5 ∼ 10 µm) with hierarchical nanostructures and superhydrophobic properties. The produced microspheres show a water contact angle up to 164° and a rolling off angle as low as 2.6°, comparable to natural lotus leaf. The effectiveness of this process has been demonstrated in waste PP from various sources, including surgical masks, syringes, take-away food boxes, and bubble tea cups. The resulting superhydrophobic material has promise for applications in superhydrophobic coating, self-cleaning, and water-oil separation.

Impact and freezing coupling processes of supercooled water droplets on cold superhydrophobic spheres at low Weber numbers

While the previous studies discussed the impact and freezing coupling processes of supercooled water droplets on cold superhydrophobic convex spheres at medium or high Weber numbers (We > 10), the cases on convex and concave spheres at low Weber numbers (We ≤ 10) are numerically studied. A numerical model using the VOF (Volume of Fluid) method and solidification-melting model is established and validated by comparing the calculated results with the experimental data in reference. The influences of the Weber number, supercooling degree, and sphere diameter (characterized by the sphere-to-droplet diameter ratio) on the dynamic behaviors, including the droplet profile, the spreading area and arc angle, and the final outcomes, are explored. As the Weber number increases and the supercooling degree decreases, the impact droplet yields a higher retraction speed and tends to rebound from the cold spheres more easily. The influence of the diameter ratio at low Weber numbers is distinct from that at medium Weber numbers. With the increase of the diameter ratio, the maximum spreading area factor on the convex sphere increases while that on the concave sphere decreases. The final droplet outcome transits from adhesion to rebound when the surface shape continuously changes from concave to convex. The rebound-adhesion morphology maps in terms of the Weber number and diameter ratio are obtained at various supercooling degrees. The rebound region of the convex sphere in the morphology map is wider than that of the concave sphere, and the adhesion region of the morphology map expands with a greater supercooling degree. A smaller diameter ratio yields a smaller critical Weber number between rebound and adhesion regions on the convex sphere but a greater one on the concave sphere. The innovation and optimization of the anti-icing surface may obtain theoretical reference and guidance from the above findings.

Superhydrophobic Sand Repels Water

A key concept in current fluid dynamics and its applications to biology and technology is a phenomenon known as wetting. Wetting is familiar from everyday life and is simply the ability of a liquid to stay in contact with a solid surface. The wettability depends on the properties of the liquid and the solid and can be characterized by the static equilibrium contact angle θ (the angle at which the liquid–gas interface meets the liquid–solid interface). A contact angle below 90° indicates favorable wetting such that a drop of the liquid would spread over a large amount of the flat solid surface, whereas a high contact angle indicates that very little of the solid is wetted (this can be seen in Fig. 1, which shows various stages of surface wetting in terms of the equilibrium contact angle). Nevertheless, this theory generally sounds quite dry or difficult to visualize when explained to students for the first time. The theory of the contact angle also contains some controversies and has undergone some recent developments. We propose a simple classroom demonstration with superhydrophobic sand that gives a concrete visualization of “superhydrophobicity” and outline how the phenomenon can be explained macroscopically with wetting theory. There are several interesting physical effects that are due to superhydrophobicity: experimental studies have found, for example, that superhydrophobic spheres always splash when they impact a body of liquid. In terms of applications, there are various possibilities for water storage with superhydrophobic sand outlined in the chemistry literature.

Interface engineering of superhydrophobic octadecanethiol-functionalized hollow mesoporous carbon spheres for alkaline oxygen reduction to hydrogen peroxide

HMCSs@ODT with a superhydrophobic interface provides an O2-rich surface layer and effectively repels the synthesized H2O2molecules from the catalyst surface, thus improving 2e−ORR performance.

Study of the drag reduction performance on steel spheres with superhydrophobic ER/ZnO coating

Superhydrophobic surface can effectively reduce the pressure drag of the falling sphere, which grants it potential application prospects in drag reduction of underwater vehicle. Herein, a superhydrophobic steel sphere was fabricated by coating a ZnO/epoxy resin solution onto the steel sphere and subsequently reacted with a mixed acetic acid/stearic acid/ethanol solution. The prepared superhydrophobic coating had a contact angle of 159.3 ± 1.7° and a sliding angle below 1°. By comparing the terminal velocity observed in the fall of smooth and superhydrophobic steel spheres, the drag reduction efficiency for superhydrophobic steel spheres can reach up to 19 %. The drag reduction effect of superhydrophobic coating depends strongly on the Reynolds number and the plastron. The plastron directly influences the fluid flow around the surface, thereby reducing the drag. Thus, this superhydrophobic coating may offer a new strategy for drag reduction of underwater vehicles.

A super-hydrophobic and antibiofouling membrane constructed from carbon sphere-welded MnO2 nanowires for ultra-fast separation of emulsion

The discharge of large quantities of oily wastewater has become a serious threat to ecosystem and human health. Superhydrophobic/superhydrophobic under-oil porous materials have received considerable attention for its applications in oil/water mixture separation. However, some challenges still exist, such as low flux, poor stability, and high production cost. Here, a simple vacuum filtration-calcination welding method was used to fabricate super-hydrophobic membranes for the ultra-fast separation of W/O emulsions. Inorganic nanowires MnO2 and carbon spheres were used as the membrane-forming material. Here, carbon spheres not only function as the pore-forming and weld agent to form welded nanowire-constructed framework, but also improve membrane surface roughness. Through this fabrication technique, the resulting composite membrane has a very stable structure. Compared with those of most previously reported advanced oil/water separation membranes, the super-hydrophobic MnO2&Carbon Sphere (MCS) composite membranes show remarkable high separation fluxes and separation efficiencies for various oil phases with different viscosities. In addition, the superhydrophobic membranes show excellent reusability, which is greatly important for long-term oil/water separation applications. In addition, the prepared super-hydrophobic MCS composite membrane shows obvious anti-bacterial properties. The high separation fluxes and long-lasting cycling stability of super-hydrophobic MCS composite membrane make it show great potential in water purification application.

Impact of superhydrophobic sphere onto a pool covered by oil layer

We experimentally investigate the impact of a millimetric superhydrophobic sphere on a water pool covered by a thin oil layer, with the aim of seeking the critical conditions for sphere entrapment at the interfaces. The interfacial tension and viscosity of the thin oil layer are found to have a significant effect on the fate of the impacting spheres that are denser than the liquids: sinking or floating. For the oil layer of low viscosity, the impact dynamic is dominated by the capillary force, and the sphere experiences more or less uniform acceleration after the impact, which is similar to a sphere impacting onto a pure water pool. For the oil layer of relatively high viscosity, the viscous dissipation inside the thin oil layer greatly hinders the descending of the sphere, and thus, it is the viscosity of the oil layer that dictates the acceleration process of the spheres at the early stage of impact. At the late stage, the sphere moves very slowly under water (particularly at the onset of sinking), and the competition between the oil–water interfacial tension and buoyancy determines whether the sphere would eventually sink or float. We then conduct the theoretical analysis of the dynamic processes of the impacting sphere and give the theoretical predictions of the respective critical conditions, which agree well with the experimental observations.

Droplet rebound and dripping during impact on small superhydrophobic spheres

While droplet impact processes on hydrophilic and hydrophobic spheres have been widely investigated experimentally and numerically, the impact behaviors of water droplets on small superhydrophobic spheres are studied numerically and theoretically in this research. The numerical model adopts the volume of fluid method (VOF) and is verified by comparing the simulation results with the experimental observations in the literature. The effects of Weber number and sphere-to-droplet diameter ratio on the droplet impact dynamics are discussed. The final outcomes of the impact droplets are classified into rebound and dripping types with the latter appearing at a larger Weber number or a smaller diameter ratio. As the Weber number and diameter ratio increase, droplet deformation during impact is reinforced with the maximum width factor of the rebound droplet becoming greater. The maximum width factor of the dripping droplet is nearly independent of the Weber number but is enlarged by the increasing diameter ratio. Moreover, a larger diameter ratio reduces the contact time of the rebound droplet but raises that of the dripping one. A theoretical model based on energy conservation is established to predict the boundary between the droplet rebound and dripping outcomes and is in good agreement with the simulation results. The diameter ratio limit for droplet dripping at a zero Weber number is also obtained. Our results and analyses provide insight into the interaction mechanism between the impact droplet and small spheres or particles.

Supercooled water droplet impacting-freezing behaviors on cold superhydrophobic spheres

The impacting-freezing behaviors of supercooled water droplets on superhydrophobic spheres are investigated. A numerical model using the VOF (Volume of Fluid) method and enthalpy–porosity technology is established and validated by comparing the droplet profiles and spreading area factors by simulations and experiments. The effects of Weber number, supercooling degree and sphere-to-droplet diameter ratio (D*) on the impacting-freezing dynamics, including the droplet profile, spreading area factor and final rebound/adhesion, are studied. The results indicate that the supercooling rarely influences the spreading stage but significantly affects the receding stage with a larger stable spreading area factor obtained at a greater supercooling degree. With the decreasing of diameter ratio, the droplet generates a larger maximum spreading arc angle, a higher receding speed and thus a shorter contact time. The maximum spreading area factor greatly increases with reducing diameter ratio for D*<10 but it remains almost unchanged for D*≥10. Three different final morphologies of full rebound, partial rebound and adhesion are obtained, revealing the competition between the fluid flow and phase change in the droplet impacting-freezing process. The morphology map of rebound and adhesion indicates that the boundaries for droplet rebound and adhesion initially move to a smaller supercooling degree and then revert to the previous value as the diameter ratio becomes smaller. The findings in this research may deepen our understanding of the mechanism of supercooled water droplet impacting and freezing on a superhydrophobic curved surface, and contribute to the design of anti-icing/frosting surface.

Many-body dissipative particle dynamics study of droplet impact on superhydrophobic spheres with different size

Droplet impact on non-flat surfaces is a common phenomenon, further understandings of this phenomenon can guide a rational design of relevant applications in daily life. The present work numerically studies droplet impact on superhydrophobic solid spheres of different sizes by using many-body dissipative particle dynamics. The simulation results indicate that the time-dependent wrapping angle highly relies on the initial impact velocity and solid sphere size. Four spreading types, bouncing, wrapping, dripping, and bouncing & dripping, can be identified under different impact conditions. It is found that bouncing type prefers to happen at low impact velocity while dripping type is more likely to happen at high impact velocity. Further, on a solid sphere with small size, the dripping type happens at a large range of impact velocity while bouncing type occupies the large range on sphere surface with large size. The contact time is found much shorter for dripping cases than bouncing cases for all sphere sizes.

Capillary-scale solid rebounds: experiments, modelling and simulations

Abstract

Bioinspired Cavity Regulation on Superhydrophobic Spheres for Drag Reduction in an Aqueous Medium.

Hydrodynamic drag not only results in high-energy consumption for water vehicles but also impedes the increase of vehicle speed. The introduction of a low-viscosity gas lubricating film is assumed to be an effective and promising method to reduce hydrodynamic drag. However, the poor stability of the gas film and massive extra energy consumption restricts the practical application of the gas lubricating method. Herein, inspired by the microhairs with low surface energy wax covering the abdomen of water spiders, superhydrophobic sphere surfaces were designed. Attributed to numerous neighboring nanoneedle branches with low surface energy chemicals, an air-entrained cavity with a large surface area was captured and stabilized by the superhydrophobic sphere, changing its shape from a sphere to a streamlined body. The cavity continued attaching to the superhydrophobic sphere without bursting at a depth of 70.0-90.0 cm underwater and reduced the hydrodynamic drag by more than 90%. This work provides a simple, cost-effective, and energy-efficient way to stabilize the underwater gas-liquid interface to achieve a reduction in the hydrodynamic drag.