Superhydrophobic Surfaces Research Articles

A bi-layer superhydrophobic coating with long-term anti-corrosion enabled by functionalized hexagonal boron nitride and PDMS-based polyurethane on Mg alloy

Two-dimensional (2D) nanocomposite coatings and superhydrophobic coatings are applied to retarding corrosion of Magnesium (Mg) and its alloys. However, stability of the delicate micro/nanostructure and corrosion durability are still intractable issues. In this work, a bi-layer superhydrophobic coating with excellent corrosion resistance has been successfully constructed on Mg alloy via a facile spray/spin assisted self-assembly technique. The polyurethane (PU) coating containing well-dispersed hydroxylated hexagonal boron nitride (OH-BN) nanoparticles acts as the barrier layer and “adhesive”, while the superhydrophobic modified h-BN (f-BN) nanoparticles serve as the water-repellent surface. In addition, the modifying agents containing dynamic disulfide bonds provide covalent bonding effect to construct a stable superhydrophobic surface. Hence, the obtained coating presents extreme non-wettability with an impressive static water contact angle (WCA) of 155° as well as possesses outstanding anti-water adhesion, anti-fouling and self-cleaning performance. Meanwhile, the designed coating shows excellent corrosion durability, which the value of |Z|f=0.01 Hz retains at 9.5 × 109 ohm cm2, higher than PU coating of two orders of magnitude. The developed coating shows promising potentials as a durable protective material for Mg alloy and other metals and alloys, shedding new lights on developing high-performance corrosion resistance technologies.

Water collection through a directional leaf vein pattern by fast laser marker ablation of stainless-steel

Inspired by the natural water harvesting mechanisms of desert beetles, cactus thorns, and leaf veins, we designed a heterogeneous wettability surface with superhydrophilic pattern integrating leaf vein as the directional water transport main channel, attached capillary triangles as auxiliary channel plus a deep rough desorption channel on an overall superhydrophobic surface for an efficient water collection. A superhydrophilic surface was initially fabricated on the stainless steel disc by laser marker ablation allowing 1 μL droplet to spread completely to 0° within 0.12 seconds, followed by fluorine-containing coating transforming superhydrophilic surface to superhydrophobic one. Directional water transport patterns were then etched on the superhydrophobic surfaces by the secondary laser marker. The surface energy gradient and Laplace pressure induced by the pattern facilitated directional fast transport and efficient desorption of droplets, thus improving water collection efficiency. The enhancement mechanism of the water harvesting behavior for such surfaces was analyzed, with one focus on enhancing collection in hydrophobic regions with capillaries to reduce bouncing off loss and the other on improving balanced cycling of the collection process. At a fog flow rate of 1500ml/h and 20cm away from the fog outlet, the directional leaf vein-patterned 19.625cm2 sized surface demonstrated a fog water collection rate (WCR) of 5.6 Kg·m-2·h-1 and first drop collection at the 49th s, an impressively short time rarely reported. Compared to the superhydrophobic, superhydrophilic samples, and the reference, WCR increased by 180%, 62%, and 59%, respectively, and the first droplet collection time decreased by 73%, 46%, and 62%, respectively. This efficient water collection method has huge potential in arid regions.

Fabrication of superhydrophobic transparent coatings prepared by spraying dual-sized silica particles

Abstract Photovoltaic modules exposed to severe wind and sand conditions over extended periods often accumulate dust, significantly impacting their power generation efficiency. Superhydrophobic self-cleaning coatings present a promising solution to mitigate this issue. In this study, a SiO2/PU coating was applied to glass substrates using a combined sol-gel spraying technique. The sol system comprised a mixture of SiO2 particles with diameters of 20 nm and 200 nm, enabling the construction of a micro-nano structure that enhances surface hydrophobicity. The SiO2/PU coatings prepared with mixed particle sizes demonstrated significant improvements in contact angle, rolling angle, self-cleaning, and antifouling properties compared to those made with a single particle size, achieving a water contact angle of 164.52°. Furthermore, the prepared SiO2/PU coatings possess excellent stability and transmittance. It is anticipated that this superhydrophobic SiO2/PU coating will be a candidate for anti- dust applications in photovoltaic modules.

Mechanochemical Damage Self‐Repairable Photothermal Anti/De‐Icing Superhydrophobic Coating by Dynamic Bonding Phase‐Change Fillers with the Matrix

AbstractThe development of robust superhydrophobic coatings with the capacity for self‐healing against mechanochemical damage is pivotal for their practical deployment. This study develops a physically and chemically self‐healing superhydrophobic coating with exceptional durability for anti‐icing applications using a simple spray coating method. This is achieved by incorporating phase‐change fillers into a dynamic cross‐linked matrix via dynamic imine bonds. Specifically, oleylamine (ODA) is encapsulated within rigid diatomite nanopores and modified with dopamine (DOA), significantly enhancing the grafting efficiency of aminopropyl‐terminated polydimethylsiloxane (NH₂‐PDMS‐NH₂) (N‐DOA). The incorporation of 15% N‐DOA increases the water contact angle (WCA) of the acrylic resin/aminopropyl‐terminated polydimethylsiloxane (AR/NH₂‐PDMS‐NH₂) matrix to 159.7° and reduces the sliding angle (SA) to 2.9°, while also improving the mechanical durability of coating to withstand 600 abrasion cycles. The dynamic imine bonds between NH₂‐PDMS‐NH₂ and trimesic acid (BTC) facilitate the mobility of N‐DOA and NH₂‐PDMS‐NH₂, enabling rapid recovery of superhydrophobic properties and low ice adhesion strength after abrasions, scratches, oxygen plasma etching, and multiple de‐icing cycles due to the synergistic phase‐change effect of ODA. Thus, the self‐healing coating produced via this simple spray method presents a novel approach for superhydrophobic coatings in anti‐icing applications.

Droplet impact dynamics on the surface of super-hydrophobic BNNTs stainless steel mesh.

The 'gas‒liquid‒solid' mechanism annealing method was used to create a superhydrophobic boron nitride nanotube (BNNT) stainless steel mesh in a tube furnace at 1250°C in an NH3 environment. Fe powder was used as a catalyst, and B:B2O3 = 4:1 was used as the raw material. The water droplets on the surface of the superhydrophobic material had a contact angle of approximately 150° and a slide angle of approximately 3°. By using molecular dynamics (MD) simulation technology, a three-dimensional braided physical model of nanodroplets and superhydrophobic BNNT mesh surfaces with the same contact angle and rolling angle was prepared via the function weaving method. The Weber number (We) was used as the entrance point to establish the relationship between macroscale experimental studies and nanoscale MD simulation analysis on the basis of these efforts. A study was conducted on the dynamic behaviour of droplets impacting a superhydrophobic BNNT filter surface. We suggest that the wettability, substrate structure, and impact velocity are connected to the impact dynamic behaviour of droplets on the basis of the data obtained at various scales. The findings demonstrate that when the droplet impact velocity increases, several droplet phenomena-such as impact-rebound, impact-spread-rebound, and impact-spread-breaking-polymerisation-spatter-appear on the substrate surface sequentially. The mechanism of impact behaviour at various scales is explained in light of these events. Furthermore, a better theoretical model is proposed to assess the droplet wetting transition at the nanoscale. This model accurately predicts the boundary Weber number that starts the wetting transition. Moreover, the connections among the impact velocity, spreading diameter, and contact time (or We) are examined. The tendencies found via MD simulations match the outcomes of the experiments. Our discoveries and outcomes broaden our understanding of how droplet impact affects the dynamic behaviour of superhydrophobic surfaces. A scientific foundation for examining the dynamic behaviour of droplets is provided by combining simulations and experiments.

Superhydrophobicity With Self-Adaptive Water Pressure Resistance and Adhesion of Pistia Stratiotes Leaf.

Superhydrophobic surfaces are promising for optimizing amphibious aircraft by minimizing water drag and adhesion. Achieving this involves ensuring these surfaces can resist high liquid pressure caused by deep water and fluid flow. Maximizing the solid-liquid contact area is a common strategy to improve liquid pressure resistance. However, this approach inevitably increases solid-liquid adhesion, making it challenging to guarantee a trade-off between the two wetting characteristics. Here, it is found that the Pistia stratiotes leaf exhibits superhydrophobicity with high water pressure resistance and low adhesion, attributed to its self-adaptive deformable microstructure with unique re-entrant features. Under pressure, these microstructures deform to increase the solid-liquid contact area, thereby enhancing water pressure resistance. The re-entrant features elevate the deformation threshold, enabling higher modulus microstructures to achieve adaptive response. This facilitates the recovery of deformed microstructures, restoring the air layer and maintaining low adhesion. Following these concepts, Pistia stratiotes leaf-inspired surfaces are fabricated, achieving an 183% improvement in water impact resistance and an ≈80% reduction in adhesion after overpressure compared to conventional superhydrophobic surfaces. The design principles inspired by Pistia stratiotes promise significant advancements in amphibious aircraft and other trans-media vehicles.

Mechanical Durability Testing and Self-Recovery of Topographically Modified Superhydrophobic Surfaces

Mechanical Durability Testing and Self-Recovery of Topographically Modified Superhydrophobic Surfaces

Supercooled Water Droplets Impacting at High Speed on Superhydrophobic Surfaces: Open Questions Related to Icephobicity

It is posited that impact ice adhesion strength on superhydrophobic surfaces is related to droplet impalement. Several gaps still exist in the understanding of how different types of textured surfaces resist the impalement of high‐speed impacting droplets (<102 μm diameter at 102–103 m s−1). A previous study reveals that a pressure balance calculation can predict the wetting state of an impacting drop (≈103 μm diameter at ≈101 m s−1). However, the ability of this model to predict the wetting state of small droplets impacting at high speed or of droplets in a supercooled state has not been reported. To better understand supercooled microdroplet impalement, and to verify the hypothesis that droplet impalement is directly related to ice adhesion strength, herein, the impact behavior of microdroplets in a cold airflow onto surfaces of varying topography and wettability in an icing wind tunnel with support of high‐speed video images are investigated. Although the results do not allow for validating the above hypothesis about impalements and ice adhesion, they show that the difference between supercooled droplet spreading and retraction can be a predictor of ice adhesion strength. However, this relation must be validated with a greater body of data from samples with very diverse surface properties.

Innovations and Insights in Superhydrophobicity: Learning from Nature to Surpass It

Abstract. Superhydrophobic surfaces, inspired by natural examples, have emerged as a promising area of research due to their unique properties. This paper reviews the characteristics and challenges of creating surfaces that can exceed the capabilities of natural counterparts. Key aspects include surface tension, contact angle, and morphology, with a focus on natural models such as lotus leaves and gecko feet. The paper highlights knowledge gaps in durability and manufacturing, and discusses design challenges in achieving roughness, hierarchy, and balance between hydrophobicity and durability. Methods for enhancing durability, such as chemical and roughness restoration, are explored, alongside the potential of AI in optimizing material and design choices. The paper concludes with recommendations for future research directions in the field.

Scalable robust photothermal superhydrophobic coatings for efficient anti-icing and de-icing in simulated/real environments

Photothermal superhydrophobic coatings are supposed promising to prevent ice accumulation on infrastructures but often experience significant performance degradation in real icing conditions and lack mechanical robustness. Here, we report design of robust photothermal superhydrophobic coatings with three-tier hierarchical micro-/nano-/nanostructures by deposition of nanosized MOFs on natural attapulgite nanorods, fluorination, controlled phase separation of a hydrophobic adhesive and spraying assembly. Phase separation degree and adhesive content significantly influence the coatings’ properties by regulating the structural parameters and morphology. In simulated/real icing environments, the coatings simultaneously show (i) high superhydrophobicity and stable Cassie-Baxter states due to their low-surface-energy, three-tier micro-/nano-/nanostructure, (ii) excellent photothermal effect primarily due to nanosized MOFs, and (iii) good mechanical robustness by the phase-separated adhesive, reinforcement with attapulgite and the coatings’ self-similar structure. Accordingly, combined with low thermal conductivity, the coatings exhibit remarkable anti-icing/frosting (e.g., no freezing in at least 150 min and almost free of frost in 25 min) and de-icing/frosting performances (e.g., fast de-icing in 12.7 min and fast de-frosting in 16.7 min) in such environments. Furthermore, we realize large-scale preparation of the coatings at reasonable costs. The coatings have great application potential for anti-icing and de-icing in the real world by efficiently using natural sunlight.

A durable self-cleaning superhydrophobic coating fabricated using the preset grid-spraying method

Herein, a new durable superhydrophobic coating was prepared using the preset grid-spraying method which was low cost and simple. To achieve the regular continuous grid structure which had protection and support function to the coatings, the sieve mesh was preset on the substrate with hot pressing process. After spraying the superhydrophobic coating on the substrate of the preset mesh followed by curing, a superhydrophobic coating was obtained. The surface morphology of the superhydrophobic coating prepared using the preset grid-spraying method was primarily composed of the regular grid structure and island-like structure with the micro-nano scale rough structure on them. Compared with the superhydrophobic coating prepared with the traditional spraying method, the mechanical stability of the superhydrophobic coating, which was prepared using the preset grid-spraying method, was considerably higher along with good hydrophobic properties. The superhydrophobic coating demonstrated superhydrophobic properties after 170 cycles of the sandpaper abrasion test, which was about 300 % higher than the traditional superhydrophobic coating. Furthermore, the superhydrophobic coating prepared using the preset grid-spraying method had excellent self-cleaning performance and weather resistance. Self-cleaning efficiencies of this superhydrophobic coatings on kaolin and sand were 99.2 % and 98.9 %, respectively. Even after 170 cycles of the sandpaper abrasion test, the self-cleaning efficiency of superhydrophobic coatings on kaolin and sand remained over 85 % and 90 %, respectively. The coated surface still showed superhydrophobic properties after 30 days of the outdoor placement experiment. Showing excellent comprehensive properties and low cost, the superhydrophobic coating prepared using the preset grid-spraying method had a board application prospect.

Wood Surface-Embedding of Functional Monodisperse SiO2 Microspheres for Achieving Robust, Durable, Nature-Inspired, Programmable Superrepellent Interfaces.

Nature-inspired, robust, durable, liquid-repellent interfaces have attracted considerable interest in the field of wood biomimetic intelligence science and technology application. However, realizing green environmental protection and low maintenance and replacement cost wood surfaces constructed with micro/nanoarchitectures is not an easy task. Aiming at the problem of poor waterproof performance of wood, a silicon dioxide/polydimethylsiloxane (SiO2/PDMS) self-cleaning programmable superhydrophobic coating was biomimetically constructed on the wood substrate by surface-embedded dual-dipping design based on the "substrates + nanoparticles" hybrid principle of the lotus leaf effect. This robust, durable, nature-inspired, self-cleaning, programmable superhydrophobic coating was found to have no observable impact on the original color and texture of the natural wood. The SiO2/PDMS/wood prepared exhibited exceptional liquid repellency and a high static water contact angle (WCA) of 158.5° and a low slide angle (SA) of 10°, including everyday general-purpose droplets, indicating that the introduction of the monodisperse SiO2 microspheres can effectively enhance the superhydrophobic properties of the hydrophilic wood. We applied this strategy to a variety of substrates, including wood-cellulose aerogel and wood-cellulose paper, and demonstrated that the liquid-repellent nature of the self-cleaning superhydrophobic coating remained unchanged. Moreover, the superhydrophobic surface of SiO2/PDMS/wood was preserved even after harsh abrasion conditions, including mechanical damage (sandpaper, sharp steel blade, and tapes), thermal damage (UV irradiation and low/high-temperature exposure such as steaming and freezing), chemical damage, and solvent corrosion (immersion in acid, alkali), demonstrating robust stability of the superhydrophobic coating. Furthermore, the SiO2/PDMS programmable superhydrophobic coating exhibits exceptional exciting self-cleaning and stain-resistant properties, making it offer greater possibilities in terms of scientific challenges and real-world problem-solving at biomimetic smart superhydrophobic interfaces in wood.

Study on the water and mold resistance of ZnO/polyurethane superhydrophobic coating on circuit boards

In this work, superhydrophobic coatings were prepared on circuit boards based on the susceptibility of circuit boards to harsh environments such as humidity and mold adhesion, leading to reduced service life. Firstly, cetyltrimethoxysilane and γaminopropyltriethoxysilane were used to modify zinc oxide nanoparticles with different particle sizes. Then polyurethane and modified nanoparticles were successively sprayed on the circuit boards with a spraying process, and the superhydrophobic coatings were finally produced. The impact of the zinc oxide mass ratio with different particle sizes, silane coupling agent content, and surface modifier content on the hydrophobicity of the coatings were investigated. The results show that when the mass ratio of zinc oxide (30nm) to zinc oxide (90nm) is 1:1, the silane coupling agent content is 12‰. The surface modifier content is 12‰, the hydrophobicity of the coating is the best, and its CA can be up to 169.5°, and its SA can be up to 2.8°. It exhibits good protection of circuit boards in the tests of adhesion, acid, and alkali corrosion resistance, self-cleaning performance, and anti-mold performance. Performance, so that the coating is in the future field of electronic equipment applications.

A surfactant-mediated dynamic protection strategy for 100% waterborne fabrication of durable superhydrophobic coatings

Waterborne preparation of superhydrophobic coatings now becomes strongly favored because of its environmental and health friendliness. However, the huge surface energy difference between low surface-energy components and water still poses a challenge for achieving their co-dispersion. Here, a surfactant-mediated protection strategy is proposed to achieve well dispersion of both SiO2 nanoparticles and perfluorosilane in water as well as block their conjugation, thereby fabricating a 100% waterborne F-FC-SiO2 suspension with high homogeneity and dispersion stability. During the painting and drying process, demulsification spontaneously triggers the deprotection and enables sufficient covalent conjugation between perfluorosilane and SiO2, thus achieving a superhydrophobic coating. This supramolecular protection/deprotection approach enables high compatibility of the F-FC-SiO2 suspension with versatile waterborne polymer emulsions (e.g., waterborne polyurethane, waterborne polysiloxane, and waterborne epoxy resin) to fabricate durable superhydrophobic coatings. The resulting composite coating can impart super-repellency to various substrates against aqueous solutions and multiple oils, displaying high antifouling and self-cleaning properties, as well as resistance to extreme mechanical abrasion and weathering. This work provides a promising approach for the convenient production of totally waterborne superhydrophobic coatings.

Bouncing dynamics of a droplet impacting onto a superhydrophobic surface with pillar arrays

A superhydrophobic surface (SHS) patterned with pillar arrays has been demonstrated to achieve excellent water repellency and is highly effective for self-cleaning, anti-icing/frosting, etc. However, the droplet impact dynamics and the related mechanism for contact time (tc*) reduction remain elusive, especially when different arrangements of pillar arrays are considered. This study aims to bridge this gap by exploring a droplet impinging on an SHS with square pillar arrays in a cuboid domain. This fluid dynamics problem is numerically simulated by applying the lattice Boltzmann method. The influences of the droplet diameter (D*), the Weber number (Wew), and the pillar spacing and height (s* and h*) on the droplet dynamics and tc* are investigated. The numerical results show that the droplet can exhibit different bouncing patterns, normal or pancake bouncing, depending on Wew, s*, and h*. Pancake bouncing usually occurs when Wew ≥1.28, h*≥1, and s* ≈ 1, yielding a small tc*. Among all cases, a small tc* can be attained when the conversion rate of kinetic energy to surface energy (ΔĖsur*) right after the impacting exceeds a critical value around 0.038. This relation broadens that given in A. M. Moqaddam et al. [J. Fluid Mech. 824, 866–885 (2017)], which reported that the large total change of surface area renders small tc*. Furthermore, the maximum impacting force remains nearly the same in all cases, regardless of the bouncing patterns.

Formation of Superhydrophobic Coatings Based on Dispersion Compositions of Hexyl Methacrylate Copolymers with Glycidyl Methacrylate and Silica Nanoparticles.

In the last decade, the task of developing environmentally friendly and cost-effective methods for obtaining stable superhydrophobic coatings has become topical. In this study, we examined the effect of the concentrations of filler and polymer binder on the hydrophobic properties and surface roughness of composite coatings made from organic-aqueous compositions based on hexyl methacrylate (HMA) and glycidyl methacrylate (GMA) copolymers. Silicon dioxide nanoparticles were used as a filler. A single-stage "all-in-one" aerosol application method was used to form the coatings without additional intermediate steps for attaching the adhesive layer or texturing the substrate surface, as well as pre-modification of the surface of filler nanoparticles. As the ratio of the mass fraction of polymer binder (Wn) to filler (Wp) increases, the coatings show the lowest roll-off angles among the whole range of samples studied. Coatings with an optimal mass fraction ratio (Wn/Wp = 1.2 ÷ 1.6) of the filler to polymer binder maintained superhydrophobic properties for 24 h in contact with a drop of water in a chamber saturated with water vapor and exhibited roll-off angles of 6.1° ± 1°.

Superhydrophobic Coating in Cement Mortar via Inherent Microstructure for Enhanced Efflorescence Resistance

Superhydrophobic Coating in Cement Mortar via Inherent Microstructure for Enhanced Efflorescence Resistance

Heat Treatment of PTFE/PDMS/Nano‐silica Deposited Layer for Preparation of Superhydrophobic Coating with Anti‐corrosion, Anti‐scaling, and Anti‐fouling Properties

AbstractPTFE‐based superhydrophobic coatings have received increasing research and the fabricated coatings exhibited excellent anti‐corrosion, anti‐scaling, anti‐fouling, and anti‐icing properties. In order to further enhance the hydrophobicity of PTFE coating, a PTFE‐based coating consisted of nano‐silica and polydimethylsiloxane (PDMS) was fabricated by deposition coating and annealing treatment. The water contact angle and sliding angle of the coating achieved 160.65° and 3.02°, respectively, demonstrating it possessed superhydrophobic property. Due to its excellent surface feature, the coating exhibited resistance to milk, coffee, and mud water, and could be easily cleaned by water flow. Besides, the water droplet on the coating had a longer freezing time and the ice droplet could be easily removed from the coating. Ascribed to the abundant nonpolar groups and the air cushion formed under water, the prepared coating had outstanding anti‐scaling and anti‐corrosion properties. The electrochemical impedance of the steel plate with the coating (above 9000 Ω·cm2) increased by two orders of magnitude compared with the non‐coating one (78.2 Ω·cm2), and the anti‐scaling rates of the coating towards BaSO4 and SrSO4 were all above 80%. It is promising that the coating will be a competitive candidate in many application fields.

Preparation of PTFE-TIO2-GO/EP superhydrophobic costing and its performance study

For many years, the issue of microbial adhesion has presented difficulties in both daily life and business. In this paper, superhydrophobic coatings were produced by adding epoxy resin (EP), butyl acetate, titanium dioxide (TiO2) nanoparticles, polytetrafluoroethylene micropowder (PTFE), and graphene oxide (GO) sequentially into a mug and mixing well, and then modifying the microscopic particles by using perfluorooctyltriethoxysilane (POTS), and lastly producing the superhydrophobic coatings applied via spraying on the aluminum sheet surface. The micro morphology of the samples was analysed by SEM and EDS, the molecular makeup of the samples was analysed by FTIR and the molecular stability, mechanical stability and algae resistance were tested, and finally the the rust unwillingness of the coatings was investigated by using an electrochemical workstation (Tafel and EIS). The outcomes demonstrated that the best GO to nanoparticle mass ratio of 10% was chosen to achieve a contact angle of 167.5° and a sliding angle of 2.5°. The coating contact angle was still superior to 150° after 7 days of immersion in strong acids and bases as well as 3.5 wt% Nacl and after 8 hours of immersion in boiling water. After 800 abrasion tests the contact angle was still 150.6°. Algae resistance tests showed that the coatings had good resistance to algae adhesion.

On interfacial deformation and resistance characteristics over superhydrophobic surfaces: A numerical study

The air/water interface, known as the plastron, entrapped in submerged superhydrophobic surfaces (SHSs), plays a crucial role in underwater drag reduction. However, the plastron can be easily deformed or collapsed by turbulent flow, leading to decreased drag reduction or even an increase in drag. Most previous numerical studies have simplified these interfaces as idealized flat or curved rigid boundaries. To thoroughly investigate the interfacial behavior of SHSs, this study presents a numerical comparison between ideal and dynamic interfaces. The plastron undergoes regular oscillations after a brief adaptation period, transitioning between convex, nearly flat, and concave shapes. During the oscillatory decay, the dynamic properties of the interface modify the surface drag of the SHSs by both affecting the viscous drag and introducing pressure drag. The viscous drag is affected in two main ways. First, the momentum exchange across the dynamic interface is enhanced due to the roughness-like effect, leading to an increased viscous drag in groove region. Second, the trailing interfaces induce step flows and secondary flows downstream of the grooves, creating regions of nonuniform shear stresses. Consequently, the viscous drag of the downstream walls is slightly reduced. Overall, for convex and nearly flat interfaces, the drag increase within the groove region outweighs the drag reduction at the downstream walls, resulting in a total drag increase in 47.3% and 29.8%, respectively. Conversely, for concave interfaces, the drag increase within the groove region is smaller than the drag reduction at the downstream walls, leading to a 9.8% decrease in total drag.