Thriving artificial underwater drag-reduction materials inspired from aquatic animals: progresses and challenges

Experimental Studies of Bioinspired Shark Denticles for Drag Reduction

Shark skin is composed of denticles, or complex scale-like features, which have been shown to reduce drag in turbulent and laminar flow. The denticle crown features undulating structures called riblets that interact with the turbulent boundary layer to reduce drag. Two mechanisms thought to contribute to the drag-reducing properties of riblets include the lifting of streamwise vortices and the hampering of spanwise vortex interactions to reduce crossflow, which could translate to similar flow mechanisms for denticles. Because of the varied morphologies of dermal denticles on different shark species, which also depend on body location, the impact of these denticle geometries on flow is of interest to the biology community, including related fields such as fluid mechanics and oceanography. This review highlights the past 15 years of manufacturing techniques and experimental measurements of drag over denticle-inspired surface structures, including real shark skin samples and engineered denticles and riblets. State-of-the-art additive manufacturing and other techniques are primarily limited to mm-length denticle scales, which have demonstrated drag reduction in lower flow speeds, under 1 m s-1. New manufacturing approaches can create sub-mm length denticles and nanotextured surface structures, which have achieved reported drag reductions of up to 31%. We synthesize results from the literature to illustrate the drag reduction properties of bioinspired denticles and riblets according to their geometry and flow conditions. Using these trends, we suggest design features and focus areas for future research, such as increasing studies of different denticle morphologies, hydrophobicity, antifouling properties, and acoustic noise reduction. Continued work on bioinspired denticles for drag reduction has wider implications in comparative biology and applications to design more energy-efficient, persistent vehicles for environmental monitoring.

Bioinspired Slippery Surfaces for Liquid Manipulation from Tiny Droplet to Bulk Fluid

Slippery surfaces, which originate in nature with special wettability, have attracted considerable attention in both fundamental research and practical applications in a variety of fields due to their unique characteristics of superlow liquid friction and adhesion. Although research on bioinspired slippery surfaces is still in its infancy, it is a rapidly growing and enormously promising field. Herein, a systematic review of recent progress in bioinspired slippery surfaces, beginning with a brief introduction of several typical creatures with slippery property in nature, is presented. Subsequently,this review gives a detailed discussion on the basic concepts of the wetting, friction, and drag from micro- and macro-aspects and focuses on the underlying slippery mechanism. Next, the state-of-the-art developments in three categories of slippery surfaces of air-trapped, liquid-infused, and liquid-like slippery surfaces, including materials, design principles, and preparation methods, are summarized and the emerging applications are highlighted. Finally, the current challenges and future prospects of various slippery surfaces are addressed.

Investigation on drag reduction on rotating blade surfaces with microtextures

To enhance the aerodynamic performance of aero engine blades, simulations and experiments regarding microtextures to reduce the flow loss on the blade surfaces were carried out. First, based on the axisymmetric characteristics of the impeller, a new simulation method was proposed to determine the aerodynamic parameters of the blade model through the comparison of flow field characteristics and simulation results. Second, the placement position and geometrical parameters (height, width, and spacing) of microtextures with lower energy loss were determined by our simulation of microtextures on the blade surface, and the drag reduction mechanism was analyzed. Triangular ribs with a height of 0.2 mm, a width of 0.3 mm, and a spacing of 0.2 mm exhibited the best drag reduction, reducing the energy loss coefficient and drag by 1.45% and 1.31% for a single blade, respectively. Finally, the blades with the optimal microtexture parameters were tested in the wind tunnel. The experimental results showed that the microtexture decreased energy loss by 3.7% for a single blade under 57° angle of attack and 136.24 m/s, which was favorable regarding the drag reduction performance of the impeller with 45 blades.

Fluid-Solid Interfacial Properties and Drag-Reducing Characterization of the Flexible Conical Microstructured Film Inspired by the Streamlined Body Surface of the Pufferfish

Given the challenges in accurately replicating the surface of the pufferfish, this study employed three-dimensional (3D) printing to create a model based on inverse modeling. The morphology of the pufferfish exhibits a streamlined configuration, characterized by a gradual widening from the anterior oral region to the central ocular area, followed by a progressive narrowing from the midabdominal region toward the caudal extremity. The RNG k-ε turbulence simulation results demonstrate that the streamlined body surface of the pufferfish diminishes differential pressure resistance. This enhancement promotes laminar flow formation, delays fluid separation, minimizes turbulence-induced vortices, and reduces frictional resistance. Moreover, the pufferfish's supple and uneven outer epidermis was simplified into a flexible, nonsmooth planar film to conduct fluid-solid coupling simulations. These revealed that the pufferfish's unique skin can absorb turbulent energy and minimize momentum transfer between the fluid and the solid film, lowering the fluid resistance during swimming. In summary, The high-efficiency swimming capacity of pufferfish stems not only from their streamlined body surface but also significantly from the unique structural characteristics and mechanical properties of their flexible skin. This research provides critical theoretical underpinnings for the design of functional bionic surfaces aimed at drag reduction.

Nature-Inspired Micro/Nano-Structured Antibacterial Surfaces

The problem of bacterial resistance has become more and more common with improvements in health care. Worryingly, the misuse of antibiotics leads to an increase in bacterial multidrug resistance and the development of new antibiotics has virtually stalled. These challenges have prompted the need to combat bacterial infections with the use of radically different approaches. Taking lessons from the exciting properties of micro-/nano-natural-patterned surfaces, which can destroy cellular integrity, the construction of artificial surfaces to mimic natural functions provides new opportunities for the innovation and development of biomedicine. Due to the diversity of natural surfaces, functional surfaces inspired by natural surfaces have a wide range of applications in healthcare. Nature-inspired surface structures have emerged as an effective and durable strategy to prevent bacterial infection, opening a new way to alleviate the problem of bacterial drug resistance. The present situation of bactericidal and antifouling surfaces with natural and biomimetic micro-/nano-structures is briefly reviewed. In addition, these innovative nature-inspired methods are used to manufacture a variety of artificial surfaces to achieve extraordinary antibacterial properties. In particular, the physical antibacterial effect of nature-inspired surfaces and the functional mechanisms of chemical groups, small molecules, and ions are discussed, as well as the wide current and future applications of artificial biomimetic micro-/nano-surfaces. Current challenges and future development directions are also discussed at the end. In the future, controlling the use of micro-/nano-structures and their subsequent functions will lead to biomimetic surfaces offering great potential applications in biomedicine.

Flexible Platform Composed of T-Shaped Micropyramid Patterns toward a Waterproof Sensing Interface

Antifouling is essential to guaranteeing the sensitivity and precision of flexible sensing interfaces. Materials and structures are the two primary strategies. However, optimizing the inherent microstructures to integrate waterproofing and sensing is rarely reported. To improve the liquid repellency of micropyramid structures, this work presents a study of the design and fabrication of T-shaped micropyramid structures. These structures are patterned uniformly and largely on polydimethylsiloxane (PDMS) skin by the new process of two-step magnetic induction. The waterproofing is related to the breakthrough pressure and the liquid repellency, both of which are a function of structural characteristics, D, and material properties, θY. At the breakthrough transition, two failure models distinguished by θY appear: the depinning transition and the sagging transition. Meanwhile, when considering D in practice, some models will shift and occur early. The D value regulates the transition of the material's wettability to the liquid repellency. The influence of the material's inherent nonwettability on liquid repellency diminishes as D decreases, and the transition from completely wetting liquids to super-repellents can be achieved. Experiments demonstrate that for D = 0.3 under water the resistance is approximately 142 times larger than the depth of the structure, considerably facilitating the waterproofing of conventional micropyramid arrays. This work provides a novel method for fabricating flexible T-shaped micropyramid array structures and opens a new window on flexible sensing interfaces with excellent waterproofing.

Looking for future biological control agents: the comparative function of the deutosternal groove in mesostigmatid mites

The physics of fluid laminar flow through an idealised deutosternum assembly is used for the first time to review predatory feeding designs over 72 different-sized example species from 16 mesostigmatid families in order to inform the finding of new biological control agents. Gnathosomal data are digitised from published sources. Relevant gnathosomal macro- and micro-features are compared and contrasted in detail which may subtly impact the control of channel- or 'pipe'-based transport of prey liquids around various gnathosomal locations. Relative deutosternal groove width on the mesostigmatid subcapitulum is important but appears unrelated to the closing velocity ratio of the moveable digit. Big mites are adapted for handling large and watery prey. The repeated regular distance between deutosternal transverse ridges ('Querleisten') supports the idea of them enabling a regular fluctuating bulging or pulsing droplet-based fluid wave 'sticking' and 'slipping' along the groove. Phytoseiids are an outlier functional group with a low deutosternal pipe flow per body size designed for slot-like microchannel transport in low volume fluid threads arising from daintily nibbling nearby prey klinorhynchidly. Deutosternal groove denticles are orientated topographically in order to synergise flow and possible mixing of coxal gland-derived droplets and circumcapitular reservoir fluids across the venter of the gnathosomal base back via the hypostome to the prey being masticated by the chelicerae. As well as working with the tritosternum to mechanically clean the deutosternum, denticles may suppress fluid drag. Shallow grooves may support edge-crawling viscous flow. Lateral features may facilitate handling unusual amounts of fluid arising from opportunistic feeding on atypical prey. Various conjectures for confirmatory follow-up are highlighted. Suggestions as to how to triage non-uropodoid species as candidate plant pest control agents are included.

Drag reduction using bionic groove surface for underwater vehicles

Introduction: The reduction of drag is a crucial concern within the shipping industry as it directly influences energy consumption. This study addresses this issue by proposing a novel approach inspired by the unique ridge structure found on killer whale skin. The objective is to develop a non-smooth surface drag reduction method that can effectively decrease drag and improve energy efficiency for ships. Methods: The study introduces a technique involving the creation of transverse bionic groove surfaces modeled after the killer whale skin's ridge structure. These grooves are aligned perpendicular to the flow direction and are intended to modify the behavior of turbulent boundary layer flows that form around the ship's hull. Numerical simulations are employed using the Shear Stress Transport k-ω model to analyze the effects of the proposed groove surface across a wide range of flow conditions. The research investigates the impact of various parameters, such as the width-to-depth ratio (λ/A), groove depth, and inlet velocity, on the drag reduction performance of the bionic groove surface. Results: The study reveals several key findings. Optimal shape parameters for the bionic groove surface are determined, enabling the most effective drag reduction. The numerical simulations demonstrate that the proposed groove surface yields notable drag reduction benefits within the velocity range of 2∼12 m/s. Specifically, the friction drag reduction ratio is measured at 26.91%, and the total drag reduction ratio at 9.63%. These reductions signify a substantial decrease in the forces opposing the ship's movement through water, leading to enhanced energy efficiency. Discussion: Comparative analysis is conducted between the performance of the bionic groove surface and that of a smooth surface. This investigation involves the examination of velocity gradient, streamwise mean velocity, and turbulent intensity. The results indicate that the bionic groove structure effectively mitigates viscous stress and Reynolds stress, which in turn reduces friction drag. This reduction in drag is attributed to the alteration in flow behavior induced by the non-smooth surface. Conclusion: The study proposes a novel approach for drag reduction in the shipping industry by emulating the ridge structure of killer whale skin. The transverse bionic groove surface, aligned perpendicular to flow direction, demonstrates promising drag reduction outcomes across diverse flow conditions. Through systematic numerical simulations and analysis of key parameters, the research provides insights into the drag reduction mechanism and identifies optimal design parameters for the groove surface. The potential for significant energy savings and improved fuel efficiency in maritime transportation underscores the practical significance of this research.

Bidirectional Underwater Drag Reduction on Bionic Flounder Two-Tier Structural Surfaces

Engineering marvels found throughout the exclusive structural features of biological surfaces have given rise to the progressive development of skin friction drag reduction. However, despite many previous works reporting forward drag reduction where the bio-inspired surface features are aligned with the flow direction, it is still challenging to achieve bidirectional drag reduction for non-morphable surface structures. Inspired by the flounder ctenoid scales characterized by tilted, millimeter-sized oval fins embedded with sub-millimeter spikes, we fabricate a bionic flounder two-tier structural surface (BFTSS) that can remarkably reduce the forward skin friction drag by ηdr = 19%. Even in the backwards direction, where the flow is completely against the tilting direction of surface structures, BFTSS still exhibits a considerable drag reduction of ηdr = 4.2%. Experiments and numerical simulations reveal that this unique bidirectional drag reduction is attributed to synergistic effects of the two-tier structures of BFTSS. The array of oval fins can distort the boundary layer flow and mitigate the viscous shear, whilst the microscale spikes act to promote the flow separation to relieve the pressure gradient in the viscous sublayer. Notably, the pressure gradient relief effect of microscale spikes remains invariant to the flow direction and is responsible for the backward drag reduction as well. The bidirectional drag reduction of BFTSS can be extensively applied in minimizing the energy consumption of ships and underwater vessels, as well as in pipeline transport.

Multistage Gradient Bioinspired Riblets for Synergistic Drag Reduction and Efficient Antifouling

Shark skin-inspired riblets have represented the tremendous potential for drag reduction (DR) and antifouling in submarine, ship, and so on. Most studies simplified the complex denticle embedded in the shark skin into the single-stage riblet with uniform parameters, ignoring the influence of riblet height gradient and material deformation on DR and antifouling. In the present study, flexible multistage gradient riblets (MSGRs) with varied heights were proposed, and their DR and antifouling effects were investigated by the experiment and numerical simulation. The experimental results showed that the maximum DR rate of flexible MSGRs with an elastic modulus of 4.592 MPa could reach 16.8% at a flow velocity of 0.5 m/s. Moreover, the dynamic adhesion measurement indicated a reduction by 69.6% of the adhesion area of Chlorella vulgaris on the flexible MSGR surface. The results identified that flexible MSGRs with low surface energy could generate steady high- and low-velocity streaks and alter the flow state of the fluid, thus lessening the average velocity gradient near the wall and the adhering selectivity of pollutants in riblet and achieving synergistic DR and efficient antifouling. Taken together, the proposed flexible MSGR surface holds promise for reducing surface friction and inhibiting particle attachment in engineering applications.

Antibacterial features of material surface: strong enough to serve as antibiotics?

Bacteria are small but need big efforts to control. The use of antibiotics not only produces superbugs that are increasingly difficult to inactivate, but also raises environmental concerns with the growing consumption. It is now believed that the antibacterial task can count on some physiochemical features of material surfaces, which can be anti-adhesive or bactericidal without releasing toxicants. It is necessary to evaluate to what extent can we rely on the surface design since the actual application scenarios will need the antibacterial performance to be sharp, robust, environmentally friendly, and long-lasting. Herein, we review the recent laboratory advances that have been classified based on the specific surface features, including hydrophobicity, charge potential, micromorphology, stiffness and viscosity, and photoactivity, and the antibacterial mechanisms of each feature are included to provide a basic rationale for future design. The significance of anti-biofilms is also introduced, given the big role of biofilms in bacteria-caused damage. A perspective on the potential wide application of antibacterial surface features as a substitute or supplement to antibiotics is then discussed. Surface design is no doubt a solution worthy to explore, and future success will be a result of further progress in multiple directions, including mechanism study and material preparation.

Bionic research on Paramisgurnus dabryanus scales for drag reduction

Drag reduction is a key problem in marine vehicles and fluid transportation industries. Reducing drag strategies and mechanisms need to be further investigated. To explore a bionic approach for reducing flow resistance, experimental and numerical simulation research was conducted to study the drag reduction characteristics of the Paramisgurnus dabryanus surface microstructure. In this study, the large-area flexible surface of the bionic loach scale was prepared by the template method of one-step demoulding. The water tunnel experiment results show that compared with the smooth surface, the drag reduction rate of the bionic surface ranges from 9.42% to 17.25%. And the numerical simulation results indicate that the pressure gradient and low-speed vortex effect created by the bionic loach scales can effectively reduce the friction drag. The results of experimental data and numerical simulation both prove that the bionic scales of Paramisgurnus dabryanus can achieve the underwater drag reduction function. This research provides a reference for drag reduction in marine industries and fluid delivery applications.

Influence of Synthetic Jets on Multiscale Features in Wall-Bounded Turbulence

This experimental research focuses on the impacts of submerged synthetic jets on a fully-developed turbulent boundary layer (TBL) under a drag reduction working case. Two-dimensional velocity vectors in the flow field are captured with the aid of a particle image velocimetry (PIV) system. Proper orthogonal decomposition (POD) analyses provide evidence that synthetic jets notably attenuate the induction effect of prograde vortex on the low-speed fluid in large-scale fluctuation velocity field, thereby weakening the bursting process of near-wall turbulent events. Furthermore, the introduced perturbance redistributes the turbulent kinetic energy (TKE) and concentrates the TKE onto small-scale coherent structures. Modal time coefficients in various orders of POD are divided into components of multiple frequency bands by virtue of complementary ensemble empirical mode decomposition (CEEMD). It is found that the turbulence signals are shifted from low-frequency to high-frequency bands thanks to synthetic jets, thus revealing the relationship between scales and frequency bands. One further method of scale decomposition is proposed, that is, the large-scale fluctuating flow field will be obtained after removing the high-frequency noise data with the help of continuous mean square error (CMSE) criterion.

Low Air Drag Surface via Multilayer Hierarchical Riblets

Riblets inspired by shark skin exhibit a great air drag reduction potential in many industries, such as the aircraft, energy, and transportation industries. Many studies have reported that blade riblets attain the highest air drag reduction ability, with a current limit of ∼11%. Here, we propose multilayer hierarchical riblets (MLHRs) to further improve the air drag reduction ability. MLHRs were fabricated via a three-layer hybrid mask lithography method, and the air drag reduction ability was studied in a closed air channel. The experimental results indicated that the maximum air drag reduction achieved with MLHRs in the closed channel was 16.67%, which represents a 52% higher reduction than the highest previously reported. Conceptual models were proposed to explain the experiments from a microscopic perspective. MLHRs enhanced the stability of lifting and pinning vortices, while vortices gradually decelerated further, reducing the momentum exchange occurring near the wall. This verified that MLHRs overcome the current air drag reduction limit of riblets. The conceptual models lay a foundation to further improve the air drag reduction ability of riblets.