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Inspired by the shark skin shield scale structure and the excellent elasticity of shark skin, an elastic texture composed of the arc grooves and the rectangular convex structure evenly arranged in the lower layer is constructed to improve the lubrication performance of the friction pair. Under different geometric parameters and speeds, the elastic deformation, stress distribution, friction coefficient, and oil film bearing capacity of the friction pair are compared to analyze the influence of sharkskin texture on the lubrication performance. Firstly, the fluid–solid coupled method establishes a 3D simulation model of the elastic hydrodynamic lubrication. Additionally, the elastomeric bearing specimens with sharkskin bionic texture are fabricated using 3D printing technology, and visualization experiments are performed to validate the simulation results. During the sliding friction process, the shark skin texture can appropriately intensify elastic deformation and uniform overall stress distribution. With the increase in the dimensionless width of the rectangular convex structures, the overall elastic deformation intensifies, the bearing capacity of the oil film thickens, and the friction coefficient decreases. In this study, when the depth-width ratio of the arc groove is 0.1 and the dimensionless width of the rectangular convex structures is 0.125, the friction coefficient of the elastic bearing is the minimum, and the maximum reduction percentage is about 15.3%. [ABSTRACT FROM AUTHOR]

The characteristic time periodicity τ* and the spatial characteristic wavelength λ of extrusion flow instabilities of a linear and a branched commercial polyethylene (PE) are characterized via capillary rheology, optical analysis and modeled. The two investigated polyethylenes have the similar weight average molecular weight (Mw). The characteristic time periodicity τ* is obtained and compared using three methods: (i) a highly sensitive pressure slit die, (ii) a new online optical analysis method based on the construction of a space–time diagrams, and (iii) an offline transmission polarization microscopy. In addition, the spatial characteristic wavelength λ is quantified by offline transmission polarization microscopy. The characteristic time periodicity τ* of the extrusion flow instabilities follows a power law behavior as a function of apparent shear rate to a power of −0.7 for both materials,τ*∝γ̇app.−0.7. A qualitative model is used to predict the spatial characteristic wavelength λ of extrusion flow instabilities as well. It is found that the characteristic spatial wavelength λ and the characteristic time periodicity τ* have an Arrhenius temperature‐dependent behavior. [ABSTRACT FROM AUTHOR]

The flow curves of polymers often reveal the onset of melt instabilities such as sharkskin, stick–slip, or gross melt fracture, in order of increasing shear rates. The focus of this work lies in the application of the Göttfert sharkskin option to the investigation of flow curves of styrene-butadiene rubber (SBR) compounds. The sharkskin option consists of highly sensitive pressure transducers located inside a slit die of a capillary rheometer. This tool allows the detection of in-situ pressure fluctuation characteristics of different melt instabilities. It is shown that the change of slope of the transition region in the flow curves is only linked to slip. Dynamic Mechanical Analysis (DMA) measurements furthermore show that the shear rate at which the change of slope can be observed shows the same temperature dependency as the viscous and elastic properties of the compounds. [ABSTRACT FROM AUTHOR]

The extrusion flow instabilities of commercial polybutadiene (PBD) are investigated as a function of the different extrusion die geometries, such as round capillary, slit, and complex cross‐section profile slit dies via capillary rheology. Qualitative models are used to fit the experimental data for the spatial characteristic wavelength (λ) of the appearing extrusion flow instabilities. A new qualitative model for the slit die geometry, rectangular cross‐section, is derived based on the theoretical concept of the "two layers" extrudate and the force balance at the die exit region. The proposed qualitative model for the slit die geometry is used to predict the spatial characteristic wavelength (λ) for extrudates obtained by complex cross‐section profile slit die geometries similar to industrial manufacturing. Correlation between the ratio of the extensional (Ys) and shear (σx) stress at the die exit area and the characteristic dimension, height H for slit dies and diameter D for round capillary dies, is presented. Moreover, a geometry‐dependent model is used to predict the spatial characteristic wavelength (λ) of the extrusion flow instabilities from a round capillary die to a slit die and vice versa. [ABSTRACT FROM AUTHOR]

The extrusion flow instabilities of two commercial styrene‐butadiene rubbers are investigated as they vary in isomer content (1,4‐cis, 1,4‐trans, and 1,2 conformation) of the butadiene monomer and the molecular architecture (linear, branched). The investigated samples have similar multimodal molecular weight distribution. Two geometries of extrusion dies, slit and round capillary, are compared in terms of the type and the spatial characteristics of the flow instabilities. The latter are quantified using three methods: a highly pressure sensitive slit die, online and offline optical analysis. The highly pressure‐sensitive slit die has three piezoelectric pressure transducers (Δt ≈ 10−3 s and Δp ≈ 10−5 bar) placed along the die length. The characteristic frequency (fChar.) of the flow instabilities follows a power law behavior as a function of shear rate to a 0.5 power for both materials, fChar.∝γ˙app.0.5. A qualitative model is used to predict the spatial characteristic wavelength (λ) of the flow instabilities from round capillary to slit dies and vice versa. Slip velocities (Vs) are used to quantify the slippage at slit and round capillary dies as well. [ABSTRACT FROM AUTHOR]

Surface roughness and sharkskin of extruded polymers, including blends are affected by the morphology and processing conditions. In this study, different effects on the roughness of the polymer blend extrudates were investigated. Co-continuous poly(ethylene) oxide/ethylene-vinyl acetate (PEO/EVA) blends with three different molecular weight (Mw) PEOs were compounded successfully. It was found that the better co-continuity of the structure and smoother surface were achieved for lower Mw PEO/EVA blend because of more effective stress transfer in the PEO phase. The effect of processing temperature was also studied with decreasing processing temperature reducing the surface roughness of the high Mw PEO/EVA blend, which was also achieved as a result of improved co-continuous morphology by adjusting the viscosity and elasticity ratio with shifting temperatures. [ABSTRACT FROM AUTHOR]

Two guide bar warp knitted fabrics are used as technical textile applications nowadays. In this, Part III of these series of papers, models for basic two guide bar warp knitted fabrics are created and given based on Parts I and II. Modeling of warp knitted fabrics started in the 1960s with some basic geometrical models and it was continued with using Non Uniform Basis Spline (NURBS) curves and surfaces. There were discontinuities in the loop parts in early created geometrical models and the NURBS models needed some control points from some sources. In this work instead of using NURBS curves, modeling of warp knitted fabric is attempted by using known curves. During creation of the models, curvature equalities are attained as much as possible, loop connection parts are modeled by having two continuously connected parabolas for the Double Tricot fabric and the loop connection parts of the other two bar fabrics are obtained by applying a sinus square curve to the connection part of the Double Tricot fabric. The models created are versatile and they can be changed and used for special cases. Models created are drawn to scale by using the 3DS-Max Computer Graphical Program. At first glance, it is seen that the model shapes are similar to the real fabric structures, which can be observed on photographs of them. [ABSTRACT FROM AUTHOR]

Natural sharkskin features staggered-overlapped and multilayered architectures of riblet-textured anisotropic microdenticles, exhibiting drag reduction and providing a flexible yet strong armor. However, the artificial fabrication of three-dimensional (3D) sharkskin with these unique functionalities and mechanical integrity is a challenge using conventional techniques. In this study, it is reported on the facile microfabrication of multilayered 3D sharkskin through the magnetic actuation of polymeric composites and subsequent chemical shape fixation by casting thin polymeric films. The fabricated hydrophobic sharkskin, with geometric symmetry breaking, achieves anisotropic drag reduction in frontal and backward flow directions against the riblet-textured microdenticles. For mechanical integrity, hard-on-soft multilayered mechanical properties are realized by coating the polymeric sharkskin with thin layers of zinc oxide and platinum, which have higher hardness and recovery behaviors than the polymer. This multilayered hard-on-soft sharkskin exhibits friction anisotropy, mechanical robustness, and structural recovery. Furthermore, coating the MXene nanosheets provides the fabricated sharkskin with a low electrical resistance of ≈5.3 Ω, which leads to high Joule heating (≈229.9 °C at 2.75 V). The proposed magnetomechanical actuation-assisted microfabrication strategy is expected to facilitate the development of devices requiring multifunctional microtextures., (© 2023 Wiley-VCH GmbH.)

Salvinia leaf and sharkskin are prime examples of nature's marvel. Salvinia leaf-inspired superhydrophobic surfaces keep themselves clean and reduce drag in fluid flow. Sharkskin also reduces drag in turbulent flow and inhibits biofouling. Therefore, the prospect of having a drag-reducing surface with both salvinia leaf and sharkskin properties is attractive. However, fabricating such a surface is difficult, and the current fabrication methods require at least two separate steps. In addition, the mechanisms of drag reduction of salvinia leaf and sharkskin are different, and their combined effect on the flow field is not well understood. In this study, we produced a PTFE surface that mimics sharkskin in its surface pattern and copies the superhydrophobic nature of the salvinia leaf in its microstructure. This surface was fabricated by laser machining and tested in a closed channel under turbulent flow conditions. We measured the pressure drop at different Reynolds numbers on this surface both in pre-wet and non-pre-wet conditions and compared the result with pressure drop data on four other PTFE samples: two types of non-superhydrophobic sharkskin inspired surface (riblets), a superhydrophobic surface, and a non-machined surface. Both the non-superhydrophobic riblets and the superhydrophobic sample reduced drag compared to the non-machined surface. However, we observed a lack of drag reduction by the superhydrophobic riblets sample. We presented a qualitative explanation for the lack of drag reduction and concluded that the modifications of the flow field by the two drag reduction mechanisms are not beneficial for overall drag reduction in our experiment. [ABSTRACT FROM AUTHOR]

The processability of different grades of polyisobutylene (PIB) was investigated using a capillary rheometer. Direct focus was given to the occurrence of melt fracture phenomena, such as sharkskin and gross melt fracture (GMF). The influence of molecular weight (MW) of PIB, temperature and die entrance angle on melt fracture was examined in detail. Due to their highly elastic nature, high MW PIBs were found to exhibit gross melt fracture instability even at low shear rates, rendering their processing an impossible task. An increase in temperature resulted in postponing both instabilities (sharkskin and gross) to higher shear rates, thus making their processing easier. Finally, decreasing the entrance angle below a critical value resulted in postponing the onset of GMF to higher shear rates. [ABSTRACT FROM AUTHOR]

The biomimetic sharkskin riblet has attracted more attention in engineering and research owing to the economic benefit of drag-reducing and antifouling properties. Although several works have optimized the parameters for a number of riblet designs, there is not yet a study dedicated to optimizing for rectangular cuboidal riblet parameters. Hence, this work proposes a new numerical optimization of riblet orientations and arrangements of a three-dimensional rectangular cuboidal riblet. The shear-stress transport k-omega was adopted as an appropriate turbulence model for the simulation. The results showed that the riblets efficiently improved drag reduction regardless of the flow state. Riblets oriented perpendicularly to the flow direction showed the greatest performance of 11.3% and 6% in the laminar and turbulent flow, respectively. Moreover, the analysis of the flow field characteristics near the wall revealed a significant improvement in terms of reduction in near-wall velocity and shear stress. This work offers new perspectives on the role of the intricate multifunctional architecture of shark-scale structures in improving swimming speed and opening new doors for marine and underwater applications. • A numerical study of 3D shark riblets with different orientation and arrangement. • The SST k − ω model was adapted and laminar and turbulent conditions were discussed. • Simplified riblets greatly improved drag reduction regardless of the flow state. • Maximum performance was 11.3% and 6% in laminar and turbulent flow, respectively. • A significant reduction in near-wall velocity and shear stress was revealed. [ABSTRACT FROM AUTHOR]

The artificial drag-reducing surface with biological sharkskin morphology was fabricated by the direct bio-replicated method, and the satisfactory drag reduction effect was validated in the water tunnel. However, the splicing step is necessary for the area larger than the entire surface of shark, the consequence of which is that the complexity of process will be increased seriously. At the meanwhile, two adverse effects may appear, namely, the stress contraction on the jointing seams and the drop-out phenomenon. Therefore, it is urgent to manufacture the continuous biomimetic sharkskin surface. In this paper, a novel and facile method to fabricate the drag-reducing surface on a semi-cured coating based on biological sharkskin is investigated and explored. Firstly, the sputtering and photo lithography processes are put into application to eliminate the wedge angle on scale's back. Secondly, the relationship between forming precision and curing degree of epoxy resin is inspected and the appropriate time-zone fitting for bio/micro-rolling is attained. Thirdly, the continuous sharkskin surface is manufactured in a large area. Lastly, for evaluating the effect of drag reduction, an experiment is performed. The experimental results show the drag reduction efficiency of the surface fabricated by the bio/micro rolling method, which can expand more applications in the fluid engineering. [ABSTRACT FROM AUTHOR]

Reducing energy consumption and protecting the environment has always been becoming the pursing goal and object for mankind, especially in the past several decades. Our living environment is all surrounded by the fluids, and the friction in turbulence has progressively developed into the important proportion of energy consumption. Therefore, how to realize the drag reduction in turbulence has turned into the imperative issue to be resolved, and many scholars have made great contributions in the field with achieving so many profits. In this paper, three different typical biological functional surfaces including sharkskin, lotus leaf and dolphin skin are reviewed and generalized systematically and comprehensively, the drag reduction mechanisms based on boundary layer control are generalized. The paper will enable the potential readers to better understand the recent progresses in exploring drag reduction technologies in turbulence, with improving the comprehension of the drag reduction mechanism and extending the relevant technologies into more applications. [ABSTRACT FROM AUTHOR]

The superiority of sharkskin is the consequence of nature selection and self-evolution. The nano/microhierarchical structure covering over sharkskin can match its living surroundings perfectly, whereas the best drag reducing effect cannot be implemented at all circumstances. Therefore, it is necessary to adjust the size and shape of sharkskin morphology to accommodate more potential flowing conditions. In this paper, the stretching deformed fabrication process of sharkskin surface is explored and investigated, and the super-hydrophobic and hydrodynamic drag reduction effect is inspected. The experimental results in the water tunnel indicate that the stretched sharkskin can expand the speed scope of applications with satisfactory drag reduction effect. Additionally, the drag reduction mechanism is explained and derived comprehensively, which has important significance to apprehend sharkskin effect. [ABSTRACT FROM AUTHOR]

With the rapid development of science and technology, increasing research interests have been focused on environment protection, global warming, and energy shortage. At present, reducing friction force as much as possible has developed into an urgent issue. Sharkskin effect has the potential ability to lower viscous drag on the fluid-solid interface in turbulence, and therefore, how to fabricate bio-inspired sharkskin surfaces is progressively becoming the hot topic. In this review, various methods of fabricating drag reduction surfaces covering biological sharkskin morphology are illustrated and discussed systematically, mainly involving direct bio-replicated, synthetic fabricating, bio/micro-rolling, enlarged solvent-swelling, drag reduction additive low-releasing, trans-scale enlarged three-dimensional fabricating, flexible printing, large-proportional shrunken bio-replicating, ultraviolet (UV) curable painting, and stretching deformed methods. The overview has the potential benefits in better acquainting with the recent research status of fabricating sharkskin surfaces covering the biological morphology. [ABSTRACT FROM AUTHOR]

This publication is concerned with the development of a novel high sensitivity instability detection die for a laboratory size extruder. The high sensitivity system consists of piezoelectric transducers placed along the die length to monitor local pressure fluctuations, having temporal and pressure resolution of up to Δ t ≈ 10−3 s and Δ p ≈ 10−5 bar, at a nominal pressure of 500 bar. The system is tested on short chain branched polyethylenes, namely on the detection and characterization of the sharkskin instability. It is shown that the determined instabilities are related to characteristic peaks in the corresponding in situ pressure Fourier-transform spectra and that other peaks are caused by different, but defined, sources. The in situ results obtained are validated through optical image analysis of the instabilities. [ABSTRACT FROM AUTHOR]

It is well-known that by increasing the flow rate in polymer extrusion, the flow becomes unstable and the smooth extrudate surface becomes wavy and disordered to an increasing degree. In order to investigate the mechanisms responsible for these instabilities we perform a linear stability analysis of the steady extrusion of a viscoelastic fluid flowing through a planar die under creeping flow conditions. We consider the Phan–Thien–Tanner (PTT) model to account for the viscoelasticity of the material. We employ the mixed finite element method combined with an elliptic grid generator to account for the deformable shape of the interface. The generalized eigenvalue problem is solved using Arnoldi's algorithm. We perform a thorough parametric study in order to determine the effects of all material properties and rheological parameters. We investigate in detail the effect of the interfacial tension and the presence of a deformable interface. It is found that the presence of a finite surface tension destabilizes the flow as compared to the case of the stick–slip flow. We recognize two modes, which become unstable beyond a critical value of the Weissenberg number and perform an energy analysis to examine the mechanisms responsible for the destabilization of the flow and compare against the mechanisms that have been suggested in the literature. [ABSTRACT FROM AUTHOR]

Natural selection, survival of the fittest. Through millions of years' evolution, shark has become one of the fastest swimming animals in the ocean, and it is very well-known for sharkskin effect, especially for "sharkskin swimsuit". Due to its great superior properties in drag reduction, anti-wear, self-cleaning and so on, the investigations on the essential mechanisms and fabricating methods have attracted so much attention from all over the world, and the achievements have been widely put into application in industry, agriculture, transportation, airspace and so on, and so lots of profits have been obtained so far. In this paper, the method of fabricating artificial composite drag reduction surface based on biological sharkskin morphology and mucus nano-long chain is investigated and studied, the chemical, mechanical and hydrodynamic properties are explored from different aspects in depth, in which, the experimental results in water tunnel showed that the drag-reducing efficiency could surpass 20% with the smooth skin as reference, and the drag reduction mechanism is systematically explained and discussed from different aspects, which has important significance to understand the recent research status and expand the applications of sharkskin in the fluid engineering. [ABSTRACT FROM AUTHOR]

It has gradually developed into an undisputable fact that sharkskin surface has the obvious drag reduction effect compared with the absolutely smooth skins, and it has been put into application widely, which has brought great advantages and profits in daily life, industry and agriculture. Because some problems in turbulence are not resolved completely and perfectly, the drag reduction mechanism of real sharkskin has also not been understood absolutely and thoroughly so far. However, many researchers have carried out lots of the relevant experiments and analyses, very plentiful and important conclusions are obtained, which can explain some certain phenomena of sharkskin drag reduction effect. An overview of exploring drag reduction mechanism of real sharkskin surface is systemically presented in detail. These mechanisms include inhibition of turbulence using micro/nano structured morphology, influence of scale's attack angles, nano-long chains and boundary layer slipping based on superhydrophobicity. This paper will improve the comprehension of the drag reduction mechanism and expand biomimetic sharkskin technology into more applications. [ABSTRACT FROM AUTHOR]

Shark is the fast swimming animal in the ocean, and it is well-known for sharkskin effect. Sharkskin is covered by the tiny and rigid scales, which can stick out of the viscous sublayer and effectively inhibit the occurrence of turbulence and reduce the wall resistance. The longitudinal sections of the scale surface are not parallel to the flowing direction, but at a particular attack angle, which could be considered as a supplement to the mechanism of sharkskin effect. At present, it is almost impossible to observe the variation of scales' attack angles during the biological shark's swimming. Although the real sharkskin surfaces with original sizes have been fabricated by the bio-replicated method, the attack angles cannot be exactly controlled, the result of which is that the drag-reducing efficiency of sharkskin with different attack angles cannot be obtained by the experimental methods. In this paper, the highly accurate three dimensional digital model is exactly built through the high-accurate scanning the biological sharkskin, and the micro flow field is investigated comprehensively and deeply, especially that, the influence of scales' attack angles on drag-reducing efficiency is analyzed, which has the important significance on exploring the sharkskin effect. [ABSTRACT FROM AUTHOR]

Employing a finite element computing scheme implemented onto the Leonov viscoelastic model, we newly describe various kinds of melt fracture for the extrudate exiting from the Poiseuille flow in the contraction channel with wall slip ignored. Four types such as sharkskin, gross melt fracture, slow surface undulation, and ripples are found depending on the flow conditions like the flow rate and liquid property, and they are expressed as 2D elastic instability in this inertialess flow regime. Even though not considered, the effect of die wall slip has to be included in the realistic modeling of melt fracture. However, here, we make the first attempt to analyzing extrudate instability in terms of purely fluid mechanical factors. As a result, each type of melt fracture is verified to result from the different origin, and thus, geometric singularities and streamline vortices at contraction corner and die exit determine this type of extrudate distortion. [ABSTRACT FROM AUTHOR]

Numerous facts have validated that sharkskin possesses the obvious drag reduction effect in certain turbulent flowing stations, and it has huge potential and important applications in the fields of agriculture, aerospace, industry, transportation, daily life and so on, which have attracted increased attention throughout the world. To meet the increasing requirements of practical applications, it has been progressively developing into an urgent problem to manufacture sharkskin surfaces with perfect forming quality and high drag-reducing effect. In this paper, the vacuum casting method is put forward to fabricate the drag-reducing surface with the real sharkskin morphology by eliminating the air bubbles from the bottom of sophisticated morphology in the pouring process. Meanwhile, a novel and facile 'marking key point' method is explored and adopted to search for the corresponding biological sharkskin and negative template, a more convincing way to evaluate the replicating precision is systematically illustrated and the hydrodynamic experiment is carried out in the water tunnel. The results indicate that wall resistance over sharkskin surface replicated by the vacuum casting method can be decreased by about 12.5% compared with the smooth skin. In addition, the drag reduction mechanism hypotheses of sharkskin are generalized from different respects. This paper will improve the comprehension of the sharkskin fabrication method and expand biomimetic sharkskin technology into more applications in the fluid engineering. [ABSTRACT FROM AUTHOR]

• The moving walls (rotating rollers) provide a flexible mean to control die swell and instabilities • The smooth convergent geometry of the rotating die prevents volume instabilities normally related to entrance effect. However very large stresses on the melt are generated in the stationary mode so much so that surface roughness instabilities, not usually reported with polystyrene, appear on the extrudate surface. • This surface roughness resembles sharkskin but the mechanism leading to it is different to that leading to sharkskin. • This observation is a first as PS is reported to never exhibit surface roughness. • The rotating die is a versatile tool to produce sheet of various thicknesses as the gap can be changed by moving the rollers up and down. • The experimental data presented are underpinned by simple theoretical considerations, showing the importance of the wall shear stress and the changes in extension at the die exit. Thermoplastics extruded from dies will always swell and above a critical flow rate display instabilities (sharkskin, stick-spurt or gross melt fracture). Prior research has shown that the best way to suppress these instabilities is to reduce the entry converging angle using a smooth convergence and induce permanent wall slip. In this research we go a step further by allowing the walls to move using a rotating roller die. Thus both extrudate swell and flow instabilities become controllable. This paper presents data to support this claim. The practical benefits are important as stable operation at higher flow rates become permissible. Also, by providing extra control variables, this device becomes a useful tool to help unravel the causes of the various instabilities that arise in polymer melt die extrusion. A first from this research, using this roller die geometry we were able to tease out surface roughness instability with polystyrene hitherto not reported. [ABSTRACT FROM AUTHOR]