The problem of contact line pinning on surfaces is pervasive and contributes to problems from ring stains to ice formation. Here we provide a single conceptual framework for interfacial strategies encompassing five strategies for modifying the solid-liquid interface to remove pinning and increase droplet mobility. Three biomimetic strategies are included, (i) reducing the liquid-solid interfacial area inspired by the Lotus effect, (ii) converting the liquid-solid contact to a solid-solid contact by the formation of a liquid marble inspired by how galling aphids remove honeydew, and (iii) converting the liquid-solid interface to a liquid-lubricant contact by the use of a lubricant impregnated surface inspired by the Nepenthes Pitcher plant. Two further strategies are, (iv) converting the liquid-solid contact to a liquid-vapor contact by using the Leidenfrost effect, and (v) converting the contact to a liquid-liquid-like contact using slippery omniphobic covalent attachment of a liquid-like coating (SOCAL). Using these approaches, we explain how surfaces can be designed to have smart functionality whilst retaining the mobility of contact lines and droplets. Furthermore, we show how droplets can evaporate at constant contact angle, be positioned using a Cheerios effect, transported by boundary reconfiguration in an energy invariant manner, and drive the rotation of solid components in a Leidenfrost heat engine. Our conceptual framework enables the rationale design of surfaces which are slippery to liquids and is relevant to a diverse range of applications.

Applying hot-embossing technology, a simple and cost-effective method for the fabrication of microstructured High Density Polyethylene (HDPE) surfaces with a robust superhydrophobic wetting state is proposed. Micro-meshes and micro-grooves in the flexible template are filled by the PE melt in the hot embossing process. Subsequently, a two-stage microstructure on the PE film surface is formed. This PE film exhibits a contact angle of 151.8? ± 2? and roll-off angle of > 90? when a 5 μL water droplet is dropped on its surface. Water pinning ability on the surface is figured out and roll-off angles are as a quadratic function of specified water droplet volume. Specifically, a 356 μN water pinng force appears on the HDPE film due to the solid–vapor composite interface on its surface. Meanwhile, self-cleaning and immersion tests reveal that the HDPE surface with micro-pillars exhibit robust Cassie impregnating wetting state against external pressure. The proposed method for facial fabrication of microstructured surfaces is an appropriate candidate for the development of droplet manipulation and functional biomimetic polymer surfaces.

Although the toughening of Calcium phosphate (CaP) scaffold by the addition of fiber has been well recognized, integrated mechanical, structural and functional considerations have been neglected in the design and fabrication of CaP scaffold implant. The emerging 3D printing provides a promising technique to construct CaP scaffold with precise size and elaborate microstructure. However, the most challenge is to extrude smoothly the CaP paste containing fibers for frequently-used extrusion-based 3D printing. In this study, frozen section and chemical dispersant (Pluronic F127, F127) were employed jointly to prepare non-aggregated polylactic-co-glycolic acid (PLGA) fibers. The injectability of CaP pastes with well dispersed PLGA fibers was more than 90% when the content of PLGA fibers was no more than 3 wt%. Meanwhile rheological property of CaP pastes with well dispersed fibers showed shear thinning, which were both beneficial to extrude CaP paste with well dispersed fibers for 3D printing. Moreover, these CaP scaffolds showed ductile fracture behavior due to the pullout and bridging effect of PLGA fibers. The cell proliferation and alkaline phosphatase (ALP) activity indicated that 3D printed CaP scaffold containing PLGA fibers possesses excellent biocompatibility and facilitate osteogenic differentiation ability. Thus, it was feasible to print CaP pastes with well dispersed PLGA fibers to construct toughening CaP scaffolds with the higher shape fidelity and complex structures, which had significant clinical potentials in osteoanagenesis due to their higher toughness and excellent biocompatibility.

Collagen is one of the most versatile tissues of living organisms that comes in many shapes and sizes, providing functions ranging from tissue matrix through, ligament formation up to enabling mineralization in teeth. The detailed light microscopy and Scanning Electron Microscopy (SEM) observations conducted in this study, allowed us to investigate morphology, sizes and crimp patterns of collagen fibers observed in crocodile skin and teeth. Moreover, the microscopy study revealed that although two completely different tissues were investigated, many similarities in their structure based on collagen fibers were observed. Collagen type I is present in crocodile skin and teeth, showing the flexibility in naturally constructed tissues to obtain various functions. The crimp size investigation of collagen fibers confirmed experimentally the theoretical 67 nm D-periodicity expected for collagen type I. The collagen in teeth provides a matrix for crystal growth and in the skin provides flexibility and is a precursor for corneous scales. Importantly, these observations of the collagen in the skin and tooth structure in crocodiles play an important role in designing biomimetic materials with similar functions and properties.

At present, biomimetic antifouling research objects are mostly concentrated on the fast-moving marine organism, but the anti-fouling effect of the low-speed or static marine equipment is not obvious. This paper describes the anti-fouling mechanism of soft coral (Sarcophyton trocheliophorum), including the physical defense mechanism and the bactericidal ability of mucus and coral powder. As a sessile organisms, soft coral lacks escape mechanism. Therefore, the study on its antibacterial strategy is significant because it can provide theoretical guidance for static antifouling. Results showed that the live soft coral would molt in unfriendly environment, and the secreted mucus could defend themselves against fouling microorganism. Then, Liquid Chromatography-Mass Spectrometry (LC-MS) analysis was conducted to identify the bioactive compounds of the coral powder and mucus. Results revealed that both powder and mucus contained a wide variety of toxic components, which had bactericidal effects. Moreover, at the same concentration, the inhibitory effect of the main components on Gram-negative bacteria was stronger than that on positive bacteria. These findings enhance the understanding about the antifouling mechanism of soft coral and provide new ideas for design and prepare novel antifouling strategy to combat biofouling under static condition.

The application of trenchless technology is the trend of underground public facilities’ installation, replacement and repairing. As the soil-engaging component during penetrating bore, the working resistance of penetration head has remarkable effect on energy consumption of the whole working process. Some typical soil-digging animals, like pangolin and earthworm, they own special micro structures on their surface. It has been widely proved that some micro geometrical structures can effectively reduce adhesion resistance. Four kinds of bionic penetration heads were designed by imitating micro geometrical structures inspired by the soil animals. In this work, the real time jacking forces of the bionic penetration heads were measured and compared with a smooth penetration head (control group) without micro geometrical structures. The result indicated that the jacking forces of the bionic penetration heads were smaller than that of the smooth penetration head. This proved that the bionic penetration heads have the ability of reducing adhesion resistance. The vertical concave penetration head got the smallest jacking force, of which the average jacking force was 18.7% lower than that of the smooth penetration head. The interaction between soil and bionic surface of penetration head was discussed on the condition of wet friction. The bionic surface reduced the adhesion resistance by disturbing the soil and braking the continuous water film between soil and the surface of the penetration head.

Redundancy facilitates some of the most remarkable capabilities of humans, and is therefore omni-present in our physiology. The relationship between redundancy in robotics and biology is investigated in detail on the Series Elastic Dual-Motor Actuator (SEDMA), an actuator inspired by the kinematic redundancy exhibited by myofibrils. The actuator consists of two motors coupled to a single spring at the output. Such a system has a redundant degree of freedom, which can be exploited to optimize aspects such as accuracy, impedance, fault-tolerance and energy efficiency. To test its potential for human-like motions, the SEDMA actuator is implemented in a hopping robot. Experiments on a physical demonstrator show that the robot’s movement patterns resemble human squat jumps. We conclude that robots with bio-inspired actuator designs facilitate human-like movement, although current technical limitations may prevent them from reaching the same dynamic and energetic performance.

The attitude control system of a flapping-wing flying robot plays an important role in the precise orientation and tracking of the robot. In this paper, the modeling of a bird-like micro flapping-wing system is introduced, and the design of a sliding mode controller based on an Extended State Observer (ESO) is described. The main design difficulties are the control law and the adaptive law for the attitude control system. To address this problem, a sliding mode adaptive extended state observer algorithm is proposed. Firstly, a new extended state approximation method is used to estimate the final output as a disturbance state. Then, a sliding mode observer with good robustness to the model approximation error and external disturbance is used to estimate the system state. Compared with traditional algorithms, this method is not only suitable for more general cases, but also effectively reduces the influence of the approximation error and interference. Next, the simulation and experiment example is given to illustrate the implementation process. The results show that the algorithm can effectively estimate the state of the attitude control system of the flapping-wing flying robot, and further guarantee the robustness of the model regarding error and external disturbance.

Bionic inspiration from human thumb and index finger was the drive to design a high-performance two-finger dexterous hand. The size of each phalanx and the motion range of each joint in the human thumb and index finger were summarized, and the features of three grasping patterns were described in detail. Subsequently, a two-finger dexterous bionic hand with 6 Degrees of Freedom (DoFs) was developed. Both the mechanical thumb and index finger were made up of three rigid phalanx links and three mechanical rotation joints. Some grasp-release tests validated that the bionic hand can perform three grasping patterns: power grasp, precision pinch and lateral pinch. The grasping success rates were high under the following cases:  (1) when power grasping was used to grasp a ring with external diameter 
20 mm – 140 mm, a cylinder with mass < 500 g, or objects with cylinder, sphere or ellipsoid shape; (2) when the precision pinch was used to grasp thin or small objects; (3) when the lateral pinch was used to grasp low length-to-width ratio of objects. The work provided a method for developing two-finger bionic hand with three grasping patterns, and further revealed the linkage between the difference in finger structure and size and the hand manipulation dexterity.

The ability to grip unhatched eggs is a skill exploited by the ants Harpegnathos venator, as they care their brood in tunneled nests, which is of extreme difficulty to keep the eggs intact while gripping. In this paper we propose a mathematical modeling method to elucidate the mechanism of such a gripping behavior in the ant mandibles. The new method can be subdivided into following steps. As a preliminary, the concavity geometry and mandible kinematics are examined experimentally. Second, coordinate transformation is used to predict the real-time spatial topology of the concavity. Third, we come up with a new method to quantify the workspace required to grip and the contact area between the concavity and ant egg. Our model indicates that the biaxial rotation fashion with specialized concavities can reduce workspace by 40% and increase contact area by 53% on average compared with the uniaxial rotation pattern, which augments success rate of gentle gripping. This methodology may have applications in evaluating mechanical performance in both natural and artificial grippers.

Most commercial spine analogues are not intended for biomechanical testing, and those developed for this purpose are expensive and yet still fail to replicate the mechanical performance of biological specimens. Patient-specific analogues that address these limitations and avoid the ethical restrictions surrounding the use of human cadavers are therefore required. We present a method for the production and characterisation of biofidelic, patient-specific, Spine Motion Segment (SMS = 2 vertebrae and the disk in between) analogues that allow for the biological variability encountered when dealing with real patients. Porcine spine segments (L1–L4) were scanned by computed tomography, and 3D models were printed in acrylonitrile butadiene styrene (ABS). Four biological specimens and four ABS motion segments were tested, three of which were further segmented into two Vertebral Bodies (VBs) with their intervertebral disc (IVD). All segments were loaded axially at 0.6 mm?min?1 (strain-rate range 6×10?4 s?1 – 10×10?4 s?1). The artificial VBs behaved like biological segments within the elastic region, but the best two-part artificial IVD were ~15% less stiff than the biological IVDs. High-speed images recorded during compressive loading allowed full-field strains to be produced. During compression of the spine motion segments, IVDs experienced higher strains than VBs as expected. Our method allows the rapid, inexpensive and reliable production of patient-specific 3D-printed analogues, which morphologically resemble the real ones, and whose mechanical behaviour is comparable to real biological spine motion segments and this is their biggest asset.

Most commercially available spine analogues are not intended for biomechanical testing, and the few that are suitable for using in conjunction with implants and devices to allow a hands-on practice on operative procedures are very expensive and still none of these offers patient-specific analogues that can be accessed within reasonable time and price range. Man-made spine analogues would also avoid the ethical restrictions surrounding the use of biological specimens and complications arising from their inherent biological variability. Here we sought to improve the biofidelity and accuracy of a patient-specific motion segment analogue that we presented recently. These models were made by acrylonitrile butadiene styrene (ABS) in 3D printing of porcine spine segments (T12–L5) from microCT scan data, and were tested in axial loading at 0.6 mm?min?1 (strain rate range 6×10?4 s?1 – 10×10?4 s?1). In this paper we have sought to improve the biofidelity of these analogue models by concentrating in improving the two most critical aspects of the mechanical behaviour: the material used for the intervertebral disc and the influence of the facet joints. The deformations were followed by use of Digital Image Correlation (DIC) and consequently different scanning resolutions and data acquisition techniques were also explored and compared to determine their effect. We found that the selection of an appropriate intervertebral disc simulant (PT Flex 85) achieved a realistic force/displacement response and also that the facet joints play a key role in achieving a biofidelic behaviour for the entire motion segment. We have therefore overall confirmed the feasibility of producing, by rapid and inexpensive 3D-printing methods, high-quality patient-specific spine analogue models suitable for biomechanical testing and practice.

The Leading-Edge (LE) serrations on owls’ wings are known to be responsible for silent flight. However, this design has rarely been applied to reduce the noise of rotational rotor propellers and the morphologies of the existing serration designs are diverse. Here, we present a comparative study of LE serrations with different morphologies in terms of the effectiveness in suppressing noise and promoting thrust forces. The performances of biomimetic propellers are investigated by Computational Fluid Dynamics (CFD) simulations and rotation experiments. The simulation results reveal that LE serrations could reduce velocity fluctuations and change the 
lamina-turbulent transition and turbulence distribution on the suction surface of propeller, but the morphology of the serrations influences its effectiveness. Rotation testing results indicate that the sawtooth propeller has the best performance on noise reduction (on average 
2.43 dB and in maximum 4.18 dB) and simultaneously enhancing the thrust forces (3.53%). The largest practical noise reductions (4.73 dB and 3.79 dB) using the sawtooth propeller are observed when the quad-rotor Unmanned-Aerial Vehicle (UAV) is hovering at heights of 5 m and 8 m, respectively. Our results indicate the robustness and usefulness of owl-inspired biomimetic serration devices for aero-acoustic control and aerodynamic performance promotion on propeller designs. This finding is expected to contribute to suppressing the sound of propeller and the rotor-based aircraft.

The aerodynamic noise generated by the centrifugal fan used in the air conditioner is related to the comfort of human living and working, which can be controlled by using the bionic design and optimization of key components of centrifugal fan. Inspired by the non-smooth leading edge of long-eared owl wing, eight kinds of volute tongues are proposed to reduce the aerodynamic noise of a centrifugal fan. The flow and sound characteristics are numerically investigated by incorporating computational fluid dynamics and computational aero-acoustics. The optimal result exhibits a noise reduction of up to 1.5 dB with a slight increase in mass flow rate. The acoustic characteristics, with respect to the sound pressure level, power spectral density, and sound directivity are discussed. The time-domain, 
frequency-domain, and root mean square values of pressure fluctuation are monitored and analyzed to assess the unsteady flow interaction between the volute tongue and impeller. The intensity and scale of vortices in the centrifugal fan are suppressed in the upstream and downstream of the bionic volute tongue, and the turbulence effect on the surface of the volute tongue becomes even and weak.

The coffee ring effect commonly exists in droplet deposition patterns, which fundamentally affects scientific research and industrial applications, like pharmaceutical purification, salt manufacturing, etc. Some researchers have tried different solutions to control the distributions of droplet deposition patterns, but most control deposits by adjusting droplet characteristics. In this work, droplet deposition patterns with different wettability are investigated by both localized and substrate heating. A whole process of droplet evaporation is recorded. The droplet generally evaporates from Constant Contact Radius (CCR) mode to Constant Contact Angle (CCA) mode, and CCR stage occupies the most of time. Experimental results show that, without any chemicals, laser induced local heating transitions particle deposition patterns from ring-like structure to dot-like patterns on a hydrophilic surface, driving most saline solvent to the center. Meanwhile, a hydrophobic surface is also investigated showing that the particles tend to assemble at the central area, but the pattern is slightly different compared to that on hydrophilic surface. In addition, physical mechanisms of local heating and heating from substrate are also explored.

Evaporation of multicomponent droplets has gained much attention nowadays because of their complex flow fields and various deposition patterns. Here we observe strong flows in evaporating sodium chloride saline droplets with suspended alumina oxide nanoparticles. The evolution of flow pattern was studied by tracking the trajectories of particles and the velocity field was investigated with Particle Image Velocimetry analysis. The non-uniform evaporation rate along the droplet surface leads to a concentration gradient which induces the convection flow. During the evaporation process before crystallization happens, evolution of the flow can be divided into two regimes. In Regime I, a centrosymmetric convection recirculation is formed gradually. In Regime II, the convection recirculation migrates to the droplet edge and evolves into several small vortexes. At the late evaporation stage, crystallization could induce strong convection flows. It is shown that the flow tends to become more chaotic with a lower salt concentration.

Wettability is known to play a major role in enhancing pool boiling heat transfer. In this context bioinspired surfaces can bring significant advantages in pool boiling applications. This work addresses a numerical investigation of bubble growth and detachment on a biphilic surface pattern, namely in a superhydrophobic region surrounded by a hydrophilic region. Surface characteristics resemble bioinspired solutions explored in our research group, mainly considering the main topographical characteristics. This numerical approach is intended to provide additional information to an experimental approach, allowing to obtain temperature, pressure and velocity fields in and around the bubble, which help to describe bubble dynamics. The model was validated based on experimental data obtained with extensive image processing of synchronized high-speed video and high-speed thermographic images. The results obtained here clearly evidence that combining enhanced direct numerical simulations with high-resolution transient experimental measurements is a promising tool to describe the complex and intricate hydrodynamic and heat transfer phenomena governing pool boiling on heated biphilic surfaces.

To improve the controllability for the evaporation process of fuel spray impinging on the cylinder wall, an experimental study on the development of morphological process of different fuel droplets on aluminium alloy surfaces is carried out. The metal surfaces with different wettability are prepared by laser etching and chemical etching for the experiments. In total, three different fuels are tested and compared under different surface temperatures, including diesel, n-butanol and dimethyl carbonate (DMC). The results show that under a lower wall temperature, the surface wettability, viscosity and surface tension of the fuels have significant effects on spreading and rebounding behaviour of the droplets. As the wall temperature rises over the boiling points of the fuel but below its Leidenfrost temperature, the contact angles between the fuels and surfaces are varying according to the surface wettability, boiling point and Leidenfrost temperature of the fuels. When the temperature of the surface exceeds the Leidenfrost temperature of all the fuels, after impacting the surfaces, different fuel droplets tend to have the same development pattern, regardless of the surface wettability. The rebound level is mainly affected by the amount of fuel vapour generated during the wall-hitting process. Viscosity, surface tension and other properties of the fuel have little effect on post-impacting behaviour of the droplet when the wall temperature is higher than the Leidenfrost temperature of the fuel.

In the present study, we investigated the evaporation process and deposition pattern of saline droplet on a copper substrate with different roughness under 40 ?C ambient temperature. These four substrates are classified as smooth surface and rough surface based on their droplet contact angles. It has been found in this study that the evaporation pattern of droplets has a strong relationship to substrate roughness. The thickness boundary of the evaporation pattern on a smooth surface is larger than that on a rough surface and the particles are closer to boundary and the tendency is more obvious on a smooth surface. The below factors contribute to the result. On the smooth surface, the contact angle of droplet increases as the roughness decreases. On the rough surface, the contact angle increases as the roughness increases. With contact angle decreasing, the evaporation rate at the boundary increases leading to the particles at the boundary more easily sedimentate. Moreover, the capillary flow is hindered by increasing the substrate roughness, while the Marangoni flow remains constant, resulting in more particles remain in the center of the droplet on the rough surface. To sum up, the coffee-ring formation is suppressed by increasing the substrate roughness on a copper substrate under 40 ?C temperature.

Super-hydrophobic surfaces are quite common in nature, inspiring people to continually explore its water-repellence property and applications to our lives. It has been generally agreed that the property of super-hydrophobicity is mainly contributed by the microscale or nanoscale (or even smaller) architecture on the surface. Besides, there is an energy barrier between the Cassie-Baxter wetting state and the Wenzel wetting state. An optimized square post micro structure with truncated square pyramid geometry is introduced in this work to increase the energy barrier, enhancing the robustness of super-hydrophobicity. Theoretical analysis is conducted based on the wetting transition energy curves. Numerical simulation based on a phase-field lattice Boltzmann method is carried out to verify the theoretical analysis. The numerical simulation agrees well with the theoretical analysis, showing the positive significance of the proposed micro structure. Furthermore, another novel micro structure of rough surface is presented, which combines the advantages of truncated pyramid geometry and noncommunicating roughness elements. Theoretical analysis shows that the novel micro structure of rough surface 
can effectively hinder the Cassie-Baxter state to Wenzel state transition, furtherly enhancing the robustness of the surface hydrophobicity.

Research into evaporating droplets on patterned surfaces has grown exponentially, since the capacity to control droplet morphology has proven to have significant technological utility in emerging areas of fundamental research and industrial applications. Here, we incorporate two interest domains – complex wetting patterns of droplets on structured surfaces and the ubiquitous coffee-ring phenomenon of nanofluids containing dispersed aluminium oxide particles. We lay out the surface design criteria by quantifying the effect of pillar density and shape on the wetting footprint of droplets, yielding complex polygon droplet geometries. Our work is not constrained to pure liquids only, as we delve into the shape selection of particle-laden droplets of different concentrations. We visualise the deposition patterns through microscopy on surfaces exhibiting different features and further establish the ordering of particles on microscale surface asperities. At a high nanofluid concentration, we observe intriguing self-assembly of particles into highly ordered intricate structures. The collective findings of this work have the potential to enhance many industrial technologies, particularly attractive for high performance optical and electrical devices.