The Lift Effects of Chordwise Wing Deformation and Body Angle on Low-Speed Flying Butterflies.

This work investigates the effects of body angle and wing deformation on the lift of free-flying butterflies. The flight kinematics were recorded using three high-speed cameras, and particle-image velocimetry (PIV) was used to analyze the transient flow field around the butterfly. Parametric studies via numerical simulations were also conducted to examine the force generation of the wing by fixing different body angles and amplifying the chordwise deformation. The results show that appropriately amplifying chordwise deformation enhances wing performance due to an increase in the strength of the vortex and a more stabilized attached vortex. The wing undergoes a significant chordwise deformation, which can generate a larger lift coefficient than that with a higher body angle, resulting in a 14% increase compared to a lower chordwise deformation and body angle. This effect is due to the leading-edge vortex attached to the curved wing, which alters the force from horizontal to vertical. It, therefore, produces more efficient lift during flight. These findings reveal that the chordwise deformation of the wing and the body angle could increase the lift of the butterfly. This work was inspired by real butterfly flight, and the results could provide valuable knowledge about lift generation for designing microaerial vehicles.

Decoupling wing-shape effects of wing-swept angle and aspect ratio on a forward-flying butterfly.

The effect of wing shape on a forward-flying butterfly via decoupled factors of the wing-swept angle and the aspect ratio (AR) was investigated numerically. The wing-shape effect is a major concern in the design of a microaerial vehicle (MAV). In nature, the wing of a butterfly consists of partially overlapping forewing and hindwing; when the forewing sweeps forward or backward relative to the hindwing, the wing-swept angle and the AR of the entire wing simultaneously change. The effects of the wing-swept angle and AR on aerodynamics are coupled. To decouple their effects, we established wing-shape models with varied combinations of the wing-swept angle and AR based on the experimental measurement of two butterfly species (Papilio polytes and Kallima inachus) and developed a numerical simulation for analysis. In each model, the forewing and hindwing overlapped partially, constructing a single wing. Across the models, the wing-swept angle and AR of these single wings varied sequentially. The results show that, through our models, the effects of the wing-swept angle and AR were decoupled; both have distinct flow mechanisms and aerodynamic force trends and are consistent in the two butterfly species. For a fixed AR, a backward-swept wing increases lift and drag because of the enhanced attachment of the leading-edge vortex with increased strength of the wingtip vortex and the spanwise flow. For a fixed wing-swept angle, a small AR wing increases lift and decreases drag because of the large region of low pressure downstream and the wake-capture effect. Coupling these effects, the largest lift-to-drag ratio occurs for a forward-swept wing with the smallest AR. These results indicate that, in a flapping forward flight, sweeping a forewing forward relative to a hindwing is suitable for cruising. The flow mechanisms and decoupled and coupled effects of the wing-swept angle and the AR presented in this paper provide insight into the flight of a butterfly and the design of a MAV.

Beneficial wake-capture effect for forward propulsion with a restrained wing-pitch motion of a butterfly.

Unlike other insects, a butterfly uses a small amplitude of the wing-pitch motion for flight. From an analysis of the dynamics of real flying butterflies, we show that the restrained amplitude of the wing-pitch motion enhances the wake-capture effect so as to enhance forward propulsion. A numerical simulation refined with experimental data shows that, for a small amplitude of the wing-pitch motion, the shed vortex generated in the downstroke induces air in the wake region to flow towards the wings. This condition enables a butterfly to capture an induced flow and to acquire an additional forward propulsion, which accounts for more than 47% of the thrust generation. When the amplitude of the wing-pitch motion exceeds 45°, the flow induced by the shed vortex drifts away from the wings; it attenuates the wake-capture effect and causes the butterfly to lose a part of its forward propulsion. Our results provide one essential aerodynamic feature for a butterfly to adopt a small amplitude of the wing-pitch motion to enhance the wake-capture effect and forward propulsion. This work clarifies the variation of the flow field correlated with the wing-pitch motion, which is useful in the design of wing kinematics of a micro-aerial vehicle.

Flight Maneuver of a Damselfly with Phase Modulation of the Wings.

We developed a numerical model for four-wing self-propulsion to calculate effectively the flight velocity generated with varied wing motions, which satisfactorily verified biological experiments. Through this self-propulsion model, we analyzed the flight velocity of a damselfly (Matrona cyanoptera) at varied phases. The results show that after phase modulation of the wings, the aerodynamic performance of the forewing (FW) is affected by the incoming flow and an effective angle of attack, whereas that of the hindwing (HW) is dominated by the vortex interaction and induced flow generated by the shed vortex of the FW. Cooperating with the flow interaction, in stable flight, the HW in the lead phase has a larger vertical velocity, whereas the FW in the lead phase has a larger horizontal velocity. Regarding the aerodynamic efficiency, the FW in the lead phase has greater horizontal efficiency, whereas the HW in the lead phase has greater vertical efficiency; the overall efficiency does not vary with the phase. This work interprets that a dragonfly adopts the HW in the lead phase to generate a larger lift, thus supporting the larger body weight, whereas a damselfly adopts the FW in the lead phase to have a greater forward velocity, which can supplement the lack of flapping frequency.

Effect of wing-wing interaction coupled with morphology and kinematic features of damselflies.

We investigated the effect of the wing-wing interaction, which is one key aspect of flight control, of damselflies (Matrona cyanopteraandEuphaea formosa) in forward flight that relates closely to their body morphologies and wing kinematics. We used two high-speed cameras aligned orthogonally to measure the flight motions and adopted 3D numerical simulation to analyze the flow structures and aerodynamic efficiencies. The results clarify the effects of wing-wing interactions, which are complicated combinations of biological morphology, wing kinematics and fluid dynamics. As the amplitude of the hindwing ofM. cyanopterais larger than that ofE. formosa, the effect of the wing-wing interaction is more constructive. Restricted by the body morphology ofE. formosa, the flapping range of the hindwing is below the body. With the forewing in the lead, the hindwing is farther from the forewing, which is not susceptible to the wake of the forewing, and enables superior lift and thrust. Because of the varied rotational motions, the different shed direction of the wakes of the forewings causes the optimal thrust to occur in different wing phases. Because of its biological limitations, a damselfly can use an appropriate phase to fulfill the desired flight mode. The wing-wing interaction is a compromise between lift efficiency and thrust efficiency. The results reveal that a damselfly with the forewing in the lead can have an effective aerodynamic performance in flight. As an application, in the design concept of a micro-aircraft, increasing the amplitude of the hindwing might enhance the wing-wing interaction, thus controlling the flight modes.

Enhanced lift and thrust via the translational motion between the thorax-abdomen node and the center of mass of a butterfly with a constructive abdominal oscillation.

Butterflies fly with an abdomen oscillating relative to the thorax; the abdominal oscillation causes body parts to undulate translationally relative to the center of mass of a butterfly, which could generate a significant effect on flight. Based on experimental measurements, we created a numerical model to investigate this effect in a free-flying butterfly (Idea leuconoe). We fixed the motions of wing-flapping and thorax-pitching, and parametrized the abdominal oscillation by varied oscillating phase. To concentrate the analysis on translational dynamics, we used a motion of a thorax-abdomen node, a joint that the thorax and the abdomen rotate about, to express the translational motion of body parts relative to the center of mass. The results show that the abdominal oscillation enhances lift and thrust via the translational motion of the thorax-abdomen node relative to the center of mass. With the abdominal oscillating phase recorded from real butterflies, the abdominal oscillation causes the thorax-abdomen node to move downward relative to the center of mass in downstroke and move upward relative to the center of mass in upstroke. This constructive movement amplifies the wing-flapping speed relative to the center of mass, which enhances the angle of attack and the strength of leading- and trailing-edge vortices on the wings. The wings thereby generate increased values of instantaneous lift and thrust by 50.32% and 32.57% compared to the case of no abdominal oscillation. Natural butterflies are stated to utilize a particular phase offset of abdominal oscillation to fly. With comparing varied oscillating phases, only the abdominal oscillating phase recorded from natural butterflies produces the best constructive effect on the translational motion of thorax-abdomen node, which maximizes the lift and thrust generated on the wings. It clarifies that butterflies use this specific range of abdominal oscillating phase to regulate the translational motion between the thorax-abdomen node and the center of mass to enhance flight. Our work reveals the translational mechanism of the abdominal oscillation, which is as important as the thorax-pitching effect. The findings in this work provide insight into the flight of butterflies and the design of micro aerial vehicles.

Visual DNA diagnosis of Tomato yellow leaf curl virus with integrated recombinase polymerase amplification and a gold-nanoparticle probe.

A visual DNA diagnosis with a rapid and simple procedure has been developed on integrating recombinase polymerase amplification (RPA) and a gold nanoparticle (AuNP) probe. The entire process is implemented in only one tube with no precision instrument and requires in total 20 min to amplify a DNA fragment with RPA and to discriminate a DNA fragment with an AuNP probe. The result in various colors is directly observable with the naked eye. Through discovering a small DNA fragment of Tomato yellow leaf curl virus (TYLCV), this system can detect one copy per microlitre of virus in a pure isolate of extracted DNA and can readily identify an infected plant with a healthy appearance. This system hence provides a highly sensitive and stable DNA diagnosis. This visual method has a potential for disease diagnosis and prognostication in the field based on advantages of simplicity, high speed, portability and sensitivity.

Effects of phase lag on the hovering flight of damselfly and dragonfly.

In this work we studied the differences in flight kinematics and aerodynamics that could relate to differences in wing morphologies of a dragonfly and a damselfly. The damselflies and dragonflies normally fly with the fore wing or hind wing in the lead, respectively. The wing of the damselfly is petiolate, which means that the wing root is narrower than that of the dragonfly. The influence of the biological morphology between the damselfly and the dragonfly on their hovering strategies is worthy of clarification. The flight motions of damselflies and dragonflies in hovering were recorded with two high-speed cameras; we analyzed the differences between their hovering motions using computational fluid dynamics. The distinct mechanisms of the hovering flight of damselflies (Matrona cyanoptera) and dragonflies (Neurothemis ramburii) with different phase lags between fore and hind wings were deduced. The results of a comparison of the differences of wing phases in hovering showed that the rotational effect has an important role in the aerodynamics; the interactions between fore and hind wings greatly affect their vortex structure and flight performance. The wake of a damselfly sheds smoothly because of slender petiolation; a vertical force is generated steadily during the stage of wing translation. Damselflies hover with a longer translational phase and a larger flapping amplitude. In contrast, the root vortex of a dragonfly impedes the shedding of wake vortices in the upstroke, which results in the loss of a vertical force; the dragonfly hence hovers with a large amplitude of wing rotation. These species of Odonata insects developed varied hovering strategies to fit their distinct biological morphologies.

The Visual Colorimetric Detection of Multi-nucleotide Polymorphisms on a Pneumatic Droplet Manipulation Platform.

A simple and visual method to detect multi-nucleotide polymorphism (MNP) was performed on a pneumatic droplet manipulation platform on an open surface. This approach to colorimetric DNA detection was based on the hybridization-mediated growth of gold nanoparticle probes (AuNP probes). The growth size and configuration of the AuNP are dominated by the number of DNA samples hybridized with the probes. Based on the specific size- and shape-dependent optical properties of the nanoparticles, the number of mismatches in a sample DNA fragment to the probes is able to be discriminated. The tests were conducted via droplets containing reagents and DNA samples respectively, and were transported and mixed on the pneumatic platform with the controlled pneumatic suction of the flexible PDMS-based superhydrophobic membrane. Droplets can be delivered simultaneously and precisely on an open-surface on the proposed pneumatic platform that is highly biocompatible with no side effect of DNA samples inside the droplets. Combining the two proposed methods, the multi-nucleotide polymorphism can be detected at sight on the pneumatic droplet manipulation platform; no additional instrument is required. The procedure from installing the droplets on the platform to the final result takes less than 5 min, much less than with existing methods. Moreover, this combined MNP detection approach requires a sample volume of only 10 µl in each operation, which is remarkably less than that of a macro system.

Importance of body rotation during the flight of a butterfly.

In nature the body motion of a butterfly is clearly observed to involve periodic rotation and varied flight modes. The maneuvers of a butterfly in flight are unique. Based on the flight motion of butterflies (Kallima inachus) recorded in free flight, a numerical model of a butterfly is created to study how its flight relates to body pose; the body motion in a simulation is prescribed and tested with varied initial body angle and rotational amplitude. A butterfly rotates its body to control the direction of the vortex rings generated during flapping flight; the flight modes are found to be closely related to the body motion of a butterfly. When the initial body angle increases, the forward displacement decreases, but the upward displacement increases within a stroke. With increased rotational amplitudes, the jet flows generated by a butterfly eject more downward and further enhance the generation of upward force, according to which a butterfly executes a vertical jump at the end of the downstroke. During this jumping stage, the air relative to the butterfly is moving downward; the butterfly pitches up its body to be parallel to the flow and to decrease the projected area so as to avoid further downward force generated. Our results indicate the importance of the body motion of a butterfly in flight. The inspiration of flight controlled with body motion from the flight of a butterfly might yield an alternative way to control future flight vehicles.

Enhanced thrust and speed revealed in the forward flight of a butterfly with transient body translation.

A butterfly with broad wings, flapping at a small frequency, flies an erratic trajectory at an inconstant speed. A large variation of speed within a cycle is observed in the forward flight of a butterfly. A self-propulsion model to simulate a butterfly is thus created to investigate the transient translation of the body; the results, which are in accordance with experimental data, show that the shape of the variation of the flight speed is similar to a sinusoidal wave with a maximum (J=0.89) at the beginning of the downstroke, and a decrease to a minimum (J=0.17) during a transition from downstroke to upstroke; the difference between the extrema of the flight speed is enormous in a flapping cycle. At a high speed, a clapping motion of the butterfly wings decreases the generation of drag. At a small speed, a butterfly is able to capture the induced wakes generated in a downstroke, and effectively generates a thrust at the beginning of an upstroke. The wing motion of a butterfly skillfully interacts with its speed so as to enable an increased speed with the same motion. Considering a butterfly to fly in a constant inflow leads to either an underestimate of its speed or an overestimate of its generated lift, which yields an inaccurate interpretation of the insect's flight. Our results reveal the effect of transient translation on a butterfly in forward flight, which is especially important for an insect with a small flapping frequency.

Detection of an amphiphilic biosample in a paper microchannel based on length.

We developed a simple method to achieve semiquantitative detection of an amphiphilic biosample through measuring the length of flow on a microfluidic analytical device (µPAD) based on paper. When an amphiphilic sample was dripped into a straight microchannel defined with a printed wax barrier (hydrophobic) on filter paper (hydrophilic), the length of flow was affected by the reciprocal effect between the sample, the filter-paper channel and the wax barrier. The flow length decreased with increasing concentration of an amphiphilic sample because of adsorption of the sample on the hydrophobic barrier. Measurement of the flow length enabled a determination of the concentration of the amphiphilic sample. The several tested samples included surfactants (Tween 20 and Triton X-100), oligonucleotides (DNA), bovine serum albumin (BSA), human albumin, nitrite, glucose and low-density lipoprotein (LDL). The results show that the measurement of the flow length determined directly the concentration of an amphiphilic sample, whereas a non-amphiphilic sample was not amenable to this method. The proposed method features the advantages of small cost, simplicity, convenience, directness, rapidity (<5 min) and requirement of only a small volume (5 µL) of sample, with prospective applications in developing areas and sites near patients for testing at a point of care (POCT).

Origami paper-based fluidic batteries for portable electrophoretic devices.

A manufacturing approach for paper-based fluidic batteries was developed based on the origami principle (three-dimension paper folding). Microfluidic channels were first created on a filter paper by a wax-printing method. Copper and aluminium sheets were then glued onto the paper as electrodes for the redox reaction. After the addition of copper sulphate and aluminium chloride, commonly available cellophane paper was attached as a membrane to separate the two electrodes. The resulting planar paper sheets were then folded into three-dimensional structures and compiled as a single battery with glue. The two half reactions (Al/Al(3+) and Cu/Cu(2+)) in the folded batteries provided an open-circuit potential from 0.82 V (one cell) to 5.0 V (eight cells in series) depending on the origami design. The prepared battery can provide a stable current of 500 µA and can light a regular LED for more than 65 min. These paper-based fluidic batteries in a set can also be compiled into a portable power bank to provide electric power for many electric or biomedical applications, such as LED lights and electrophoretic devices, as we report here.

Analysis of the aerodynamic force in an eye-stabilized flapping flyer.

Experimental methods and related theories to evaluate the lift force for a flyer are established, but one can traditionally acquire only the magnitude of that lift. We here proffer an analysis based on kinematic theory and experimental visualization of the flow to complete a treatment of the aerodynamic force affecting a hovering flyer that generates a lift force approximately equal to its weight, and remains nearly stationary in midair; the center and direction of the aerodynamic force are accordingly determined with some assumptions made. The principal condition to resolve the problem is the stabilization of the vision of a flyer, which is inspired by a hovering passerine that experiences a substantial upward swing during downstroke periods while its eye remains stabilized. Viewing the aerodynamic force with a bird's eye, we find that the center and direction of this aerodynamic force vary continuously with respect to the lift force. Our results provide practical guidance for engineers to enhance the visual stability of surveillance cameras incorporated in micro aerial vehicles.

DNA diagnosis in a microseparator based on particle aggregation.

A novel aggregation-based biosensing method to achieve detection of oligonucleotides in a pinched-flow fractionation (PFF) microseparator was developed. Employing functionalized polystyrene microspheres, this method is capable of the direct detection of the concentration of a specific DNA sequence. The label-free target DNA hybridizes with probe DNA of two kinds on the surface of the microspheres and causes the formation of an aggregate, thus increasing the average size of the aggregate particles. On introducing the sample into a PFF microseparator, the aggregate particles locate at a specific position depending on the size of the aggregate. Through a multi-outlet asymmetric PFF microseparator, the aggregate particles become separated according to outlets. Because the size of the aggregate particles is proportional to the concentration of the target DNA, a rapid quantitative analysis is achievable with an optical microscope. A biological dose-response curve with concentration in a dynamic range 0.33-10nM has been achieved; the limit of detection is between 33 and 330 pM. The specificity of the method and the potential to detect single-nucleotide polymorphism (SNP) of known concentration were examined. The method features simple, direct and cheap detection, with a prospect of detecting other biochemical samples with distinct aggregation behavior, such as heavy-metal ions, bacteria and proteins.

Ventral-clap modes of hovering passerines.

Some small birds typically clap their wings ventrally, particularly during hovering. To investigate this phenomenon, we analyzed the kinematic motion and wake flow field of two passerine species that hover with the same flapping frequency. For these two birds, the ventral clap is classified as direct and cupping. Japanese White-eyes undertake a direct clap via their hand wings, whereas Gouldian Finches undertake a cupping clap with one wing overlaying the other. As a result of their morphological limitation, birds of both greater size and wing span cup their wings to increase the wing speed during a ventral clap because of the larger wing loading. This morphological limitation leads also to a structural discrepancy of the wake flow fields between these two passerine species. At the instant of clapping, the direct clap induces a downward air velocity 1.68 times and generates a weight-normalized lift force 1.14 times that for the cupping clap. The direct clap produces a small upward jet and a pair of counter-rotating vortices, both of which abate the transient lift at the instant of clapping, but they are not engendered by the cupping clap. The aerodynamic mechanisms generated with a ventral clap help the small birds to avoid abrupt body swinging at the instant of clapping so as to maintain their visual stability during hovering.

Particle chain display--an optofluidic electronic paper.

A particle-based display medium and a driving mechanism insensitive to the charge polarity of those particles, based on the transformation of particle chains, are developed for reflective electronic paper displays. Particle chains are formed by dipole-dipole interactions between polarized particles with an appropriate electric field applied across the tested display medium, i.e. the solution that regulates the light in the field of display technology, containing neutral polystyrene (PS) particles dispersed in water. Formation of the particle chains results in a large change in optical transmittance and reflectance of the display medium. The performance of the particle chain displays (PCD) was evaluated according to macroscopic (device), microscopic (particle) and optical (reflectance) points of view. A display medium (thickness 100 µm) containing colored PS particles (3 µm, 2.5% w/v) was polarized to display the fixed images of the directly driven electrodes and programmable images of arrayed (5 × 5) electrodes with electric fields (0.48 MV m(-1) and 0.09 MV m(-1), 500 kHz, respectively). The formation of particle chains under electric fields (0.2 MV m(-1) and 0.4 MV m(-1), 500 kHz) was observed in the microscopic images of a display medium (thickness 100 µm) with fluorescent PS particles (5 µm, 1%). Images recorded with a confocal microscope demonstrated the particle chains. The opacity, a common parameter serving to characterize a display medium, was derived by measuring the reflectance ratio of a black background to a white background of the display medium with varied thickness and particle concentration. The temporal response of a display medium (thickness 50 µm) with black PS particles (3 µm, 5%) was tested. When an electric field (0.6 MV m(-1), 500 kHz) was applied, the reflectance increased twice at the first data point in 0.7 s, attaining a contrast ratio of 2. Application of a voltage (20 s) yielded a contrast ratio of 10. The performance of a tested display medium, composed of simple PS particles and water and driven to form particle chains by polarization, is reported.

Locally enhanced concentration and detection of oligonucleotides in a plug-based microfluidic device.

We propose a novel technique that allows oligonucleotides with specific end-modification within a plug in a plug-based microfluidic device to undergo a locally enhanced concentration at the rear of the plug as the plug moves downstream. DNA was enriched and detected in situ upon exploiting a combined effect underlain by an entropic force induced through fluid shear (i.e. a hydrodynamic-repellent effect) and the interfacial adsorption (aqueous/oil interface) attributed to affinity. Flow fields within a plug were visualized quantitatively using micro-particle image velocimetry (micro-PIV); the distribution of the fluid shear strain rate explains how the hydrodynamic-repellent effect engenders a dumbbell-like region with an increased concentration of DNA. The concentration of FAM (6-carboxy-fluorescein)-labeled DNA (FC-DNA) and of TAMRA (tetramethyl-6-carboxyrhodamine)-labeled DNA (TC-DNA), respectively, and the hybridization of probe DNA (modified with FAM) with target DNA (modified with TAMRA) were investigated in devices; a confocal fluorescence microscope (CFM) was utilized to monitor the processes and to resolve the corresponding 2D patterns and 3D reconstruction of the DNA distribution in a plug. TC-DNA, but not FC-DNA, concentrating within a plug was affected by the combined effect so as to achieve a concentration factor (C(r)) twice that of FC-DNA because of the lipophilicity of TAMRA. Using fluorescence resonance-energy transfer (FRET), we characterized the hybridization of the DNA in a plug; the detection limit of a system, improved by virtue of the proposed technique (the locally enhanced concentration), for DNA detection was estimated to be 20-50 nM. This technique enables DNA to concentrate locally in a nL-pL free-solution plug, the locally enhanced concentration to profit the hybridization efficiency and the detection of DNA, prospectively serving as a versatile means to accomplish a rapid DNA detection in a small volume for a Lab-on-a-Chip (LOC) system.

A passerine spreads its tail to facilitate a rapid recovery of its body posture during hovering.

We demonstrate experimentally that a passerine exploits tail spreading to intercept the downward flow induced by its wings to facilitate the recovery of its posture. The periodic spreading of its tail by the White-eye bird exhibits a phase correlation with both wingstroke motion and body oscillation during hovering flight. During a downstroke, a White-eye's body undergoes a remarkable pitch-down motion, with the tail undergoing an upward swing. This pitch-down motion becomes appropriately suppressed at the end of the downstroke; the bird's body posture then recovers gradually to its original status. Employing digital particle-image velocimetry, we show that the strong downward flow induced by downstroking the wings serves as an external jet flow impinging upon the tail, providing a depressing force on the tail to counteract the pitch-down motion of the bird's body. Spreading of the tail enhances a rapid recovery of the body posture because increased forces are experienced. The maximum force experienced by a spread tail is approximately 2.6 times that of a non-spread tail.

Aerodynamic trick for visual stabilization during downstroke in a hovering bird.

We provide physical insight into how a small hovering bird attains stabilized vision during downstroke. A passerine generates a lift force greater than its body weight during downstroke, leading to a substantial swing of the bird body, but the bird's eyes are nearly stable. Employing digital particle-image velocimetry, we demonstrate that a hovering passerine generates a lift force acting dorsal to the center of mass, concurrently resulting in rotational and translational displacements of the bird's body. The most notable finding is that the rotational and translational displacements at the bird's eyes almost cancel each other; the displacement of the eye is ~8% that of the trailing tip of the tail. This aerodynamic trick enables a bird to attain stabilized vision beneficial for the inspection of the environment.