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Concept: Fluid dynamics


Fire whirls are powerful, spinning disasters for people and surroundings when they occur in large urban and wildland fires. Whereas fire whirls have been studied for fire-safety applications, previous research has yet to harness their potential burning efficiency for enhanced combustion. This article presents laboratory studies of fire whirls initiated as pool fires, but where the fuel sits on a water surface, suggesting the idea of exploiting the high efficiency of fire whirls for oil-spill remediation. We show the transition from a pool fire, to a fire whirl, and then to a previously unobserved state, a “blue whirl.” A blue whirl is smaller, very stable, and burns completely blue as a hydrocarbon flame, indicating soot-free burning. The combination of fast mixing, intense swirl, and the water-surface boundary creates the conditions leading to nearly soot-free combustion. With the worldwide need to reduce emissions from both wanted and unwanted combustion, discovery of this state points to possible new pathways for reduced-emission combustion and fuel-spill cleanup. Because current methods to generate a stable vortex are difficult, we also propose that the blue whirl may serve as a research platform for fundamental studies of vortices and vortex breakdown in fluid mechanics.

Concepts: Fluid dynamics, Hydrogen, Tornado, Fire, Combustion, Vortices, Fuel, Flame


Small-diameter (<4 mm) vascular constructs are urgently needed for patients requiring replacement of their peripheral vessels. However, successful development of constructs remains a significant challenge. In this study, we successfully developed small-diameter vascular constructs with high patency using our integrally designed computer-controlled bioreactor system. This computer-controlled bioreactor system can confer physiological mechanical stimuli and fluid flow similar to physiological stimuli to the cultured grafts. The medium circulating system optimizes the culture conditions by maintaining fixed concentration of O(2) and CO(2) in the medium flow and constant delivery of nutrients and waste metabolites, as well as eliminates the complicated replacement of culture medium in traditional vascular tissue engineering. Biochemical and mechanical assay of newly developed grafts confirm the feasibility of the bioreactor system for small-diameter vascular engineering. Furthermore, the computer-controlled bioreactor is superior for cultured cell proliferation compared with the traditional non-computer-controlled bioreactor. Specifically, our novel bioreactor system may be a potential alternative for tissue engineering of large-scale small-diameter vascular vessels for clinical use.

Concepts: Fluid dynamics, Cell culture, Engineering, Fluid, Design, The Culture, Chemical engineering, Bioreactor


Microactuation of free standing objects in fluids is currently dominated by the rotary propeller, giving rise to a range of potential applications in the military, aeronautic and biomedical fields. Previously, surface acoustic waves (SAWs) have been shown to be of increasing interest in the field of microfluidics, where the refraction of a SAW into a drop of fluid creates a convective flow, a phenomenon generally known as SAW streaming. We now show how SAWs, generated at microelectronic devices, can be used as an efficient method of propulsion actuated by localised fluid streaming. The direction of the force arising from such streaming is optimal when the devices are maintained at the Rayleigh angle. The technique provides propulsion without any moving parts, and, due to the inherent design of the SAW transducer, enables simple control of the direction of travel.

Concepts: Fundamental physics concepts, Fluid dynamics, Fluid mechanics, Liquid, Wave, Fluid, Field, Surface acoustic wave


We investigate the influences of expansion-contraction microchannels on droplet breakup in capillary microfluidic devices. With variations in channel dimension, local shear stresses at the injection nozzle and focusing orifice vary, significantly impacting flow behavior including droplet breakup locations and breakup modes. We observe transition of droplet breakup location from focusing orifice to injection nozzle, and three distinct types of recently-reported tip-multi-breaking modes. By balancing local shear stresses and interfacial tension effects, we determine the critical condition for breakup location transition, and characterize the tip-multi-breaking mode quantitatively. In addition, we identify the mechanism responsible for the periodic oscillation of inner fluid tip in tip-multi-breaking mode. Our results offer fundamental understanding of two-phase flow behaviors in expansion-contraction microstructures, and would benefit droplet generation, manipulation and design of microfluidic devices.

Concepts: Psychology, Fluid dynamics, Fluid mechanics, Viscosity, Liquid, Surface tension, Shear stress, Microfluidics


Unlike current chemical trace detection technology, dogs actively sniff to acquire an odor sample. Flow visualization experiments with an anatomically-similar 3D printed dog’s nose revealed the external aerodynamics during canine sniffing, where ventral-laterally expired air jets entrain odorant-laden air toward the nose, thereby extending the “aerodynamic reach” for inspiration of otherwise inaccessible odors. Chemical sampling and detection experiments quantified two modes of operation with the artificial nose-active sniffing and continuous inspiration-and demonstrated an increase in odorant detection by a factor of up to 18 for active sniffing. A 16-fold improvement in detection was demonstrated with a commercially-available explosives detector by applying this bio-inspired design principle and making the device “sniff” like a dog. These lessons learned from the dog may benefit the next-generation of vapor samplers for explosives, narcotics, pathogens, or even cancer, and could inform future bio-inspired designs for optimized sampling of odor plumes.

Concepts: Fluid dynamics, Aerodynamics, Olfaction, Odor, Artificial insemination, Dog, Aroma compound, Body odor


Dense active matter, from bacterial suspensions and microtubule bundles driven by motor proteins to cellular monolayers and synthetic Janus particles, is characterized by mesoscale turbulence, which is the emergence of chaotic flow structures. By immersing an ordered array of symmetric rotors in an active fluid, we introduce a microfluidic system that exploits spontaneous symmetry breaking in mesoscale turbulence to generate work. The lattice of rotors self-organizes into a spin state where neighboring discs continuously rotate in permanent alternating directions due to combined hydrodynamic and elastic effects. Our virtual prototype demonstrates a new research direction for the design of micromachines powered by the nematohydrodynamic properties of active turbulence.

Concepts: Fluid dynamics, Symmetry, Group, Fluid, Standard Model, Symmetry group, Spontaneous symmetry breaking, Higgs mechanism


Soft robots possess many attributes that are difficult, if not impossible, to achieve with conventional robots composed of rigid materials. Yet, despite recent advances, soft robots must still be tethered to hard robotic control systems and power sources. New strategies for creating completely soft robots, including soft analogues of these crucial components, are needed to realize their full potential. Here we report the untethered operation of a robot composed solely of soft materials. The robot is controlled with microfluidic logic that autonomously regulates fluid flow and, hence, catalytic decomposition of an on-board monopropellant fuel supply. Gas generated from the fuel decomposition inflates fluidic networks downstream of the reaction sites, resulting in actuation. The body and microfluidic logic of the robot are fabricated using moulding and soft lithography, respectively, and the pneumatic actuator networks, on-board fuel reservoirs and catalytic reaction chambers needed for movement are patterned within the body via a multi-material, embedded 3D printing technique. The fluidic and elastomeric architectures required for function span several orders of magnitude from the microscale to the macroscale. Our integrated design and rapid fabrication approach enables the programmable assembly of multiple materials within this architecture, laying the foundation for completely soft, autonomous robots.

Concepts: Fluid dynamics, Fluid, Robotics, Robot, Autonomous robot, Humanoid robot, Pneumatics, Evolutionary robotics


Large ears enhance perception of echolocation and prey generated sounds in bats. However, external ears likely impair aerodynamic performance of bats compared to birds. But large ears may generate lift on their own, mitigating the negative effects. We studied flying brown long-eared bats, using high resolution, time resolved particle image velocimetry, to determine the aerodynamics of flying with large ears. We show that the ears and body generate lift at medium to cruising speeds (3-5 m/s), but at the cost of an interaction with the wing root vortices, likely reducing inner wing performance. We also propose that the bats use a novel wing pitch mechanism at the end of the upstroke generating thrust at low speeds, which should provide effective pitch and yaw control. In addition, the wing tip vortices show a distinct spiraling pattern. The tip vortex of the previous wingbeat remains into the next wingbeat and rotates together with a newly formed tip vortex. Several smaller vortices, related to changes in circulation around the wing also spiral the tip vortex. Our results thus show a new level of complexity in bat wakes and suggest large eared bats are less aerodynamically limited than previous wake studies have suggested.

Concepts: Fluid dynamics, Aerodynamics, Bat, Drag, Spiral, Fixed-wing aircraft, Lift, Wingtip vortices


Phase-change materials, such as meta-stable undercooled (supercooled) liquids, have been widely recognized as a suitable route for complex fabrication and engineering. Despite comprehensive studies on the undercooling phenomenon, little progress has been made in the use of undercooled metals, primarily due to low yields and poor stability. This paper reports the use of an extension of droplet emulsion technique (SLICE) to produce undercooled core-shell particles of structure; metal/oxide shell-acetate (‘/' = physisorbed, ’-‘ = chemisorbed), from molten Field’s metal (Bi-In-Sn) and Bi-Sn alloys. These particles exhibit stability against solidification at ambient conditions. Besides synthesis, we report the use of these undercooled metal, liquid core-shell, particles for heat free joining and manufacturing at ambient conditions. Our approach incorporates gentle etching and/or fracturing of outer oxide-acetate layers through mechanical stressing or shearing, thus initiating a cascade entailing fluid flow with concomitant deformation, combination/alloying, shaping, and solidification. This simple and low cost technique for soldering and fabrication enables formation of complex shapes and joining at the meso- and micro-scale at ambient conditions without heat or electricity.

Concepts: Fluid dynamics, Metal, Solid, Viscosity, Liquid, Fluid, Supercooling, Melting point


Archer fish knock down insects anchored to vegetation by hitting them with a precisely aimed jet of water. The striking force of the jet at the impact is such to overcome the strong anchoring forces of insects. The origin of the effectiveness of such hunting mechanism has been long searched for inside of the fish, in the unsuccessful attempt to identify internal structures dedicated to the amplification of muscular power. Here we perform a kinematic analysis of the jet emitted by two specimens of Toxotes jaculatrix. We estimate that at the impact the jet conveys a typical specific power of about 3000 W/kg, which is well above the maximum specific power of the order of 500 W/kg deliverable by a vertebrate muscle. Unexpectedly, we find that the amplification of muscular power occurs outside of the fish, and is due to a hydrodynamic instability of the jet akin to those occurring in Drop-on-Demand inkjet printing. The investigated fish are found to modulate the velocity of the jet at the orifice to favor the formation of a single, large, water drop that hits the prey abruptly with a large momentum. The observed mechanism represents a remarkable example of use of an external hydrodynamic lever that does possibly not entail the high evolutionary cost needed for the development of highly specialized internal structures dedicated to the storing of mechanical energy.

Concepts: Energy, Fundamental physics concepts, Fluid dynamics, Muscle, Force, Classical mechanics, Inkjet printer, Water jet cutter