Oscillation and self-propulsion of Leidenfrost droplets enclosed in cylindrical cavities.

Leidenfrost droplets can be considered as soft engines capable of directly transforming heat into mechanical energy. Despite remarkable advancements in understanding the propulsion of Leidenfrost droplets on asymmetric structures, the complex dynamics of droplets in enclosed structures is not fully understood. To address this fundamental gap, we investigated the dynamics of Leidenfrost droplets restricted by metal disks. The disk alters the accumulation and release of the vapour generated by the droplet, and substantially changes its dynamic characteristics. Our experiments reveal the formation of oscillating multi-lobed structures when restricting the droplet within a disk. In comparison, patterning offset radial grooves on the surface of the disk rectifies the vapour flow and facilitates the self-propulsion of the droplet along the edge of the disk. Our work offers opportunities for developing soft and short-living actuators, which can operate at high temperatures.

Signal or cue: the role of structural colors in flower pollination.

Angle dependent colors, such as iridescence, are produced by structures present on flower petals changing their visual appearance. These colors have been proposed to act as signals for plant-insect communication. However, there is a paucity of behavioral data to allow for interpretations of how to classify these colors either as a signal or a cue when considering the natural conditions under which pollination occurs. We sampled flowers from 6 plant species across various viewpoints looking for changes in the visual appearance of the petals. Spectral characteristics were measured with different instruments to simulate both the spectral and spatial characteristics of honeybee's vision. We show the presence of color patches produced by angle dependent effects on the petals and the calyx of various species; however, the appearance of the angle dependent color patches significantly varies with viewpoint and would only be resolved by the insect eye at close distances. Behavior experiments with honeybees revealed that pollinators did not use angle dependent colors to drive behavior when presented with novel flower presentations. Results show that angle dependent colors do not comply with the requirements of a signal for plant-pollinator communication since the information transmitted by these colors would be unreliable for potential, free-flying pollination vectors. We thus classify angle dependent colors produced by micro- and ultra-structures as being a cue (a feature which has not evolved for communication), and observe no evidence supporting claims of these angle dependent colors having evolved as visual signal.

Dynamics of high viscosity contrast confluent microfluidic flows.

The laminar nature of microfluidic flows is most elegantly demonstrated via the confluence of two fluids forming two stable parallel flows within a single channel meeting at a highly stable interface. However, maintenance of laminar conditions can become complicated when there is a large viscosity contrast between the neighbouring flows leading to unique instability patterns along their interface. Here, we study the dynamics of high viscosity contrast confluent flows - specifically a core flow made of highly viscous glycerol confined by sheath flows made of water within a microfluidic flow focusing system. Our experiments indicate the formation of tapered core structures along the middle of the channel. Increasing the sheath flow rate shortens the tapered core, and importantly induces local instability patterns along the interface of core-sheath flows. The dynamics of such tapered core structures is governed by the intensity of instability patterns and the length of the core, according to which the core structure can experience stable, disturbed, broken or oscillated regimes. We have studied the dynamics of tapered core structures under these regimes. In particular, we have analysed the amplitude and frequency of core displacements during the broken core and oscillating core regimes, which have not been investigated before.

The gust-mitigating potential of flapping wings.

Nature's flapping-wing flyers are adept at negotiating highly turbulent flows across a wide range of scales. This is in part due to their ability to quickly detect and counterract disturbances to their flight path, but may also be assisted by an inherent aerodynamic property of flapping wings. In this study, we subject a mechanical flapping wing to replicated atmospheric turbulence across a range of flapping frequencies and turbulence intensities. By means of flow visualization and surface pressure measurements, we determine the salient effects of large-scale freestream turbulence on the flow field, and on the phase-average and fluctuating components of pressure and lift. It is shown that at lower flapping frequencies, turbulence dominates the instantaneous flow field, and the random fluctuating component of lift contributes significantly to the total lift. At higher flapping frequencies, kinematic forcing begins to dominate and the flow field becomes more consistent from cycle to cycle. Turbulence still modulates the flapping-induced flow field, as evidenced in particular by a variation in the timing and extent of leading edge vortex formation during the early downstroke. The random fluctuating component of lift contributes less to the total lift at these frequencies, providing evidence that flapping wings do indeed provide some inherent gust mitigation.

Continuous transfer of liquid metal droplets across a fluid-fluid interface within an integrated microfluidic chip.

Micro scale liquid metal droplets have been hailed as the potential key building blocks of future micro-electro-mechanical systems (MEMS). However, most of the current liquid metal enabled systems involve millimeter scale droplets, which are manually injected onto the desired locations of the microchip. Despite its simplicity, this method is impractical for patterning large arrays or complex systems based on micro scale droplets. Here, we present a microfluidic chip, which integrates continuous generation of micro scale galinstan droplets in glycerol, and the hydrodynamic transfer of these droplets into sodium hydroxide (NaOH) solution. Observation via high-speed imaging along with computational fluid dynamics (CFD) analysis are utilised to comprehend the lateral migration of droplets from the glycerol to NaOH fluid. This platform is simple, can be readily integrated into other microfluidic systems, and creates flexibility by separating the continuous phase for droplet generation from the eventual target carrier fluid within a monolithic chip.

Differentiating Biological Colours with Few and Many Sensors: Spectral Reconstruction with RGB and Hyperspectral Cameras.

BACKGROUND: The ability to discriminate between two similar or progressively dissimilar colours is important for many animals as it allows for accurately interpreting visual signals produced by key target stimuli or distractor information. Spectrophotometry objectively measures the spectral characteristics of these signals, but is often limited to point samples that could underestimate spectral variability within a single sample. Algorithms for RGB images and digital imaging devices with many more than three channels, hyperspectral cameras, have been recently developed to produce image spectrophotometers to recover reflectance spectra at individual pixel locations. We compare a linearised RGB and a hyperspectral camera in terms of their individual capacities to discriminate between colour targets of varying perceptual similarity for a human observer. MAIN FINDINGS: (1) The colour discrimination power of the RGB device is dependent on colour similarity between the samples whilst the hyperspectral device enables the reconstruction of a unique spectrum for each sampled pixel location independently from their chromatic appearance. (2) Uncertainty associated with spectral reconstruction from RGB responses results from the joint effect of metamerism and spectral variability within a single sample. CONCLUSION: (1) RGB devices give a valuable insight into the limitations of colour discrimination with a low number of photoreceptors, as the principles involved in the interpretation of photoreceptor signals in trichromatic animals also apply to RGB camera responses. (2) The hyperspectral camera architecture provides means to explore other important aspects of colour vision like the perception of certain types of camouflage and colour constancy where multiple, narrow-band sensors increase resolution.

Liquid metal enabled pump.

Small-scale pumps will be the heartbeat of many future micro/nanoscale platforms. However, the integration of small-scale pumps is presently hampered by limited flow rate with respect to the input power, and their rather complicated fabrication processes. These issues arise as many conventional pumping effects require intricate moving elements. Here, we demonstrate a system that we call the liquid metal enabled pump, for driving a range of liquids without mechanical moving parts, upon the application of modest electric field. This pump incorporates a droplet of liquid metal, which induces liquid flow at high flow rates, yet with exceptionally low power consumption by electrowetting/deelectrowetting at the metal surface. We present theory explaining this pumping mechanism and show that the operation is fundamentally different from other existing pumps. The presented liquid metal enabled pump is both efficient and simple, and thus has the potential to fundamentally advance the field of microfluidics.

Electrochemically induced actuation of liquid metal marbles.

Controlled actuation of soft objects with functional surfaces in aqueous environments presents opportunities for liquid phase electronics, novel assembled super-structures and unusual mechanical properties. We show the extraordinary electrochemically induced actuation of liquid metal droplets coated with nanoparticles, so-called "liquid metal marbles". We demonstrate that nanoparticle coatings of these marbles offer an extra dimension for affecting the bipolar electrochemically induced actuation. The nanoparticles can readily migrate along the surface of liquid metals, upon the application of electric fields, altering the capacitive behaviour and surface tension in a highly asymmetric fashion. Surprising actuation behaviours are observed illustrating that nanoparticle coatings can have a strong effect on the movement of these marbles. This significant novel phenomenon, combined with unique properties of liquid metal marbles, represents an exciting platform for enabling diverse applications that cannot be achieved using rigid metal beads.