Phred Petersen

Liquid metal enabled pump

Significance The utilization of small-scale pumps is presently hampered by their limited flow rates with respect to the input power or their rather complicated fabrication process. These issues arise as many conventional pumping effects rely on moving elements. Here, we demonstrate the concept of a liquid metal enabled pump with no mechanical parts by simply incorporating droplets of Galinstan. The liquid metal enabled pump creates high flow rates (>5,000 µL/min) at exceptionally low powers (<15 mW) by electrowetting/deelectrowetting the surface upon application of electric field. The presented pump is both efficient and simple; hence, it has the potential to advance the field of actuation in small-scale systems. 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.

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.

Influence of semiconducting properties of nanoparticle coating on the electrochemical actuation of liquid metal marble

Semiconducting properties of nanoparticle coating on liquid metal marbles can present opportunities for an additional dimension of control on these soft objects with functional surfaces in aqueous environments. We show the unique differences in the electrochemical actuation mechanisms of liquid metal marbles with n- and p-type semiconducting nanomaterial coating. A systematic study on such liquid metal marbles shows voltage dependent nanoparticle cluster formation and morphological changes of the liquid metal core during electrochemical actuations and these observations are unique to p-type nanomaterial coated liquid metal marbles.

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.

Hydrodynamic directional control of liquid metal droplets within a microfluidic flow focusing system

Here, we investigate the directional control of Galinstan liquid metal droplets when transferring from the high-viscosity glycerol core into the parallel low-viscosity NaOH sheath streams within a flow focusing microfluidic system. In the presence of sufficient flow mismatch between the sheath streams, the droplets are driven toward the higher velocity interface and cross the interface under the influence of surface tension gradient. A minimum flow mismatch of 125 μl/min is required to enable the continuous transfer of droplets toward the desired sheath stream. The response time of droplets, the time required to change the direction of droplet transfer, is governed by the response time of the syringe pump driven microfluidic system and is found to be 3.3 and 8.8 s when increasing and decreasing the flow rate of sheath stream, respectively.

Influence of large-scale freestream turbulence on the performance of a thin airfoil

Micro air vehicles are typically small in size and are remotely controlled or autonomous aircraft that fly slowly (50; 000 < Reynolds number < 200; 000) and very close to the Earths surface. Due to the combined influences of low Reynolds number effects and the high levels of freestream turbulence, present within the atmospheric boundary layer, micro air vehicles are very difficult to control/fly outdoors.

The gust-mitigating potential of flapping wings

Natures 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.

A smoke visualisation technique for wake flow from a moving human manikin

An experimental technique using the chemical reaction between acetic acid (CH3COOH) and cyclohexylamine (C6H13N) generated smoke to visualise wake flow from a moving object. A 1/5th scaled manikin was dabbed with cyclohexylamine on specific locations and entered an acetic acid saturated chamber. Smoke was generated via the chemical reaction as the manikin moved through the chamber. High-speed photography and image processing techniques were used to determine whether qualitative and quantitative data could be produced for (1) better understanding the effects of trailing wakes on particle exposure induced by human movement and (2) validation data for computational fluid dynamic (CFD) modelling results. Image analysis showed three phases of manikin movement: peak velocity, deceleration, and stationary. Detailed flow separation images showed that regular vortices were produced at the left shoulder, while flow separating at the hand swirled behind and inwards. Analysis of flow over the head revealed how the separation point shifted from the back of the head to the front as the velocity decreased. The results demonstrated that the experimental method was feasible in producing meaningful results for wake flow phenomena behind a manikin and validation data for CFD simulations.Graphical abstract

Flow Characteristics of Compressed Natural Gas Delivery for Direct Injection Spark Ignition Engines

High pressure ratio (PR) compressed natural gas (CNG) jets are studied using optical diagnostics under quiescent but realistic spark-ignited (SI) engine conditions. CNG jets were issued impulsively from a bespoke direct injector. Observations into the delivery characteristics for a large pressure ratio (PR) range were carried out (8.3<PR<400). Non-dimensional scaling laws show that the penetration rate can be represented by a single relationship. The penetration rate is the primary determinant of fuel targeting. Additionally, the jet growth rate and spread angle are also presented. The evolution of the transient underexpanded jet features: the barrel length and Mach disc diameter are detailed and a strong dependency on the injector needle-lift is shown. These nearfield compressible features are recognised as flow structures which influence the mixing rate of a free jet and are also shown to control the size of the incompressible regions. The highly dynamic jet features are captured with high-fidelity schlieren high-speed video. Particle image velocimetry (PIV) maps taken at multiple after start of injection (aSOI) instances detail the flow characteristics within the jet core and the ambient regions. Using the jet core centerline velocity, the Mach disc location (barrel length) is measured to within 7% of schlieren measurements. PIV measurements acquired at late injection (timing) conditions reveal detail on the influence of fuel delivery on the flow within the ignition zone. The mean flow velocity and kinetic energy are consequences of the jets proximity to the ignition region with influences largely originating from the encroaching shear-layer and/or the rolling vortex.

Evaluation of the aging process of composite insulator based on surface charaterisation techniques and electrical method

Long term reliability is one of the most important performance measures for high voltage insulators in the power transmission and distribution networks. To evaluate the long-term performance of insulators, properties such as hydrophobicity, surface roughness, images from Scanning Electron Microscopy and dielectric breakdown measurements are often studied. In this paper, a new evaluation method based on the oscillating period of water droplet, namely Oscillating Water Droplet (OWD) method is proposed. Using a high-speed camera, the full sequence of the water droplet making contact with the composite surface was recorded and analyzed. The OWD was performed on specimens of different ageing levels. Further investigation on hydrophobicity using the optical contact angle device is performed to study the relationship between the oscillating water droplet over the surface and the contact angle. To study the loss of hydrophobicity, surface roughness tests were conducted using BRUKER ContourGT Optical Profiler with eight different roughness spots per specimen. Our results show that more time is needed for the water droplet to settle over the surface when the hydrophobicity is decreased due to aging. Scanning Electron Microscopy (SEM) images showing the level of degradation on the surface of the specimens are included. The results obtained are closely correlated to the decrease in dielectric breakdown voltage over the insulator surface as the period of aging cycle increases. This work also indicates that the OWD method can be applied as a new method for evaluating the hydrophobicity of polymeric insulators.