Shaping drops with magnetic fields

The control of small volumes of fluids (or drops) is important for a wide range of applications, including lab-on-chip devices, where drops are transported and merged for sensing and chemical mixing; liquid lenses, where drops are shaped to set optical properties; and printing, where drops are generated by nozzles. Electric techniques are widely used to generate, transport, split and merge drops. Equivalent magnetic techniques are less well-known. Similarly to electric dipoles in electric fields, magnetic dipoles experience a force in magnetic fields. This effect, called magnetophoresis, is used to shape ferrofluids in magnetic valves and seals. Interest in shaping drops with magnetic fields for microfluidics has recently increased, and ferrofluids and paramagnetic salt solutions have been studied. The rich phenomenology of the interaction of magnetic fields and fluids offers ample opportunities for exploration. Diamagnetic fluids for example have no natural electric equivalent and are rarely studied as a tool for microfluidics. In this thesis, I study the shaping of drops with magnetic fields. My research focus is on para- and diamagnetic salt solutions. Deformation of drops using external fields and induced magnetism has not been fully explored in the literature. I study here how induced magnetism can shape the liquid-vapour interface of drops and control solids that float on them. This thesis includes (i) an introduction to the background of the interaction of electromagnetic fields and fluids; (ii) a derivation of an expression for the shape of drops in electromagnetic fields; (iii) experimental validation of this expression through the measurement of the shape of para- and diamagnetic axisymmetric sessile drops in homogeneous magnetic fields; (iv) demonstration of the transport of para- and diamagnetic drops in magnetic field gradients; (v) explorations of the use of shaping drops with magnetic fields for rheological measurements, and for the controlled driving of objects floating on drops. In summary, I explore how drops can be shaped in homogeneous magnetic fields, and how the drops can be transported by magnetic field gradients. These fundamental investigations may help stimulate novel applications of the controlled shaping of drops with magnetic fields. In particular, I explore how this technique can be used in rheology for food or medical research

Magnetic control of graphitic microparticles in aqueous solutions

Graphite is an inexpensive material with useful electrical, magnetic, thermal, and optical properties. It is also biocompatible and used universally as a substrate. Micrometer-sized graphitic particles in solution are therefore ideal candidates for novel lab-on-a-chip and remote manipulation applications in biomedicine, biophysics, chemistry, and condensed-matter physics. However, submerged graphite is not known to be amenable to magnetic manipulation, the optimal manipulation method for such applications. Here, we exploit the diamagnetism of graphite and demonstrate contactless magnetic positioning control of graphitic microflakes in diamagnetic aqueous solutions. We develop a theoretical model for magnetic manipulation of graphite microflakes and demonstrate experimentally magnetic transport of such particles over distances [Formula: see text] with peak velocities [Formula: see text] in inhomogeneous magnetic fields. We achieve fully biocompatible transport for lipid-coated graphite in NaCl aqueous solution, paving the way for previously undiscovered biomedical applications. Our results prove that micrometer-sized graphite can be magnetically manipulated in liquid media

Tribology of Microball Bearing MEMS

This dissertation explores the fundamental tribology of microfabricated rolling bearings for future micro-machines. It is hypothesized that adhesion, rather than elastic hysteresis, dominates the rolling friction and wear for these systems, a feature that is unique to the micro-scale. To test this hypothesis, specific studies in contact area and surface energy have been performed. Silicon microturbines supported on thrust bearings packed with 285 µm and 500 µm diameter stainless steel balls have undergone spin-down friction testing over a load and speed range of 10-100mN and 500-10,000 rpm, respectively. A positive correlation between calculated contact area and measured friction torque was observed, supporting the adhesion-dominated hysteresis hypothesis. Vapor phase lubrication has been integrated within the microturbine testing scheme in a controlled and characterized manner. Vapor-phase molecules allowed for specifically addressing adhesive energy without changing other system properties. A 61% reduction of friction torque was observed with the utilization of 18% relative humidity water vapor lubrication. Additionally, the relationship between friction torque and normal load was shown to follow an adhesion-based trend, highlighting the effect of adhesion and further confirming the adhesion-dominant hypothesis. The wear mechanisms have been studied for a microfabricated ball bearing platform that includes silicon and thin-film coated silicon raceway/steel ball materials systems. Adhesion of ball material, found to be the primary wear mechanism, is universally present in all tested materials systems. Volumetric adhesive wear rates are observed between 4x10^-4 µm^3/mN*rev and 4x10^-5 µm3/mN*rev were determined by surface mapping techniques and suggest a self-limiting process. This work also demonstrates the utilization of an Off-The-Shelf (OTS) MEMS accelerometer to confirm a hypothesized ball bearing instability regime which encouraged the design of new bearing geometries, as well as to perform in situ diagnostics of a high-performance rotary MEMS device. Finally, the development of a 3D fabrication technique with the potential of significantly improving the performance of micro-scale rotary structures is described. The process was used to create uniform, smooth, curved surfaces. Micro-scale ball bearings are then able to be utilized in high-speed regimes where load can be accommodated both axially and radially, allowing for new, high-speed applications. A comprehensive exploration of the fundamental tribology of microball bearing MEMS has been performed, including specific experiments on friction, wear, lubrication, dynamics, and geometrical optimization. Future devices utilizing microball bearings will be engineered and optimized based on the results of this dissertation

NIAC Phase II Orbiting Rainbows: Future Space Imaging with Granular Systems

Inspired by the light scattering and focusing properties of distributed optical assemblies in Nature, such as rainbows and aerosols, and by recent laboratory successes in optical trapping and manipulation, we propose a unique combination of space optics and autonomous robotic system technology, to enable a new vision of space system architecture with applications to ultra-lightweight space optics and, ultimately, in-situ space system fabrication. Typically, the cost of an optical system is driven by the size and mass of the primary aperture. The ideal system is a cloud of spatially disordered dust-like objects that can be optically manipulated: it is highly reconfigurable, fault-tolerant, and allows very large aperture sizes at low cost. This new concept is based on recent understandings in the physics of optical manipulation of small particles in the laboratory and the engineering of distributed ensembles of spacecraft swarms to shape an orbiting cloud of micron-sized objects. In the same way that optical tweezers have revolutionized micro- and nano-manipulation of objects, our breakthrough concept will enable new large scale NASA mission applications and develop new technology in the areas of Astrophysical Imaging Systems and Remote Sensing because the cloud can operate as an adaptive optical imaging sensor. While achieving the feasibility of constructing one single aperture out of the cloud is the main topic of this work, it is clear that multiple orbiting aerosol lenses could also combine their power to synthesize a much larger aperture in space to enable challenging goals such as exo-planet detection. Furthermore, this effort could establish feasibility of key issues related to material properties, remote manipulation, and autonomy characteristics of cloud in orbit. There are several types of endeavors (science missions) that could be enabled by this type of approach, i.e. it can enable new astrophysical imaging systems, exo-planet search, large apertures allow for unprecedented high resolution to discern continents and important features of other planets, hyperspectral imaging, adaptive systems, spectroscopy imaging through limb, and stable optical systems from Lagrange-points. Furthermore, future micro-miniaturization might hold promise of a further extension of our dust aperture concept to other more exciting smart dust concepts with other associated capabilities. Our objective in Phase II was to experimentally and numerically investigate how to optically manipulate and maintain the shape of an orbiting cloud of dust-like matter so that it can function as an adaptable ultra-lightweight surface. Our solution is based on the aperture being an engineered granular medium, instead of a conventional monolithic aperture. This allows building of apertures at a reduced cost, enables extremely fault-tolerant apertures that cannot otherwise be made, and directly enables classes of missions for exoplanet detection based on Fourier spectroscopy with tight angular resolution and innovative radar systems for remote sensing. In this task, we have examined the advanced feasibility of a crosscutting concept that contributes new technological approaches for space imaging systems, autonomous systems, and space applications of optical manipulation. The proposed investigation has matured the concept that we started in Phase I to TRL 3, identifying technology gaps and candidate system architectures for the space-borne cloud as an aperture

Medical robots for MRI guided diagnosis and therapy

Magnetic Resonance Imaging (MRI) provides the capability of imaging tissue with fine resolution and superior soft tissue contrast, when compared with conventional ultrasound and CT imaging, which makes it an important tool for clinicians to perform more accurate diagnosis and image guided therapy. Medical robotic devices combining the high resolution anatomical images with real-time navigation, are ideal for precise and repeatable interventions. Despite these advantages, the MR environment imposes constraints on mechatronic devices operating within it. This thesis presents a study on the design and development of robotic systems for particular MR interventions, in which the issue of testing the MR compatibility of mechatronic components, actuation control, kinematics and workspace analysis, and mechanical and electrical design of the robot have been investigated. Two types of robotic systems have therefore been developed and evaluated along the above aspects. (i) A device for MR guided transrectal prostate biopsy: The system was designed from components which are proven to be MR compatible, actuated by pneumatic motors and ultrasonic motors, and tracked by optical position sensors and ducial markers. Clinical trials have been performed with the device on three patients, and the results reported have demonstrated its capability to perform needle positioning under MR guidance, with a procedure time of around 40mins and with no compromised image quality, which achieved our system speci cations. (ii) Limb positioning devices to facilitate the magic angle effect for diagnosis of tendinous injuries: Two systems were designed particularly for lower and upper limb positioning, which are actuated and tracked by the similar methods as the first device. A group of volunteers were recruited to conduct tests to verify the functionality of the systems. The results demonstrate the clear enhancement of the image quality with an increase in signal intensity up to 24 times in the tendon tissue caused by the magic angle effect, showing the feasibility of the proposed devices to be applied in clinical diagnosis

MEMS Conveyance: Piezoelectric Actuator Arrays for Reconfigurable RF Circuits

An array of piezoelectric cantilevers was designed, fabricated, and characterized for use as a micromanipulation surface in a reconfigurable RF circuit micro-factory. The project, known as RFactory, is an effort by the U.S. Army Research Laboratory to create environmentally adaptable, rapidly upgradeable RF systems. The RFactory actuator surface uses unimorph lead zirconate titanate cantilevers with metal posts at the tip that exaggerate the horizontal deflection produced by out-of-plane bending. The motion of a circuit component on the surface has been modeled and observed experimentally. By varying the waveform, voltage amplitude, and frequency of the drive signal, as well as the actuator length and width, the speed and precision of the motion can be controlled. From these characterization efforts, operating conditions that create speeds above 1 mm/s and low positional error (<200 microns after 5 mm translation) have been identified. Finally, full system RF reconfigurability has been demonstrated

Bio-inspired Magnetic Systems: Controlled Swimming, Fluid Pumps, and Collective Behaviour

This thesis details the original experimental investigations of magnetically actuated and controlled microscopic systems enabling a range of actions at low Reynolds number. From millimetre-robots and self-propelled swimmers to microfluidic and lab-on-a-chip technology applications. The main theme throughout the thesis is that the systems reply on the interactions between magnetic and elastic components. Scientists often take inspiration from nature for many aspects of science. Millimetre to micrometre machines are no exception to this. Nature demonstrates how soft materials can be used to deform in a manner to create actuation at the microscale in biological environments. Nature also shows the effectiveness of using beating tails known as flagella and the apparent enhancements in flow speeds of collective motion. To begin with, a swimmer comprised of two ferromagnetic particles coupled together with an elastic link (the two-ferromagnetic particle swimmer), was fabricated. The system was created to mimic the swimming mechanism seen by eukaryotic cells, in which these cells rely on morphological changes which allows them to propel resulting in approximate speeds of up to 2 body lengths per second. The aim of this system was to create a net motion and control the direction of propagation by manipulating the external magnetic field parameters. It was shown that the direction of swimming has a dependence on both the frequency and amplitude of the applied external magnetic field. A key factor discovered was that the influence of a small bias field, in this case, the Earth’s magnetic field (100 orders of magnitude smaller than the external magnetic field) resulted in robust control over the speed (resulting in typical swimming speeds of 4 body lengths per second) and direction of propulsion. Following this work, swimmers with a hard ferromagnetic head attached to an elastic tail (the torque driven ferromagnetic swimmer) were investigated. These systems were created to be analogous to the beating flagella of many natural microscopic swimmers, two examples would be sperm cells and chlamydomonas cells. These biological cells have typical speeds of 10s of body lengths per second. The main focus of this investigation was to understand how the tail length affects the swimming performance. An important observation was that there is an obvious length tail (5.7 times the head length) at which the swimming speed is maximised (approximately 13 body lengths per second). The experimental results were compared to a theoretical model based on three beads, one of which having a fixed magnetic moment and the other two non-magnetic, connected via elastic filaments. The model shows sufficient complexity to break time symmetry and create a net motion, giving good agreement with experiment. Portable point-of-care systems have the potential to revolutionise medical diagnostics. Such systems require active pumps with low power (USB powered devices) external triggers. Due to the wireless and localisation of magnetic fields could possibly allow these portable point-of-care devices to come to life. The main focus of this investigation was to create fluid pump systems comprising from the previously investigated two-ferromagnetic particle swimmer and the torque driven ferromagnetic swimmer. Building on the fact that if a system can generate a net motion it would also be able to create a net flow. Utilising the geometry of the systems, it has been demonstrated that a swimmer-based system can become a fluid pump by restricting the translational motion. The flow structure generated by a pinned swimmer in different scenarios, such as unrestricted flow around it as well as flow generated in straight, cross-shaped, Y-shaped and circular channels were investigated. This investigation demonstrated the feasibility of incorporating the device into a channel and its capability of acting as a pump, valve and flow splitter. As well as a single pump system, networks of the previously mentioned pump systems were fabricated and experimentally investigated. The purpose of this investigation was to utilise the behaviour of the collective motion. Such networks could also be attached to the walls or top of the channel to create a less invasive system compared to pump based within the channel system. The final investigation involved creating collective motion systems which could mimic the beating of cilia - known as a metachronal wave. Two methods were used to create an analogous behaviour. The first was using arrays of identical magnetic rotors, which under the influence of an external magnetic field created two main rotational patterns. The rotational patterns were shown to be controllable producing useful flow fields at low Reynolds numbers. The second system relied on the magnetic components having different fixed magnetisation to create a phase lag between oscillations. The magnetic components were investigated within a channel and the separation between the components was shown to be a key parameter for controlling the induced flow. In both cases, a simple model was produced to help understand the behaviour. Finally, a selection of preliminary investigations into possible applications were conducted experimentally. These investigations included, measuring the effective surface viscosity of lipid monolayers, created cell growth microchannels, as well as systems which could be used for blood plasma separation. The properties of lipid monolayers vary with the surface density, resulting on distinct phase transitions. Slight differences in the molecular lattice are often accompanied by significant changes in the surface viscosity and elasticity. The idea was to use a swimmer as a reporter of the monolayer viscosity, resulting in a less invasive method compared to current techniques to monitor monolayer viscosity, for example torsion pendulums and channel viscometers. The reported effective surface viscosity closely matched the typical Langmuir trough measurements (with a systematic shift of approximately 17 Ų/molecule). The blood plasma separation preliminary work shows the previously investigated two-ferromagnetic particle swimmer mixing a typical volume (100 μm) blood sample with a buffer solution in 21 seconds. The system was also able to create locations with a high population of red blood cells. This resulted in a separation between the blood plasma and red blood cells. Two other preliminary results of future investigations were presented; the collective motion of free swimmers, and the fabrication of ribbon-like structures with fixed magnetic moment patterns.European CommissionEngineering and Physical Sciences Research Council (EPSRC

NASA patent abstracts bibliography: A continuing bibliography. Section 1: Abstracts (supplement 41)

Abstracts are provided for 131 patents and patent applications entered into the NASA scientific and technical information system during the period Jan. 1992 through Jun. 1992. Each entry consists of a citation, an abstract, and in most cases, a key illustration selected from the patent or patent application