The Acoustic Hologram and Particle Manipulation with Structured Acoustic Fields

This book shows how arbitrary acoustic wavefronts can be encoded in the thickness profile of a phase plate - the acoustic hologram. The workflow for design and implementation of these elements has been developed and is presented in this work along with examples in microparticle assembly, object propulsion and levitation in air. To complement these results, a fast thermographic measurement technique has been developed to scan and validate 3D ultrasound fields in a matter of seconds

Nonlinear Acoustics in Underwater and Biomedical Applications: Array Performance Degradation and Time Reversal Invariance

This dissertation describes a model for acoustic propagation in inhomogeneous flu- ids, and explores the focusing by arrays onto targets under various conditions. The work explores the use of arrays, in particular the time reversal array, for underwater and biomedical applications. Aspects of propagation and phasing which can lead to reduced focusing effectiveness are described. An acoustic wave equation was derived for the propagation of finite-amplitude waves in lossy time-varying inhomogeneous fluid media. The equation was solved numerically in both Cartesian and cylindrical geometries using the finite-difference time-domain (FDTD) method. It was found that time reversal arrays are sensitive to several debilitating factors. Focusing ability was determined to be adequate in the presence of temporal jitter in the time reversed signal only up to about one-sixth of a period. Thermoviscous absorption also had a debilitating effect on focal pressure for both linear and nonlinear propagation. It was also found that nonlinearity leads to degradation of focal pressure through amplification of the received signal at the array, and enhanced absorption in the shocked waveforms. This dissertation also examined the heating effects of focused ultrasound in a tissue-like medium. The application considered is therapeutic heating for hyperther- mia. The acoustic model and a thermal model for tissue were coupled to solve for transient and steady temperature profiles in tissue-like media. The Pennes bioheat equation was solved using the FDTD method to calculate the temperature fields in tissue-like media from focused acoustic sources. It was found that the temperature-dependence of the medium's background prop- erties can play an important role in the temperature predictions. Finite-amplitude effects contributed excess heat when source conditions were provided for nonlinear ef- fects to manifest themselves. The effect of medium heterogeneity was also found to be important in redistributing the acoustic and temperature fields, creating regions with hotter and colder temperatures than the mean by local scattering and lensing action. These temperature excursions from the mean were found to increase monotonically with increasing contrast in the medium's properties.Office of Naval Research (Code 321-TS

Transducers for measuring acoustic transients

This thesis is concerned with the design and development of measuring devices for the characterisation of acoustic transients with high temporal and spatial resolution. Three new techniques are demonstrated characterising acoustic transients generated by Nd-YAG laser (1060nm, 30ns, 55mJ) assisted breakdown of water and air. The first technique demonstrates the use of a high power semiconductor laser in a high speed multiple exposure imaging system. This system developed is capable of illuminating an event with up to 10 pulses of light at a maximum repetition rate of 5MHz, with a timing accuracy of ≈5ns. Each semiconductor laser light pulse has a FWHM duration of 50ns, peak power of 30W, and a wavelength of 860nm. Images of individual acoustic transients are displayed on the same CCD camera frame, and it was found that this is best achieved using a dark field imaging technique such as Schlieren imaging. [Continues.

Acoustic modelling of bat pinnae utilising the TLM method

This thesis describes the numerical modelling of bioacoustic structures, the focus being the outer ear or pinnae of the Rufous Horseshoe bat (Rhinolophus rouxii). There have been several novel developments derived from this work including: • A method of calculating directionality based on the sphere with a distribution of measuring points such that each lies in an equal area segment. • Performance estimation of the pinna by considering the directionality of an equivalent radiating aperture. • A simple synthetic geometry that appears to give similar performance to a bat pinna. The outcome of applying the methods have yielded results that agree with measurements, indeed, this work is the first time TLM has been applied to a structure of this kind. It paves the way towards a greater understanding of bioacoustics and ultimately towards generating synthetic structures that can perform as well as those found in the natural world

A study of the application of ultrasonic standing waves to the segregation of fine biological particles from liquids

This thesis describes research to evaluate the application of megahertz (1 to 10 MHz) ultrasonic standing waves to the segregation and separation of fine biological particles, in the size range of 0.1 to 10 μm, from liquids. Research has focused on the development of an alternative separation technique through the ability to selectively manipulate delicate, highly hydrated particles typical of many biological process streams where the sedimentation characteristics of the particles preclude traditional centrifugation-based separation methods and the requisite for non-invasive in line processing rules out filtration. A survey of both acoustic and ultrasonic research concentrating on the application of ultrasonic energy to processes involving biological particles has been carried out. An in-depth analysis of the theories of ultrasonics in relation to the stated aims of the work is presented in which the mechanisms controlling the migration of fine particles under the influence of a megahertz frequency standing wave field are discussed. Results of investigations to determine the feasibility of concentrating micron-sized particles in a standing wave field arc presented. These confirm that the small-scale separation of biological particles is achievable. The subsequent design of an experimental separation device and detailed experiments to elucidate the parameters of importance in determining the segregation of biological particles from liquids using this apparatus are described. Ultrasonic power input and fluid velocity were found to be the most critical process parameters and operational constraints as functions of particle size and ultrasonic frequency were identified. The design and development of a novel laser scanning technique for the monitoring of the migration of particles in an ultrasonic standing wave field is presented. Data obtained using this equipment has been used when discussing the design of large-scale continuous solid-liquid separation devices. Details of an ultrasonic system for the non-invasive, in-situ sample preparation of material for dynamic laser light scattering analysis of particle size distributions in the monitoring and control of bioprocesses are presented together with data from experimental trials. Results showed this to be a promising method for rapid and controlled sample preparation and well suited to handling process streams containing heterogeneous particle sizes. The thesis concludes by giving consideration to the necessary future work and to the application of the techniques described in the thesis to relevant biological separation problems

Synthesis, characterization and magneto-rheological properties of biopolymer based magnetosensitive nanocomposites

Un materiale biocomposito con caratteristiche magnetiche è stato creato e caratterizzato inglobando nanoparticelle di ossido si ferro magnetico in una matrice di alginato; le nanoparticelle sono state preventivamente funzionalizzate in superficie con specifici ligandi per favorire il legame con la matrice biopolimerica e con le molecole di fluoroforo, usate per imitare il comportamento dei farmaci. Un tale materiale è pensato per applicazioni di trasporto controllato di farmaci nel corp