Small-amplitude, high-frequency (ultrasound) forcing of fluid/particle systems is being used in a number of applications associated with non-destructive fluid mixing and the movement/manipulation of particles in suspension. Of most importance in this context are the second-order, steady, effects arising from the nonlinear interaction of a leading-order oscillatory field with itself. In this thesis we consider some of these steady effects in both incompressible and compressible fluids. We first consider the axisymmetric steady streaming generated in an incompressible, viscous fluid contained between two (radially) infinite parallel plates, each oscillating in a direction normal to its own plane. In the limit of small-amplitude, high-frequency oscillations, we show that the steady-streaming flow in the fluid bulk is driven by thin streaming sublayers at the plates, at which the normal velocity is zero and the radial velocity varies linearly with distance from the axis of rotational symmetry. Effectively, in the bulk flow, the bounding plates appear as (no-slip) impermeable walls that stretch radially. This bulk-flow problem is extended to allow for the analogous steady flow of two immiscible, incompressible, viscous fluids, each undergoing a radial-stretching motion appropriate to high-frequency steady streaming. For a flat interface between the fluids, a self-similar solution reduces the Navier--Stokes equations to a nonlinear boundary-value problem, the solution of which exhibits an interesting structure in the limit of large Reynolds number. In this limit, solutions can be found using matched asymptotic expansions, but the location of the interface between the fluids can only be determined if terms that are exponentially small in the Reynolds number are included. It is shown that for fluids of almost-equal densities, exponentially-small differences can have a leading-order effect on the observed flow. The second part of the thesis is concerned with the (steady) acoustic radiation force on a rigid sphere submerged in a compressible, inviscid fluid, when the wavelength of the incident acoustic field is large compared to the radius of the sphere. In this limit, a matched asymptotic expansion method is used to derive an expression for the acoustic radiation force, on both fixed and free rigid spheres, due to a range of incident fields. For incident acoustic fields that are appropriate to planar and circular waveguides/channels, expressions are derived for the scattered field and the radiation force on a rigid sphere in the long-wavelength limit. Fixed and free spheres located both on and off the axis of symmetry of these incident fields are considered. This is an extension to the current literature, in which numerical methods are used to examine the scattering from spheres in an off-axis position, and problems are restricted to the consideration of fixed spheres only. It is shown that there are stable and unstable positions within the waveguide where any off-axis acoustic radiation force vanishes, leaving only an along-channel component. For free spheres, these positions are shown to be dependent on the relative particle density and it is suggested that this may allow for a mechanism to sort such small particles radially in a circular waveguide, if secondary scattering effects are neglected.

The fish ear contains three dense, bony bodies (otoliths) surrounded by fluid (the endolymph) and tissue. Under acoustic stimulation, the surrounding fluid and tissues oscillate relative to the otoliths, stimulating the endolymph as well as the array of hair cell cilia adjacent to the otolith and embedded in tissue. It is believed that the hair cell cilia move with the surrounding fluid. This doctoral thesis studied the steady streaming (i.e., time-independent) component of the acoustically induced fluid motion inside of the fish ear to determine how the hair cell cilia displacements due to the steady streaming could provide acoustically relevant information to the fish. This research characterizes the fluid flow around oscillating model otoliths, namely spheroids, grooved spheroids, and a 350% scale model of a cod saccular otolith. This study models the otolithic endorgan as an oscillating body in a Newtonian fluid. The model ignores the surrounding tissues and assumes that the hair cell cilia move like the surrounding fluid. Particle pathline visualizations and particle-image velocimetry (PIV) are used to characterize the flow fields at various oscillation orientations, frequencies and amplitudes. These data are used to determine the location of the stagnation points on the body surface and at the boundaries of the inner rotating region of the flow. Studies are also conducted on bodies sinusoidally oscillated at both a single frequency and two (simultaneous) frequencies along the same direction. Both the steady streaming flow patterns and velocity fields are found to contain acoustically relevant information, but given the very small displacements associated with these flows, it is unclear if the steady streaming flows can be sensed by the fish ear.

Parmi les différents mécanismes physiques permettant d'actionner des échantillons liquides au sein de labopuces, l'électromouillage sur diélectrique (EWOD) s'impose peu à peu comme une solution fiable permettant de manipuler en particulier des gouttes (labopuces digitaux). Bien que des modèles énergétiques permettent d'expliquer la plupart des fonctions fluidiques élémentaires obtenues par effet EWOD, il demeure certains phénomènes hydrodynamiques en goutte dont la compréhension à l'échelle locale présente des enjeux en termes scientifiques et applicatifs. En particulier, la maîtrise des oscillations de goutte et des écoulements électrohydrodynamiques induits par des signaux électriques alternatifs (AC-EWOD) pourrait permettre l'insertion de nouvelles fonctions dans les labopuces digitaux (brassage, séparation d'espèces). Dans la thèse proposée, une configuration optimale a été retenue consistant en une goutte reposant sur deux électrodes coplanaires passivées. Un banc de caractérisation a été développé ainsi que des logiciels spécifiques dédiés à l'analyse du mouillage et des oscillations de la goutte. Une première étude permet de caractériser l'électromouillage d'une goutte en configuration d'électrodes coplanaires. En particulier, la modélisation des contraintes électriques surfaciques permet de conclure sur leurs rôles dans l'injection normale et tangentielle de quantité de mouvement. Les oscillations de la goutte et les écoulements induits sont ensuite étudiés de manière expérimentale. Suivant la géométrie des électrodes, des configurations d'écoulements axisymétriques et quadripolaires sont observées. Un modèle basé sur le concept de courant de dérive est développé ; sa résolution numérique par éléments finis permet de retrouver les configurations d'écoulement. Des applications biologiques de l'actuation EWOD en signal alternatif sont finalement proposées et discutées pour le développement de laboratoires sur puces. / Among numerous physical mechanisms enabling liquids actuation, electrowetting on dielectric (EWOD) is increasingly considered as a trustful solution enabling discrete iquid sample handling by using electrical fields. Although energetic models can explain most basic fluidic functions, some electrowetting phenomena still remain to be considered as scientific issues. So is the case for droplet oscillations and hydrodynamic flows induced by electrowetting when AC voltage is applied (AC EWOD). The understanding and mastering of these phenomena represent the objectives of this thesis. An open electrode configuration has been used in which the droplet straddles two insulated coplanar electrodes. An experimental setup and dedicated software for the measurement of droplet wetting and droplet oscillations have been developed. A first experimental and numerical study allowed us the characterization of electrowetting in the coplanar electrode configuration. The calculation of the electrical stress distribution delivers criteria which help to conclude about the involvement of tangential electrical stresses in drop convective flow. Then, droplet oscillations and induced flows are experimentally and theoretically investigated. Depending on electrode designs, axisymetrical and quadripolar flows as well can be observed. A model based on steady streaming is implemented and structure flows are numerically reproduced using finite element method. Some practical applications of this research within the framework of lab-on-chips are proposed and discussed.