On the experimental part of the programme, new test rigs have been conceived, set-up and utilised for obtaining a wide range of measurements. Laser-induced individual bubble dynamics, high speed imaging, high-frequency needle hydrophones for shock wave measurements, spectrometer for luminescence detection and oxygen monitoring probes for gas content evaluation have been utilised to test the collapse of air, argon, carbon dioxide, nitrogen and ammonia bubbles for the first time. It was found that the maximum size of the bubble and the rebound bubble size are independent on the type and amount of dissolved gas, while the vapor content of the bubble does not condense totally during the collapse phase, as commonly believed. The influence of viscoelastic materials around bubbles was also investigated, revealing that the viscoelasticity of tissue is dramatically different in the MHz driving regime from that known at low strain rates; a novel rheology method was developed to measure these properties. With regards to the interaction of bubbles with soft materials, it has been demonstrated that extremely high acoustic images can reveal the details of non-spherical bubble collapses; these lead to the propagation of the strain field as a shear wave, which allowed measurements of the strain fields in non-transparent media and thus, estimation of the properties of the tissue. Moving towards more complex configurations relevant to cavitation in tissue mimicking materials (TMM), the cavitation threshold has been quantified in three potential TMM containing scatterers; the variations in concentration and addition of scatterers allowed the acoustic and cavitation properties to be tailored to specific tissues. Based on the findings, the tissue mimics were used to build a “drug delivery” phantom intended for use in the study of the effect of different ultrasound exposure parameters on the transport of dyes across vessel walls. Finally, acoustic vaporization was discovered as new physical mechanism of cavitation nucleation for endovascular liquid core high boiling point PFOB microdroplets, involving the dynamic redistribution of the dissolved gases (oxygen and nitrogen) at the hydrophobic-hydrophilic interface under the effect of the negative pressure.
On the modelling side, a wide range of simulations approaches have been developed. The dependence of behavior collapse on the contact angle was explained using the impulse potential flow theory for short times, which shows the presence of a singularity on the initial acceleration of the contact line. The collapse of multiple cavitation bubbles nucleating and collapsing in contact with a rigid wall was also studied to understand the interaction among the bubbles and its consequences. Moreover, numerical simulations utilising real-fluid thermodynamics equations of state for estimating the properties of both the liquid and gas as function of pressure and temperature have been developed, allowing for more accurate prediction of both the gas and liquid temperatures to be predicted during bubble collapses. The interaction of cavitation bubbles with free surfaces was also investigated using a compressible all-Mach number flow solver with interface tracking capability. This shed light to the mechanism of bubble entrainment process. It was found that the oscillatory flow inside the nozzle, naturally accompanied by alternating gradients of pressure, induces flow focusing at the curved meniscus via the baroclinic torque. The misalignment of the density and pressure gradients across the interface induces the generation of vorticity, which in turn, focuses the flow at the level of the meniscus resulting in a bubble pinch-off governed at the last stage by capillary forces. With the same flow solver, immersed boundary methods allowed for fluid-structure interactions and thin elastic capillary to be computed considering surface tension effects. With regards to the interaction of cavitation bubbles with soft material, two different approaches have been developed. The first novel numerical framework has employed a Diffuse Interface Method (DIM) accounting for the interaction across fluid-solid-gas interfaces. It usilised an Adaptive Mesh Refinement (AMR) framework for unstructured grids. The numerical framework was validated for cases of bubble dynamics, under high and low ambient pressure ratios, shock-induced collapses, and wave transmission problems across a fluid-solid interface, against theoretical and numerical results. The detailed collapse dynamics, jet formation, solid deformation, rebound, primary and secondary shock wave emissions, and secondary collapse that govern the near-solid collapse and tissue sonoporation mechanisms was revealed for the first time. Finally, a mess less Smooth Particle Hydrodynamics (SPH) method for liquids was combined by a TL SPH method for solids where the 1st Piola-Kirchhoff stress tensor was considered. This enabled simulations of collapsing cavities (“vacuum bubbles”) in a liquid above or close to a deformable solid object. The developed monolithic approach was successfully tested against reference cases and was applied to simulate the sonoporation process of a collapsing bubble close to a cell membrane, including the breaking of the membrane due to the liquid jet.
