We have erred in our identification of the l 0 spheroidal modes. Figure 3 of our Letter is replotted based on a refit of our Brillouin data using the correct eigenvalue equation [Eq. (59) in Ref.[13] of the original Letter] for the l 0 modes and Eq. (1), of the Letter, for the l > 0 modes. The fitting yielded respective longitudinal and transverse acoustic mode velocities of V L 5042 m=s and V T 2097 m=s.Equation (3) now becomes 10

The vibrational modes in three-dimensional ordered arrays of unembedded SiO2 nanospheres have been studied by Brillouin light scattering. Multiple distinct Brillouin peaks are observed whose frequencies are found to be inversely proportional to the diameter (approximately 200-340 nm) of the nanospheres, in agreement with Lamb's theory. This is the first Brillouin observation of acoustic mode quantization in a nanoparticle arising from spatial confinement. The distinct spectral peaks measured afford an unambiguous assignment of seven surface and inner acoustic modes. Interestingly, the relative intensities and polarization dependence of the Brillouin spectrum do not agree with the predictions made for Raman scattering.

A boundary element method based on the incompressible potential flow theory is adopted to investigate the interaction between two identical oscillating bubbles and a free surface in a tank. An axisymmetric numerical model is established, and certain numerical techniques are proposed to address coefficient matrix singularity and fluid-structure intersection. Experiments with spark-generated bubbles in a cylindrical tank recorded by a high-speed camera are conducted, and the numerical results are validated. On this basis, a typical case of bubbles interacting with a free surface in a tank with relatively small inter-bubble and bubble-free surface distances is carefully studied. A crown-shaped water column at the free surface is observed both numerically and experimentally. The maximum volume of the lower bubble is found to be much larger than that of the upper one. The effects of the inter-bubble and bubble-wall distances on bubble dynamics and free surface motion are analyzed. The results can provide a useful reference for underwater explosion experiments in the confined fluid field.

Dynamics behavior for a submerged oscillating bubble is closely related to the physical properties of boundaries adjacent to the bubble. For an incomplete boundary, such as a ship structure broken by the shock wave from underwater explosion attack, the bubble load formed after the shock wave may cause secondary damage to it, and the jet characteristics generated during the bubble collapse are worth exploring. Based on the incompressible potential flow method, a boundary integral method with axisymmetric code is employed to investigate the jet features of the bubble under the combined action of a free surface above and a fixed plate with a centric circular opening below. For the numerical divergence caused by the expanding bubble infinitely approaching the rim of the circular opening, the bubble walls above and below the opening are, respectively, fused with the rigid wall, and the initial infinite flow field surrounding the bubble is then divided into two independent basins that are both required to be solved. Several experiments of a spark-generated bubble oscillating near a free surface and a rigid plate with a circular opening are conducted and the numerical results are validated. Following that, two typical cases with a dominant jet, respectively, directing upward and downward are analysed in detail, and some new phenomena are discovered. Additionally, another particular case that the upward and downward jet collides in the vicinity of the opening centroid is also presented. Finally, the initial bubble-free surface distance effects upon the bubble movement and the jet developments are analysed and discussed.

High-intensity focused ultrasound (HIFU) is a non-invasive technique and widely used in medical and dental treatment for reaching and affecting deep seated tissues within the body, such as the removal of fat tissues under the skin, destruction of malignant tumour cells or target volume for the purpose of tissue debulking. This study aims to explore the damage characteristics of elastic material (regarded as soft tissue) by HIFU separated by a thin membrane all immersed in a water medium, in which the latter is used to mimic the human skin in medical treatments. Measurements of acoustic pressure along the vertical central line of HIFU transducer with and without this membrane were conducted and effects of membrane on the focal region were investigated. Results showed that the existence of the membrane serves primarily to reflect partial energy of incident ultrasound wave, and has no effect on phases of wave propagating in water. Next, Polydimethylsiloxane (PDMS) was selected as the elastic material for the experiments. The process of PDMS damage through the membrane by HIFU were recorded and the distribution of temperature around and within the ‘destroyed’ PDMS block was captured by an infrared camera. The very high temperature measured within the PDMS leading to the formations of cavity and crack within the PDMS block, can be attributed to the accumulation of the energy from the ultrasound source. Finally, the effects of PDMS with different elasticity on their damage characteristics are discussed.