Interferometric particle imaging (IPI) is used to measure both the size distribution and concentration of microbubbles (with a diameter less than 100 micron) in water. Using a new method for calibration makes it possible to obtain quantitative results for the concentration of microbubbles. The results are validated using imaging with a long-range microscope shadowgraph (LMS). Estimates of the size distribution and concentration from both IPI and LMS agree within uncertainty limits. The relative uncertainty in the IPI concentration estimation is about 10% and is mostly due to the finite number of detected bubbles. It is shown that the performance of the bubble-image detection algorithm needs to be quantified to obtain a reliable estimate of the concentration obtained with IPI.

Since the industrial revolution the ocean has become noisier. The global increase in shipping is one of the main contributors to this. In some regions, shipping contributed to an increase in ambient noise of several decibels, especially at low frequencies (10 to 100 Hz). Such an increase can have a substantial negative impact on fish, invertebrates, marine mammals and birds interfering with key life functions (e.g. foraging, mating, resting, etc.). Consequently, engineers are investigating ways to reduce the noise emitted by vessels when designing new ships. At the same time, since the industrial revolution (starting around 1760) greenhouse gas emissions have increased the atmospheric carbon dioxide fraction x(CO₂) by more than 100 μmol mol^-1. The ocean uptake of approximately one third of the emitted CO₂ decreased the average global surface ocean pH from 8.21 to 8.10. This decrease is modifying sound propagation, especially sound absorption at the frequencies affected by shipping noise lower than 10 kHz, making the future ocean potentially noisier. There are also other climate change effects that may influence sound propagation. Sea surface warming might alter the depth of the deep sound speed channel, ice melting could locally decrease salinity and more frequent storms and higher wind speed alter the depth of the thermocline. In particular, modification of the sound speed profile can lead to the appearance of new ducts making specific depths noisier. In addition, ice melting and the increase in seawater temperature will open new shipping routes at the poles increasing anthropogenic noise in these regions. This review aims to discuss parameters that might change in the coming decades, focusing on the contribution of shipping, climate change and economic and technical developments to the future underwater soundscape in the ocean. Examples are given, contrasting the open ocean and the shallow seas. Apart from the changes in sound propagation, this review will also discuss the effects of water quality on ship-radiated noise with a focus on propeller cavitation noise.

The mechanism of bubble capture in a vortical flow is investigated using a Lagrangian bubble tracking method. The motion of bubbles and the factors influencing their movement are examined. Detailed analysis is conducted on the roles played by each force component, such as the lift, added mass, and centrifugal forces, in the bubble capture process. An interesting finding is the identification of the stabilizing effect of the azimuthal lift force on the bubble capture mechanism. Furthermore, a model for capture time based on the radial force balance is also developed, and validated with existing experimental data. These findings, including the force mechanism and capture time model, provide a foundation for understanding the bubble capture process and can potentially inform future studies on tip vortex cavitation inception such as determining the cavitation hotspot.

Rare earth oxyhydrides REO_xH_(3-2x), with RE = Y, Sc, or Gd and a cationic FCC lattice, are reversibly photochromic in nature. It is known that structural details and anion (O^2-:H^-) composition dictate the efficiency of the photochromic behavior. The mechanism behind the photochromism is, however, not yet understood. In this study, we use ¹H, ²H, ¹⁷O, and ⁸⁹Y solid-state NMR spectroscopy and density functional theory (DFT) calculations to study the various yttrium, hydrogen, and oxygen local environments, anion oxidation states, and hydride ion dynamics. DFT models of YO_xH_(3-2x) with both anion-ordered and anion-disordered sublattices are constructed for a range of compositions and show a good correlation with the experimental NMR parameters. Two-dimensional ¹⁷O-¹H and ⁸⁹Y-¹H NMR correlation experiments reveal heterogeneities in the samples, which appear to consist of hydride-rich (x ≈ 0.25) and hydride-poor domains (x ≈ 1) rather than a single composition with homogeneous anion mixing. The compositional variation (as indicated by the different x values in YO_xH_(3-2x)) is determined by comparing static ¹H NMR line widths with calculated ¹H-¹H dipolar couplings of yttrium oxyhydride models. The 1D ¹⁷O MAS spectrum demonstrates the presence of a small percentage of hydroxide (OH^-) ions. DFT modeling indicates a reaction between the protons of hydroxides and hydrides to form molecular hydrogen (H⁺ + H^- → H₂). ¹H MAS NMR indicates the presence of a mobile component that, based on this finding, is attributed to trapped molecular H₂ in the lattice.

In this paper, we investigate by ab initio DFT how the O:H ratio influences the formation and lattice energy, metastability, and optical properties of Y and La anion-disordered ROxH3-2x oxyhydrides. To achieve this, a set of special quasirandom structures (SQS) is introduced to model anion-disorder along the whole RH3-R2O3 composition line. A comparison with an extensive set of anion-ordered polymorphs of the same composition shows the comparable energy of the anion-disordered phase, which, in particular, in the H-rich composition interval showed the lowest relative energy. In turn, the metastability of the anion-disordered phase depends on the cation size (Y versus La), which determines the maximum H content above which the CaF2-type structure itself becomes unstable. To overcome the accuracy limitations of classical DFT, the modified Becke-Johnson (mBJ) scheme is employed in the study of the electronic properties. We show that major differences occur between H-rich and O-rich R oxyhydrides, as the octahedral H- present for x<1 form electronic states at the top of the valence band, which reduce the energy band gap and dominate the electronic transitions at lower energies, thus increasing the refractive index of the material in the VIS-nIR spectral range. Comparing the DFT results to experimental data on photochromic Y oxyhydride films reinforces the hypothesis of anion-disorder in the H-rich films (x<1), while it hints towards some degree of anion ordering in the O-rich ones (x>1). Our paper exemplifies a strategy to calculate ab initio the electronic/optical properties of a wide range of materials with occupational disorder.