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We study single bubble deformation statistics in an homogeneous and isotropic turbulent flow by means of direct numerical simulations. We consider bubbles at low Weber number (
$We <3$) that have not been broken. We show that we can reproduce bubble deformations with a linear dynamics for each spherical harmonic mode. Inferring the coefficients of the linear model from the DNS data, we find that the natural frequency corresponds to the Rayleigh frequency, derived in a quiescent flow. However, the effective damping increases by a factor 7 compared with the quiescent case, at Taylor Reynolds number 
$\textit {Re}_\lambda = 55$. Looking at the flow structure around the bubble, we argue that the enhanced damping originates from a thick boundary layer surrounding the bubble. We demonstrate that the effective forcing, originating from the turbulent flow forcing on the bubble surface, is independent of bubble deformability. Therefore, the interface deformations are only one-way coupled to the flow. From this model we conclude that bubbles break rather from turbulent fluctuations than from a resonant mechanism. Eventually, we investigate the pressure modes’ statistics in the absence of bubbles and compare them with the effective forcing statistics. We show that both fields share the same probability distribution function, characterized by exponential tails, and a characteristic time scale corresponding to the eddy turnover time at the mode scale.
We present experiments on large air cavities spanning a wide range of sizes relative to the Hinze scale 
$d_{H}$, the scale at which turbulent stresses are balanced by surface tension, disintegrating in turbulence. For cavities with initial sizes 
$d_0$ much larger than 
$d_{H}$ (probing up to 
$d_0/d_{H} = 8.3$), the size distribution of bubbles smaller than 
$d_{H}$ follows 
$N(d) \propto d^{-3/2}$, with 
$d$ the bubble diameter. The capillary instability of ligaments involved in the deformation of the large bubbles is shown visually to be responsible for the creation of the small bubbles. Turning to dynamical, three-dimensional measurements of individual break-up events, we describe the break-up child size distribution and the number of child bubbles formed as a function of 
$d_0/d_{H}$. Then, to model the evolution of a population of bubbles produced by turbulent bubble break-up, we propose a population balance framework in which break-up involves two physical processes: an inertial deformation to the parent bubble that sets the size of large child bubbles, and a capillary instability that sets the size of small child bubbles. A Monte Carlo approach is used to construct the child size distribution, with simulated stochastic break-ups constrained by our experimental measurements and the understanding of the role of capillarity in small bubble production. This approach reproduces the experimental time evolution of the bubble size distribution during the disintegration of large air cavities in turbulence.
In this era of spatially resolved observations of planet-forming disks with Atacama Large Millimeter Array (ALMA) and large ground-based telescopes such as the Very Large Telescope (VLT), Keck, and Subaru, we still lack statistically relevant information on the quantity and composition of the material that is building the planets, such as the total disk gas mass, the ice content of dust, and the state of water in planetesimals. SPace Infrared telescope for Cosmology and Astrophysics (SPICA) is an infrared space mission concept developed jointly by Japan Aerospace Exploration Agency (JAXA) and European Space Agency (ESA) to address these questions. The key unique capabilities of SPICA that enable this research are (1) the wide spectral coverage 
$10{-}220\,\mu\mathrm{m}$, (2) the high line detection sensitivity of 
$(1{-}2) \times 10^{-19}\,\mathrm{W\,m}^{-2}$ with 
$R \sim 2\,000{-}5\,000$ in the far-IR (SAFARI), and 
$10^{-20}\,\mathrm{W\,m}^{-2}$ with 
$R \sim 29\,000$ in the mid-IR (SPICA Mid-infrared Instrument (SMI), spectrally resolving line profiles), (3) the high far-IR continuum sensitivity of 0.45 mJy (SAFARI), and (4) the observing efficiency for point source surveys. This paper details how mid- to far-IR infrared spectra will be unique in measuring the gas masses and water/ice content of disks and how these quantities evolve during the planet-forming period. These observations will clarify the crucial transition when disks exhaust their primordial gas and further planet formation requires secondary gas produced from planetesimals. The high spectral resolution mid-IR is also unique for determining the location of the snowline dividing the rocky and icy mass reservoirs within the disk and how the divide evolves during the build-up of planetary systems. Infrared spectroscopy (mid- to far-IR) of key solid-state bands is crucial for assessing whether extensive radial mixing, which is part of our Solar System history, is a general process occurring in most planetary systems and whether extrasolar planetesimals are similar to our Solar System comets/asteroids. We demonstrate that the SPICA mission concept would allow us to achieve the above ambitious science goals through large surveys of several hundred disks within 
$\sim\!2.5$ months of observing time.
