There are two main mysteries in bulk nanobubbles which are cavitation nuclei. One is the mechanism of stability of a bulk nanobubble. The other is the problem whether OH radicals are produced from bulk nanobubbles without a dynamic stimulus. For the former problem, several proposed models are briefly reviewed. The dynamic equilibrium model is discussed in details that a bulk nanobubble is stabilized by a partial coverage of the bubble surface by a hydrophobic material. The TEM images of bulk nanobubbles seem to support the dynamic equilibrium model. For the latter problem, numerical simulations of dissolution of an air nanobubble are reviewed, which suggest that no OH radical is produced from a dissolving nanobubble. A possible role of HO generated during bulk nanobubble production using hydrodynamic cavitation is briefly discussed in relation to the experimental results of "OH radical" detection.

The dynamic equilibrium model for a bulk nanobubble partly covered with hydrophobic material in water is theoretically and numerically studied. The gas diffusion into a bubble near the peripheral edge of the hydrophobic material on the bubble surface balances that out of the bubble from the other part of the uncovered bubble surface. In the present model, gas diffusion in quiescent liquid is assumed and there is no liquid flow. The total changes of energy and entropy are both zero as it is a kind of equilibrium state. The main origin of the dynamic equilibrium state is the gradient of chemical potential of gas near the peripheral edge of the hydrophobic material. It is caused by the permanent attractive potential of a hydrophobic material to gas molecules dissolved in liquid water as there is permanent repulsion of a hydrophobic material against liquid water. Thus, the gas supply will not terminate. It is numerically shown that stable nanobubble could be present when the fraction of surface coverage by hydrophobic material is from about 0.5 to 1. The stable size of a nanobubble changes with the liquid temperature as well as the degree of gas saturation of water. In slightly degassed water, not only a nanobubble but also a microbubble could be stable in mass balance when the fraction of surface coverage for a microbubble is on the order of 10 or less. For hydrophilic materials, however, a bubble could not be stable unless the fraction of the surface coverage is exactly 1. It is suggested that in many experiments of bulk nanobubbles there could be aggregates of nanobubbles.

This article reviews the analytic techniques for Raman spectroscopic imaging with emphasis on chemometrics. Key information included in Raman spectra is often distributed broadly throughout the dataset. It is possible to condense the information into a very compact matrix representation by a chemometric technique of factor analysis such as principal component analysis (PCA) or self-modeling curve resolution (SMCR). PCA yields two matrices called scores and loadings which complementarily represent the entire features broadly distributed in the dataset. This concept can be further extended to other forms of data transformation schemes, including bilinear data decomposition based on SMCR analysis. SMCR offers a firmer model which is chemically or physically interpretable. The information derived from these techniques readily brings useful insight into building a mechanistic model for understanding complex phenomena studied by Raman spectroscopy. Illustrative examples are given for applications of both PCA and SMCR to Raman imaging of pharmaceutical tablets.

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The dynamic-equilibrium model for stabilization of a nanobubble on a hydrophobic surface by Brenner and Lohse [M. P. Brenner and D. Lohse, Phys. Rev. Lett. 101, 214505 (2008)] has been modified taking into account the van der Waals attractive force between gas molecules inside a nanobubble and solid surface. The present model is also applicable to a nanobubble on a hydrophilic surface. According to the model, the pressure inside a nanobubble is not spatially uniform and is relatively higher near the solid surface. As a result, there is gas outflux near a hydrophilic surface, while near a hydrophobic surface there is gas influx which has been already suggested. In the present model, the radius of curvature for a nanobubble depends on the distance from the solid surface because the pressure depends on it. The shape of the micropancake, which is a nearly-two-dimensional bubble, is reproduced by the present model due to the strong dependence of the radius of curvature on the distance from the solid surface. The effect of temperature on the stability of a nanobubble or micropancake is also discussed.