すまん不純物の由来については考察されてた
＞These contaminants were naturally included in ultrapure water because they were below the impurity threshold [32], [33], [37], or coming from the glassware, laboratory atmosphere, or other instruments with which the sample solutions contact.
＞In the untreated solution, these contaminants were almost dissolved in the solution, or their size was out of the limit of the detection by the dark-field microscopy. However, in the process of nanobubble generation, the interface of shrinking microbubbles accumulates these contaminants, and eventually, they form nanoparticles after the disappearance of the nanobubbles [25], [38].

しかしこれは「ナノバブルの正体は非ガス粒子」という話とは違うような……