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A hydrodynamic analog of critical phenomena : an uncountably infinite number of universality classes
When a solid object starts falling into a viscous fluid from air-liquid interface, air is entrained into the liquid and eventually detaches from the solid. Such detachment could occur with or without topological change. Recently, it was found that the former case (i.e., breakup, a form of singular transitions) is observed in a confined geometry but the topology change is suppressed in a more confined geometry, by falling a metal disk in a vertically stood Hele-Shaw cell filled with a viscous liquid with the disk axis perpendicular to the direction of gravity. In this talk, we discuss the results when we tune a confinement parameter, the thickness difference between the cell and the disk, with fixing another confinement parameter, the disk thickness. As a result, we find that the present hydrodynamic case possesses a strikingly close analogy with critical phenomena. Critical phenomena have widely been observed in nature, and the concept of universality class, which has emerged from our understanding of critical phenomena, has guided the recent development of physics. Accordingly, identifying a rich variety of universality class is a major issue in modern physics. Here, we remarkably find the present hydrodynamic analog of critical phenomena reveals the existence of an uncountably infinite number of universality classes, by showing critical exponents that define a universality class depend on continuous numbers characterizing the confinement.