Reflections on cavitation nuclei in water

Abstract

The origin of cavitation bubbles, cavitation nuclei, has been a subject of debate since the early years of cavitation research. This paper presents an analysis of a representative selection of experimental investigations of cavitation inception and the tensile strength of water. At atmospheric pressure, the possibility of stabilization of free gas bubbles by a skin has been documented, but only within a range of bubble sizes that makes them responsible for tensile strengths up to about 1.5 bar, and values reaching almost 300 bar have been measured. However, cavitation nuclei can also be harbored on the surface of particles and bounding walls. Such nuclei can be related to the full range of tensile strengths measured, when differences of experimental conditions are taken into consideration. The absence or presence of contamination on surfaces, as well as the structure of the surfaces, are central to explaining why the tensile strength of water varies so dramatically between the experiments reported. A model for calculation of the critical pressure of skin-covered free gas bubbles as well as that of interfacial gaseous nuclei covered by a skin is presented. This model is able to bridge the apparently conflicting results of the many scientists, who have been working in the field over the years.

Cavitation inception from bubble nuclei

“…they are spherical caps, and they are taken to be covered by an amphiphilic skin which allows them to be in diffusion balance with the surrounding water. Thus, the initial pressure inside such a bubble p g,0 þ p v ¼ p 1,0 , where p g,0 is the gas pressure, p v is the vapour pressure and p 1,0 is the initial far-field pressure, the skin giving the bubble an effective initial surface tension coefficient of g eff , 0 ¼ 0 [15]. Such a skin-stabilized interfacial cavitation bubble is shown in figure 8a, and an angle of attachment u 0 ¼ 178 is assumed.…”

“…values commonly found for plain water, by assuming diffusion balance across the skin at stabilization and applying the Blake formulae [1] to these bubbles [15].…”

Cavitation nuclei in water exposed to transient pressures

“…Calculations show that such a skin-stabilized bubble has a critical radius in principle equivalent to that of a skin-free bubble of the same gas content. Although the latter is smaller and unstable, their critical radii and pressures can be determined from the same equations (Blake 1949;Mørch 2007). Undoubtedly, cavitation nuclei at normal solid boundaries as well as on the surface of particles in the bulk of plain water are also skin-stabilized, and an analysis by Ducker (2009) supports this expectation.…”

“…Surface nanobubbles can be observed with atomic force microscopy and are always found to be very flat, with contact radii of up to 1000 nm and heights of up to 50 nm (Zhang, Quinn & Ducker 2008;Seddon & Lohse 2011). Let us apply the considerations of Mørch (2007) to a planar solid-water interface with a stable interfacial gas bubble, shaped as a spherical cap of attachment radius R n and height h 0 , the bubble being covered by an amphiphilic skin that allows gas diffusion balance at the water-gas interface, i.e. surface tension γ eff ,0 = 0, figure 1(a).…”

Numerical simulation of unsteady three-dimensional sheet cavitation

“…For homogeneous nucleation in pure water the theoretical tensile strength is approximately equal to 1320 bar at T = 298 K [65,141]. However, in experiments, due to the problem of cleaning and degassing of the water and equipment, this value is not found.…”