Cavitation inception by almost spherical solid particles in water

Abstract

The tensile strength of water increases when solid particles are filtered out, and it becomes greater the smaller the remaining particles are. Natural particles are of random shape, making parametric studies on the relationship between tensile strength and particle characteristics difficult. In this investigation, using degassed tap water from which natural particles larger than about 1 μm had been filtered out, the tensile strength was measured before and after seeding with almost spherical solid balls of diameters from 3 up to 76 μm. The smallest balls, one type being hydrophobic, the other hydrophilic, had no measurable influence on the tensile strength, though they were notably larger than the remaining natural nuclei. Seeding with the larger balls, hydrophilic as well as hydrophobic ones, reduced the tensile strength compared with that measured for unseeded, filtered water, but at most down to 1/3. On this basis it is concluded that a greater tensile strength is connected to the almost spherical solid balls than that due to natural particles of the same size. The critical cavities developed from the larger balls had radii much smaller than those of the balls themselves. This supports the hypothesis that cavitation nuclei are related to the fine scale surface structures observed on the balls, and in dependence of their global radii of curvature, critical cavities are developed. A model of this development is presented.

Reflections on cavitation nuclei in water

“…in which the tensile strength exceeded 10 bar in only a single case. The measurements of Marschall et al 19 and Arora et al, 20 carried out with artificial, almost perfectly spherical, smooth 30 m particles from the same batch, are illustrating. When seeding these particles into tap water filtered of natural particles larger than 1 m, Marschall et al obtained a tensile strength of ϳ0.9 bar in a nozzle flow of filtered tap water.…”

“…This means that perfect spheres should be highly cavitationresistant, the more so the smaller they are. The hypothesis was tested and actually supported in flow experiments with a vortex nozzle by Marschall et al 19 By filtration, natural particles ͑and thus also stabilized free gas bubbles͒ larger than 1 m were removed from tap water of low gas content. The filtered water had a tensile strength of ϳ1.3 bar, i.e., notably smaller than the equilibrium tensile stress for vapor bubbles of 1 m diameter.…”

Cavitation Inception on Microparticles: A Self-Propelled Particle Accelerator

“…The discussion of their nature -stabilized spherical gas bubbles or surface nuclei on particles -has been ongoing. Though [3,5,8,9] point to the latter ones, the direct observation of cavity-particle separation has been missing until now. Further, the particle ejection suggests that accelerated particles may penetrate into nearby soft surfaces, e.g., biological tissue or cells during exposure to strong, focused sound fields.…”

“…3(a)], while particles with a smooth surface [ Fig. 3(b), for details see [9] ] did not cause cavitation.…”

Inhibition of the ultrasonic microjet-pits on the carbon steel in the particles-water mixtures

“…2,3 For the water-particles mixture, the gas nuclei in crevices on the surface of particle grow into cavitation bubbles when they are irradiated by the ultrasound wave. 4,5 Therefore, in contrast to the pure water, the cavitation threshold of particles-water mixture is decreased, which results in an increased cavitation level. 6,7 Meanwhile, because the cavitation erosion is due to the collapse of cavitation bubbles, the particles-water mixture causes stronger cavitation erosion than the pure water does.…”