Ultrasound homogenises suspensions of hydrophobic particles

TL;DR

This study demonstrates that ultrasound can suspend hydrophobic particles in water by inducing cavitation, confirming their role as cavitation nuclei and revealing the transient nature of their gaseous shell.

Contribution

It provides experimental evidence that ultrasound can suspend hydrophobic particles by cavitation, a novel insight into their behavior as cavitation nuclei.

Findings

Hydrophobic particles of C65 and ZnO can be suspended using ultrasound.

High-speed observations confirm cavitation activity around hydrophobic particles.

Gas layers around particles dissolve after initial cavitation, stopping further cavitation.

Abstract

Hydrophobic particles inherently resist being suspended. Hydrophobic particles might be regarded as tiny solid particles surrounded by a thin gaseous shell. It has been hypothesised that hydrophobic particles act as cavitation nuclei. This cavitation behaviour would explain the translation speeds observed when hydrophobic polystyrene microspheres were driven through a liquid medium by means of ultrasound. These translation speeds corresponded to those observed with gas microbubbles of similar sizes. If hydrophobic particles do have a thin gaseous layer surrounding the solid cores, a sound field of sufficient pressure amplitude might force the gas layer to form and inertial cavity and subsequently fragment during the collapse phase. In this study, we investigated whether hydrophobic particles can be forced to suspend by using ultrasound. Hydrophobic particles of the materials C65 and ZnO can be forced to be suspended in water using ultrasound. The high-speed observations confirm that hydrophobic particles can act as cavitation nuclei. The lack of cavitation after the first pulse indicates that the gas layer surrounding the hydrophobic particle dissolves after inertial cavitation.