Cavitation Inception on Microparticles: A Self-Propelled Particle Accelerator

Corrugated, hydrophilic particles with diameters between 30 and $150\mu \mathrm{m}$ are found to cause cavitation inception at their surfaces when they are exposed to a short, intensive tensile stress wave. The growing cavity accelerates the particle into translatory motion until the tensile stress decreases, and subsequently the particle separates from the cavity. The cavity growth and particle detachment are modeled by considering the momentum of the particle and the displaced liquid. The analysis suggests that all particles which cause cavitation are accelerated into translatory motion, and separate from the cavities they themselves nucleate. Thus, in the research of cavitation nuclei the link is established between developed cavitation bubbles and their origin.

Figure 1

Sketch of the experimental setup: The microscope, embedded partly in a cylindrical glass housing, is operated at a working distance of 45 mm from the focus of the shock wave generator. The flask is positioned with an $xyz$-translation stage.

Figure 2

Pressure, $P(t)$, versus time recorded with a fiber optical hydrophone.

Figure 3

Scanning electron microscope (SEM) pictures of the particles taken from batches where (a) cavitation inception was observed (copolymer:divinylbenzol, diameter distribution $30$ to $150\mu \mathrm{m}$), and (b) no inception was achievable (monodisperse 30 $\mu \mathrm{m}$ dynospheres EXP-SS-42.3-RSH, see Ref. 9). Please note the different magnifications.

Figure 4

Two three-frame sequences (a) and (b) depicting the explosive growth of cavitation bubbles from particles and their later separation. The timing of the individual frames relative to the start of the tensile wave (see Fig. 2) are indicated at the top of each frame (the length of the bar is $200\mu \mathrm{m}$).

Figure 5

Position and size of the particle-cavity system versus time (note the logarithmic scale). Left and right dotted lines show calculated positions of the cavity and particle centers, respectively; left and right shaded regions represent the diameters of the cavity and the particle, respectively. The bars indicate the position and the size of the cavity (left) and the particle (right) from the experiment in Fig. 4b (lower two frames). The measured pressure profile is shown along the right edge of the graph. The inset depicts the measured velocity of the particles for multiple experimental runs before and after separation from the cavity. The size of the disk symbols scales linearly with the particle diameter from 56 to $108\mu \mathrm{m}$.