Browsing by Subject "Cavitation Erosion"
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(2021)High-intensity and -amplitude focused ultrasound has been used to induce cavitation for decades. Well known applications are medical (lithotripsy and histotripsy) and industrial ones (particle cleaning, erosion, sonochemistry). These applications often use low frequencies (0.1-5 MHz), which limits the spatial precision of the actuation, and the chaotic nature of inertial cavitation is rarely monitored or compensated for, constituting a source of uncertainty. We demonstrate the use of high-frequency (12 MHz) high-intensity (ISPTA=90 W/cm2 ) focused-ultrasound- induced cavitation to locally remove solid material (pits with a diameter of 20 µm to 200 µm) for non- contact sampling. We demonstrate breaking cohesion (aluminium) and adhesion (thin film on a substrate, i.e. marker ink on microscope glass). The eroded surfaces were analyzed with a scanning acoustic microscope (SAM). We present the assembly and the characterization of a focused ultrasound transducer and show quantification of the effect of different sonication parameters (amplitude, cycle count, burst count, defocus) on the size and shape of the resulting erosion pits. The quantitative precision of this method is achieved by systematic calibration measurements, linking the resulting erosion to acoustic parameters to ensure repeatability (sufficient probability of cavitation), and inertial cavitation monitoring of the focal echoes. We discuss the usability of this method for localized non-contact sampling.
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(2024)This thesis contributes to the ongoing development of a novel, environmentally friendly e-waste recycling technology. We utilize high-intensity focused ultrasound to locally extract gold from the surface of printed circuit boards via cavitation erosion. Acoustic cavitation erosion is the phenomenon in which the acoustically driven violent collapse of gas bubbles in liquid cause damage to nearby solids. Bubble collapse is preceded by its dramatic growth, which is driven by the rarefactive phase of the acoustic wave. In this work, I investigate the effect of ultrasound frequency on the efficiency of gold extraction. Gold extraction experiments were conducted with three custom-built transducers, with different resonant frequencies [4.2, 7.3, 11.8] MHz. The geometries of the transducers are identical, as were the electrical driving parameters. With each transducer, a sequence of gold extraction experiments was conducted with an increasing number of acoustic bursts (ranging from 100k to 1.9M). The results demonstrate that the lowest frequency (4.2 MHz) is 3.8 and 4.5 times more efficient at extracting gold compared to [7.3, 11.8] MHz, respectively. This dramatic improvement is likely due to larger cavitation bubbles associated with lower frequencies. Larger bubbles in the cavitating zone would be expected to undergo more bubble coalescence due to a higher gas volume ratio. Since the energy of bubble collapse increases with bubble size, increased bubble coalescence should augment the energy of bubble collapse. These results provide valuable insights relating to cavitation research and will guide the ongoing development of our novel e-waste recycling technology.
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