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Safety of biomedical ultrasound largely depends on controlling cavitation bubbles in vivo, yet bubble nuclei in biological tissues remain unexplored compared to water. This study evaluates the effects of elastic modulus (E) and impurities on bubble nuclei available for cavitation in tissue-mimicking polyacrylamide (PA) hydrogels. A 1.5 MHz focused ultrasound transducer with f# = 0.7 was used to induce cavitation in 17.5%, 20%, and 22.5% v/v PA hydrogels using 10-ms pulses with pressures up to peak negative pressure (p−) = 35 MPa. Cavitation was monitored at 0.075 ms through high-speed photography at 40 000 fps. At p− = 29 MPa for all hydrogels, cavitation occurred at random locations within the −6 dB focal area [9.4 × 1.2 mm (p−)]. Increasing p− to 35 MPa increased bubble location consistency and caused shock scattering in the E = 282 MPa hydrogels; as the E increased to 300 MPa, bubble location consistency decreased ( p = 0.045). Adding calcium phosphate or cholesterol at 0.25% w/v or bovine serum albumin at 5% or 10% w/v in separate 17.5% PA as impurities decreased the cavitation threshold from p− = 13.2 MPa for unaltered PA to p− = 11.6 MPa, p− = 7.3 MPa, p− = 9.7 MPa, and p− = 7.5 MPa, respectively. These results suggest that both E and impurities affect the bubble nuclei available for cavitation in tissue-mimicking hydrogels.
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Effect of dissolved gas on the tensile strength of water
While theoretical estimates suggest that cavitation of water should occur when pressure falls much below −25 MPa at room temperature, in experiments, we commonly observe conversion to vapor at pressures of the order of 3 kPa. The commonly accepted explanation for this discrepancy is that water usually contains nanometer-sized cavitation nuclei. When the pressure decreases, these nuclei expand and become visible to the naked eye. However, the origin of these cavitation nuclei is not well understood. An earlier work in this field has mainly focused on the inception of nuclei which are purely composed of water vapor, whereas experimental data suggest that these nuclei are mainly composed of air. In this Letter, we develop a theoretical approach to study the inception of cavitation nuclei in water with uniformly dissolved air, using a diffuse interface approach. We derive equations which govern the transition of water with uniformly dissolved air to a critical state. Our results show that the dissolved air decreases the free energy barrier from the initial to the critical state, thereby aiding the formation of cavitation nuclei. This study opens up possibilities to explore cavitation inception in fluids containing dissolved gases.
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- Award ID(s):
- 1805817
- PAR ID:
- 10482441
- Publisher / Repository:
- AIP
- Date Published:
- Journal Name:
- Physics of Fluids
- Volume:
- 34
- Issue:
- 12
- ISSN:
- 1070-6631
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0131165
