Ultrahigh-Speed Dynamics of Micrometer-Scale Inertial Cavitation from Nanoparticles

Direct imaging of cavitation from solid nanoparticles has been a challenge due to the combined nanosized length and time scales involved. We report on high-speed microscopic imaging of inertial cavitation from gas trapped on nanoparticles with a tunable hemispherical depression (nanocups) at nanosecond time scales. The high-speed recordings establish that nanocups facilitate bubble growth followed by inertial collapse. Nanoparticle size, acoustic pressure amplitude, and frequency influence bubble dynamics and are compared to model predictions. Understanding these cavitation dynamics is critical for applications enhanced by acoustic cavitation.

Figure 1

Schematic of the experimental setup. The ultrasound (US) transducer is confocally aligned to both a passive cavitation detector (PCD) and the optical focus of the ultra-high-speed camera Brandaris 128. The nanoparticles are contained in an optically and acoustically transparent Opticell culture cell. The light source used was a cool white (CW) lamp. The three images to the right display transmission electron microscopy (TEM) images of small ($174\pm 2\mathrm{nm}$), medium ($425\pm 6\mathrm{nm}$), and large ($609\pm 8\mathrm{nm}$) nanocups are shown.

Figure 2

Single images from the ultrahigh-speed recordings. (a) These snapshots show the response of nanocups exposed to ultrasound driven at 0.5 MHz and 3.5 MPa as a function of size. (b) Bubble behavior at constant size exposed to a 1.6- or 3.3-MHz driving frequency are also shown. The driving pressure amplitudes are 6.0 and 7.0 MPa, respectively. Across all test conditions, the recordings are dividable into two regimes, the inertial growth phase and the bubble collapse phase. The initial time point ($0.0\mu \mathrm{s}$) is the frame before the first bubble is observed. The scale bars in all images are $100\mu \mathrm{m}$.

Figure 3

Comparison of hypothesized mechanism and experimental observation. (a) An illustration of a surface-trapped nanobubble undergoing inertial collapse. (b) Evidence for a (1) nanocup likely with a trapped nanobubble, (2) nucleation, and (3) detachment from a single large nanocup is shown. Dashed lines indicate the initial position of the nanocup. Scale bars represent $100\mu \mathrm{m}$. The arrow points to the 600-nm-diameter nanocup. The time stamps on the images are based on the onset of cavitation. The color bar is the difference in pixel intensity (of arbitrary units) in each frame of the video compared to the same frame number of the no-ultrasound control video.

Figure 4

A comparison between the predicted and observed cavitation behavior. (a),(b) A good agreement is found between the diameter-time curve predicted by the modified Rayleigh-Plesset crevice model and representative experimentally observed bubble diameter at (a) 0.5 MHz [at 2 MPa (black) and 3.5 MPa (red)] and (b) 1.6 MHz [at 3 MPa (black) and 6 MPa (red)] for medium-sized nanocups. The dashed line in the location of the time axis break marks the bubble transition from spherical collapse (left of the line) to nonspherical collapse (right of the line). (c)–(e) Peak diameters achieved during inertial expansion for (c) small, (d) medium, and (e) large nanocups at 0.5 MHz (black) and 1.6 MHz (red) are shown. Shaded regions are predicted peak diameters. The single open square in (e) is from measurements taken at concentrations 10 times lower than those for the closed squares. Values without error bars are due to only a single bubble captured at that particular acoustic setting.

Figure 5

Recorded number of bubbles per first recording ($N_{\text{bubbles}}$) for small, medium, and large nanocups at (a) 0.5-MHz, (b) 1.6-MHz, and (c) 3.3-MHz driving frequencies. The legend in (a) is valid for all figures, and LC represents the low concentration (tenfold dilution) of large nanocups.