Bubble cavitation is important in technologies such as noninvasive cancer treatment and diagnosis, surface cleaning, and waste-water treatment. The cavitation threshold is the critical external tensile pressure that induces unstable growth of the bubble. Surface nanobubbles have been previously shown experimentally to be stable down to −6 MPa, in disagreement with the Blake threshold, which is the classical cavitation model that predicts bulk bubbles with radii ∼100 nm should be unstable below −0.6 MPa. Here, we use molecular dynamics to simulate quasi-two-dimensional (2D) and three-dimensional (3D) nitrogen surface nanobubbles immersed in water, subject to a range of pressure drops until unstable growth is observed. We propose and assess new cavitation threshold models, derived from mechanical equilibrium analyses for both the quasi-2D and 3D cavitating bubbles. The discrepancies from the Blake threshold are attributed to the pinned contact line, within which the surface nanobubbles grow with constant lateral contact diameter, and consequently a reduced radius of curvature. We conclude with a critical discussion of previous experimental results on the cavitation of relatively large surface nanobubbles.

Surface nanobubbles have potential applications in manipulation of nanoscale and biological materials, waste-water treatment and surface cleaning. These spherically capped bubbles of gas can exist in stable diffusive equilibrium on chemically patterned or rough hydrophobic surfaces, under supersaturated conditions. Previous studies have investigated their long-term response to pressure variations, which is governed by the surrounding liquid's local supersaturation, however, not much is known about their short-term response to rapid pressure changes, i.e. their cavitation dynamics. Here, we present Molecular Dynamics simulations of a surface nanobubble subjected to an external oscillating pressure field. The surface nanobubble is found to oscillate with a pinned contact line, while still retaining a mostly spherical cap shape. The amplitude frequency response is typical of an underdamped system, with a peak amplitude near the estimated natural frequency, despite the strong viscous effects at the nanoscale. This peak is enhanced by the surface nanobubble's high internal gas pressure, a result of the Laplace pressure. We find that accurately capturing the gas pressure, bubble volume and pinned growth mode is important for estimating the natural frequency, and we propose a simple model for the surface nanobubble frequency response, with comparisons made to other common models for a spherical bubble, a constant contact angle surface bubble, and a bubble entrapped within a cylindrical micropore. This work reveals the initial stages of growth of cavitation nanobubbles on surfaces, common in heterogeneous nucleation, where classical models based on spherical bubble growth break down.

The collapse of cavitation bubbles often releases high-speed liquid jets capable of surface damage, with applications in drug delivery, cancer treatment, and surface cleaning. Spherical cap-shaped surface nanobubbles have previously...

Our understanding of homogeneous vapour bubble growth is currently restricted to asymptotic descriptions of their limiting behaviour. While attempts have been made to incorporate both the inertial and thermal limits into bubble growth models, the early stages of bubble growth have not been captured. By accounting for both the changing inertial driving force and the thermal restriction to growth, we present an inertio-thermal model of homogeneous vapour bubble growth, capable of accurately capturing the evolution of a bubble from the nano- to the macro-scale. We compare our model predictions with: (a) published experimental and numerical data, and (b) our own molecular simulations, showing significant improvement over previous models. This has potential application in improving the performance of engineering devices, such as ultrasonic cleaning and microprocessor cooling, as well as in understanding of natural phenomena involving vapour bubble growth.

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