Controlled Generation of Single Microbubble at Solid Surfaces by a Nanosecond Pressure Pulse

We study the controlled generation of single microbubbles (approximately 100 mu m) at solid surfaces in water. By focusing a nanosecond pressure pulse (<100 ns), the single microbubble is deterministically generated over the focal area, which is rather immune to surface heterogeneity that is typically responsible for uncontrolled bubble nucleation. Such controlled bubble nucleation is realized by strong negative pressure generated by pulsed optical excitation of a carbon-nanotube-polymer composite and its tight focusing (approximately 100 mu m). The negative-pressure amplitude (>50 MPa), which is further increased by acoustic interference at an impedance-mismatched boundary, even exceeds the cavitation threshold of distilled water in a free-field (boundary-free) condition. This strong pressure can activate ubiquitous submicron nuclei in both water and surfaces, making the nucleation process controllable but irrelevant to a pit array (<10 mu m in diameter). The ability to control bubble formation can lead to new applications in cell-level therapy, e. g., ablation of individual cancerous cells (microhistotripsy).