A setup dedicated to high-speed photography of ultrasound-generated microjets

A bubble collapsing near an interface may result in the formation of a liquid jet protruding from the distal bubble side, through the bubble, towards the interface. Given the requirement of acoustic amplitudes above the inertial cavitation threshold, potential application of jets for ultrasound-guided bubble-assisted drug and gene delivery are limited. Ultrasound contrast agents comprise stabilised microbubbles that may act as inertial cavities at acoustic amplitudes substantially lower than the cavitation threshold. Microjets generated through ultrasound contrast agent microbubbles have been observed when coupling high-speed cameras to microfluidic microscopy setups. Still, microjetting is quite a rare phenomenon, as the main interfaces of the sample under sonication are parallel to the field of view. In this study, a optically transparent partial interface was added perpendicular to the field of view in a sonication compartment underneath a water tank to facilitate jetting without interfering with the sound propagation through the sample. Several commercial and experimental contrast agents were subjected to 3-cycle bursts of 1-MHz focussed ultrasound and recorded with a 10-MHz high-speed camera. The field of view in the sonication tank was 125×78 μm². Microjets were observed with all agents used, at acoustic amplitudes as low as 400 kPa.