Visualization of the ultrasound-induced behaviour of gas pockets entrapped on a patterned surface. Application to inspection of sodium-cooled fast reactors

The presence of microscopic gas pockets trapped on a non-wetting solid/liquid interface induces a significant decrease of the transmitted ultrasonic energy. This phenomenon can reduce the performance of ultrasonic inspection in sodium-cooled fast reactors (SFR) for instance. To explain this event, a hypothesis was formulated in a previous study: under the effect of ultrasound, the gas pockets could coalesce, forming a gas film which would prevent the passage of the ultrasound. This coalescence hypothesis can be studied by visual observation of the phenomenon. This paper is presenting an original experiment which simulates this phenomenon in water. This experiment consists in observing the ultrasound-induced behaviour of gas pockets over various time scales. The results allow dismissing the coalescence hypothesis. Our conclusion indicates how future works are reoriented to improve the design of ultrasonic transducers dedicated to SFR inspection.