Nucleation and subsequent cavitation in a hydraulic oil poppet valve

If bubble nuclei are the cause of cavitation, how are they initially produced? According to what Washio et al. have found out so far, there are two possible ways for cavitation nuclei to be generated in liquid flows: separation of flow and a relative motion between solids contacting in liquid. The present article intends to reinforce that assertion by observing the cavitation occurring in an oil hydraulic poppet valve. At a certain flowrate, a microscopic cavity suddenly emerged on the valve seat where the flow separated. As the flowrate increased, the cavity developed extending circumferentially on the seat and discharged bubbles by splitting. A collision of the poppet with the valve seat also caused the generation of a cavity. As the poppet was away from the seat after the collision, the cavity shrunk leaving behind a bubble. Cavities generated on the seat by flow separation regularly repeated a process of growth and shrinkage accompanied by bubble discharge, which induced flow pulsation and consequently vibration of the poppet supported by a spring as well. Moreover, these cavities brought about so-called 'choking' in the poppet-seat constriction and acted to increase the pressure loss there by narrowing its cross-section.