Vortex formation and fluid entrainment of high-speed spore discharge

For explosive launch found in Sphagnum spores, which is considered as air gun mechanism, the formation of a turbulent vortex ring was found to be the key transport mechanism of spore particles. In this transport mechanism, we can find two origins of vortex formation. One is the jet from the spore capsule which is similar to that generated by a conventional piston-cylinder configuration for vortex ring formation studies. The other is the wake generated behind the cap-like structure called as operculum, corresponding to the vortex ring formed behind a moving bluff body. Past studies on the launch mechanism of the Sphagnum spores have focused only on pressurized jet from the spore capsule. However, the wake behind the operculum has significant effects on the flow characteristics and the performance of fluid transport. In this study, we conduct PIV and LIF experiments using a simple scaled-up mechanical model to observe how the vortex structures from two different origins interact each other and affect the entrainment process in which fluid initially inside the capsule eventually follows the moving cap.