  We report on the first observation of fine structure of latent tracks in
rutile TiO2, which changes from cylinder to dumbbell-shape and then to
sandglass-shape as a function of the ion path length. Based on inelastic
thermal spike model, we show that Hagen-Poiseuille flow of molten phase
produces the hillocks on surface and the void-rich zone near surface after
epitaxial recrystallization due to material deficit, while at a deep depth, the
lack of efficient outflow and recrystallization result in the absence of
tracks. We propose that core-shell duration of transient molten phase induced
by swift heavy ion and parabolic distribution of fluid velocity are
radial-dependent. Moreover, the various morphologies of tracks are a
consequence of the molten phase outflow and recrystallization during rapid
cooling down. Our perspective provides a new interpretation in the track
formation.
