Subsurface vortex rings forming from droplets impacting liquid surfaces have been known for more than 150 years. While the phenomenon has been studied extensively, the effects of different liquid densities have garnered less attention.
Zhang et al. characterized the evolution of heavy impacting droplets into lower density liquid surfaces from an unstable vortex ring to what the authors describe as a flowering bifurcation form.
By capturing, with a high-speed camera, a series of droplets of a glycerol solution impacting water pools, the group detailed how droplet impact evolves. The resulting vortex ring morphs stunningly into a bifurcation flower, resembling the process of a growing plant, complete with a stem, petals and stamens.
“To me, the whole process is beautiful, like 4D artwork,” said author Yanju Wei. “We systematically analyzed the characteristics of the behaviors and found the corresponding mathematical controlling equations and finally revealed the underlying physics of the whole phenomenon.”
The experiments modified different parameters of droplet impact, including droplet viscosity, solution density, droplet diameter and the impacting velocity. They determined equations relating these factors to penetration velocity and depth and the critical height of disintegration.
They found the vortex ring undergoes a two-stage evolution with the disintegration as the turning point, during which the viscous drag and gravity alternatingly dominates the penetration through a combination of gravitational acceleration and Rayleigh-Taylor instability.
Wei hopes to continue investigating this phenomenon on the droplets with different parameters to see the coupling effects of gravity and buoyancy and Rayleigh-Taylor instability in the mixing of different liquid-liquid pairs.
Source: “Evolution of the heavy impacting droplet: via a vortex ring to a bifurcation flower,” by Yajie Zhang, Zhiqiang Mu, Yanju Wei, Huzaifa Jamil, and Yajing Yang, Physics of Fluids (2021). The article can be accessed at https://doi.org/10.1063/5.0064072.
