The origin and dynamics of vortex rings during drop splashing were elucidated by Prof. Jung Ho Je and scientists at POSTECH in a paper published in the September edition of Nature Communications.
Vortical flow is a phenomenon commonly observed in nature, such as circular winds in a hurricane or a tornado, or what is commonly known as a whirlpool in water bodies. Prof. Je’s team delved further into the unclear causes behind the formation of the vortex ring during drop splashing by using ultrafast X-ray imaging to obtain clear visuals of the vertical flows.
The x-ray’s precise images provide a look into how the pool fluid rapidly climbs up the drop wall and sharply penetrates into the drop, forming a spiral. Such highly vivid imagery enabled the team to quantitatively analyze the evolution dynamics of the vortices.
Among the discoveries the team made was the long-standing problem of determining when the vortex ring was formed. They revealed that the formation of vortex rings originates from the energy transfer by capillary waves generated at the moment of the drop impact. Interestingly, the team found a row of vortex rings along the drop wall with different power-law dependencies of the angular velocities on the Reynolds number. Moreover, the vorticity behavior and the spiral geometry of the vortex rings were characterized in detail.
This study offers substantial insight for further analytical, numerical, and experimental work on vortex ring dynamics in fluid mechanics.
Formation of a vortex ring during drop impact. (a) Illustration of the vortex ring formation during drop impact on a pool surface. (b) Schematic of X-ray imaging coupled with the experimental set-up for the drop impact. (c) Sequential X-ray images (of the red square in a) in ethanol drop impact (with diameter ~1.9mm) from 80mm height showing a positive vortical flow. The interfacial boundaries between the drop fluid (dark contrast) and pool fluid are clearly resolved in a high temporal resolution. Scale bar, 100 μm long.