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Title: Helical instability of a rotating liquid jet - 2008 DFD

Description: Rotating oil jets with a unique combination of viscosity and surface tension exhibit distinct modes of instability at different rotation rates. This video, acquired using a high speed camera at 5000 frames per second and slowed to 50 frames per second, illustrates the dynamic characteristics of the first four modes of instability. At relatively low rotation rates, the n = 2 mode naturally arises as indicated by an elliptical, flattening cross-section, which forms a helix as the unstable disturbances grow. Increasing the rotation rate to 2500 rpm produces the n = 3 mode represented by three distinct azimuthal lobes. Again, the helical structure is apparent. Further increases to 3500 and 4500 rpm reveal the n = 4 and n = 5 modes, respectively. In all cases, secondary instabilities eventually arise in the axial development resulting in breakup of the liquid jet. For higher rotation rates producing larger growth rates, the onset of these secondary instabilities occurs more rapidly. It is these secondary instabilities that form droplets which are initially connected by ligaments. Due to centrifugal forces, the ligaments are subsequently stretched and ultimately severed, resulting in the production of remarkably similar-sized droplets.

Credits: J. P. Kubitschek and P. D. Weidman


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