Quantifying the Spatial-Temporal Evolution of Rim/Ligament in Drop Breakup via Digital In-Line Holography

A liquid drop undergoes aerodynamic deformation and breakup when it is exposed into a gas stream. Many techniques were used to measure the size and velocity of the secondary droplets while quantifying the rim/ligament still remains a challenge. An automatic method to extract the 3D properties of the toroidal rim in the bag breakup was recently developed based on digital in-line holography (DIH). To reduce the uncertainty caused by the out-of-focus overlap, a DIH configuration with a slightly rotated view is adopted here. The entire rim is reconstructed by stitching all the sections together. Holograms are recorded with a high-speed camera operated at 20 kHz to study the dynamic evolution of the rim in the bag breakup of an ethanol drop. Both the 3D visualization and z–y view reflect the rim’s structure development within 5.2 ms. The rim expands followed with disintegration into ligaments and relatively larger droplets. The volume of the rim is measured ∼ 95 % and that of the secondary droplets is ∼ 5 % of the initial drop volume before rim breakup. Then the volume of rim/ligament decreases after rim breakup which on the other hand increases the volume fraction of secondary droplets. The total volume of the rim/ligament and fragments is very close to the initial drop volume in most measurements except when the instant swelling happens in local atomization. Then the total measured volume decreases rapidly as the relatively large fragments move out of the field of view.