Ultra-Fast Optical Imaging of Bubbly Flows

A complete understanding of cavitation phenomena requires a complete characterization of bubble statistics. Experimentally quantifying bubble statistics fails with increased optical thickness (τ), characterized by smaller bubble diameters. To address the current missing link between τ=1~10, a new Ultra-fast optical (UFO) imaging method is being developed to measure the statistics of bubbles on the order of 1 mm diameter.

Incident light on a bubble plume is scattered due to the change in refractive index and bubble curvature. With this change of direction and speed, there will be an inherent time delay between scattered and unscattered light. Such a delay produces a sub-picosecond temporal distribution in intensity received by a detector, where the initial peak corresponds to light passing directly through a bubble's optical central axis. Experimental measurement, therefore, employs the non-linear Optical Kerr Effect (OKE) to produce sub-picosecond time-resolved images of light scatted by cavitation bubbles. Using a 1 kHz 800 nm Ti:Sapphire laser generating 70 fs Q-switched pulses, UFO images have been acquired with a temporal spacing of 33 fs, potentially reaching 0.67 fs.