High-Speed Accelerometry of Atomizing Sprays using the FRAME concept

Capturing fast transient spray phenomena, such as droplet breakup and secondary atomization, requires both high temporal and spatial resolution over extended durations, which is difficult to achieve with existing imaging methods. Burst imaging on one hand offers ultra-high frame rates but for short windows in time, while conventional high-speed cameras extend duration but are currently unable to track sub- microsecond events. Thus, these technologies either miss the atomization dynamics or the long-term effects of these dynamics. In this work, we demonstrate high-speed droplet accelerometry in an atomizing spray using an image multiplexing technique known as FRAME (Frequency Recognition Algorithm for Multiple Exposures). The FRAME method illuminates the sample with a burst of three laser pulses, each imprinted with a unique coded intensity profile. This coding enables the pulse intervals to be shorter than the inter-frame period of a high-speed camera, yielding a single multi-exposed image that captures information from all three illumination events. Fourier analysis is then applied during post-processing to separate and temporally arrange the individual images based on their coded structures. In our study, we combine a high-speed camera operating at ~20 kHz with FRAME image multiplexing to track small-scale motion at MHz acquisition rates over an entire injection event. This approach allows us to measure the acceleration of droplets in an atomizing spray in 2D at the frame rate set by the camera and to observe the forces acting on the liquid during breakup.