Motion tracking and analysis of spray water droplets studied by high-speed photography using an iPhone X

Smartphones are not only becoming an inseparable part of our daily lives, but also a low-cost, powerful optical imaging tool for more and more scientific research applications. In this work, we hypothesized that some smartphones could be used as a low-cost, high-speed, photographic alternative to expensive equipment, such as those typically found in scientific research labs, to accurately perform motion tracking and analysis of fast-moving objects. This demonstrative experiment was done at home at very low cost by using a low-power green laser (<0.1 W) purchased from Amazon, an iPhone X, and a water spray bottle. The sprayed water droplets were illuminated by a divergent green laser beam and recorded with a smartphone using the Slow-Motion Video function (240 frames per second), which allows us to record high-speed events. The Slow-Motion Video was converted to a series of consecutive images taken every 1/240 second (or 4.16 ms), enabling tracking and motion analysis of the sprayed water droplets. By analyzing these consecutive images, the speed and flight trajectory of water droplets in the air were obtained, thereby enabling us to estimate the area of the water droplets landing on the ground.