Optical effects from free falling plates in fluid

We experimentally study how thin plates fall freely in quiescent fluids, and find three distinct regimes. When the Reynolds number is small, plates descend stably with arbitrary angles. At large Reynolds numbers, however, plates swing back and forth while descending. Between these two regimes, all plates fall horizontally despite their initial orientations. These different motions of plates are closely related to the stability of structure-flow interaction. Collectively, the three regimes render different optical effects. Our observations can be used to explain certain intriguing optical phenomena caused by snowflakes.