When snowflakes and volcanic ash fall, they tumble. Historically, it’s been too hard to observe this behavior first hand — the particles are too small to easily follow with a camera — so scientists instead looked at larger particles falling through water. That change preserves important characteristics of the physics, but it misses out on one key feature: in air, the density of the falling particle is much higher than air’s.
To account for that, researchers built a special apparatus that drops particles one-at-a-time through the field of view of four high-speed cameras. This setup gave them a narrow 1-mm band where they could track a falling particle’s orientation — provided the particle fell through the band, which happened about 20% of the time. Their results show that particles in air tumble and oscillate back and forth around their stable orientation more than in water experiments. This difference affects how quickly particles settle, which, in turn, affects how much they tend to clump and grow. (Image credit: snow – A. Burden, experiment – T. Bhowmick et al.; research credit: T. Bhowmick et al.; via APS Physics)