Hourglasses are very common objects in our daily life. Their working mechanism is apparently pretty simple: grains pass through a neck driven by gravity. But it turns complicated when you look more carefully. In this video we will tell you what is so fascinating about such simple objects, and what do they have in common with membranes, filters or any system in which particles suspended in a liquid pass through constrictions. And more importantly, our results confirm that the hourglass behavior is universal: any type of passive objects passing through constrictions behave in the same way, no matter if we are talking about grains, particles, sheep or human beings [see for example A. Garcimartín et al., Phys. Rev. E, 2015 or Zuriguel et al., Scientific Reports, 2014]. As a result of this, we can devise anti-congestion strategies via simulations or model experiments [Helbing et al., Nature 2000] which actually work!
Important note: this video contains audio that narrates the action (voices by Angela Walenta and Alvaro Marin) and we strongly recommend to watch and listen. Nonetheless, the audio is complementary and not necessary to follow the video.