Jumping beads

When you put a glass of water into a freezer, the water becomes colder until, at around zero degrees Celsius, it suddenly turns into ice—a substance with very different properties. If you heat water instead, it turns into steam at about one hundred degrees. Such sudden qualitative changes, caused by gradually changing external conditions, are called phase transitions. Water, for example, exists in three phases: solid, liquid, and gaseous. 

A similar phase transition occurs in the experiment with the jumping beads. The beads sit on a vibrating membrane driven by a speaker. Under most conditions, the beads jump randomly between the left and right compartments, resulting in roughly equal numbers on both sides. However, at a specific vibration frequency and amplitude, the system undergoes a transition: almost all beads end up on the same side, revealing an image beneath the other. 

How? Initially, bead motion is random, but small fluctuations can create a slight imbalance. In most settings of the speaker, this is not important, because the floor is shaking enough to keep the beads jumping in both directions. However, under the special settings, beads on the more crowded side collide more often, lose energy, and are less likely to jump to the emptier side. Meanwhile, beads on the emptier side move more easily toward the crowded one, amplifying the imbalance until all beads collect on one side.

• Phase transition: Wikipedia 
• This experiment first appeared in the 2016 International Physics Olympiads, which took place at the University of Zurich. There, high school students had to tune the settings of the speaker to find and characterize the phase transition. You can find the experiment description starting on page 17 of the exam: PDF