A handful of sand on a vibrating table is like a crowd on a dance floor that still holds the rhythm of the old song for a second after the DJ has switched to a new one. Scientists made a thin layer of grains bounce on a vertically shaking platform and abruptly changed the shaking intensity. Right after the jump, the granular temperature, a measure of the random motion of the grains, behaved differently than it should have under the new intensity. It remembered the old setting.
This effect is called Kovacs memory: a system that has undergone two temperature jumps, on its way to equilibrium, passes through a point where its state matches equilibrium, but then 'changes its mind' and returns. In the granular layer, researchers saw only a brief flash of such memory — literally in the first moments after the vibration change. After that, the system became amnesic: its behavior was determined only by the current temperature, as if the past had never happened.
The authors built a kinetic theory that splits the process into two stages: a fast one, where memory still exists, and a slow one, where it is gone. The first corresponds to the so-called kinetic regime, the second to the hydrodynamic regime. Computer simulations using molecular dynamics confirmed both the experiment and the theory. The main conclusion: in such a system, the full Kovacs effect is impossible due to the coupling between horizontal and vertical temperatures — memory fades before the system could 'remember' everything at once.
A thin layer of sand is a simple-looking system, but it teaches us how materials with memory lose it under non-equilibrium conditions. This matters not only for granular physics, but also for understanding glasses, foams, and other media where the past influences the present.
The Kovacs effect was first described in 1963 for polymers: if a sample is heated, then cooled to a target temperature, its volume initially changes in the unexpected direction, then reverses. Sand turned out to be too 'fast' for such a trick.