Shake a granular raft gently, repeatedly. It gets denser — logarithmically.
A granular raft is a two-dimensional packing of particles floating at an air-oil interface, held together by capillary attraction. Unlike dry granular materials, the raft is cohesive: particles stick to their neighbors. And unlike crystalline solids, it's amorphous — disordered, with a distribution of local environments.
Gokhale, Pérez-Barba, and Pashine (arXiv:2603.07852) cyclically load this raft with small uniaxial strain and watch it age. The packing fraction increases logarithmically with cycle number. Not exponentially (which would imply a constant fractional improvement per cycle). Not linearly (which would imply constant absolute improvement). Logarithmically — each additional unit of densification requires exponentially more cycles than the last.
The structural heterogeneity decreases in parallel. Regions that were initially loose pack down; regions that were already dense change little. The raft becomes more uniform, but the approach to uniformity slows. The easy rearrangements happen first. Each subsequent cycle faces a landscape with fewer available improvements.
This is the same logarithmic aging seen in glasses, soils, and compacting powders — but in a two-dimensional system with direct imaging access. Every particle is tracked. The rearrangement events that drive densification are individually resolved: small, cooperative, and increasingly rare as the raft ages.
The deformation behavior changes too. Early cycles show irreversible strain (plastic flow). Late cycles show reversible strain (elastic response). The raft transitions from a material that flows to a material that bounces, with the transition driven not by temperature or chemistry but by accumulated history of deformation.
Gokhale, Pérez-Barba, and Pashine, "Structural aging of a cohesive and amorphous granular solid under cyclic loading," arXiv:2603.07852 (2026).