When you press your finger on a piece of soft putty, it obediently dents and then slowly returns to its original shape. But if you press on a window pane, it either holds or cracks — no smooth deformation. Both materials are amorphous: their atoms are not arranged in a strict crystal lattice like in metal. Yet they behave completely differently.
The secret lies in what happens at the microscopic level. Inside an amorphous body, there are tiny regions where atoms can shift from their positions, like microscopic 'clicks' or 'jumps.' Each such jump creates a stress field around it, resembling four lobes: in two directions atoms are compressed, in two others they are stretched. Scientists call this a quadrupolar singularity.
It turns out that everything depends on how large the impact is. If the push is small (for example, you press a needle against glass) then 'clicks' occur only in the immediate vicinity of the tip. Their density there is high, and they indeed change the material's stiffness: it becomes anomalously soft or, conversely, stiff. But step a little further away, and the material again behaves like an ordinary elastic body.
Standard Model of elasticity, used by engineers, assumes that deformation spreads uniformly in all directions. New data show that for amorphous materials this is true only at large distances. Near the source of load, special zones arise where the laws change. It is as if around a stone thrown into water, the ripples spread evenly, but right at the point of impact the water churns and behaves unpredictably.
Understanding this mechanism will help create stronger glasses, plastics, and gels, as well as more accurately predict when and how they will break. After all, it is the anomalous elasticity at the crack tip that determines whether it will stop or propagate further.
Amorphous materials such as obsidian (volcanic glass) were used by humans to make knives as early as the Stone Age. Their cutting edge can be thinner than a steel scalpel because glass lacks crystalline grains that would interfere with chipping.
In Kurt Vonnegut's novel 'Cat's Cradle,' the fictional substance 'ice-nine' instantly causes any water to solidify at room temperature. This is an example of an amorphous material with extremely anomalous elasticity — though, so far, fictional.