Ion sputtering from a flat surface is well-characterized: an ion hits, transfers energy to surface atoms through collisions, and some atoms escape. The angular distribution of ejected atoms, the energy spectrum, the yield per incident ion — all are predictable from the ion energy, angle, and surface material.
Loose powders are different (arXiv:2603.20251). A powder is not a surface. It's a three-dimensional landscape of grains with voids between them. An ion entering a powder can penetrate between grains, ricochet off internal surfaces, and sputter atoms from grains it never directly struck. The geometry of the void network — not just the chemistry of the material — determines what comes out.
Multiscale Monte Carlo simulations of krypton ions striking copper powders reveal three departures from flat-surface behavior. First, backward-directed ejection dominates at oblique incidence angles — atoms preferentially escape back toward the ion source. Second, the peak angular distribution is substantially lower than for flat slabs. Third, the yield depends on porosity through a relationship that can be captured by universal fitting functions.
The backward preference arises from geometric shadowing. Atoms sputtered forward from a grain encounter neighboring grains and are reabsorbed. Atoms sputtered backward escape into the void that the ion entered through. The void network acts as a directional filter — it selects for backward emission by blocking forward paths.
The structural insight: sputtering from a powder is not a scaled version of sputtering from a surface. The void structure introduces a new physical mechanism — geometric selection of ejection direction — that has no counterpart in flat-surface sputtering. The powder's architecture is as important as its composition. What escapes when you bombard a powder reveals the geometry of its internal landscape as much as the properties of its material. The void speaks as loudly as the grain.