2D companion to the 3D BET/VSSA scene: the same gas-adsorption physics, read off three live plots instead of a rendered particle bed. The EU's nanomaterial definition (2011/696/EU, used in REACH nanoform dossiers and referenced by ISO/TC229) has two alternative criteria: 50%+ of particles by number between 1–100 nm, or a volume-specific surface area (VSSA) above 60 m²/cm³, measured directly by gas adsorption rather than imaging.
BET equation (multilayer adsorption):
θ(p/p0) = C·x / [(1-x)(1-x+Cx)], x = p/p0
Geometric surface area (ideal sphere, D in nm):
SSA_geo [m²/g] = 6000 / (ρ[g/cm³] · D[nm])
VSSA [m²/cm³] = f · SSA_geo · ρ = f · 6000 / D[nm]
- Particle diameter — smaller particles pack far more surface per gram; VSSA of an ideal smooth sphere scales as 1/D and is independent of density.
- Material / density — sets the mass-specific SSA (m²/g), the figure quoted on a REACH technical dossier.
- Roughness / porosity f — real powders are never perfect spheres; necking, internal pores and surface fractality inflate the BET-measured area above the geometric value, exactly like a real instrument reading.
- p/p₀ slider & Sweep — scrubs the N₂ relative pressure through the BET linear range (0.05–0.35); the dot ring around the cross-section particle visualizes adsorbed-gas coverage θ climbing toward a full monolayer.
- VSSA vs. diameter chart — traces VSSA = f·6000/D across the whole diameter slider range against the 60 m²/cm³ threshold line, with the current bed marked; for a perfectly smooth sphere (f = 1) the line crosses 60 m²/cm³ almost exactly at D = 100 nm, the same number the EU's size-based and surface-area-based criteria converge on independently.