Locard's Exchange Principle: Trace Evidence Transfer Simulator

Every contact between two surfaces leaves a trace — that is the whole of Locard's Exchange Principle, and it is the foundation forensic trace-evidence analysis is built on. This simulator renders two real surfaces in 3D: a donor plate scattered with hundreds of trace particles, and a recipient plate you bring down into contact with it. Pressing "Make Contact" runs a probabilistic transfer model driven by contact pressure and recipient surface roughness, so only a fraction of the donor's trace jumps across — exactly as real fibers, dust and residue transfer unevenly rather than completely. A second, independent model then governs persistence: every transferred particle gets its own random retention threshold, and as you scrub forward through simulated days and recipient activity level (undisturbed, normal wear, or washing/abrasion), the exponential survival curve peels particles away, showing live how quickly real trace evidence can disappear before it is ever collected.