Locard's Exchange Principle: 2D 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 2D simulator renders a top-down contact area scattered with hundreds of trace particles, alongside a live survival curve and a retention histogram tracking the very same population. 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) — or press Play to watch it run — the exponential survival curve peels particles away in real time, with the histogram panel showing exactly which particles are lost and why.