Locard's Exchange Principle: Trace Evidence Transfer Simulator

Contact Parameters

Contact pressure 55%

Recipient surface roughness 50%

Two surfaces at rest — press Make Contact.

Trace Persistence

Days since contact 0.0 d

Recipient activity level

Live Readouts

Transfer probability

Particles transferred

Still detectable now

How it works

Locard's Exchange Principle (1910): "every contact leaves a trace" — when two surfaces touch, material is transferred both ways. Real trace evidence (fibers, dust, glass, gunshot residue) transfers probabilistically, not universally, and the transferred trace then decays with time and handling.

Transfer probability:
  P = 1 − exp(−k · pressure · roughness)

Persistence over time:
  S(t) = exp(−λ · t)
  particle lost once S(t) drops below
  its own random retention threshold

Contact pressure — how hard the two surfaces are pressed together; higher pressure drives more particles across the interface.
Recipient surface roughness — a textured/fibrous recipient (e.g. wool) catches and holds more trace than a smooth one (e.g. glass).
Make Contact — animates the recipient plate down onto the donor plate; at the moment of contact each donor particle is transferred with probability P.
Days since contact / activity level — every transferred particle is assigned a random retention threshold at the moment it transfers; as simulated time and activity (washing, abrasion, brushing) increase, the survival curve S(t) drops below more thresholds and those particles are marked lost — this is why trace evidence has to be collected quickly.

Real-world relevance: this probabilistic transfer-and-persistence model is the same reasoning crime labs use to judge whether an absence of trace evidence means "no contact occurred" or simply "the trace didn't survive" — a distinction at the heart of countless real forensic cases.

Donor trace (unmoved) Transferred trace Lost / decayed

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