Meaning
Classical mass transport models describe solute and solvent concentration gradients within solid matrices through concentration-dependent molecular motion. Theoretical behavior governed by Fickian diffusion kinetics assumes that penetrant flux across a boundary remains proportional to local concentration gradients while diffusion coefficients remain independent of stress history. Forensic document examiners and material testing laboratories in China apply these principles to quantify ink solvent evaporation and polymer degradation rates over time.
Rate Equation
Linear relationship between penetrant sorption mass and the square root of elapsed time characterizes early-stage molecular transport. Ink solvents migrating into paper fibers follow Fickian diffusion kinetics when polymer relaxation rates exceed solvent diffusion velocity within the substrate. Mathematical modeling of solvent concentration profiles allows forensic laboratories to calculate elapsed time since ink deposition on official commercial contracts.
Deviations occur when polymer swelling or structural relaxation alters matrix diffusivity, shifting transport behavior into non-Fickian regimes.
Forensic Application
Relative peak area ratios of residual volatile solvents measured via gas chromatography determine ink age. Quantitative analysis evaluates solvent loss curves against established reference standards to verify claimed document execution dates during judicial proceedings.
Transport Limit
Mass transfer calculations fail when environmental humidity or extreme temperature alters paper fiber porosity. Substrate damage invalidates standard diffusion models, establishing a strict physical boundary for forensic age determination in legal disputes.
