Dynamics and mechanism of the physical developer process for visualization of latent fingerprints on paper

Abstract

We present a detailed mechanistic study of the PD process, focused on the nucleation and growth dynamics of silver particles on fingermarks deposited on a paper surface, from macroscopic (whole fingermark) and microscopic (particle level) perspectives. Conceptually, we separate the outcomes into aspects that precede exposure of the exhibit (relating to the reagent formulation), that relate to the development of the fingermark during immersion in the PD formulation, and that characterise the fully developed mark subsequent to immersion. Initially, dynamic light scattering shows the silver particles in solution to be relatively monodisperse, with a peak particle size of 880 nm. In the second instance, the issue is whether the particles grow to final size in solution then deposit on the surface or deposit as relatively small particles then grow on the surface. To the naked eye, silver deposition is evident after 2 min; corresponding optical profilometry images show evidence of surface-bound particles (mean diameter 2.13 µm) after 30 s. Across the development time (15 min) the particle population density (2.36 ( ± 0.52) x 105 cm−2), is independent of time. During this time, the mean particle diameter increases with the square root of development time to 16.09 µm. The dynamics suggest essentially instantaneous (shorter than observation time) nucleation and diffusionally controlled growth. Surface analysis (EDS) shows the expected high (low) levels of silver on ridge detail (in furrows) but no evidence of iron (from the redox component of the formulation) entrapment at any point on the surface.