Nucleation and Growth Phenomena of Silver in Physical Developer for Latent Fingerprint Visualisation
thesisposted on 2018-09-03, 09:56 authored by Jodie Louise Coulston
Fingerprints are used to identify individuals in both criminal and civil cases based on their unique pattern of characteristic ridge details. Latent, i.e. non-visible, fingerprints are the most commonly exploited type of fingermark, since they are frequently left at a crime scene. However, they must be visualised via an enhancement method, the nature of which is dependent on the substrate. Physical developer (PD) is a widely used effective chemical enhancement method for the visualisation of fingerprints on porous surfaces, notably paper. The principle of the technique is based on a ferrous/ferric redox couple reducing silver ions to colloidal silver metal, which is stabilised by a surfactant formulation. Recent environmental legislation banning a crucial surfactant component – Synperonic N – motivates the design of a new formulation. This requires determination of the mechanism of the PD process and the roles of the components; previous work is macroscopic and empirical. This thesis describes solution and interfacial measurements and surface imaging that provide mechanistic insights and real-time dynamics of the process. Dynamic light scattering and microscopy reveal the particle size in solution. These particles deposit selectively on the fingermark, interacting with chemical constituents identified via spot tests, and grow to a size over an order of magnitude greater than in solution. Silver nucleation and growth on the surface are independent of the age of the mark. Neutron reflectivity measurements revealed that the cationic surfactant component, n-dodecylamine acetate, adsorbs more strongly to a silver surface in the presence of a non-ionic surfactant. Working solution stability variations with surfactant formulation are characterised, together with the effect on latent mark development times. The mechanistic and structural insights are used to design an alternative PD formulation with greater stability at no loss of the image quality.