Advances in Fingerprint Technology.pdf

surrounds these negatively charged particles, forming micelles. The surfactant
molecules are long-chain alkyl compounds with a positive end (an
R–NH 3 + group). They arrange themselves on the negatively charged silver
particle in a staggered way: one with the positive end pointing toward the
particle and an adjacent one with the positive end pointing outward (see
Figure 7.1). 12 These micelles have two properties: they are positively charged
(because the molecules arrange themselves in a staggered way) and they form
a thick cover. Both of these prevent positive silver ions from approaching
them and thus prevent their growth. Saunders 21 formulated a simplified
physical developer (SPD) that contains no surfactant. For it to have any
stability, he had to significantly reduce the amount of AgNO 3 (by 72%) and
the ferrous/ferric redox couple (by 25%). It performs well during its stable
period (about 1 hr) and thus helps to understand the role of the colloidal
silver with or without using surfactants.
Silver Image Formation
The reduction of the silver ions to silver metal by the oxidizing agent occurs
on nucleation or catalytic sites. In photographic chemistry, where silver physical
developers were first applied, the catalytic nucleation sites are on the
photo-exposed silver bromide crystals of the photographic emulsion. The silver
specks and silver sulfide specks are the actual sites. For visualizing latent prints,
there is one explanation for the silver physical development of latent prints that
have not been wetted and several hypotheses for those that have been wetted. 1
• For the non-wetted (dry) residue, Morris 15 contends that the sodium
chloride in the latent print residue triggers the physical development.
The silver ions form an insoluble salt with the chloride ions and the
resulting silver chloride gets photoreduced to silver with ambient light,
thus creating the silver nucleation sites.
• For the wetted residues, a hypothesis being proposed here is that the
silver physical developer actually provides the nucleation sites as spontaneously
formed silver particles attach themselves electrostatically on
the latent print residue.
A Hypothesis for the Silver Physical Development
of Latent Prints Residue
This chapter section expands on the hypothesis mentioned above that
requires the Ag-PD to contain spontaneously formed silver particles. Cantu 1
has discussed several proposed explanations for the silver physical development
of latent prints as well as the reasons why such development occurs
better on porous than on nonporous surfaces. The most likely of the reasons
for physical development is based on the work by Morris 15 and the ideas of