Advances in Fingerprint Technology.pdf

and this leads to stability; but for particles formed near the residue, particle
attachment is preferred. The attachment or binding process is not immediate
and probably requires that the silver particle not grow significantly during
the process (more on this below). The attachment process neutralizes the
particles and makes them catalytic nucleation sites for silver physical development.
Another way of depositing negatively charged silver particles on the
positively charged residue is by the “surfactant stripping” process suggested
by Morris. This involves already-formed silver micelles in the vicinity of the
print residue and the process is probably slower because both the micelle and
the residue are positively charged.
Formation of Silver Physical Developer Particles
on Latent Print Residue: Silver Image Formation
Once the print residue acquires the catalytic nucleation sites, silver physical
development occurs on these sites. The final particles that grow on these sites
are spherical, about 5 to 40 µm in diameter, and are made up of strands of
silver (see Figure 7.2). The gray to black color of these particles is attributed
to their size and configuration. The print image is made up of a dense
accumulation of these particles.
Substrates
Porous vs. Nonporous Surfaces
Silver physical developers visualize latent prints better if they are on porous
or semiporous surfaces rather than on nonporous surfaces. On porous surfaces,
latent print residue enters into and spreads throughout the porous
structure. It thus has a greater surface area and is more exposed than on a
nonporous surface. Because of this, more nucleating sites are formed on such
residue and silver physical development (silver particle deposition and
growth) occurs sooner and to a greater extent. Another reason why Ag-PDs,
whether (cationic) surfactant stabilized or not, do not visualize latent prints
on nonporous surfaces well is because newly formed, negatively charged silver
particles grow. With a cationic surfactant, they also grow but after a certain
point their growth is reduced and their charge is reversed. That is, these
developers cannot retain particles of a fixed size and concentration long
enough for a sufficient number of them to adhere to the latent print residue.
Although the use of colloidal gold to create nucleation sites for subsequent
silver physical development is discussed later, it is sufficient to say that treatment
with negatively charged, fixed-size colloidal gold particles (at sufficiently
low pH to create a positive residue) does bring about sufficient particle