to make indentations of different dimensions. Thus, Vickers hard-
ness was considered. The Vickers pyramid has a square base withan
angle between opposite faces of 1361, so a correlation between the
side length of the square (L) and the depth (d) can be computed and
is approximately equal to 5 (L d 2 tg(1361/2)) (Fig. 3).
Fig. 4 shows the matrix of defects produced in steel, aluminium
and copper as shown in micrographs presented in Fig. 4b–d,
respectively. Distance between indentations was equal to 300 mm.
in line and in column. The letters correspond to different defect
dimensions (Fig. 4a). This matrix con guration aimed to guarantee
that near a small defect there was always a large one to guarantee
that the suspension of bacteria that did not entered a small defect,
but entered a nearby large one, was not due to a lack of suspension
availability, but due to a physical limitation of the technique,
namely, size and motility of bacteria, as well as capillarity phenomena. Thus, a detection limit can be estimated.