6. Conclusions
A new NDT technique based on bacterial cell films for micro surface defects identification was presented and validated. A suspension of R. erythropolis bacterial cells proved to be adequate to reveal very small size surface defects in both steel and aluminium.
An experimental validation of the basic assumptions of this methodology was performed. It was seen that bacteria cells
penetrate and adhere preferentially to defects, remaining inside the defect after mechanically removing the excess of bacteria. This was achieved even in the absence of external additional mobility means and using bacteria without electric or magnetic properties.
Penetration of bacterial suspension inside the defects was controlled only by capillarity phenomena, for the R. erythropolis bacteria under study, and the time of penetration necessary to detect defects depended on the material.
A detection limit was estimated for each material, under the conditions tested, together with the correspondent time of penetration and these were: 4.3 mm depth in aluminium, 2.9 mm in steel and 6.8 mm in copper, for an optimum penetration time of 4 min, except in copper which was 3 min.
It is possible to apply a revelation stage, by forcing the bacteria to grow inside defects under a medium rich in carbon. This allowed to identify, by naked eye, small size defects in steel, such as a defect with 4.1 mm depth. However, bio corrosion effects in steel were identified after 24 h of exposure to bacterial cells.
Each material interacts differently with bacteria, so the optimum penetration time and bacteria have to be experimentally
determined to establish the detection limit. As observed, the bactericidal behaviour of copper for the R. erythropolis cells limits the use of this bacterium with this material.
Electric and magnetic fields could be applied to proper bacteria to improve the limit of detectability by increased penetration stage efficiency.
Acknowledgments
The authors would like to acknowledge Fundação para a Ciência e a Tecnologia (FCT) for funding the project “Defects
Detection in Microfabrication With Bacterial Cells”—(MicroBac) (PTDC/EME-TME/118678/2010). TS and RM acknowledge Pest OE/ EME/UI0667/2011 and the Portuguese company 2AB—Auto Acessórios da Benedita, Lda. CCCRC thanks FCT for financial support(“Ciência2007” and “FCT Investigator” programs).