Quantitative Scanning Probe Microscopy for Nanomechanical Forensics

Atomic force microscopy (AFM) was used to assess the indentation modulus M_s and pull-off force F_po in four case studies of distinct evidence types, namely hair, questioned documents, fingerprints, and explosive particle-surface interactions. In the hair study, M_s decreased and F_po increased after adding conditioner and bleach to the hair. For the questioned documents, M_s and F_po of two inks were markedly different; ballpoint pen ink exhibited smaller variations relative to the mean value than printer ink. The fingerprint case study revealed that both maximum height and F_po decreased over a three-day period. Finally, the study on explosive particle-surface interactions illustrated that two fabrics exhibited similar M_s, but different F_po. Overall, it was found that AFM addresses needs in forensic science as defined by several federal agencies, in particular an improved ability to expand the information extracted from evidence and to quantify its evidentiary value.