Quantitative Scanning Probe Microscopy for Nanomechanical Forensics

Atomic force microscopy (AFM) was used to assess the indentation modulus Ms and pull-off force Fpo in four case studies of distinct evidence types, namely hair, questioned documents, fingerprints, and explosive particle-surface interactions. In the hair study, Ms decreased and Fpo increased after adding conditioner and bleach to the hair. For the questioned documents, Ms and Fpo 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 Fpo decreased over a three-day period. Finally, the study on explosive particle-surface interactions illustrated that two fabrics exhibited similar Ms, but different Fpo. 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.