Scanning probe microscopy techniquesin particular atomic force microscopy (AFM)provide a means for imaging objects and measuring mechanical properties at the nano- and micro-scales in a non-destructive manner. In this paper, the previous use of AFM in forensic evidence applications is briefly reviewed before demonstrating the power of AFM to quantify mechanical properties such as stiffness and adhesion in four case studies of different evidence types. Specifically, the plane strain modulus E* and pull-off force Fpo were assessed for hair as a function of treatment type, questioned documents with different ink types, fingerprint residue as a function of time, and explosive particle-surface interactions as a function of fabric type. In the study on hair, E* decreased and Fpo increased after adding conditioner and bleach to the hair, with the extent of the variations dependent on the position on the hair and the thickness of the treatment. For the questioned documents, E* and Fpo of the two inks were decidedly different; the ballpoint pen ink demonstrated much smaller variations relative to the mean values than the printer ink, consistent with topographic observations that show ballpoint ink is dense and smooth and printer ink is granular and rough. The case study on the fingerprint residue indicated that the maximum height and Fpo both decreased over a 3 day period, most likely due to the loss of water via evaporation. Finally, the study on explosive particle-surface interactions illustrated that the two fabrics exhibited very similar E*, but markedly different Fpo; in detail, the pull-off force between the particle and rayon was two times greater than that between the particle and cotton. In all, it was found that AFM directly addresses needs in forensic science as defined by several federal agencies, in particular an improved capability to expand the information extracted from evidence and to quantify its evidentiary value.
Citation: Experimental Mechanics
Pub Type: Journals
scanning probe microscopy, trace evidence, questioned documents, impression evidence, controlled substances