Examining the backspatter of blood during bullet impacts using biological simulant materials*

Bloodstain pattern analysis is vital to criminal investigation and provides crucial forensic information. However, much of the existing literature on backspatter uses very simplified geometries consisting of blood soaked sponges or penetration into liquid cavities. To better simulate cranial geometries, experiments used three configurations of materials subjected to ballistic impacts, including the effects of a brain and tissue surrogate on the spray pattern. Optical diagnostics include simultaneous capture of the cavity formation in ballistic gels and the resulting spray pattern and digital inline holography on downstream droplets to infer their size and velocity from high-speed images. These images provide comprehensive information on the time scales of cavity collapse and backspatter events. Bloodstains were also collected on butcher paper downstream of the test targets. Size distribution from the blood stain patterns are analyzed, and different stain patterns are seen for various configurations of surrogate materials. These experiments aim to integrate a fluid dynamics component into the modeling used in pattern-based forensic studies to allow for improved trajectory models, and aim to provide sufficient datasets to the BPA forensics community to establish confidence in these models.