Bloodstain size, shape and formation: implications for the bloodstain pattern analyst

[Truncated abstract] Bloodstains and bloodstain patterns are some of the most common forms of physical evidence encountered during the forensic investigation of incidents involving violence against a person. Bloodstain Pattern Analysis (BPA) is the forensic discipline that integrates these physical evidence records into complex forensic matrices with a view to offering reconstructive dialogue on events where blood has been shed. Whilst fingerprint or DNA analysis can supply information on "who" may have been involved in a violent event, BPA can provide information on "how" they were involved. Impact spatter patterns are formed when force applied to a source of blood results in the distribution of small blood droplets through the air that subsequently land on adjacent surfaces. Using a fundamental mathematical relationship that exists between the length and width axis of these spattered bloodstains and the angle at which the blood droplet impacted the surface, it is possible to determine within a 3D spatial context the location of the contributing blood source. The level of confidence that can be held in any such blood source determination is dependant upon two criteria; selection of the most physically appropriate bloodstains; being those caused by fast upward moving droplets, and the accurate measurement of those bloodstains selected. ... Following the analysis of bloodstains created by small droplets falling vertically onto angled surfaces and spatter deposited on vertical surfaces following an impact to a blood source, an apparent shift in the relative influence of inertia, surface tension and viscosity on the formation of small bloodstains was detected. As the angle of droplet-surface impact angle became increasingly oblique vi (<20.0), divergence between the experimentally calculated angle and that angle theoretically expected, was found. When comparative analysis of multiple data sets for larger bloodstains was undertaken, no such divergence from theoretical expectations was seen. Results from this study suggest that a shift in the relative influence between the forces responsible for stain formation; namely, inertia, surface tension and viscosity occurs as the size of a blood droplet and its surface contact angle becomes increasing small. Specifically, this research suggests that as droplet size and impact angle diminish, there is a corresponding increase of the retarding influence exerted by surface tension on fluid spreading. This thesis makes a significant and original contribution towards qualifying and quantifying some of the fundamental principles upon which the forensic discipline of bloodstain pattern analysis is based. It builds upon work already contributed to the discipline by previous researchers and practitioners and has generated new scientific data for examination by others. A number of these principles have previously been applied arbitrarily by practitioners without due consideration of possible reconstructive variance; examination results guarded by "estimation only" caveat. The studies described in this thesis have contributed to the advancement of knowledge surrounding the discipline of BPA and have assisted in the identification of specific areas of limitation and error, thus adding robustness to the discipline within the scientific, legal and judicial environments.