Blood is thicker than water: Physical limitations of bloodstain pattern analysis.

Scientific bloodstain pattern analysis was appeared at the end of the 19th century in Kraków. Nowadays, bloodstain pattern analysis is the forensic science involving analysis of blood traces on the crime scene. One topic is an estimation of the point of origin of given blood spatter. The processes behind the formation of the blood spatter are quite complicated, some simplifications are necessary. For example, the true trajectory of the blood drop is rather the ballistic curve than the line, though the line is often used as a sufficient model of the trajectory. Next, the blood is a non-Newtonian complex fluid, it differs from common fluids like water. Moreover, qualities of the surface are also involved in the formation of the final spatter. Some of these properties of blood must be neglected in order to make the modeling of the formation of the spatter possible. The crucial question is how to determine the angle of impact of the blood drop. The widely used method is the ellipse fitting method which is based on the assumption that the blood drop is ball-shaped and the spatter is chiefly the ellipse-shaped imprint of the falling drop. Other methods are using rather in experiments than in practice. Unfortunately, all these simplifications lead to the increasing uncertainty and, therefore, the point of origin is rather estimated than determined. Selection of an appropriate model of the behavior of the blood drop and estimation of uncertainty of obtained results should be based on related knowledge close to the physics of blood. Keywords: bloodstain pattern analysis - biomechanics.