3D Morphology of Blood Stain Can Provide Critical Information Missed In Standard Bloodstain Analysis
James Bird
Major goals and objectives:
Forensic scientists have long used blood stains to reconstruct crime scenes. In situations in which blood may be dripping from a wound, the size and shape of the stain is used to determine whether the wounded was walking or running; whereas in situations in which blood is spattered, the sizes of blood stains are often correlated with force of the trauma and the shape can provide insight into origin location, such as whether the victim was sitting or standing. To establish the origin of a blood stain, forensic analysts use a “stringing” technique. Specifically, using the width and length of the stain, an impact angle can be calculated. When an event leads to several nearby stains, the angles for these stains are each calculated with the assumption that where the tangential trajectories associated with these converge is the source of the splatter. Recent software packages have aimed to reduce the uncertainty in these estimates by accounting for parabolic paths and, if the size of the drop is known, drag from the air. To find these trajectories, information is needed on the drop size, impact angle, and impact velocity. However, it is less clear how to relate the final stain size and shape to these impact conditions. In particular, the role of gravity and substate wettability are typically neglected. Models typically assume that bloodstains are pinned to the substrate at largest point where they have spread; yet recent experiments have demonstrated that microscopic residues, such as superhydrophobic coatings and the oil resulting from fingerprints, can fundamentally change the shape of the final stain.
The goal of this project was to collect data on which common coatings and residues – often invisible to the eye – can dramatically alter the shape and size of bloodstains. A secondary goal was to investigate 3D morphological features to see if these contain the necessary information to detect the presence of the residues. The project was primarily experimental with a goal of generating extensive photographic data of human blood drop impact and drying under various conditions so that a fundamental understanding of the underlying fluid dynamics could be pursued.
Research questions:
The two major research questions that this project aimed to address are:
- How does the advancing and receding contact angles affect the stain size, and are these angles are influenced by hematocrit?
- Is there a difference between oblique and inclined impact on a blood stain pattern?