Forensic Serology: Blood Spatter
Dakota Emery, Physics 211, Fall 2014 Section F04
How Does Blood Spatter Analysis Work?
Blood is composed of both, liquid (plasma and serum) and solids (red blood cells, white blood cells, platelets, and proteins). like all other matter, gravity and other forces act upon the blood, as a result, the trajetory of a droplet of blood from a bloodstain can be traced back to the place where the blood left an injured person's body.
When a droplet of blood is in free fall, the droplet forms a sphere due to surface tension. A sphere is a solid shape with the smallest surface area per unit of volume. Surface tension is created by an unbalance of the van der Waal forces in the molecules. Each molecule is attracted to all of the neighboring molecules in every direction. These forces result in all of the molecules being pulled inward, forming a sphere.
Upon hitting a surface, the droplet becomes a bloodstain. On a smooth surface, the droplet forms a circular stain with few satellites (such as hardwood floors or linoleum). However, rougher surfaces will result in more satellites (eg. Concete or carpet). Along with this, the angle of impact will determine the length and width of the stain, and gives the droplet directionality. This principle is the foundation that allows scientists to characterize and obtain data from blood spatter.
