When a crime results in bloodshed, the blood left behind functions as evidence for investigators. However, a bloodstain pattern analyst can't simply glance at drips and smears of blood and immediately tell you the who, what and when of a crime scene. Blood spatter analysis takes time and provides only a few pieces of the total crime puzzle. Yet such analysis can corroborate other evidence and lead investigators to seek additional clues. After close analysis, blood spatters can provide important clues to aspects of the violent act such as [sources: Dutelle, James et al., Murray]:
- Type of weapon
- Velocity of blood
- Number of blows
- Position and movements of the victim and assailant during and after the attack
- Which wounds were inflicted first
- Type of injuries
- How long ago the crime took place
- Whether death was immediate or delayed
Blood spatters can guide the recreation of a crime thanks to the same laws of motion and gravity, physics and chemistry that govern all liquids. Blood travels in spherical drops because of surface tension, the tendency of liquids to minimize surface area because their molecules are attracted to one another — it's cohesive [sources: DLS, Rosina et al.]. Also, its drops behave in predictable ways when they strike a surface or when a force acts on them.
Consider what happens when you spill water: The liquid falls to the ground and makes a puddle. The shape and size of the puddle depends on the amount of liquid, the height of the container, and whether you spill on carpet, wood, linoleum or some other surface. In general, more liquid, or a fall from a greater height, will make a larger puddle. Moreover, droplets striking a hard surface will retain a more circular shape than those landing on carpet, which partially absorbs the liquid and causes the edges to spread [sources: Dutelle, Murray, Wonder]. These are but some of the many factors a blood spatter analyst must consider.