What is blood spatter?
Blood spatter is any group of blood stains found at a crime scene. Blood spatter analysis can be used in criminal cases to determine various aspects of a crime.
There are three main types of blood stains:
- impact
- passive
- transfer stains.
Impact Stains
Impact spatter, sometimes called projected spatter, is created as a result of a force that is applied to a liquid (in most cases blood).
The size of blood drops depends on the distance from the surface and the speed at which the droplet hit the surface. If a blood drop comes from a low force impact, it is likely that the drop will be large (~4mm) and circular.
Normally, low force impact spatters are a result of a fall from an open wound or a highly-saturated surface and the force of the blood is around 5 ft per second or less. On the other hand, high velocity impact stains are caused by gunshot wounds or any other impact of around 100 feet or more per second.
The blood drops created by high impact forces are very small(~0.1mm) and spread in a mist-like fashion.
Additional factors include the angle of impact and direction. If an impact is caused at a steeper angle, a more elongated drop can be expected.
Additionally, looking at the “tail” of a blood droplet can indicate what direction the blood drop originated from.
Passive Stains
Passive stains are a result of an indirect impact to a blood stain. These stains are heavily affected by gravity because of which they become elongated.
There are a couple of different types of passive stains: drops, pools, clots, and various different drip patterns. Examples of causes of passive stains include drops from a weapon or dripping from a cut or wound.
Transfer Stains
Transfer stains are a result of coming in contact with another surface.
These stains could come from smudges, wiping hands or legs with an article of clothing, a shoe print, or even a swipe on a surface.
Void Patterns
A void pattern is created when someone blocks a natural path of blood.
Void patterns are significant to forensics because it can help investigators determine if anything has been moved or altered at a scene or even where different people were at the time of a crime.
Collection of Bloodstain Evidence
Investigators collect blood samples by using thread or cotton swabs. At a minimum, investigators hope to collect at least 4 threads to create a substantial evidence profile.
After collecting the samples, they place the threads in a secured space and let them air-day.