The patterns left by falling or projected drops of blood can help investigators determine where a crime took place. Therefore,
blood drops and stains should be examined closely before the evidence is collected.
The shape of a blood drop can indicate the distance from which the blood fell and the angle of its impact. However, very
few studies have been done on the patterns produced when blood impacts a surface. Therefore, a thorough forensic scientist
will carry out his or her own experiments on the shape of blood droplets. To be accurate, this scientist will conduct the
tests under conditions very much like those found at the crime scene.
While a droplet is falling, it is primarily spherical shape. This is suprising to some people, who may have visualized
droplets as tear shaped, as cartoonists often draw them. The smaller the droplet, the more spherical its shape during fall.
As a drop falls through the air, it accelerates until it reaches a constant or terminal velocity. Measurements have
shown that a blood drop, resulting from dripping at a height of 15 feet, has a volume of about 0.05-ml and falls at a velocity
of about 25 feet per second. Smaller drops have a terminal velocity that is less than 25 feet per second, and larger drops
have a terminal velocity that is greater than 25 feet per second. Therefore, and individual blood droplet can give an investigator
the following useful information:
- The droplet’s speed at time of impact
- The direction of the droplet’s travel
- The approximate size of a blood drop.
If you examine a blood droplet that struck a surface straight-on (at a 90º angle from the surface), the droplet is generally
round. Straight-on impacts on hard, smooth surfaces produce round droplets with smooth edges. Higher velocities and rougher
surfaces produce drops with more ragged edges.
The angle of impact of a droplet affects the droplet’s shape. When the angle of impact is 90º, the droplet is round.
However, droplets that fall on surfaces at an angle that is greater than 90º have elongated shapes. The larger the angle,
the more elliptical the droplet.
Dripping and Spraying
Blood drops can be produced in several ways. A droplet that forms slowly, as in a dripping
wound, has a volume of about 0.05-ml. However, smaller droplets are produced during active situations, such as fights and
beatings. Blood droplets as small as an aerosol spray indicate that the wound was produced by a powerful force, such as a
gunshot or an explosion.