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Blood Drop Analysis
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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.

Shapely Drops

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.

 

Drop Acceleration

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:

  1. The droplet’s speed at time of impact
  2. The direction of the droplet’s travel
  3. The approximate size of a blood drop.

 

Round Drops

If you examine a blood droplet that struck a surface straight-on (at a 90 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.

 

Elongated Drops

The angle of impact of a droplet affects the droplet’s shape. When the angle of impact is 90, the droplet is round. However, droplets that fall on surfaces at an angle that is greater than 90 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.

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