Transcript Blood: Drops and Spatter

BLOOD
Drops and Spatter
Surface Tension
The liquid in a drop of blood is held
together by a cohesive force known as
surface tension
 Surface tension keeps a drop of blood in a
spherical shape as it falls
 If any of the blood overcomes the
cohesive force and separates from the
main drop of blood, it will form secondary
droplets known as satellites

Satellite
Blood Drop
Blood Drops
Blood drops are subject to the same
principles of physics as any free-falling
object
 The force of a free-falling object can be
expressed by the formula:

F = ma where F = force
m = mass
a = acceleration due to gravity
(9.8 m/s2)
Blood Drops (cont.)
The longer an object is falling, the greater
the speed of descent and thus the greater
the kinetic energy
 In actual crime scene analysis, the most
important variable to consider and try to
control is the contact surface itself
 When striking a surface, the blood will
leave a pattern that is very much
dependent on the type and nature of the
landing surface

Blood Drops (cont.)
The blood drop will not break apart on a
hard and less porous surface (i.e. glass)
 A soft and porous surface (i.e. concrete)
causes the blood drop to break apart

GLASS
CONCRETE
CARDBOARD
Blood Drops (cont.)
The shape of a blood drop can indicate
the distance from which the blood fell
and the angle of impact
 Round drops:

◦ The drop struck the surface at a 90o angle
◦ If the edges of the drop are smooth, it
indicates a low velocity
◦ If the edges are ragged, it indicates a high
velocity
Blood Drops (cont.)

Elongated drops:
◦ The drop struck the surface at an angle
greater than 90o
◦ The larger the angle, the more elliptical the
droplet
◦ The tail of a drop indicates the direction the
blood travels
◦ Smaller drops are produced during active
situations (i.e. struggle)
◦ Very small drops indicate that the wound was
produced by a powerful force (i.e. gunshot)
Directionality of Blood Drops



When blood comes into contact with another
surface, the blood tends to stick or adhere to
it
As a result, the point of impact may appear to
be darker and wider than the rest of the drop
Momentum tends to keep blood moving in
the direction it was traveling, but cohesion
keeps most of the blood as one drop
Directionality of Blood Drops
(cont.)
As blood droplets move away from their
source, they elongate and produce a
thinner tail-like appearance
 The tail points in the direction of blood’s
movement
 Satellites may appear in front of the
moving droplet of blood

Direction
Of Blood
Movement
Lines of Convergence
If there are at least two drops
of blood spatter, the location of
the source of blood can be
determined
 By drawing straight lines down
the long axis of the blood
spatter and noting where the
lines intersect, a point of
convergence can be
determined

Lines of Convergence (cont.)
Height of Blood Drop
Once the point of convergence has been
determined, the angle of the blood drop
can be used to determine the height at
which the blood drop originated
 Measure the length and width of the drop
 Use the following formula to
find the angle:
Length
sin x = width/length

Width
Height of Blood Drop (cont.)
Once the angle has been determined, use
this and the length from the point of
convergence to the blood drop to
determine height
 Use the following formula:

◦ tan x = height/distance to pt of convergence
◦ where x = angle of blood drop
Example

A drop of blood found at a crime scene measures 5 mm
by 15 mm. The point of convergence is found to be 120
cm away. At what height, in cm, was the wound that
caused the blood droplet? What is the height in ft?