Blood Spatter Scenario
Forensic Science 1/6/14
Drill
• Describe blood:
– drips
– smears
– pools
– splashes
– spray
• When might each happen?
• HW: Review Qs #22-25
Objectives
• IWBAT
– Reconstruct a crime based on blood
spatter analysis.
Blood spatter
• Why does blood spatter the way it
does?
• Most of this is review, but presented
in a new way—I’d recommend
getting out your old notes and
adding to them.
Surface tension
• Blood has properties similar to water.
• Surface tension is an upwards force that
enables insects such as a water strider to
“walk on water”.
Image: Water strider: David Cappaert, www.insectimages.org
Surface tension
• Surface tension enables
blood droplets to maintain a
sphere shape.
Image courtesy UWA PhD research student Mark Reynolds.
Traveling blood droplets
• When a force is applied to a
mass of blood, the blood breaks
into droplets.
• The droplets fly through the air
as “perfect” spheres due to
surface tension.
Image used with permission from Tom Bevel & Ross Gardner, June 2006.
Stages of impact
Stage 1: contact &
collapse
Image used with permission from Tom Bevel & Ross Gardner, June 2006.
Stage 2: displacement
Image used with
permission from Tom
Bevel & Ross Gardner,
June 2006.
Stage 3: dispersion
Image used with permission
from Tom Bevel & Ross
Gardner, June 2006.
Stage 4: retraction
Image used with
permission from Tom Bevel
& Ross Gardner, June
2006.
Height and size of blood
drops
• A blood droplet released from a
1m height will be smaller than a
blood droplet released from a
1.5m height.
• This is because the velocity of the
blood droplet released from a
higher height is greater.
Surface and shape of
blood drops
Images courtesy DUIT Multimedia: Paul Ricketts.
Low impact
• Low impact is really blood under the
influence of gravity - it just falls.
Image courtesy UWA PhD research student Mark Reynolds.
Medium impact
• Medium impact occurs when a force such as a bat
is applied.
Image courtesy UWA PhD research student Mark Reynolds.
High impact - fine
mist of droplets
Image
courtesy
Stuart
James,
February
2007
Angle of impact
Image used with
permission from
Stuart James,
February 2007.
Origin of blood
Image used with permission from Tom Bevel & Ross Gardner, June 2006.
Reconstructing the crime
• With the evidence that is collected, the
crime scene investigator attempts to
reconstruct the crime.
• This involves trying to work out what events
happened and the order that they happened.
Image courtesy UWA
PhD research student
Mark Reynolds.
Angle of Blood Spatter
• Let’s take a look at the Application
and Critical Thinking questions.
• Now, we need to calculate the angle
of impact for each droplet.
Calculations
•
•
•
•
•
•
Angle = sin-1(W/L)
Let’s do an example:
#1
Length?
Width?
Calculate W/L – this is the sine of
the angle
• Look at sine table – what angle?
Crime Scene
• You and a partner (or trio) will be
investigating a crime scene to
determine what happened.
• Mr. Imavictim was found beaten and
bloody at the elementary school by
his friend, Mr. Mopandbrush. They
are both janitors there.
• Mr. Imabaddy was seen leaving the
area by Mr. Mopandbrush.
Analysis
• First, determine the area of convergence
(AOC) (front of pg. 1)
• Then, calculate the angles for each blood
droplet. (front of pg. 2) Fill the distance and
angle in on the table (back of pg.1)
• Next, read the statements from each man
and take notes on what they say
happened.
• Finally, Sketch on the blank crime scene
map where the crime took place
– Use the angles and distances calculated to
determine where each impact occurred.
Materials
• Each group will need
–
–
–
–
Ruler (measure in cm)
Protractor
Various pencils (colored and normal)
Packet of information (colored paper)
• Interviews
• Sin/Cos/Tan table
• Sign this out!
– Scenario packet (white paper)
•
•
•
•
Wall with blood stains (step 1)
Table to record data (step 3)
Angle of Impact calculations (step 2)
Crime scene layout (step 4)
Solve the crime!
• Who is telling the truth? Only the
blood will tell…