Blood Spatter Scenario Forensic Science 1614 Drill Describe

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 Ph. D 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 1 m height will be smaller than a blood droplet released from a 1. 5 m 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 Ph. D research student Mark Reynolds.

Medium impact • Medium impact occurs when a force such as a bat is applied. Image courtesy UWA Ph. D 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 Ph. D 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…