First we will let you in on a little secret. When you or your brother or sister do something wrong, your parents have to figure out who did it. To parents this is like a whodunit game and in fact, they have a name for it that they have sworn never to reveal to the kids (breaking this rule can result in loss of their parenting card). It’s called the “which kid do we smack” game. And no, they don’t roll dice or draw straws to figure it out the winner. Parents have an eerie, almost superhero-like ability that allows them to scan faces to find the guilty party. It worked so well that a long time ago, policemen decided to use the technique to figure out which bad guy committed the crime. They would line all the bad guys against the wall and stare at them. This worked well until they ran across this guy.
After this, police went back to using scientific evidence to figure out whodunit.
Blood spatter analysis
Police use many different scientific techniques to figure out who the bad guy is. They dust for fingerprints, use special lights to look for blood, use chemicals to find out what mysterious substances are, and sample DNA to figure out whodunit. And in addition to figuring out who did it, they have to figure out how he did it. And to do that, they often use a technique called blood spatter (not “splatter”) analysis. Blood is primarily made of water and as such, it behaves much like water and therefore must obey the laws of motion and gravity. Particles in the blood are attracted to each other and this cohesive property causes blood that falls onto a surface to leave behind droplets rather than just spreading out all over the place. The blood droplets are pretty much round when travelling through the air but deform into various shapes and patterns when they land on a hard surface. The size and shape of these droplets tells detectives quite a bit about how the person lost the blood. From blood spatter, detectives can determine what type of weapon was used, how many times a person was hit with it, if the bad guy was right or left handed, the position of the victim and how they moved during the attack, how long ago the crime was committed, and how long the person lived after their injuries.
The size of the bloodstain depends on how much blood was lost. If a lot of blood was lost, the blood droplets will be quite large. Detectives use this information when determining whether a person died or not. Of course two fingers on the side of the neck works well too.
The shape of the blood stain depends on the angle of the blood when it hit the surface and the speed at which the blood drop was travelling. A droplet that has fallen vertically will be more round than a droplet that has hit the surface at an angle which will be oblong shaped.
Drops that hit at an angle form a tail which typically points in the direction that the drop traveled. If you measure the width and length of the drop, then divide the width by the length, you can determine at what angle the blood struck the surface. If this ratio comes out to be .5 then the blood struck the surface at a 30-degree angle. If the ratio comes out to .25, the droplet struck the surface at a 14-degree angle. Here’s a table that will help you out the angle.
Width / length (for the geeks, the formula is the arcsin of width/length)
.1 = 6 degree angle
.2 = 11 degree angle
.3 = 17 degree angle
.4 = 24 degree angle
.5 = 30 degree angle
.6 = 37 degree angle
.7 = 45 degree angle
.8 = 53 degree angle
.9 = 64 degree angle
1 = 90 degree angle
The texture of the surface that the blood lands on affects the shape of the blood pattern too and detectives have to take that into consideration. Glass, metal, and other smooth, hard surfaces tend to yield blood stains that are rounded. Hard rough surfaces like wood or paper tend to produce blood stains with irregular, jagged edges. Soft surfaces, like clothing or carpet, absorb the blood and make the edges spread more.
Here is what the blood spatter pattern would look like on a smooth surface.
And here is the pattern the same blood droplet would produce on a rough surface.
Detectives also look at how dry the blood is. Blood begins to dry and clot within 15 minutes (depending on factors such as heat and humidity). The outer edges of droplets tend to dry first and a completely dry droplet may leave a dark ring. How dry the blood is gives the detectives an idea of how long ago the attack happened.
Detectives look at other factors too such as whether or not the blood contains bone or tissues matter (which indicate a high impact splatter). They look for voids or blank spots which may indicate something else (a person maybe) absorbed the blood spatter and then left. They call this shadowing or ghosting.
The Blood Spatter Experiment
You have to be careful with this one. You can hurt yourself with the hammer and screwdriver (which come to think of it, would add a bit of realism to the experiment). You can also get the fake blood on your clothes, furniture, or walls which won't make mom or dad very happy (which come to think of it, could also add a bit of realism to the experiment).
- If stage blood is not available, you can make your own blood using cornstarch and food coloring. Mix 44 g of cornstarch with 80 mL of water. Add 160 mL of corn syrup and mix. Add 2-3 teaspoons of red food coloring and 2 drops of green food coloring. Don't get it on your clothes no matter how funny you think it would be to douse yourself in it and sprawl out on the kitchen floor.
- Tape large sheets of paper on a wall. Make sure the paper reaches the floor and goes pretty high up.
- Lay paper on the floor underneath the papered wall.
- Pour two tablespoons of blood onto a damp sponge. The sponge should not be dripping wet but rather just moist.
- Lay the sponge on the floor about 2 feet from the wall.
- Strike the sponge with the hammer.
- Note the shape of the spatter, measure the length and width of each drop (don’t include the tail in your measurement), take pictures of the spatter pattern. The droplets will be ellipse shaped with tails pointing towards the sponge.
- Replace the paper on the floor and wall. Position it exactly like you did the first time.
- Pour two tablespoons of blood onto the damp sponge and place it back on the floor about 2 feet away from the wall (in the exact same position as you placed it before.
- Strike the sponge with the baseball bat.
- Repeat with a screwdriver by stabbing the screwdriver point into the sponge. Then set a mousetrap and apply blood to the metal and trip it with a pencil. Place a block of wood or an empty bottle between the sponge and wall and notice the void that is produced and splatter something again.
- Write your findings on an index card. Display your pictures on a poster board along with the index cards explaining your findings.
You may also want to try placing the sponge at various distances from the wall and take measurements.
Types of Blood Spatter (Patterns)
Detectives use the knowledge they gain from blood spatter analysis to figure out how the blood pattern was made and hence, how the person who bled was injured, where they were standing when injured, how long ago the crime was committed, and so on. They take pictures of the blood stains, write down lots of notes, and measure the blood stains that they find. All of this information (called evidence by the CSI buffs) is taken to a lab where it is studied so the detectives can reconstruct the crime. Below are the types of blood spatter patterns the detectives look for.
Low velocity spatter
– results from low impact blows. These often occur after the victim is injured, for instance when they are walking around after the attack and dripping blood on the floor. Droplets are about .16 to .31 inches. May be larger if they are tall or walking on a tightrope.
– results from attacks with a blunt object (like a bat) or a stabbing or when the injury is close to the surface of the skin and blood spurts out of the wound. Droplets are typically no more than 4 millimeters.
– occurs with gunshot wounds. The droplets look like a fine spray less than 1 millimeter in diameter.
– fairly obvious, see example below.
Parent/Teacher/Advanced Notes [click to expand]
We provide a table to use to determine the angle but the mathematics to calculate it yourself is pretty simple if you have a calculator handy. Below are the formulas that blood spatter analysts use to determine the angle.
Angle of impact = Arcsin (c/a)
θ = sin-1(c/a)
* θ = impact angle
* c = width of spatter (mm)
* a = length of spatter (mm)
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