Thursday, 2 May 2013

Blood Spatter Solving Crimes

You may have seen the last episode of CSI, Monk, The Shield, Dexter, Criminal Minds or some other crime show that I am not aware.  The technology is advanced for understanding blood splattering?  Right?  Wrong.

You can understand blood stain pattern analysis to help you recreate the scene of the crime which may interpret the origin or help with identifying the weapon.  

Several kinds of information can be found from blood stain analysis.  For example:

1. Distance from the blood source to the target
2. Direction of travel and impact angles
3. Nature of the force used to cause the bloodshed
4. The object used to cause the bloodshed
5. Sequencing of multiple bloodshed events
6.Interpretation of contact or transfer patterns
When properly documented, bloodstain patterns found at the crime scene, or on a particular person's clothing, can be used to:

What does this have to do with surface tension?  Since blood is an aqueous fluid information information would apply like viscosity and surface tension as well as gravity acting on the fluid (not in space).  Blood can spray like ink around a room and depending on the outgoing force.  Like ink from an ink jet nozzle spraying on paper a bullet passed through the body at a high enough speed can create a similar spray that can flow onto the carpet.  With both the droplet size is important.  For ink the droplet size relates to the pixelation and to get a certain size droplet a specific surface tension needs to be reached.  In the same way the droplet size of a blood stain relates to smaller and smaller droplets like an atomized spray or mist which may relate to faster and faster speeds.  The blood will not break up unless it is acted upon by a force like a bullet.  For the blood to break up the force must be great enough to overcome the surface tension of blood ( 56  mN/m).  The blood when sprayed across the carpet forms a nearly perfect circular shape due to the surface tension of the blood.  This spherical shape will be maintained until the blood droplets hit the surface of the carpet.  The blood spray would depend on the size, and speed of the bullet.    One might be able to calculate the speed and size of the bullet and likely the location of the shooter.  Bullets can reach speeds from 180-1500 m/s.    So with that range you likely would have different blood stains from an exit wound.