The Bleeding Edge of Forensics

Scientists who have spent their careers studying the behaviors of liquids are putting forensic techniques to the test. They're working to test the rigor of modern forensic science by improving our understanding of the secrets hidden in blood. "There's been a huge amount of basic research done on droplets and sprays," said Sanjeev Chandra, an engineer at the University of Toronto who helps General Motors develop better ways to spray paint its cars. "A lot of the physics is exactly the same for blood." In soon to be published research, Chandra and his team have revisited the techniques and software packages that forensic experts have developed over the years to reconstruct the origin of blood splatters. By testing this software scientifically on splatters of pig blood in the lab, they've shown that there is significant room for improvement in the models, which typically use straight lines to trace the path of blood droplets a surface back to their point of origin.

"They aren't very accurate," said Chandra. "They don't consider the effects of gravity on blood droplets. They ignore air drag, which can be very significant." Calculating the speed at which drops of blood leave the body during an attack is an important measurement for blood pattern analysis. The physics behind the velocity and size of a blood drop gives investigators an idea of what kind of wound was inflicted. A drop of blood falling from a cut finger, for instance, is a battle primarily between the force of surface tension, which keeps it stuck to the body, and gravity, which pulls it downwards. Solve the equations, and you'll find that a typical drop released this way has a volume of less than one percent of a teaspoon. Blood released from a wound by a violent impact -- such as a bullet -- tends fly in even smaller drops. That's because the force of a bullet is much stronger than gravity and easily overcomes the surface tension, flinging tiny drops away at high speeds.