Explosive gas from a gun can mask crime scene blood spatter patterns

By Leah Crane

Inspired by a famous murder case, a team of researchers has figured out how the gas coming from a gun that has just fired can affect blood spatter in close-range shootings. Not only can this vortex of gas slow down blood droplets, it can even turn their trajectories around completely.

Alexander Yarin at the University of Illinois at Chicago and his colleagues were inspired by a famous case in which record producer Phil Spector was suspected of shooting actress Lana Clarkson at close range, a crime for which he was later convicted. He was wearing white clothes at the time, and the fact that he had very little blood on his outfit was a key piece of evidence in the case.

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When a gun is fired, its muzzle shoots out a vortex of gas due to the combustion reaction that propels the bullet. To determine how Spector’s clothing stayed relatively free of blood, the researchers performed simulations of the fluid dynamics of blood droplets reacting with that vortex, as well as experiments using a foam container full of swine blood. They used a type of image called a shadowgram that could record the movement of the gas and the blood drops.

When the bullet hit a foam container, blood droplets splashed back towards the shooter, which was what the researchers expected. But when the gust of gas from the gun hit this blood, it split some of the drops into droplets that were up to 10 times smaller.

The drops also slowed down when they encountered the gas vortex, some reversing direction and even landing behind the target instead of in front of it. This could explain why Spector had so little blood on his clothes.

Understanding how this effect changes the patterns in blood spatter could help investigators reconstruct crime scenes more accurately, says James Michael at Iowa State University, one of Yarin’s collaborators. “In some forensic analyses, you can look at the distribution of droplets to infer what happened at a crime scene, and this alters the distribution significantly.”

The researchers also tested this effect with different guns to see whether this would influence the pattern. They found that the type of gun and bullet changed the blood’s behaviour, as did the distance from the target and the use of a suppressor or other muzzle modification.

“This is a unique measure that corresponds to any gun, so guns can be tested and categorised,” says Yarin. “If you have the predictions for different guns, you might be able to do some backwards engineering to find the specific weapon used in a shooting.”

Journal references: Physics of Fluids, DOI:10.1063/5.0045214, DOI:10.1063/5.0045219