Blood droplets travel in all directions in crime scenes

Studying a crime scene isn’t easy, especially because blood droplets don’t just fly away from the wound, according to new research.

The pattern of blood droplets can reveal a lot of information about a crime scene. Blood pattern analysis (BPA), a field of forensic science, relies on physical and mathematical analyses to understand how blood and other fluids behave in different scenarios. This evidence can be used to reconstruct crime scenes in court cases and trials.

In general, a gunshot wound produces small blood droplets that fly away from the wound and hit surrounding objects – including the shooter, if they’re at close range. This can help forensic scientists work out how people were positioned when they were shot.

But a paper published in the journal Physics of Fluids has shown that the gases produced by a gunshot can reverse the direction of these blood droplets, forcing them to fly back towards the wound.

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Scenarios for the trajectories of droplets at three different inclination angles. Credit: Gen Li, Nathaniel Sliefert, James B. Michael, and Alexander L. Yarin

The researchers had previously worked on modelling the trajectories of blood droplets after gunshot wounds.

“We used proper fluid dynamics models for the aerodynamic drag, which led to significant improvements, but concluded muzzle gases should also be included and can dramatically alter the formation and size of blood droplets,” says Alexander Yarin, a professor at the University of Illinois, US, and co-author on the paper.

In this study, the researchers examined the effect of a vortex ring – the burst of gases that comes from the muzzle of a gun as it is fired. They conducted a series of experiments at an indoor firing range, using foam cavities filled with swine blood, and combined their images and recordings of experiments with mathematical analyses and models.

They found that the vortex ring from the gunshots could have a large effect on the blood droplets – breaking them up into smaller drops and even changing their direction.

“This means that such droplets can even land behind the victim, along with the forward splatter being caused by a penetrated bullet,” says Yarin. “With a certain position of the shooter relative to the victim, it is possible for the shooter’s clothing to remain practically free of bloodstains.”

“BPA is based on physics, but ultimately it is an opinion-based art of forensic science,” says Adrian Linacre, a professor of forensic science at Flinders University, who was not involved on the paper.

“The paper shows some fascinating images of the gasses as they exit the muzzle and how the calibre alters the speed at which the gasses move towards the source of blood.”

Induced droplet bag breakup at ~40 milliseconds after bullet impact due to muzzle gas influence.
Induced droplet bag breakup at ~40 milliseconds after bullet impact due to muzzle gas influence. Credit: Gen Li, Nathaniel Sliefert, James B. Michael, and Alexander L. Yarin

He adds: “There are valuable data on how the distance between the point of discharge and impact into blood has an effect – the faster and shorter distance between the muzzle and impact point, the greater the expected effect on the back spatter.

“The only data lacking in my opinion is more on the effect on the resulting bloodstain pattern. Given any pattern an analyst encounters at a scene, what deduction could be made after reading this paper; i.e. how close was the muzzle when it was discharged?”

Linacre also points out that “papers like this state ‘a source of blood’ – there is though a real understanding that this means someone was shot with a firearm and therefore suffered.”

The Royal Institution of Australia has an Education resource based on this article. You can access it here.

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