How short-range shooters stay clean of blood, according to fluid dynamics

A US research team has found that the tiny droplets of blood produced by a gunshot wound can travel in unexpected directions, which may have implications for forensic science in shooting cases.

Bloodstain pattern analysis – reconstructing crime scenes based on the blood splatter – can be used to figure out where people were, and how they moved, while violence was taking place.

Forensic scientists have been suggesting that if someone connected to a shooting has very small blood droplets on their clothes and skin, they were likely very close to a victim when they were shot.

But a new study published in Physics of Fluids has shown how a shooter might stay clean of blood droplets, even if they’ve shot someone at close range.

The US researchers used fluid modelling and lab experiments to study the “secondary atomisation” of blood droplets from bullet wounds.

“Primary atomisation of blood is caused by a gunshot (bullet). It results in multiple drops spattered in the air,” explains co-author Professor Alexander Yarin, a researcher at the University of Illinois Chicago.

“Some of these drops are big enough to be significantly distorted and torn apart by the air drag forces acting on a drop in flight. Smaller droplets arise during this process, which is called secondary atomisation,” adds Yarin.

The researchers have for several years been investigating the way gases come out of the muzzle of a firing gun, and how these gases interact with blood droplets.

In this study, they used computer modelling, followed up with experiments, to see how different blood droplet sizes might interact with the muzzle gas from a gun.

“Muzzle gases form a turbulent vortex ring which moves toward a victim from a shooter and pushes the blood droplets from the shooter back to the victim,” says Yarin.

“Droplets are also deflected aside, and our predictions showed that some can even land behind the victim, even though initially they were moving from the victim toward the shooter.”

Six frames from high-speed imagery showing blood spatter travelling forwards then backwards
High-speed images show a rifle bullet impacting blood, followed by back spatter (toward the shooter) which interacts with the oncoming muzzle gases. This interaction leads to a change in direction where droplets can be directed back to the target, or even beyond the target. Credit: James Michael, Iowa State University

The results show how a short-range shooter might still avoid getting spattered with bloodstains from their victim.

A lack of bloodstains is sometimes used as evidence for the defence in trials, such as in the case of Phil Spector’s murder of actress Lana Clarkson.

“The results reveal the usefulness of multiphase flow fluid mechanics for the forensic discipline of back spatter analysis,” says Yarin.

“Hopefully, code based on the present results would be used in future crime scene investigations.”

Next, the researchers are interested in looking at brain tissue spatter, and whether it can be used to spot the difference between a suicide and a homicide.

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