Inspired by the conviction of music producer Phil Spector for the 2003 murder of Lana Clarkson, researchers have discovered that when a gun is fired, gas from the muzzle escapes in a series of turbulent vortex rings that can cause blood spatter to reverse direction, splattering away from the shooter instead of toward them.
The work was published April 20 in Physics of Fluids in two related papers: one on the theoretical fluid physics and one on the experiments performed to prove the concept. The studies focused on muzzle gas, which escapes the gun immediately after a bullet is fired.
"[This] physics of the interaction between muzzle gases and back spatter hadn't been considered in a thorough way," said Jacob Michael, an assistant professor of mechanical engineering at Iowa State University. "I think one of the implications is that the predictive modeling of such an interaction can be implemented, potentially as a tool for forensics."
Producing the amount of energy required for a gunshot requires a rapid succession of chemical and physical reactions, starting with a small ignition of gunpowder that produces muzzle gas. This process occurs "very rapidly," explained senior author Alexander Yarin, a distinguished professor in engineering at University of Illinois Chicago. The whole thing takes less than one hundred-thousandth of a second.
This rapid expulsion of gas is what causes the typical splatter of blood back toward the shooter.
"Depending on the distance to the target, what happens is the bullet penetrates and it actually generates back spatter," Michael said. "What we're looking at now is the interaction of this jet of gas that comes out of the firearm with that blood that's being sort of sprayed backwards."
The researchers were inspired to undertake their project by a true crime mystery: the 2003 murder of actress Lana Clarkson.
In 2009, Phil Spector was convicted for the murder. However, it took several years to convict Spector, who died in January, because he claimed she had died by suicide. And — unusually, for a gunshot homicide — there was no blood spatter on Spector's clothing.
"The clothes of Phil Spector, which was a white outfit, was practically clean," Yarin said. "At that time I thought that maybe he wasn't guilty because he was clean."
Ultimately, an autopsy determined that the gun had been forced into her mouth, leading to Spector's conviction. However, how Spector's clothes stayed so clean if he had shot Turner remained a mystery, one that the researchers used physics to crack.
For the theoretical study, the team analyzed the interaction of blood with turbulent vortex rings using a mathematical model they had already developed about the interaction of fluids of different density.
"In our previous work, we determined the physical mechanism of backward spatter as an inevitable instability triggered by acceleration of a denser fluid, blood, toward a lighter fluid, air," Yarin said. "This is the so-called Rayleigh-Taylor instability, which is responsible for water dripping from a ceiling."
They predicted that blood droplets can become engulfed in the vortex rings from the muzzle and turned around, leading to unexpected directions of blood spatter.
"This means that such droplets can even land behind the victim, along with the forward splatter being caused by a penetrated bullet," said 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."
The researchers verified this model with experiments using swine blood. They placed it in a container where it could be held between two thin membranes, and set up an imaging system to capture the moment of impact.
They found that the gas from the muzzle had a significant impact on the direction of blood spatter, even finding that the vortex rings can cause secondary breakup of blood drops into smaller drops.
The researchers are next going to analyze the spatters produced by these muzzle gas interactions. In the meantime, they hope their work will see applications in forensic investigation, as so much scientific research has before.
"There is no miracle that physics interacts with forensic sciences," Yarin said. "Forensic science progress will always be driven by physics or biology or whatever. Thinking about DNA analysis and blood spatter analysis, all this is driven by science."
The studies, "Blood backspatter interaction with propellant gases" and "Experimental and numerical study of blood backspatter interaction with firearm propellant gases," published April 20 in Physics of Fluids, were authored by Nathaniel Sliefert and James B. Michael, Iowa State University; and Gen Li and Alexander L. Yarin, University of Illinois at Chicago.