If investigators find a crime scene demands it, they will bring in bloodstain pattern analysis (BPA) experts to help infer details about the nature of the bloodstains and how that information fits into the overall crime puzzle. Estimating the originating position and droplet size distribution of blood spatter is a common expectation of BPA experts; however, depending on the technique used, the error ratio can be substantial. For example, a well-cited 2011 study showed estimates that neglect the effect of drag and gravity on the initial position of blood droplets can overestimate the distance by 50 percent.
Inspired by the real-life murder of actress Lana Clarkson, researchers from the University of Illinois at Chicago and Iowa State University employed physics-based modeling to better understand the analysis of blood backspatter in gunshot wounds.
On Feb. 3, 2003, Clarkson was found dead in the mansion belonging to record producer Phil Spector. She was shot in the face at very close range. While a witness said Spector came out of the house holding a gun and saying “I think I shot her,” Spector would later say it was an “accidental suicide.” Although Spector was dressed in white clothes and investigators noted significant backward blood spatter, no bloodstains were found on his clothing, which was puzzling.
In two new papers published in Physics of Fluid, the research team presents both theoretical and experimental results that seemingly explain how the convicted murderer escaped the shooting with zero bloodstain evidence on his person.
When a gun is fired, propellant gases from the gunpowder are released from the barrel at high speed, forming turbulent vortex rings. The research team honed in on how blood droplets that typically fly from the victim toward the shooter are affected by this turbulent vortex ring of muzzle gases.
Experiments at an indoor shooting range employed swine blood and a 22-caliber rifle with a muzzle flash suppressor. The turbulent vortex rings were imaged with a high-speed shadowgraphy system, like those used to capture flow structures of supersonic aircraft or spacecraft.
Firing from a range of standoff distances up to 1.5 m, the researchers demonstrated that backward blood spatter droplets can completely reverse course as result of the turbulent vortex ring of muzzle gases that move from the shooter toward the victim.
"This means that [blood] droplets can even land behind the victim, along with the forward splatter being caused by a penetrated bullet," said Alexander Yarin, distinguished professor at the University of Illinois at Chicago. "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 high-speed imaging also revealed that blood droplets can breakup in mid-flight as a result of high-speed muzzle gases.
"The size of droplets can be dramatically impacted through the secondary breakup induced by the muzzle gas wind," said Yarin.
The collaborative team is now working to analyze this mid-flight breakup behavior using bloodstain patterns.
"Presumably, many forensic puzzles of this type can be solved based on sound fluid mechanical principles," said Yarin.