The Fascinating Physics of Blood Splatters

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During the 2007 murder trial of music mogul Phil Spector, forensic experts for the defense and prosecution disagreed on how to interpret the blood spatter patterns on clothing worn by both Spector and his victim, among other disputed physical evidence. The end result was a hung jury, forcing the presiding judge to declare a mistrial.

Spector was convicted after a retrial in 2009 and is serving a sentence of 19 years to life. But the case serves as an example of the uncertainty that still surrounds blood pattern analysis. Now a team of physicists has developed a more general model of blood drop formation that accurately predicts the speed, sizes, and trajectories of blood drops produced by gunshot wounds, based on experiments shooting bullets through sponges soaked in pig’s blood. They describe their work in a new paper in Physical Review Fluids.

When investigating a bloody crime scene, analysts first look at the patterns made by the blood stains: size, shape, distribution, where they are located, and so forth, taking into account the texture of the surface and other factors. It’s their job to interpret those patterns and use that data for the reconstruction phase of the process.

They use the direction and angle of the blood stains to pinpoint where the bloodshed began. Typically this is done with string, creating straight lines to trace the trajectories back from individual drops to the region of origin. This process tells the analyst where the victim was likely located when the blood was shed. The National Forensic Science Technology Center (NSFTC) has a video explaining all the details:

Forensic laboratories have conducted many experiments over the years to help law enforcement and lab technicians make better interpretations of that data, but there’s still a degree of subjectivity that comes into play—a key finding of a 2009 report on forensic sciences by the National Academy of Sciences.

That’s where physicists can help, because so little is known about the precise physics of how blood behaves under various circumstances—plus the same patterns can be produced in different ways.

“We are creating quantitative models that will allow us to generalize those experiments,” Daniel Attinger, a professor of mechanical engineering at Iowa State University and co-author of the new paper, told Gizmodo. “At a crime scene everything has happened already, so you’re left with the solution of a problem that you can in theory solve forwards [in time], but it’s much harder to solve backwards [in time].”

The patterns produced from gunshots are particularly difficult to interpret. Blood dripping from a cut finger will form large, spherical drops as it hits the ground; that formation is about the interplay between gravity and surface tension. But a gunshot wound is a violent impact, and the force of the bullet is much stronger than gravity and can overcome any surface tension. Because the blood is denser than the air through which it travels, it breaks up into smaller drops (atomization), forming a cloud that sprays out from the wound. During their flight, these clouds of tiny drops are strongly affected by air drag, as well as gravity.

That’s why the straight-line trajectories currently used in blood spatter analysis are plagued by uncertainty, according to Attinger, who concluded that this only makes sense for drops that travel short distances. Drops from gunshot wounds travel longer distances and actually follow a curved trajectory, thanks to the combined effects of gravity and drag.

Other variables matter too, from changes in temperature as blood flies through colder air, to the caliber weapon used, to how the stains dry out as the liquid evaporates.

“Nobody really knows the exact cascade of physical mechanisms, even now, in the case of a gunshot because it occurs so fast,” said Attinger. The model he and his colleague, Alexander Yarin of the University of Illinois, Chicago, have proposed is the first to predict the atomization mechanism and the distances drops of blood can travel from a gunshot.

How far blood drops travel, and their size, are key factors in determining both a bullet’s trajectory and in pegging the original source of the blood relative to the stains. Why does this matter? Well, the Spector trial hinged on whether or not the victim, Lana Clarkson, had shot herself in the mouth, and the blood pattern analysis was a key piece of evidence—with very different expert interpretations.

The sheriff’s criminalist testifying for the prosecution thought it would only travel two to three feet. But the defense’s forensic expert testified it could travel as far as six feet. Clarkson had blood spatter on her dress, but not her outstretched legs, while Spector’s trousers had no spatter, but his jacket did. So determining how far blood spatter from a gunshot can travel was critical to the arguments of both prosecution and defense. If spatter could travel six feet, Spector could have been telling the truth when he claimed he was too far away from Clarkson to have shot her, yet still close enough to get stains on his jacket.

“Blood is probably the most complicated fluid we know,” said Attinger. In humans, red blood cells can stick together into long chains, like a necklace, which can become so large that blood takes on the consistency of sludge. Blood also starts to coagulate once it leaves the body, but the blood used in such experiments is usually treated with an anticoagulant.

In 2011 experiments, physicists at Washington State University used Ashanti chicken wing sauce and Ivory dish soap to mimic the consistency of actual blood. In the past, Attinger’s group used horse blood (since human blood can be a vector of disease), dropping, launching, spraying, and splattering the liquid onto white paper—all captured on a high-speed camera.

This time, they used pig’s blood, soaking sponges in the stuff before firing guns of various calibers at those targets at a local police firing range. It’s the closest thing to human blood, right down to the ability of the red blood cells to form chains.

Once again, the action was captured via high-speed camera, and the resulting patterns displayed on sheets of white paper. And the group’s model accurately predicted the number of resulting drops, their sizes and distribution, and initial velocities, in keeping with the spatter patterns observed in the experiments.

The new model better accounts for the effects of gravity and air drag. Attinger and his colleagues even identified a mechanism for the travel of blood drops from a gunshot: The cloud of drops carries a significant mass of air within it, which reduces the air drag acting on the individual drops. Attinger likens it to geese flying in formation, or the tight “peloton” of bikers in the Tour de France.

The ultimate goal is to understand the physics so well that this knowledge can be integrated into the image processing capability of a handheld device—much like an over-the-counter diagnostic kit—to analyze blood spatter at crime scenes. This way crime scene investigators could easily and accurately determine when the blood was spilled, where the spatter originated, and even what kind of weapon was used.

If Attinger and his fellow physicists are successful, the day may come when it’s possible to revisit past cases like the Spector murder trial, and determine which of the disputing experts was right.

[Physical Review Fluids]