With two recent papers, physicists continue to contribute their unique insight into the forensic analysis of blood spatter patterns. The first, published in Physics of Fluids, provides a useful mathematical model of the relationship between the shape and velocity of a sphere and the patterns created by the resulting blood spatter. The second, published in the Journal of Fluid Mechanics, examines how gases shot at firing from a firearm can affect the distribution of blood spatter and shot-wreckage (GSR). Murder case Phil Spector. The music producer was finally convicted in 2009 to shoot 40-year-old Lana Clarkson in 2003. His first trial in 2007, however, ended with a hanging jury, in part because the prosecutor and defense forensic scientists did not agree to interpreting the blood spatters on Spector and Clarkson's clothing. Physics can tell if he was really guilty or innocent.
At a bloody crime scene, analysts typically look for patterns of size, shape, location and distribution of blood spots. They interpret these patterns and use the data to reconstruct the event as well as possible. For example, it is possible to measure the direction and angle of blood spots to determine the point of origin. Fans of the Showtime series Dexter – whose fictional protagonist was a serial killer / forensic expert with experience in analyzing blood stains – may remember that this often happens with strings, which makes tracing the trajectory easier with single drops.
However, according to a 2009 National Academy of Sciences forensic report, the analysis of blood spatter is still associated with great uncertainty and subjectivity. Blood is a surprisingly complicated fluid, in part because the red blood cells in human blood can form long chains that give it the consistency of mud. And blood begins to clot as soon as it leaves the body. Technically, blood is not a Newtonian fluid: use more force and reduce viscosity rather than staying constant (like Newtonian fluids). As a result, it flows faster. Blood is also viscoelastic: not only does it slowly deform when exposed to an external force, but once that force has been removed, it returns to its original configuration. Add coagulation and a variety of other factors (including the type of surface it lands on), and the correct interpretation of the resulting spray patterns will be incredibly difficult.
Splashes from gunshot wounds are particularly hard to interpret. The power of the ball can overcome any surface tension. Because blood is denser than air and also accelerates towards the air, it breaks down into smaller drops and forms a cloud that sprays out of the wound. This process is called atomization and is not well understood in terms of the underlying physical mechanism. The drops of blood usually move over long distances and follow a curved trajectory thanks to gravity and aerodynamic drag.
Blood drops also interact with each other as they move through the air, further increasing their range, similar to birds flying in a V formation. There is also a difference between forward blood spatter moving in the same direction as the ball motion and backward spattering in the direction against the ball motion. Alexander Yarin, a physicist at the University of Illinois in Chicago, specializes in fluid dynamics and co-author of the two recent works.
Yarin and several colleagues, including Daniel Attinger, a mechanical engineer in the US state of Iowa University, have conducted laboratory experiments and developed quantitative theoretical models to better predict the behavior of blood spatter, especially during shots. In many of these experiments, bullets were shot through sponge-soaked sponges, the liquid sprayed onto white paper, and the entire process recorded on high-speed cameras. The scientists then analyzed the spray patterns and compared their experimental results with their theoretical predictions of how many drops were produced, along with their size, distribution and initial velocity.
In 2016, they released a model that could better explain the effects of gravity and drag drag impacts, which allow the first prediction of the sputtering mechanism and determine how far drops of blood can move from a gunshot wound. The ultimate goal is to use these mathematical models to create a plot of the blood spatter patterns that on-site forensic analysts can use. Maybe one day there will even be an app that will be able to scan and analyze the spills at a crime scene to create a likely scenario, including the type of weapon and the balls used (if no bullets found at the scene and where they were involved (whether they were gunmen or victims).
The current research builds on Yarin's previous work with Attinger and others. For example, an article published in Forensic Science International this year looked at splashes from various shooting scenarios with rifles and handguns of various types of ammunition. Yarin et al. developed a model to describe the effects of certain projectile types on forward and backward spraying: slender and blunt projectiles and ovoid projectiles. Now they have developed a generalized model for forward splashes that is applicable to any type of sphere, as described in the publication Physics of Fluids. They also identified the physical mechanism responsible for the formation of blood droplets. The technical term is "the Rayleigh-Taylor instability". This happens when a dense fluid such as blood is heading for a lighter fluid such as air. "A bullet accelerates the blood toward the air, and that's how the atomization begins," Yarin said.
Another aggravating factor is the effect of the gas escaping from the mouth of a weapon when fired at the blood droplets in the backward scattering. A better propellant propagation model after leaving the muzzle could help to determine the distance of a shooter to the target at the scene and to illuminate its role in the transport of weft waste (GSR). This is the focus of the journal Journal of Fluid Mechanics, which Yarin co-authored with his graduate student Patrick Comiskey from the University of Illinois.
They specifically studied the effect of turbulent vortex gas rings generated by a cannon after a shot and found that the effect on the resulting rearward blood spatter patterns can be significant. "Turbulent vortex rings of propellant gases distort the distribution of blood spots [in backward spatter] on the ground and can either turn blood droplets farther from the target or even backwards to the target," the authors wrote.
This could be significant for the Spector case. The prosecutor's expert believed that the spatters would fly only two to three feet. The defense expert believed he could drive up to two meters. This was critical in case Spector claimed he was too far from Clarkson to shoot her and insisted that she shoot herself in the mouth. There were a good deal of blood spatters on Clarkson's dress and 18 drops of blood on Spector's jacket. But his pants or jacket cuffs were empty. The defense argued that if he had shot Clarkson, his clothes would have been covered in blood spatter. The analysis of Yarin and Comiskey shows that spattering blood can cover longer distances as well as overtake a shooter and return to the target. Thus, Spector's jacket could remain flawless, while drops of blood either brushed past him or turned back to the victim, even if he was the shooter.
Spector is currently serving 19 years of his life, but there has been an ongoing cloud of uncertainty over the issue of his guilt or innocence. "His case is just the backward splash on the shooter," Yarin said. An HBO film from 2013, Phil Spector written and directed by David Mamet, contained a scene depicting a fictional forensic experiment that strongly suggests the music producer's innocence.
"This latest analysis raises some physics-based doubts about this scenario," Yarin said.
DOI: Physics of Fluids, 2019. 10.1063 / 1.5111835 (About DOIs).
DOI: Journal of Fluid Mechanics, 2019. 10.1017 / jfm.2019.56 (About DOIs).