Synopsis: Recreating the Scene of a Crime

A new theory accurately predicts the speed, sizes, and trajectories of blood drops resulting from gunshot wounds.

Analyses of bloodstain spatter from gunshot wounds typically assume that the paths of falling drops follow straight lines from the wound to the floor. A new theory, which describes how drops form when a bullet strikes, brings this assumption into question. Alexander Yarin, from the University of Illionois at Chicago, and colleagues found that the formation of drops in gunshot blood spatter arises from an acceleration-driven destabilization of the blood stream exiting the wound. The theory predicts that the drag between air and the resulting cloud of drops significantly affects the drops’ paths.

The assumption that drops travel in straight lines is only reasonable if the drops travel short distances. However, blood spatter from gunshot wounds can travel long distances, meaning that the bending caused by gravity and air drag can’t be ignored. Factors like initial drop size and velocity, along with the number of drops—not accounted for by existing models—introduce additional uncertainties in drop trajectories.

The model developed by Yarin and co-workers considers the impulse imparted by the bullet to blood that emanates from the entrance wound. This impulse causes a stream of blood to accelerate out of the body in the direction opposite to the bullet’s motion (back spatter). As the higher-density blood flows into the lower-density air it destabilizes, breaking up into drops. A cloud of drops develops, which collectively lowers the drag each drop experiences. Using their model the group predicted the drops’ sizes, trajectories, and characteristic stain patterns for real gunshot scenarios. They verified their predictions using data collected from pig’s-blood-soaked sponges shot with a bullet.

This research is published in Physical Review Fluids.

–Katherine Wright

Katherine Wright is a Contributing Editor for Physics.


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