The spatter pattern of blood caused by a gunshot wound contains a wealth of information that forensic scientists want to tap into in order to reconstruct the scene of a crime. Current methods for analysing blood spatter patterns are relatively simply however, largely ignoring effects such as gravity and air resistance — assumptions that may not be valid when blood drops travel large distances. Patrick Comiskey and colleagues now present a method for determining the origin of blood back-spatter patterns from a gunshot, which takes into account the motion of droplets through air.
Using the Rayleigh–Taylor instability framework, Comiskey et al. predicted the initial distribution of drop sizes and velocities, showing that the interaction of the resulting drop cloud with air helps to reduce the drag experienced by each droplet. This information can be used to accurately calculate the trajectory and subsequent impact dynamics of the droplets, giving rise to distinct stain distributions. Using their model, they predicted characteristic stain patterns for realistic gunshot scenarios, which agree well with preliminary experiments conducted on sponges soaked in swine blood.