Can the tails of bloodstain help forensic investigation?

In the field of forensic science, elongated elliptical stains, called spatter stains, provide valuable insights into the oblique impact of blood droplets. For low-impact angles, the stain can include an asymmetric tail that bloodstain analysts can use qualitatively to establish directionality. However, quantitative analysis of these bloodstain tails, and any insight that they can provide in the impact dynamics, is lacking. Here we carry out systematic experiments in which small human blood droplets impact a horizontal surface at impact angles ranging from 16 to 65 degrees. High-speed imagery confirms that the tail is not part of a prompt splashing event, but rather forms at the last moments of spreading. For each stain, we link the tail length and elliptical geometry to the blood drop size and impact velocity vector that created it. Additionally, we report a power-law correlation of the dimensionless tail length with the angle and Weber and Reynolds numbers, and we describe how this correlation in conjunction with other existing correlations can improve reconstruction of the droplet size and impact velocity.