Bloodstain tails: Asymmetry aids reconstruction of oblique impact

In the field of forensic science, elongated bloodstains formed from oblique impact of droplets are sometimes used to reconstruct aspects of a bloodletting event. For low impact angles, these stains can include an asymmetric tail that bloodstain analysts can use qualitatively to establish directionality. Quantitative analysis of these bloodstain tails, and any insight that they can provide into the impact dynamics, is lacking due to experimental challenges. Previous studies of bloodstain shapes have predominantly focused on relatively large drip drops that fall vertically on flat and inclined surfaces; yet, for inclinations that produce low impact angles, gravitational effects can reshape and obscure tails, preventing insight into how tails develop on horizontal surfaces. Here, we carry out systematic experiments in which human blood droplets with diameters less than a millimeter impact a horizontal surface at impact angles ranging from 16° to 65°. High-speed imagery confirms that the tail is not part of a prompt splashing event, but rather forms at the last moments of spreading, maintaining its shape as it dries. 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.