Nick Laan ( Institut de Recherche Criminelle de la Gendarmerie Nationale) - Impact velocity of blood droplets for crime scene reconstruction

Bloodstain Pattern Analysis is a forensic tool used to determine the position of a victim at a crime scene in which blood has been shed. To determine where the blood source was investigators use a straight line approximation for the trajectory, ignoring effects of gravity and air resistance and thus overestimating the height of the source. To determine a more accurate trajectory the impact velocity of the blood droplet is required. We conducted impact experiments for a range of reference fluids and blood, different substrates and impact angles, whereas we show that the stain shape is determined by a balance of capillary, viscous and inertial forces that goes beyond commonly used approximations. Based on the relations found, we were able to determine a general formula for the impact velocity as a function of stain size and volume. By taking the effect of gravity into account, we show that the location of the blood source can be determined one order of magnitude more accurate.