Characterizing drip patterns in bloodstain pattern analysis: An investigation of the influence of droplet impact velocity and number of droplets on static pattern features.

This work characterizes fundamental features of static drip patterns simulated for forensic bloodstain pattern analysis. The purpose of this study was to determine if and how two independent variables, impact velocity and droplet number, influence the parent stain size, shape and satellite spatter distribution of drip patterns created with whole ovine blood. To do this, 500 drip patterns were created in a controlled environment at five varying impact velocities and ten different droplet numbers on a hard paper surface. Digital images of the dried patterns were processed and analyzed using Fiji (ImageJ). The data collected from each pattern support the hypotheses that drip patterns contain predictable and reproducible elements based on impact velocity and droplet number. Basic fluid dynamic principles demonstrate that the size of the parent stains, as well as the number and distribution of satellite stains increase with increasing Weber number. A decrease in the circularity of the parent stains was also noted. This study provides fundamental qualitative and quantitative data on observable elements of drip patterns that can be used practically by bloodstain pattern analysts for pattern identification and classification.