Since the underlying physics in bloodstain formation on fabrics is not well understood, despite its importance in bloodstain pattern analysis (BPA), this paper presents a fundamental study of the formation of drip bloodstains on fabrics, by focusing on blood droplet impact and wicking dynamics.
The bloodstains were created on plain woven fabric by the perpendicular impact of a single blood drop with seven different impact velocities. The whole droplet impact and wicking processes were captured by multiple cameras. Fabric properties were characterized in detail at different levels. The bloodstain formation process was classified into distinct stages, including the inertial impact, initial absorption, first wicking and second wicking stages. The subsequent wicking process greatly alters the impact-induced bloodstains, in terms of bloodstain area. The dimensionless impact-induced stain factor (βi,e) is strongly dependent on the impact velocity while the final stain factor (βf,e) after the second wicking stage is not. The contribution of the subsequent wicking in altering the stain factor (or stain area) is quantified and found to decrease with increasing impact velocity. The blood wicking dynamics on the fabric in the majority of the first wicking stage can be well described by a simple scaling: (βe-βi,e)/βi,e=C[(t-ti/ti)] where ti marks the end of the inertial impact stage. The wicking coefficient C, which represents the influence of droplet impact on the subsequent droplet wicking, is found to scale as C∼We-0.34. In the end, brief comments are provided regarding (1) the influence of the evaporation on the blood drop post-impact wicking dynamics and (2) the shape of bloodstains formed on fabrics, with a few suggested research directions for future work. (Publisher Abstract)