Surface coatings including fingerprint residues can significantly alter the size and shape of bloodstains.

When conducting a blood pattern analysis (BPA) the size, shape, distribution, and location of bloodstains found at a crime scene may be critical in forming a hypothesis as to what transpired during a bloody event. Prior studies have demonstrated that the size and shape of a bloodstain on a smooth surface are determined from impact dynamics and to a lesser degree by the target material itself. Yet, these studies have relied on clean surfaces, and it is unclear whether the presence of microscopic coatings and residues could significantly alter the size or shape of the dried stain. Here, in the present work, experiments are conducted to demonstrate that various coatings, such as the sebaceous residue from a latent fingerprint, can dramatically alter the size and shape of the stain from the moment of impact through the drying process. These experiments also highlight that a drop impacting a tilted superhydrophobic-coating glass substrate can cause the blood drop to completely recoil without leaving a stain. Relying on a combination of high-speed and time-lapse photography, the specific stages in the stain evolution responsible for the deviations from the current models are identified. At a relatively low impact velocity, the stain sizes on the coated glass surfaces were 35-72% smaller than on the clean glass surface. At a higher impact velocity, the stains on the coated surfaces were not only smaller, but also contained drop spatter around the primary stain that was not observable in the absence of the microscopic coatings. The reduction in bloodstain size did not appreciably change when a chemical was added to deactivate the anticoagulant and allow the blood to clot.