NCJ Number
251757
Date Published
June 2018
Length
2 pages
Annotation
This summarized NIJ-funded project examined the behavior of individual blood droplets deposited on inclined surfaces, so as to determine whether the current standard method of examining the behavior of blood droplets deposited at a right angle to surfaces is also accurate for blood droplets deposited to inclined surfaces.
Abstract
This research studied the fluid dynamics of the impact and spreading of a blood droplet on flat surfaces of variable roughness, wetness, and absorbency oriented at various angles. The measurements were made by using a high-speed video camera to track the droplets as they landed on glass, bathroom tile, and paper. A total of 182 experiments were performed with five different impact angles that varied from a right angle to 180 degrees. Instead of using animal blood, which can vary in unexpected ways between experiments, researchers used a standardized mixture of water, glycerin, and alcohol that matched the viscosity and surface tension of blood. The video images were analyzed to obtain the maximum spreading width and length of the droplet and the number of irregularities or projections in the shape of the droplet. These data were then analyzed to determine any specific relationships with the main fluid parameters of movement. The study found that for shallow impact angles of less than about 40 degrees, the "common practice" measurement produced significant error. A new correlation was found to correct the error. Although this correlation is based on all of the experiments on three different surfaces, the correction is greater for rougher surfaces. The researchers recommend further study of oscillations to assist in the development of simple correlations or computations that could be used by forensic practitioners in the field.