Researchers Develop Insight Into Blood Droplet Behavior for Bloodstain Pattern Analysis

After analyzing individual blood droplets dropped onto inclined surfaces, researchers in this NIJ-supported project said their measurements show that the current standard method is accurate for drops that hit at right angles but has significant errors at shallow impact angles.

June 11, 2018

Bloodstain pattern analysis is a technique used in crime scene reconstruction to determine the point of origin of a blood droplet, as well as whether it resulted from blunt trauma, a gunshot wound, or some other impact to a victim. This research, conducted by scientists at Georgia Tech, studied the fluid dynamics of the impact and spreading of a blood droplet on flat surfaces of variable roughness, wettability, and absorbency, oriented at various angles.

Sketch of droplet impact defining the angle of impact

Sketch of droplet impact defining the angle of impact.

NIJ image (see reuse policy).

The measurements were made using a high-speed video camera to track the droplets as they landed on glass, bathroom tile, and paper. The researchers performed 182 experiments with five different impact angles varying from a right angle (90 degrees or straight up and down) to 180 degrees (completely flat). Instead of using animal blood, which can vary in unexpected ways between experiments, they used a standardized mixture of water, glycerin, and alcohol that matched the viscosity and surface tension of blood.

Each impact was measured using a high-speed video camera mounted on the target assembly. The camera was held above the target surface, and its view angle of the impact plane was always 90 degrees, even when the target surface was inclined.

“The video images were analyzed to obtain the maximum spreading width and length of the droplet as well as the number of irregularities or projections in the shape of the droplet that may be present,” the researchers said. “These data were then analyzed to determine any specific relationships with the main fluid dynamical parameters that govern this impact process.”

The experimental findings from the tests demonstrated that for shallow impact angles of less than about 40 degrees, the “common practice” measurement that is the current standard “is in significant error,” the researchers said. They found a new correlation to correct the error and said, although “this correlation is based on all of the experiments on three different surfaces, the data indicate that the correction is greater for rougher surfaces.”

The discovery of the error for shallower angles “is a significant result,” they noted, and worthy of further investigation. Because of the complex fluid dynamics involved in the spreading and splashing of a blood droplet, they said, “the current simple relations used to predict the droplet impact velocity and angle from the appearance of the final bloodstain do not work well in all cases.”

These results led the researchers to improve the method used for analysis of bloodstains on angled surfaces. They found a new correlation, and although “this correlation is based on all of the experiments on three different surfaces, the data indicate that the correction is greater for rougher surfaces.” Although the new method is not currently available, the rapidly falling costs of computation may mean it could be available in the near future.

They also noted that the limitation to their findings is variability seen in all of the data, “which may be a result of how the droplets were generated. Any method of droplet generation will cause some droplet oscillations.”

Although the results correlated with mathematical modeling of droplet behavior, the researchers called for further study of oscillations to “assist in the development of simple correlations or computations that could be used by forensic practitioners in the field.” Such work, they concluded, would be justified on scientific grounds and would provide a rational measure of expected error.”

About This article
The research described in this article was funded by NIJ award 2013-DN-BX-K003, awarded to the Georgia Tech Research Corporation. This article is based on the grantee report “The Fluid Dynamics of Droplet Impact on Inclined Surfaces With Application to Forensic Blood Spatter Analysis” (pdf, 30 pages) by G. Paul Neitzel and Marc Smith.

This research is part of a broader portfolio of trace evidence projects managed by NIJ Physical Scientist, Gregory Dutton., Ph.D. Find more information about other trace evidence research projects.

Cite This Article
National Institute of Justice, “Researchers Develop Insight Into Blood Droplet Behavior for Bloodstain Pattern Analysis.” June 11, 2018, NIJ.gov: https://nij.gov/topics/forensics/evidence/impression/Pages/insight-into-blood-droplet-behavior-for-bloodstain-pattern-analysis.aspx

Date Created: June 11, 2018