A Physics-based Approach to Blood at the Crime Scene

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 A Physics-based Approach to Blood at the Crime Scene

Locard’s exchange principle tells us that if blood is present at a crime scene, it was transferred. There’s a myriad of reasons why it may not be detectable once crime scene investigators arrive—it could have been expertly cleaned up, it could have been removed from the scene, methods of detection may not be sensitive enough to find the small evidence, or perhaps personnel have not looked in the correct place.

Understanding the dynamics of how blood droplets fall on and through different materials can help crime scene units investigate a scene and generate a better picture of said scene for investigators and forensic scientists.

While many studies have focused on the spread of droplets on smooth surfaces, little has been done regarding droplet spreading on fabrics. So, in a new study, Thijs de Goede and his team from the University of Amsterdam decided to take a physics-based approach to blood droplet analysis both experimentally and computationally.

The team compared droplet impact on three polyester materials with different pore sizes and yarn thicknesses, as well as a solid steel surface and a steel surface patterned similarly to one of the fabrics. They identified a number of key parameters in determining how much a droplet will spread, including its velocity, the fabric’s wettability, and the size and density of the pores in the fabric.

For even the simplest of fabrics, the researchers found fabric geometry plays a significant role.

“Our results also show that if the impact velocity becomes too large, the ink can push itself through the fabric and spread out in between the fabric and substrate as well, which can have undesirable effects on the other side of the fabric,” the researchers explain in their paper, published in Physics of Fluids.

After generating experimental results, the scientists compared those with lattice Boltzmann simulations, which is an oft-used method for fluid simulation in the physics arena. According to the paper, when droplets occur at high-impact velocities, the spreading ratio actually decreases.

“[This] is due to increased viscous losses inside the droplet, which originates from the roughness of the fabric and the droplet pushing itself through the fabric or in between the fabric and substrate during droplet spreading,” explain the researchers.

Importantly, de Goede and his team concluded that blood spreads similar to a Newtonian fluid, meaning the pre-existing model for droplet spreading on smooth surfaces is still effective in the evaluation of blood droplets on fabrics.

Still, there is more work to be done. This study only examined one specific type of weaving. The research team has now set their sights on additional studies that will include different types of mesh fabrics.

“The question remains how our findings can be generalized to any type of clothing or fabric—there are often blood stains on carpets and too—and how this can be used in practice by forensic scientists at a crime scene,” said study co-author and University of Amsterdam professor Daniel Bonn.