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A new study analyzes the way that bloodstains behave on cotton fabrics in the hopes of furthering forensic analysis.
The shapes and intensities of bloodstains can provide investigators information about the angle and speed at which the blood was moving before it stained.
Plain-woven cotton makes it easier to estimate the velocity of blood splatter.
In film, a splotch of crimson blood hitting a white square of cotton is often visual shorthand for “this is not good.” Obviously, in real life, bloodstains aren’t usually positive omens either, but it turns out that they may at least be helpful.
In a new study published in the journal Forensic Science International, a team of North Carolina State University researchers analyzed the dynamics of blood droplets falling on three different types of cotton fabrics. The goal was to work backwards from a bloodstain to the splatter of blood that created it, all by better understanding how the blood behaves when it hits fabric. The hope is that the work could be helpful in forensic investigations.
“When blood strikes fabric, it leaves a stain,” Tiegang Fang, one of the authors of the study, said in a statement. “But it can be difficult to accurately assess things like how quickly blood was traveling when it struck the fabric. Was it going fast? Slow? Did someone just brush up against the blood? It’s hard to tell, because once the blood comes into contact with the fabric, it wicks across the surface of the fibers in the fabric, spreading out.”
Five fabric surfaces were analyzed throughout the study—plain-woven cotton (which has the same surface characteristics on both the front and back), the front and back of cotton twill, and the front and back of jersey knit. Once the fabrics had been selected, the team applied pig blood (treated to ensure consistent behavior across testing) to the materials at 12 different velocities. High-speed cameras—using a rate of four frames per millisecond—captured the blood strikes and documented how the blood traveled across the fabrics after making contact. By analyzing the images, the team identified patterns in the bloodstains correlating to the velocity of the blood splatter.
“One key finding relates to the ‘fingers’ of the blood stain,” Fang said. “When you look at a bloodstain on fabric, you will sometimes see thin tendrils that spread out from the center of the stain. Those tendrils are referred to as fingers. We found that the more fingers a bloodstain has, the faster the blood was moving when it struck the fabric. However, over time, these fingers may spread out and run together.”
Additionally, the faster the blood was traveling when it hit the fabric, the more likely it was that there would be additional tiny stains—known as satellite droplets—surrounding a central stain.
The study text noted that when blood drops hit a fabric, they spread and form rims. Then, the rim breaks up and retracts, allowing the blood to enter the inter-yarn spaces in the fabric structure and stop acting like an independent droplet. From there, blood starts spreading within the fabric and begins wicking across the surface.
But not every fabric behaved the same. Researchers concluded that plain-woven cotton provided the most clues regarding estimated velocity, largely because the stains were the biggest. Conversely, twill was the trickiest to understand, and knit offered the smallest stains up for analysis.
Fang called the results of the study “promising,” and plans additional research with a wider variety of fabrics, weaves, and yarns. “It’s clear,” he said, “that the specific structures of each surface play a critical role in how these bloodstains form and what we can learn from them.”
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