IIT Bombay Study Finds Blood Droplet Cracks Can Reveal Volume, Impact Angle; Offers New Forensic Insights

Mumbai: A new study by researchers at the Indian Institute of Technology Bombay (IIT-B) has shown that cracks in dried blood droplets can provide important clues about how the blood was deposited—particularly the volume of blood and the angle at which it landed. The findings hold promise for forensic science and medical diagnostics alike.

The team examined how blood droplets dried on surfaces tilted at different angles, ranging from flat to 70 degrees, and tested droplets of varying volumes—from one microlitre (about the size of a pinhead) to 10 microlitres. As the droplets dried, high-speed cameras captured the process, and the resulting patterns were studied using microscopes and surface height measurement tools.

Typically, blood drying on a flat surface forms a ring-shaped deposit, known as a "coffee-ring" effect, with cracks radiating outward due to stress caused by shrinkage. However, when the surface is inclined, gravity alters the drop’s shape. Larger particles like red blood cells are pulled downhill to the advancing side of the droplet, while smaller components such as proteins and salts accumulate at the upper or receding edge.

This asymmetrical distribution results in uneven drying and varied cracking patterns. Cracks tend to be wider and more spaced out on the advancing side, where the deposit is thicker, and finer on the receding side, where it is thinner. The researchers explained this using the Griffith criterion, a principle in fracture mechanics that relates to the energy balance required for cracks to form.

The study developed a simplified model to link the cracking behaviour with deposit thickness, demonstrating how initial droplet conditions influence the final pattern. At steep angles or with larger drops, the liquid may even slide slightly before settling, leaving behind a faint tail that may not crack at all due to its thinness.

According to the authors, understanding how cracks form depending on droplet volume and surface angle could help forensic experts reconstruct the events at a crime scene by analysing dried blood. The team also highlighted the potential for future research into distinguishing blood patterns of healthy and diseased individuals, as well as exploring the role of surface properties like wettability.

Although some features, such as the occasional formation of a central bulge in larger drops, remain unexplained, the study makes significant strides in decoding the complex physics behind bloodstain patterns.