Scientists have long been interested in how droplets of liquids, such as water, paint, or coffee, dry and leave behind intricate patterns. However, the drying process is more complex when it comes to blood. Blood is a suspension of red blood cells, plasma proteins, salts, and other molecules, which interact uniquely during evaporation. This leads to fascinating, complex patterns as the blood dries, and these patterns can be influenced by various factors, including the size of the droplet and the angle of the surface it sits on.

A recent study explored how different droplet sizes and surface inclinations affect how blood dries, leaving behind different patterns. Researchers used a range of droplet sizes, from tiny 1-microliter drops to larger 10-microliter ones, and varied the surface angles from flat to 70 degrees. The team observed how the droplets dried, shrank, and cracked using advanced tools like optical microscopes, high-speed cameras, and surface profilers. The study, published in Langmuir, provided new insights into the physics behind blood evaporation and its resulting patterns.

On flat surfaces, the dried blood formed familiar “coffee-ring” patterns, where a ring of dried material forms at the edge of the droplet, surrounded by cracks. However, gravity pulled the red blood cells downhill when the surface was tilted, while surface tension worked to keep them in place. This created asymmetric deposits and elongated patterns, resembling a biological landslide frozen in time. Interestingly, the cracks were different on the two sides of the droplet. The cracks were thicker and spaced further apart on the “advancing” side (downhill), where more dried blood accumulated. The cracks were thinner and more closely spaced on the “receding” side (uphill). Larger droplets intensified this effect, as gravity played a more significant role.

To better understand these observations, the researchers developed a theoretical model based on the principle of energy conservation. The model showed that mechanical stress built up unevenly on both sides of the droplet, leading to the asymmetric cracking patterns observed. This model helped explain why the patterns differed based on droplet size and surface tilt.

These findings have practical implications, particularly in forensic science. Forensic investigators use bloodstain pattern analysis (BPA) to help reconstruct events at crime scenes. The study’s results suggest that both the blood droplet’s size and the surface’s tilt can significantly alter the resulting patterns. This new understanding could help forensic scientists avoid misinterpretations in bloodstain analysis, ensuring more accurate conclusions in criminal investigations.

References

• Kumar, B., Chatterjee, S., Agrawal, A., & Bhardwaj, R. (2025b, April 30). Blood droplets on inclined surfaces reveal new cracking patterns. Phys.Org; Phys.org. https://phys.org/news/2025-04-blood-droplets-inclined-surfaces-reveal.html
• Kumar, B., Chatterjee, S., Agrawal, A., & Bhardwaj, R. (2025a). Asymmetric deposits and crack formation during desiccation of a blood droplet on an inclined surface. Langmuir, 41(19), 11818–11837. https://doi.org/10.1021/acs.langmuir.4c03767