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Drying droplets have fascinated scientists for decades. From water to coffee to paint, these everyday fluids leave behind intricate patterns as they evaporate. But blood is far more complex—a colloidal suspension packed with red blood cells, plasma proteins, salts, and countless biomolecules.

As blood dries, it leaves behind a complex microstructural pattern—cracks, rings, and folds—each shaped by the interplay of its cellular components, proteins, and evaporation dynamics. These features form a kind of physical fingerprint, quietly recording the complex interplay of physics that unfolded during the desiccation of the droplet.

In our recent experiments, we explored how blood droplets dry by varying both their size—from tiny 1-microliter drops to larger 10-microliter ones—and the angle of the surface, from completely horizontal to a steep 70° incline. Using an optical microscope, a high-speed camera, and a surface profiler, we tracked how the droplets dried, shrank and cracked.

Our study is in the journal Langmuir.

On flat surfaces, blood droplets dried predictably, forming familiar coffee-ring-like deposits surrounded by networks of radial and azimuthal cracks. But as we increased the tilt, gravity pulled the red blood cells downhill, while surface tension tried to hold them up. This resulted in asymmetric deposits and stretched patterns—a kind of biological landslide frozen in time.

Cracking patterns were different on the advancing (downhill) and receding (uphill) sides. On the advancing side, where the dried blood mass accumulated more, the cracks were thicker and more widely spaced. On the receding side, where the deposit thinned out, the cracks were finer. Larger droplets (10 microliter) exaggerated the asymmetry even more, with gravity playing a bigger role as the droplets grew heavier—leaving behind a long, thin "tail" of blood that dried and showed scattered dried red blood cells.

To explain what we observed, we developed a first-order theoretical model showing how mechanical stresses build up unevenly on either side of the droplet—a difference that helps explain the asymmetric cracking patterns we saw.

These findings have real-world implications. In forensic science, for example, investigators use bloodstain pattern analysis—or BPA—to reconstruct events at crime scenes. Our results suggest that both the tilt of the surface and the size of the droplet can significantly alter the resulting patterns. Ignoring these factors could lead to misinterpretations, potentially affecting how such evidence is read and understood.

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Bibek Kumar is a Ph.D. candidate in the Department of Mechanical Engineering at I.I.T. Bombay, Mumbai, India. Sangamitro Chatterjee is an Assistant Professor in the Department of Â鶹ÒùÔºics at DIT University, Dehradun, India. Amit Agrawal and Rajneesh Bhardwaj are Professors in the Department of Mechanical Engineering at I.I.T. Bombay.