Blood droplets on inclined surfaces reveal new cracking patterns

Summary

by Bibek Kumar, Sangamitro Chatterjee, Amit Agrawal, Rajneesh Bhardwaj Dried deposit of a 5 μL blood droplet on a glass surface inclined at 35° to the horizontal, showing differential cracking between the advancing (downhill) and receding (uphill) fronts. The arrow indicates the direction of gravitational acceleration (g). Credit: Bibek Kumar, Sangamitro Chatterjee, Amit Agrawal, Rajneesh Bhardwaj 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 published 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 ...