Researchers at IIT Bombay studied how blood drops dried on tilted surfaces and what the cracks that are left behind can tell us about the blood drop.
The scene is often depicted in crime dramas: forensic scientists analyse bloodstains left behind, trying to piece together what happened. But beyond the dramatic flair, the science of how blood dries and the patterns it leaves is an area of research with implications far beyond the courtroom, extending into medical diagnostics and bioengineering. Picture a tiny drop of blood, no bigger than a raindrop, slowly drying on a surface. As the water evaporates, the solid material inside, mainly red blood cells, proteins, and salts, gets left behind, forming intricate patterns. Scientists call this desiccation, and the patterns aren't random; they hold clues to how the blood got there in the first place, specifically, how much of it and the angle it fell.
Although desiccation patterns on flat surfaces have been studied since the early days of forensic sciences, there are still questions about what happens to a drop of blood when that surface is tilted. A team of researchers from the Indian Institute of Technology (IIT) Bombay tackled that question in a recent study exploring how tilting the surface changes how the blood dries and cracks.
The researchers wanted to understand the combined effect of how much blood is in the drop and how steep the surface is. They systematically tested different droplet volumes, from a tiny one-microliter drop the size of a pinhead up to 10 microliter drops the size of small buttons. The droplets were placed on glass surfaces tilted at various angles, from flat (0 degrees) to steep (70 degrees). They watched the drops dry in real time using high-speed cameras and then examined the dried patterns up close with microscopes and tools that measured surface height.
Drying blood drops on a flat surface typically form a ring-like deposit, much like the familiar coffee ring left by a spilt coffee drop. This happens because liquid evaporates fastest at the edge, pulling particles from the centre outwards. The dried blood ring, or corona, is dense with red blood cells and often shows cracks radiating outward.
But introduce a tilt, and gravity starts to pull the drop downhill, fighting against the surface tension that tries to keep it in a neat little dome. This battle deforms the drop, stretching it out and making the liquid surface uneven. The advancing edge, or the bottom half of the drop, tries to move downward, and the receding edge or the top half remains stationary. This leads to an asymmetric deposit, with the larger, heavier red blood cells affected more by gravity, settling towards the advancing side. The smaller components, like proteins and salts, are more influenced by the evaporation-driven flow, which is stronger where the contact angle is smaller, typically at the receding edge. So, on a tilted surface, we get a sorting effect with more red blood cells on the advancing side and smaller materials on the receding side.
This uneven distribution of material leads to striking differences in the cracks that form as the deposit dries completely. Cracking happens because the drying material shrinks, creating stress, like mud drying in the sun. When this stress gets too high, the material breaks, releasing the tension and creating new surfaces, in this case, the crack walls.
The researchers used a fundamental concept called the Griffith criterion, which says that cracking occurs when the energy released by the breaking material is enough to create the energy needed for the new crack surfaces. They developed a simple model based on this principle, considering the energy stored in the dried blood material and the energy required to create new surfaces at the interface between the blood and air, as well as the blood and the glass.
Their model showed that the stress needed to cause cracking depends on the thickness of the dried deposit. On the advancing side, where the dried blood mass accumulated more, the cracks were thicker and more widely spaced. The cracks were finer on the receding side, where the deposit thinned out. These differences are attributed to thicker and thinner deposit heights at advancing and receding fronts.
The cracking patterns are also asymmetric since the tilted surface causes an asymmetric deposit with different thicknesses on the advancing and receding fronts. At very steep angles and larger volumes, the drop might even slide a bit before settling, leaving a thin tail of dried material behind, which, being very thin, might not crack at all.
The study fills a crucial gap by exploring the factors influencing cracking and desiccation patterns. It systematically investigates the combined effects of volume and inclination and shows how these factors alter the resulting crack morphology. Moreover, their model establishes a relationship between the cracks that occur and the initial condition of the droplet before drying, which can be exploited for forensic analysis.
According to the study's authors, cracks can tell us the initial droplet volume and impact angle. This finding can help to figure out the initial impact angle of the droplet, i.e., help in the reconstruction of crime in forensics.
While their model successfully explains the relationship between deposit thickness and cracking patterns qualitatively, the study has not explored the exact mechanism behind some observed features. The occasional and apparently random formation of a central bulge instead of central plaques in larger drops remains a subject for future investigation, highlighting a limitation in our current understanding of this complex process. The central bulge formation is not universal but occurs stochastically across different experimental runs. This could be attributed to flow instabilities during drying. The work, nevertheless, considerably improves our understanding of bloodstain patterns on different surfaces. Knowing how volume and surface angle influence these patterns in forensics can help investigators understand the blood splatters on non-flat surfaces.
Looking ahead, the team wants to study other differences in patterns. According to the authors, studies have shown subtle differences between the deposits of healthy and diseased persons’ blood. Different patterns as a function of the surface's wettability are helpful from a forensics point of view. These could be possible biomarkers to study in the future