Insights into the mechanics of pure and bacteria-laden sessile whole blood droplet evaporation

We study the mechanics of sessile blood drop evaporation using optical diagnostics, theoretical analysis, and micro/nano-characterization. The transient evaporation process has three major phases (A, B, and C) based on the evaporation rate. Phase A is the fastest, where edge evaporation dominates and forms a gelated three-phase contact line. Gelation results from sol-gel phase transition that occurs due to the accumulation of red blood cells, as they get transported due to outward capillary flow generated during drop evaporation. The intermediate phase B consists of a gelation front propagating radially inwards due to the combined effect of outward capillary flow and drop height reduction evaporating in pinned mode, forming a wet gel phase. We unearthed that the gelation of the entire droplet occurs in Phase B, and the gel formed contains trace amounts of water that are detectable in our experiments. Phase C is the final slowest stage of evaporation, where the wet gel transforms into a dry gel and leads to desiccation induced stress, forming diverse crack patterns in the dried blood drop precipitate. We observe radial and orthoradial cracks in the precipitate's thicker region, mud-flat cracks in the drop center, and the outer contact line where thickness and curvature are relatively small. We also study the evaporation of bacteria-laden droplets to simulate bacterial infection in human blood and show that the drop evaporation rate and final dried residue pattern do not change appreciably within the parameter variation of the bacterial concentration typically found in bacterial infection of living organisms.