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

We study the mechanics of evaporation and precipitate formation in pure and bacteria-laden sessile blood droplets in the context of precipitate patterns as a disease diagnostics marker. Using optical diagnostics, theoretical analysis, and micro/nano-characterization techniques, we show that the transient evaporation process has three stages based on the evaporation rate. In the first stage, edge evaporation dominates, forming a gelated three-phase contact line. The radially outward capillary flow inside the evaporating droplet causes an accumulation of red blood cells, resulting in a sol-gel phase transition. The intermediate stage consists of the gelation front propagating radially inwards due to the combined effect of capillary flow and droplet height reduction evaporating in pinned mode, forming a wet gel phase. We unearthed that the gelation of the entire droplet occurs in the second stage, and the wet gel formed contains trace amounts of water that are detectable in our experiments. Further, we show that the precipitate thickness profile computed from the theoretical analysis conforms to the optical profilometry measurements. In the final slowest evaporation stage, the wet gel transforms into a dry gel, leading to desiccation-induced stress forming diverse crack patterns in the precipitate. We 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. However, at exceedingly high bacterial concentrations, the cracks formed in the coronal region deviate from the typical radial cracks found in lower concentrations.