Blood pools can spread on several types of substrates depending on the surrounding environment and conditions. Understanding the influence of these parameters on the spreading of blood pools can provide crime scene investigators with useful information. The focus of the present study is on phase separation, that is, when the serum spreads outside the main blood pool. For this purpose, blood pools with constant initial masses on wooden floors that were either varnished or not were created at ambient temperatures of $$21~^{\circ }\hbox {C}$$ 21 ∘ C , $$29~^{\circ }\hbox {C}$$ 29 ∘ C , and $$37~^{\circ }\hbox {C}$$ 37 ∘ C with a relative humidity varying from 20 to 90%. The range $$21~^{\circ }\hbox {C}$$ 21 ∘ C to $$37~^{\circ }\hbox {C}$$ 37 ∘ C covers almost all worldwide indoor cases. The same whole blood from the same donor was used for all experiments. As a result, an increase in relative humidity was found to result in an increase in the final pool area. In addition, at the three different experimental temperatures, the serum spread outside the main pool at relative humidity levels above 50%. This phase separation is more significant on varnished substrates, and does not lead to any changes in the drying morphology. This phenomenon is explained by the competition between coagulation and evaporation.

The drying of human whole blood pools has been studied experimentally to improve the understanding of drops and pools of human whole blood. Here blood is assimilated to a complex fluid made of several bio-colloids. The blood pools were created inside a glove box under controlled conditions. We performed experiments at temperatures of 21 °C, 29 °C, and 37 °C, and relative humidity ranging from 20 % to 70 %. The air inside the glove box was still, and the evaporation process was mainly purely diffusive. A human blood diffusion coefficient of (1.08 ± 0.02) × 10 −9 m 2 /s was obtained. This value is in agreement with literature values for polymers and colloidal films. Moreover, we elaborated an updated model using Page's model to describe the drying kinetics of human blood pools. The effect of the surface area of pools was considered using a diffusive characteristics time. These findings are of interest for biomedical and forensic applications.

The study of blood drop drying has attracted the attention of several research groups for two main reasons:(1) for biomedical purposes as a rapid and cheaper disease detector, and (2) for forensic applications for the interpretation of bloodstain at crime scenes. Also, it presents an interdisciplinary and challenging research subject. However, the mechanisms related to the formation of patterns at the end of the drying process are not fully understood, so this topic still represents an active branch of research that requires further efforts. When a drop of blood is deposited on a non-porous substrate in an unsaturated medium, it spreads and at the same time evaporates. The spreading is described by two regimes, a first regime controlled by the viscous-capillary balance and a second one controlled by the viscous-evaporation balance. The drop spreads until it reaches equilibrium, subsequently it keeps evaporating. The drying process is described by five distinct stages, and the final pattern is divided into three regions: the periphery, the corona, and the central region. The corona region results from the existence of a capillary flow inside the drop that transports the components to the edge of the drop (coffee ring effect). During the drying process, cracks form on the central region and the corona, this is explained in the literature by the competition between evaporation in the gel phase and adhesion to the substrate. These mechanisms (from spreading to final pattern formation) are influenced by the blood composition, the drop size, the substrate nature (wettability and contact angle), the relative humidity, and the temperature. In this chapter, we highlight the significant advances in the drying of human whole blood drops and the influence of these parameters.