Influence of relative humidity on spreading, pattern formation and adhesion of a drying drop of whole blood

The drying of a drop deposited on a solid substrate has been the subject of several investigations including biomedical and forensic fields. These studies also focus on the complex final drop pattern observed at the end of the evaporation process. Our experimental work aims to investigate the effect of relative humidity (RH) on the spreading behavior and on the pattern formation of a dried drop of whole blood at the end of the evaporation process. A range of RH between 13.5% and 78.0% is studied. Drops of blood of same volume are gently deposited on ultraclean microscope glass substrates. A top-view camera allows for the monitoring of the drying regime (deposition, gelation and fracturation). We show that RH influences the contact angle, and the final wetting diameter and consequently, the final deposition pattern at the end of the evaporation process. A good agreement has been observed between our results for whole blood and the experimental work of Chhasatia et al. [Appl. Phys. Lett. 97, 231909 (2010)] performed an aqueous drop of 1.1 μm colloids. Our experimental measurements are in a good agreement with the purely diffusive model where the wetting diameter and the contact angle are function of RH. Our experimental results also show that the transition between the purely convective evaporation phase and the gelation phase occurs always at 65% of the total drying time whatever is the RH levels.