Human blood drop impact and retraction driven by ambient humidity

Although the receding (retraction) of a drop after its impact on a solid substrate is an important phenomenon, it has received less attention than spreading. We investigated the receding dynamics for a drop of whole human blood with different impact velocities for both varnished and unvarnished wooden floors. The effect of the room's relative humidity and temperature were considered by performing experiments under different surroundings conditions. The obtained results show that the receding phenomena occurred on the varnished wooden floor (smooth substrate) and were absent on the unvarnished wooden floor (rough substrate). For the smooth substrate, the retraction occurred only at low relative humidity. This is explained by the decrease in the surface tension with relative humidity level. In this presentation, the obtained results on the blood drop receding after impact are provided by discussing its dynamics and the influence of relative humidity.