Influence of the relative humidity on the impact of blood drops on solid substrates

Although the receding (retraction) of a drop after its impact on a solid substrate is an important phenomenon, it receives less attention than spreading. We investigate the retraction dynamics for a drop of whole human blood with different impact velocities on wooden floors that were either varnished or not. The effect of relative humidity and temperature are considered by carrying out extensive experiments under different surroundings conditions. The obtained results show that retraction is present on the varnished wooden floor (smooth substrate), in contrast to the unvarnished wooden floor (rough substrate). For the first one, the retraction occurred only at low relative humidity. This was explained by the decrease in the surface tension with relative humidity level. We will present our results on blood drops retraction after impact by discussing its dynamics and the influence of relative humidity.