Influence of the impact energy on the pattern of blood drip stains

The maximum spreading diameter of complex fluid droplets has been extensively studied and explained by numerous physical models. This research focuses therefore on a different aspect, the bulging outer rim observed after evaporation on the final dried pattern of blood droplets. A correlation is found between the inner diameter, the maximum outer diameter, and the impact speed. This shows how the drying mechanism of a blood drip stain is influenced by the impact energy, which induces a larger spreading diameter and thus a different redistribution of red blood cells inside the droplet. An empirical relation is established between the final dried pattern of a passive bloodstain and its impact speed, yielding a possible forensic application. Indeed, being able to relate accurately the energy of the drop with its final pattern would give a clue to investigators, as currently no such simple and accurate tool exists.

Figure 1

Blood droplet impact recorded with a high-speed camera $(U=1.85\mathrm{m}{\text{s}}^{-1}$ and $D_0=3.34\mathrm{mm})$. (a) Blood droplet before its impact showing its initial diameter $D_0$. (b) Impact and spreading of the droplet. (c) Spreading reaches its maximum spreading diameter $D_{\text{max}}$. (d) The capillary and viscous forces act on the droplet, but the maximum diameter is unchanged due to self-pinning, and now evaporation starts.

Figure 2

Rescaled maximum spreading ratio as a function of $\text{We}{\text{Re}}^{-4/5}$ according to the solution proposed by Clanet et al. [13] for maximum spreading diameters of droplets on solid substrates.

Figure 3

Rescaled maximum spreading ratio as a function of Weber number according to universal solution proposed by Lee et al. [7] for maximum spreading diameters of droplets on solid substrates.

Figure 4

Rescaled maximum spreading ratio as a function of Weber number according to the solution presented by Wildeman et al. [15]. The dashed line shows the tendency that our experimental data set follows, which is similar to the no-slip model in the inelastic regime.

Figure 5

(a) View of dried blood drip stains with bulging outer rim, from a healthy volunteer. Droplets are of initial diameters $D_0=2.4\mathrm{mm}$ and impact velocities, from left to right, of 0.74, 1.87, and 4.$75\mathrm{m}{\text{s}}^{-1}$. On the left side with photography and on the right side with confocal microscopy that gives the 3D features by means of radial cross sections. (b) Cross-section profiles of three droplets of the same volume with impact velocities of 0.74 (green), 1.87 (blue), and 4.$75\mathrm{m}{\text{s}}^{-1}$ (red).

Figure 6

(a) Motion of RBCs under standard ambient evaporation conditions inside a drip stain that impacted at low velocity. Differential evaporation rates at the edge and in the middle drive the particles to the edges, leading to coffee-stain formation. (b) Motion of RBCs undergoing the influence of Marangoni flows for drip stains that impacted at higher velocities. As previously, RBCs tend to move toward the edges, but a higher surface tension on top engenders a Marangoni flow from the contact line toward the top of the droplet. This induces a recirculating flow pattern as represented above. The recirculating flow homogenizes the particle concentration, overcoming coffee-stain formation and leading to a more uniform deposition.

Figure 7

(a) Maximum spreading ratio as a function of $U$ (in red) and its fitted function, and internal spreading diameter as a function of $U$ (in blue) and its fitted function. (b) Calculated impact velocity using Eq. (4) versus measured impact velocity.

Figure 8

(a) Rescaled maximum spreading ratio as a function of Re (in red) and its fitted function, and rescaled internal spreading diameter as a function of Re (in blue) and its fitted function. (b) Calculated impact velocity using Eq. (9) versus measured impact velocity.