 Welcome to this activity, Spatter Drop Dynamics on Impact. Four distinct phases are involved in the reaction of a moving blood droplet on impact with the surface. Phase one, contact collapse. Phase two, displacement. Phase three, dispersion. Phase four, retraction. Contact and collapse is the first phase where upon contact with the surface, the droplet begins to collapse from the bottom upward. The collapse occurs at the portion of the droplet currently contacting the surface while the other portions remain intact. As this collapse progresses, the displaced blood is pushed outward. It then forms an outward and advancing rim around the collapsing droplet. A blood droplet at 90 degrees results in the blood flowing into the rim equally on all sides. The resulting stain is circular. At acute angles, the rim forms on the side of the droplet opposite the angle of impact and is pushed in that direction. This directional outflow explains the elliptical shape of the stain. Displacement is the second phase. In this phase, the droplet has finished collapsing. Most of the blood that has not yet been pushed into the rim is carried there by the remaining outward inertia. Slight dimples and spines begin to form on the edge of the pool. These are determined by the irregularities on the impact surface because the surface tension has not been broken. The droplet still forms a single mass despite having undergone considerable changes in shape. A 90-degree impact results in development of a symmetrical rim. In the displacement phase, droplets appear symmetrically surrounding the rim. Acute angle impacts lead to protuberances only in the forward edge of the developing rim. A single developing droplet, as found in 10 to 20-degree impacts, results in wave cast-off stains. The third phase, dispersion, shows the most elegant behavior of the droplet during impact. Blood is forced into the boundary rim and rises upward. As the volume of blood in the rim becomes stable, inertia begins to separate the structures, which results in some satellite spatter. The result is a crown or blossom effect at impacts of 80 or 90 degrees. The rim will rise upward around the entire stain. The displacement motion of the droplet is lateral, and the dispersion is upward and outward. Acute angles in the impact stains blossom, built more like a wave on one side of the rim. The wave creates one or more spine-like structures. As the wave builds, surface tension pulls the liquid together. A droplet will form at the end of the wave. At the acute angles, a wave cast-off occurs in the side of the rim that is farthest from the angle of impact. Retraction is the fourth and final phase in the development of the stain. The surface tension will overcome the blood's momentum, and the tension pulls the droplet back into a single form. The spines between the satellite spatters and the parent stain narrow as the blood is drawn into one or the other. Satellite spatters that have enough momentum are able to break off completely in this phase, forming separate stains. The angle of impact has some effect on the stain. In the case of 80 to 90 degree impacts, no part of the blossom structure is responsible for the stain's length or width. In more acute angle impacts, the blood present in the spine retracts the parent stain, and the spine simply falls to the target. The spines create the tadpole tails and scalloped claws that we see in elliptical stains. You have completed this activity, spatter drop dynamics on impact.