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 a 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 protrude rinses 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 Bloods momentum and the tension pulls the
droplet back into a single form the spines between the satellites patters
and the parents taint narrow as the blood is drawn into one or the other
satellites patters 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 stains 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.