Physical Properties of Blood


Kinematics

 Blood Spatter evidence occurs when blood leaves a physical object, such as a knife, or container, such as a person, and lands on a surface.  As such, we can use two dimensional kinematics to plot the trajectory of any number of patterns so long as the stain is not pooled blood or too similar to one.  Seeing a small amount of blood on the wall, for example, one could identify the approximate velocity of that drop by measuring the size of it.  From there, knowing the vertical acceleration is gravity (9.8 m/s^2) and in free fall, initial velocity is zero, and our calculated approximation of final velocity, we can then determine how long it took to hit the surface using the kinematic equation:

Vfy = Viy + ay (delta t)

This equation is especially important if there is a limited amount of evidence to work with.  A kinematic equation is an equation of motion, and this one shows only vertical motion.  Since horizontal and vertical motion are not connected we can solve for one of them at a time.  There are other kinematic equations, but most would rarely be used given an analyst's physics and mathematical tool set.  This one equation can tell us so much about the victim's and the assailant's position.
Surface Tension

 A water drop hitting a surface is a very nice picture to see.  If you were to look at many pictures in quick sequence you will notice that some parts of the drop stick to together.  This is caused by the surface tension.  It's what allows insects to walk on water and why beads of water roll off of your skin.  Blood has a significantly higher surface tension than water which is what allows a blood drop to look like the picture to the right.  We can look at the surface tension of any liquid as a sort of elasticity to a liquid and, according to physics.bu.edu/, can be found with the following equation:

Surface Tension = y = F/L
Which states that "surface tension can also be defined as the force F per unit length L tending to pull the surface back", essentially the force per unit of surface area that tries to maintain tension.
Directionality

 Perhaps the most important of blood spatter analysis is understanding it's directionality.  There is a great deal of difference between a circular stain and an elliptical one-inch stain.  The shape of the stain caused by the angle at which the drop struck the surface.  A spherical drop of blood striking a surface at 90 degrees will have a circular stain, whereas if the same drop hit the surface at a 10 degree angle, it would be longer and will trail off to a point.  it is important to note that the blood drop travels in the direction of the point.  The image to the left illustrates this point.