Presentation of Adopted blood splatters:)

Hi! In our most recent bio lesson, different groups came up to present what they thought happened in other murders (by other groups) based on the blood splatters.  We discussed and came up with a few things on how to tell what happened, mainly these:

Velocity
The higher the velocity, the smaller the width of the blood splatter.  For example, if the blood splatter was very long and narrow, it was likely to have been made with very fast strokes.  If the stroke had been slow, the splatter would most probably have been very big and splotchy.  Another way to identify the velocity is also by seeing whether the end of the blood splatter ended in increasingly separated dots of blood or continued as one solid line.  If the velocity was low, the latter would be observed and vice versa.

Right handed/Left handed
This is debatable and may not be very reliable.  The theory was that right handers tended to draw upward strokes from the left to right whereas left handers tended to draw upward from right to left.  Hence, by observing where the blood splatter started and ended (the tail end of the slash would mean the end of the slash), a forensic scientist would be able to tell if the murderer/victim was right or left handed.  However, there were a lot of different views expressed as different people, right or left handed, preferred different ways of "slashing".  For instance, as a right hander, I usually draw upward left to right strokes or downward left to right strokes, but some of my other right hander friends preferred downward right to left strokes.  I guess it really depends and this method of identifying the murderer is not very reliable!

Angle
As done in Experiment 2 during the blood splatter practical, we made use of the knowledge gathered to interpret the blood splatters.  Most of them were correct.  If a blood splatter is very symmetrical and circular, it was probably made at a 90 degree angle.  If it is oval shaped, it was probably made at a steeper angle; the greater the length of the oval, the steeper the angle.  This angle of blood splatter can be very essential in recreating the scene of the murder, by using the point and area of convergence method with strings from one end of the room to another.  Something about surface tension was mentioned as the reason for the 90 degree angle ones to be so circular, and this is what I found out:

Liquid particles exert electromagnetic force of attraction on each other.  These forces are called cohesive forces, which affect the properties of liquid extensively.  On the surface of liquids, cohesive forces become stronger than particles in the inner layers of the liquid because there are no neighboring atoms above.  Therefore, it exhibits stronger attractive forces upon their nearest neighbors on the surface.  This is called surface tension which is the tendency of the surface of a liquid to contract to possess smallest area possible.  So how does surface tension play a role in the blood spatter analysis?  First, gravitational force acting on blood must exceed its surface tension before a drop of blood can fall.  Otherwise, there will be no blood drop since they will behave like slime or sludge.  Second, no matter how high a drop of blood falls, surface tension minimizes spattering before impacting a smooth, hard surface such as glass.  In other words, a drop of blood will scatter away into many small spatters without surface tension.  


So essentially what it's saying is that the reason why the blood is almost perfectly round is because the blood does not splatter as after it makes contact with the surface, the gravitational force present is no longer larger than the amount of surface tension!:)