The size, quantity and distribution of blood spatters depend on:
The quantity of blood subjected to impact,
The force of the impact and
The texture of the surface impacted by the blood
The identification and interpretation of spattered blood are significant for the following reasons:
Spattered blood may allow the determination of an area or location of origin of the blood source.
If found on a suspect's clothing, spattered blood may place that person at the scene of the altercation.
Spattered blood may allow the determination of the mechanism by which the pattern was created
Discussion :
The shape and size of the
depends on
• how much water you pour,
• how high you hold the stopper
and
• whether you're spilling it on
carpet, wood, linoleum or
some other surface.
A lot of water makes a larger
puddle.
If the water falls from a
distance, the puddle will be
smaller in diameter.
A hard surface will retain
more of a circular shape,
while carpet absorbs some of
the water and makes the
edges spread.
Surface Tension:
Blood, whether a single drop or large volume, is held together by strong cohesive molecular forces that produce surface tension within each drop and on the external surface. Surface tension is defined as the force that pulls the surface molecules of a liquid towards its interior, decreasing the surface area and causing the liquid to resist penetration.
The surface tension of blood is slightly less than that of water. To create splatters of blood, an external force must overcome the surface tension of the blood. In order to create smaller blood droplets or spatter from a volume of blood, this surface tension must be disturbed in some manner. Although a single drop of blood falling through air is affected by the forces of gravity and air resistance, these forces do not overcome the surface tension of the blood. No matter how far a drop of blood falls, it will not break into smaller droplets or spatters unless something disrupts the surface tension.
One factor in breaking the surface tension of blood is the physical nature of the target surface the drop strikes. Generally, a hard, smooth, nonporous surface such as clean glass or smooth tile, will create little if any spatter in contrast to a surface with a rough texture such as wood or concrete that can create a significant amount of spatter. Rough surfaces have protuberances that rupture the surface tension of the blood drop and produce spatter and irregularly shaped parent strains with spiny or serrated edges.
Viscosity:
Viscosity is defined as resistance to change of form or flow. The more viscous a fluid, the more slowly it flows. Blood is approximately six times more viscous than water and a density slightly higher than water. The SI unit for viscosity is the Pascal second and water has a viscosity of one.
Blood Droplets
The application of a force to a mass of blood causes the mass to break up into droplets. As a blood droplet travels through the air it retains a spherical shape due to surface tension. Smaller drops (1mm diameter and less) are almost perfect spheres while larger drops oscillate due a range of other forces acting on the droplet. [Smaller droplets do oscillate but the time required to dampen the oscillations is far less than larger droplets.] Droplets do not “break up” whilst in motion; another force would need to be applied to cause the droplets to further divide. The oscillations generally have no effect on the resulting spatter pattern except for instances where there are only a few stains and they are present on surfaces less than 100cm from the source.
Impact
When a droplet of blood strikes a horizontal surface at 90o it produces a circular stain. If the surface texture is smooth, such as glass or a polished tile, the surface tension will hold the droplet in the circular pattern. Essentially the surface influences the outflow. Surface tension ensures that the droplet collapses uniformly however the smooth surface means that the rim outflow is uniform. If the droplet falls onto a rough surface such as cardboard, carpet or concrete it will produce an irregular and distorted stain pattern. The rough surfaces results in an irregular rim outflow.
Phases of impact
There are 4 distinct phases of impact:
1) Contact /collapse
The droplet contacts the target surface and collapses from the bottom up. The part of the drop that has not yet collided with the surface remains as part of the sphere. As the collapse occurs, the blood that has come in contact with the surface is forced outward creating a rim. The rim gets bigger as more of the droplet comes in contact with the surface and more blood is forced into the rim.
The angle of impact affects the collapse as it defines the nature of the rim and the blood flow into it. For example: if the droplet impacts at 90o the blood flow into the rim is equal on all sides. If the impact angle is more acute, the blood flows into the area of the rim opposite the direction from which the droplet came.
2) Displacement
In this stage, the blood droplet has collapsed against the target surface and nearly all of the blood has moved from the centre of the droplet to the rim. The
actual area of displacement will be the same size as the eventual stain.
At the edge of the rim will be dimples or short spines. In this stage the movement of the blood is lateral or to the sides.
The surface texture is important. Surface tension is responsible for keeping the shape of the droplet as it moves through the air. When the droplet hits the
target surface, the ‘skin’ of the droplet, created by surface tension shifts its shape. The droplet doesn’t actually burst.
If the surface is rough, the blood flows irregularly into the rim so the spines or dimples that form will also be irregular in shape. This will result in a distorted or asymmetrical shape.
3) Dispersion
In this phase, most of the blood is forced into the rim. The spines and dimples continue to rise upward and in a direction opposite to the original momentum. As the amount of blood in the rim and spines increases they become unstable.
4) Retraction
The last phase results from the effect of surface tension attempting to pull the droplet back. If the forces trying to pull the droplet apart are overcome by
surface tension, the resulting stain will be reasonably circular and symmetrical in shape. If the forces pulling the droplet apart overcome the surface tension, the droplet will ‘burst’ and create an irregular stain pattern.
An excellent animation showing the impact behaviour of a blood droplet (November 2006).
Height
The higher the droplet falls from the ‘more’ blood satellite spatter occurs. Blood spatter is a broad term essentially meaning blood distributed through the air in the form of droplets. Satellite spatter, or spatter on the receiving surface may or may not be formed. If two similar sized droplets fall from different heights the resulting stains have different sizes. E.g. a droplet falling from 10cm will produce a different stain than a droplet falling from 100cm. The stain diameter from the 100cm height will be larger than the pattern from the 10cm height. The reason is that the velocity of the droplet will be greater the longer the droplet is airborne [until it reaches terminal velocity.] Above a fall distance of 2.2m there is little change in the diameter of the blood spot.
Force, Velocity and Droplet Size
The size and appearance of the bloodstains depends on the force that was used to create them. When an object comes into contact with blood, the force of the object moves the blood. The blood must respond to this energy transfer in some fashion. The response is often by the distribution of blood through the air in the form of droplets.
Velocity is measured in meters per second. At a crime scene there may be evidence of low, medium or high velocity blood spatter or a combination of these. For example, dripping blood (low velocity) has a velocity of 1.5 metres per second. Blood droplets produced from a bullet shot from a gun will have much greater energy and will travel faster.
Blood stain analysis-Velocity of blood splatter
Low velocity Blood Splatter:
-usually the result of dripping blood.
-The force of impact is five feet per second or less, and the size of the droplets is
somewhere between four and eight millimetres (0.16 to 0.31 inches).
• This type of blood spatter often occurs after a victim initially sustains an injury, not during the infliction of the injury itself.
• For example, if the victim is stabbed and then walks around bleeding, the resulting drops are a type of low-velocity spatters known as passive
spatters.
Medium velocity Blood Splatter:
• A medium-velocity spatter is one that had a force of anywhere from 5 to 100 feet
per second, and its diameter is usually no more than four millimetres.
• This type of spatter can be caused by a blunt object, such as a bat or an intense beating with a fist. It can also result from a stabbing.
• Unlike with a low-density spatter, when a victim is beaten or stabbed, arteries can be damaged If they're close to the skin, the victim bleeds faster and blood can spurt from wounds as his or her heart continues to pump. This results in a larger amount of blood and a very distinctive pattern. Analysts call this phenomenon projected blood.
High velocity Blood Splatter:
High-velocity spatters are usually caused by gunshot wounds, although they can be caused by other weapons if the assailant exerts an extreme amount of force.
• They travel more than 100feet per second and usually look like a fine spray of tiny
droplets, less than one millimetre in diameter.
• Bullet wounds are unique because they can have both back and front spatters, or
just back spatters.
• This depends on whether the bullet stopped after entering the victim's body or traveled through it. In most cases, the back spatter is much smaller than the front spatter because the spatter travels in the direction of the bullet.
Bloodstain pattern analysis can support other evidence and lead
investigators to seek additional clues.
After close analysis, blood spatters can indicate important
information such as:
• Type and velocity of
weapon
• Number of blows
• Handedness of assailant (assailants tend to strike with their dominant hand on the
opposite side of the victim's body)
• Position and movements of the victim and assailant during and after the attack
• Which wounds were inflicted first
• Type of injuries
• How long ago the crime was committed
• Whether death was immediate or delayed
• Blood spatters can lead to the recreation of a crime because of how blood behaves. Blood leaves the body as a liquid that follows the laws of motion and gravity. It travels in spherical drops due to surface tension. Blood molecules are very cohesive, or attracted to each other, so they squeeze against each other until they form a shape with the smallest area possible. These drops behave in predictable ways when they strike a surface or a force acts upon them. In the actual spatter analysis, a forensic investigator determines the trajectory of the blood (where the blood came from and how it spread over the surface). By measuring the shape of the bloodstain on a surface, the direction of movement can be determined. As well, the speed at which the blood contacted the surface can be approximated. This can help distinguish, for example, between the rapid movement of blood that can be produced by a gunshot or the severing of an artery in opposition to the slower movement of blood from
a minor cut.
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