FSB05
blo od spatte r
Properties of blood
Teacher Background Information
Blood is considered to be a fluid. A fluid is a substance
with no fixed shape and is subject to external
pressure. A fluid can be either a liquid or a gas. A
liquid is a fluid that has a fixed volume while a gas is a
fluid that can expand indefinitely.
Viscosity
Viscosity is defined as a fluid’s resistance to flow. The
more viscous a substance is, the more slowly it will
flow. The SI unit for viscosity is the Pascal second. Fluid
viscosity is compared to water that has a viscosity of
one. Blood is thicker than water and is viscous primarily
due to the cellular component (see FSB04). The
viscosity of some common substances, including blood:
Liquid
Viscosity (mP·s-1)
Milk (25oC)
3
Blood (37 oC)
3-4
Glycerin (20 oC)
1420
Mercury (15 oC)
1.55
Water (20 oC)
1.0
Water (100 oC)
0.28
http://hypertextbook.com/physics/matter/viscosity/
Surface tension
Surface tension is the force that pulls the surface
molecules towards the interior of a liquid, decreasing
the surface area and causing the liquid to resist
penetration or separation.
Surface tension is the tendency of the surface of a
liquid to contract to the smallest area possible. The
fluid is able to do this as the cohesive forces are
stronger on the surface of liquids as there are no
neighbouring molecules above. As a result there are
stronger attractive forces between molecules and their
nearest neighbours on the surface; the surface tension
force actually exerts an upward force. Surface tension is
like having an elastic film over the surface.
Figure 1: A water strider standing on water.
Citation: Water strider: David Cappaert, www.insectimages.org
In Figure 1, the surface tension of the water allows the
water strider to walk on the water without sinking.
This is because the upward force from surface tension
balances the insect’s weight.
Definition of surface tension: the surface tension γ is
the magnitude F of the force exerted parallel to the
surface of a liquid divided by the length L of the line
over which the force acts:
γ =
_F
L
Surface tension is measured in force per unit length:
newtons per metre: (N·m-1). The old unit is dynes per cm.
The surface tension of some common liquids:
Liquid
Surface tension N·m-1
Benzene (20oC)
0.029
Blood (37 oC)
0.058
Glycerin (20 oC)
0.063
Mercury (20 oC)
0.47
Water (20 oC)
0.073
Water (100 oC)
0.059
http://www3.interscience.wiley.com:8100/legacy/college/
cutnell/0471713988/ste/ste.pdf
FSB05
blo od spatte r
Properties of blood
Surface tension is important in bloodstain pattern
analysis as;
•
the gravitational force must overcome
the surface tension of blood before
a drop of blood can fall, and
•
drops of blood remain intact as they move
through the air due to surface tension.
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.
Figure 2: Complimentary effects of adhesion,
cohesion and surface tension on a single blood
droplet.
Image courtesy UWA PhD research student Mark Reynolds.
Density
Density is defined as mass per unit volume. The
density of water is 1000 kg/m3. The density of blood
is proportional to the total protein concentration or
cellular component of blood and is influenced only
to a minor extent by other ions, gases etc. that are
dissolved in the plasma.
The density of blood plasma is approximately 1025
kg/m3 and the density of blood cells circulating in
the blood is approximately 1125 kg/m3. The average
density of whole blood for a human is about 1060
kg/m3.
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
Figure 3: Several blood droplets that have fallen onto
a rough surface.
Image courtesy DUIT Multimedia: Paul Ricketts.
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.
FSB05
blo od spatte r
Properties of blood
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.
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.
Figure 4: Flight of a single blood droplet.
Image used with permission from Tom Bevel & Ross Gardner, June 2006.
Figure 5: A diagram showing blood being pushed
Figure 6: Displacement phase of a blood droplet in a
into a rim on contact with a receiving surface.
90o impact.
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
Image used with permission from Tom Bevel & Ross Gardner,
June 2006.
FSB05
blo od spatte r
Properties of blood
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).
http://www.nfstc.org/links/animations/images/
blood%20spatters.swf
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
Figure 7: Early dispersion phase of a blood droplet
impacting at 90o.
Image used with permission from Tom Bevel & Ross Gardner,
June 2006.
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.
FSB05
blo od spatte r
Properties of blood
Low velocity blood spatter
A low velocity force is usually the result of blood
dripping from a person who is still, walking or
running. Blood drops may be free falling and only
moving due to the force of gravity. At low velocities
larger bloodstains are produced. Sometimes low
velocity bloodstains are a result of weapon cast-off of
from blood dripping from a victim.
Dripping blood often falls at a 900 angle and forms
a round bloodstain that is often 4mm in diameter or
larger: up to approximately 10mm. If droplets are,
however, falling from a moving object or person
(walking or running) they fall to the ground at an
angle (see angle of impact) and the direction of the
movement can be established.
Identifying Blood Trail Motion
Figure 9: Passive bloodstains falling onto a smooth
surface at approximately 90°
Image courtesy UWA PhD research student Mark Reynolds.
Medium velocity blood spatter
A medium velocity force moves blood between
five and 50 metres per second and the resulting
bloodstains at 90o are between one and three
millimetres in size. The size of the bloodstain depends
on the angle of impact with the receiving surface. An
oblique stain can be greater than 10mm but would be
long and thin. Medium velocity blood spatter might
result from blunt force trauma, for example, beating
with fists, baseball bats, whips, bricks or hammers.
Medium velocity blood spatter can also occur when a
body collides with rounded or edged surfaces.
Droplets dripping from a moving object or person do not
drop straight down. As they are in motion themselves, they
fall to the ground at an angle.
Blood-trail motion is defined by considering the
directionality of the individual droplets present in the blood
trail pattern.
Figure 8: A blood-trail pattern.
Image used with permission from Tom Bevel & Ross Gardner, June 2006.
Figure 10: Spatter deposited on a wall as a result of a
‘blunt force’ beating.
Image courtesy UWA PhD research student Mark Reynolds.
FSB05
blo od spatte r
Properties of blood
High velocity blood spatter
A high velocity force moves blood greater than 50
metres per second and the bloodstains are usually
smaller than 1mm and appear as fine spray or misting.
High velocity blood spatter can be caused by highspeed machinery such as chain saws and wood
chippers.
Figure 12: Spines, scallops and satellite spatter help
to identify the path of the blood droplet.
Image used with permission from Tom Bevel & Ross Gardner, June 2006
Figure 11: Spatter deposited on a wall as a result of a
gunshot.
Image courtesy Stuart James, February 2007.
Direction
Crime scene investigators can determine the direction
that a blood droplet was travelling in as droplets
impact surfaces in a consistent manner. The droplet
will keep moving along the same path that it was
travelling before hitting the surface. When it impacts
a surface, the blood in the droplet moves outwards
during the collapse phase creating either an elliptical
or circular stain.
A crime scene investigator will look at other features
of the bloodstain to determine which direction the
blood droplet was travelling in. Bloodstains also
usually have features such as satellite stains, scallops
or spines. The stain will have a higher number of these
features on one side. This is due to the way the droplet
collapses on impact. As discussed previously, blood
flows into the area of the rim opposite the direction
from which the droplet came. In many instances the
dimples on the rim break slightly from the droplet
structure creating spines, scallops or if it breaks
entirely away, satellite stains.
The long axis of the stain (major axis) provides an
indication of the direction the droplet was travelling in
prior to contact with the receiving surface and hence
the direction that it came from. The droplet always
travels in the long axis, but it is sometimes difficult to
tell the actual direction as shown in Figure 12.
Figure 13: Scallops, spines and satellite stains are
always in the direction of travel.
Image used with permission from Tom Bevel & Ross Gardner, June 2006
The pointed end of the bloodstain always points in
the direction of travel.
FSB05
blo od spatte r
Properties of blood
Angle of Impact
There is a relationship between the length and
width of a bloodstain and the angle at which the
droplet impacts on a surface. It is therefore possible
to calculate the angle of impact on a flat surface by
measuring the length and width of a stain.
The angle of impact is the acute angle that is
formed between the direction of the blood drop
and the surface it strikes. This is an important
measure because it is used to determine the area of
convergence and the area of origin.
Figure 15: The measurement of the length and width
of stains.
Image used with permission from Tom Bevel & Ross Gardner, June 2006
Calculating the angle of impact
The angle of impact formula relies on the relationships
that exist between the angles of a right triangle
and the length of its sides. These are trigonometric
functions called sine, cosine and tangent.
Figure 14: The angle of impact of a blood droplet on
a receiving surface.
Imagine a right triangle formed between the droplet
and the target surface as the droplet strikes. A blood
droplet in flight is the same shape as a sphere.
Therefore, the width of the stain is equal to the length.
By measuring the length and width of the stain,
the droplet’s impact angle, i can be calculated. NB:
convention is to refer to the impact angle as the alpha
angle.
Image used with permission from Tom Bevel & Ross Gardner, June 2006
When a droplet of blood impacts a surface at 90o,
the bloodstain will be circular. The more the angle of
impact decreases, the more the stain is an ellipse. The
angle of impact can be measured by the degree to
which the shape of the drop changes from a circle to
an ellipse.
An excellent animation showing the angle of impact
(November 2006).
http://www.nfstc.org/links/animations/images/
blood%20spatters.swf
When measuring the length and width of a stain, no
part of the spines, tails or satellite spatter are included
in the measure. Round the stain to an elliptical shape
when making measurements.
Figure 16: The relationship of the droplet to an
imagined right angle.
Image used with permission from Tom Bevel & Ross Gardner, June 2006
FSB05
blo od spatte r
Properties of blood
The diagram below (Figure 17) represents a stain that
has impacted on a surface.
An example
Width = 3mm
Length = 5mm
Sine i = width / length
Sine i = 3mm / 5mm = 0.6
Figure 17: The width and length of a bloodstain can
be used to calculate the angle of impact.
Image used with permission from Tom Bevel & Ross Gardner, June 2006
As a result, we have two known quantities from the
crime scene, the width and length of a bloodstain,
which can be applied to the following formula:
The sine of the angle of impact = width divided by
the length.
Sine i = Width (ab) / Length (bc)
The result of the division is a ratio.
Look for the ratio on a trigonometric function table
– the closest angle will be identified, OR
Angle = 37o
The “inverse sine” or the arc sine function on a
scientific calculator (ASN) converts the ratio to an
angle.
Inverse Sine (ASIN) i = Angle of Impact
The steps are:
•
Accurately measure the width and length
of a given bloodstain. This should be
measured to the nearest millimetre.
•
Divide the width of the stain by the length of the
stain in order to obtain the width to length ratio.
•
Calculate the inverse sine of this ratio.
•
This value is the angle of impact.
Using the calculator
Inverse Sine i (0.6) = 36.8
The angle of impact is between 36-37o
It is important to note that this method gives an
estimate of the impact angle rather than a precise
result. The accepted variance is between 5-7o.
Computer fitting of theoretical ellipses has refined
the measurement process to sub-degree levels of
accuracy.
FSB05
blo od spatte r
Properties of blood
Area of Convergence
Consider a simplified crime scene where there are
two elliptical bloodstains on a floor, forty centimetres
apart. Lines are drawn from the centre of the long axis
of each bloodstain and extended until the two lines
from the separate stains meet. The point where the
lines meet is called the Area of Convergence. (NB _
this is also called the POINT of convergence – for our
purposes the term will be the AREA of convergence
as accuracy is not sufficient to determine the actual
POINT).
Figure 19: Measuring the distance from the
bloodstain to the area of convergence.
Before drawing lines it is important to determine the
directionality of the bloodstain. The lines must be
drawn away from the direction of travel towards the
origin.
Always work via the centre of the long axis and extend
the line from the back of the bloodstain.
In addition to two stains having a coincidental intersecting
point, it is also possible to have several patterns overlap.
If this condition is not considered it might well result in a
mistaken point of convergence.
Figure 18: The area of convergence.
Image used with permission from Tom Bevel & Ross Gardner, June 2006
Figure 20: Establishing the direction of travel.
This area of convergence is possibly the source of
both bloodstains, but the path crossover may also be
completely coincidental if the two stains were created
by unrelated events.
In the figure below there are 3 stains with different
angles of impact. When lines are drawn from the
stains, (the centre of the long axis of the stain) the
lines converge at an area (of convergence).
Area of Origin
At a crime scene with several bloodstains, crime
scene investigators attempt to determine the origin
of the blood. In essence the investigator is trying to
determine from which location in a 3-dimensional
space the blood originated, from 2- dimensional
measurements. Figure 21 below attempts to show the
point in space where the paths converge.
FSB05
blo od spatte r
Properties of blood
Defining the Area of Origin by Graphing
A graph is prepared that has the following features.
. The X-axis represents the target plane and
graphs the distance from the back-edge
of the stain to the area of convergence.
2. The Z-axis represents the height above
the target plane – in this example
the target plane is the floor.
3. The scales of both axes, X and Z
are scaled the same (cm).
Do the following.
The base of each stain’s present position, the point in twodimensional space where the paths converge (c), and their
point of origin (o), define another right angle.
Figure 21: A representation of the area of origin
established from 2-dimensional calculations.
4. Mark on the X-axis of the graph the
convergence distance (cm) for each stain.
5. Using a protractor, draw a line from the
mark on the X-axis, at the calculated
angle of impact, to the Z-axis.
Image used with permission from Tom Bevel & Ross Gardner, June 2006
6. Repeat this procedure for each stain.
Calculation Methods
7. The area at which the lines from the X-axis
converge on the Z-axis establishes the probable
height of the area of origin. See below.
The angle of impact and length of the convergence
line can be graphed for each stain and the area of
origin (of the blood) established OR it can be done
mathematically through the relationships that exist in
a right triangle OR it can be done using a protractor
and string.
Whichever method is used for the calculation, the
initial steps for all methods are the same:
•
Identify stains that have a common
area of convergence.
•
Draw lines through the central long axis of
the stain away from the direction of travel.
•
Identify the area of convergence.
•
Measure the distance (cm) from the back edge
of the stain to the area of convergence.
•
Calculate the angle of impact of each of
the stains (measure width and length of
stains in mm and apply the formula).
•
Use a minimum of 3 stains.
Once the angle of impact has been calculated and the
distance from each stain to the area of convergence
has been measured, either of 3 methods can be used.
Figure 22: A graph showing the method for
estimating the area of convergence.
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FSB05
blo od spatte r
Properties of blood
Defining the Area of Origin with
the Tangent Function.
The same steps as above are followed to determine
the:
Defining the Area of Origin by Triangulation.
The same steps as above are followed to determine
the:
•
Area of convergence (AOC)
•
Area of convergence (AOC)
•
Distance from the stain to the AOC
•
Distance from the stain to the AOC
•
•
Angle of impact of the selected
stains – a minimum of 3 stains.
Angle of impact of the selected
stains – a minimum of 3 stains.
Apparatus
The following formula is used to determine the point
of origin.
•
Ring stand
•
Protractor
TANi = H/D
•
String
i = angle of impact
•
Masking tape
D = distance from stain to area of convergence
•
1m rule
•
pencil
H = unknown distance above target surface
Method
Line bc = H - height above the target: unknown.
Line ca = D – distance to the area of convergence:
known.
•
Place the ring stand on the area of convergence
•
Write the calculated angle of
impact next to each stain.
•
Using string, masking tape and a protractor,
raise the string to the calculated angle
and attach it to the ring stand.
•
Do the same for a minimum of 3 stains.
•
The place on the ring stand where the string
from each stain meets is the ‘area of origin’.
•
Measure the height of the area of origin.
i = angle of impact: known.
TAN i = H/D
To solve for unknown H
H = TAN i * D
b
c
i
a
For example:
Distance to the AOC = D = 30cm
Angle of impact = i = 35o
H = TAN i * D
H = 0.7002 * 30cm
= 21cm
NB: the value for the TAN of angle 35 can be found
from the Table of Trigonometric Function or by using a
scientific calculator.
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FSB05
blo od spatte r
Properties of blood
Limitations
The methods described above have limitations but are
able to give an investigator a good approximation of
the origin. This helps to identify the general location
where an application of force to a source of blood
occurred.
Crime scene investigators now tend to use computer
software applications to analyse blood stain patterns
including area of origin calculations however many
investigators prefer to use traditional methods. Their
choice of method depends on a range of factors.
The information from area of origin calculations can
be used to verify or refute various claims made about
a crime scene. For example, if all of the blood spatter
evidence points to a certain height that equates to
a area low to the ground this would not back up
a suspect’s claim that it was self-defence from a
standing position.
The scenario in this program of work requires students
to analyse stains and calculate the area of origin of
bloodstains on one target surface which is a wall.
Students will then review 3 statements: suspect,
victim and witness and determine which statement
verifies the forensic evidence.
Bloodstains in other parts of the room (floor, walls,
stove and ceiling) are not measured in the activity
but the characteristics of the spatter can be used to
further support or refute a statement.
References
http://www3.interscience.wiley.com:8100/legacy/college/cutnell/0471713988/ste/ste.pdf
http://hypertextbook.com/facts/2004/MichaelShmukler.shtml
http://hypertextbook.com/physics/matter/viscosity/
Bevel, T. & Gardner, R.M. 1997 Bloodstain pattern analysis. CRC Press Ltd, LLC.
James S.H., Kish, P.E. & Sutton, T.P. 2005 Principles of bloodstain pattern analysis : theory and practice. Boca Raton,
CRC Press LLC.
Thanks to Mark Reynolds, UWA PhD student for verification of information and supplying a number of images.
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