Notes Blood Spatter
Bloodstains are a source of important forensic evidence regarding:
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Origin(s) of bloodstain
Distance of bloodstain from target
Direction from which blood impacted
Speed with which blood left its source
Position of victim & assailant
Movement of victim & assailant
Number of blows/shots
Characteristics of liquid blood(Slide 2)
1. Specific gravity,
2. Viscosity ,
3. Surface Tension (slide 3): is a characteristic of liquids whereby they are resistant to penetration or
separation. The surface acts to reduce surface area. A small mass of liquid which becomes
detached becomes rounded into a sphere; the shape gives the minimum service area for any
given volume; a falling blood drop is not tear-drop shaped.
For instance, blood flowing downwards towards the fingertips from a cut on the hand accumulates on the
fingertip and increases in volume until the gravitational pull (its weight) exceeds surface tension (slide 4).
Such droplets will have a fairly uniform of volume of 50 ml (0.05 ml). More rapid bleeding may result in
slightly larger drops but slower bleeding does not result in smaller drops. Blood cast from a moving
source will tend to consist of smaller droplets (slide 5)
Blood behaves as a projectile in motion and obeys the laws of physics and mathematics.
Terminal velocity of falling droplets(slide 6,7,8)
Terminal velocity of the falling blood drop depends on its size.
Smaller drops have a lower terminal velocity and acquire it after a shorter fall distance.
Larger droplets accelerate over a greater distance and therefore attain a greater terminal velocity.
Size of the bloodstain (slide 9)
The size of the blood stain depends more on the nature of the target surface than on the distance fallen.
The size of the resulting blood spot will vary only slightly with increasing fall distance and above a fall
distance of seven feet there is little change in the diameter of the blood spot (slide 10)
The size of the falling blood drop will obviously affect the size of the resulting blood stain.
Shape of the blood spot
The shape of the blood spot depends mostly on the texture of the target surface (slide 11). Blood spots
landing on smooth glass will be consistent and uniform in shape (circular) where mass blood spots
landing on textured surfaces such as paper and wood will be of variable shape and will have irregular
edge characteristics.
Target surface characteristics and splatter (slide 12, 13)
Experiments with falling droplets are easily performed. Close up pictures of bloodstains impacting various
surfaces after falling from various heights can be accessed by clicking on the right arrow at top right of
slide 13. Click the left arrow to return to slide 14 in the main presentation.
Drops striking hard, non-porous surfaces result in less peripheral spatter as the surface film of the
spreading droplet remains intact. Drops striking irregular porous surfaces result in a greater degree of
spatter since the spreading drop is disturbed by surface irregularities. This situation is analogous to a
water-filled rubber balloon which will bounce off a smooth surface but will rupture on an irregular surface.
Estimation of distance fallen based on measurements of spot diameter will be erroneous if the target
surface texture is not considered.
Angled impact: vertical drop onto tilted target (slide 14)
Blood striking a smooth surface at 90 degrees results in an approximately circular blood stain.
As the angle of impact becomes a more oblique the blood stain becomes more elongated and its length
to width ratio increases.
Impact Angle = arc sin (width of stain/ length ofstain)
Accurate measurement of the blood stain is required
Effect of horizontal motion (slide 15)
A blood spot striking a horizontal surface with a horizontal component to its motion will have a
characteristic pattern due to a small drop being cast off from the far edge of the main droplet. This
appearance is quite different to a drop falling vertically onto an angled or tilted surface. With experience it
is possible to tell whether blood fell vertically on to tilted surface obliquely from a person walking or
running
Point of convergence (slide 16, 17)
Drawing lines along long axis of several stains will show a point of convergence at their origin.
Point of origin (slide 18)
Lies at a point in space above the point of convergence. Measurement of impact angle allows translation
of 2-D image (convergence) into 3-D (origin) (slide 19). In practice (at a crime scene), strings, tapes and
protractors are commonly used. Computer programs are also used.
Blood spatter patterns are classified according to the velocity with which the blood is struck
(slide
20)
Experiments performed by Herb MacDonell (slide 21)
Diagrams reproduced from "Bloodstain Pattern Interpretation" with kind permission of Prof MacDonell
Projected and cast-off blood spatter
Blood projected at velocity exceeding that of gravity often results in a spatter pattern wave irregular spiny
edges around the central bloodstain. Running or stepping into a pool of blood is typical of this pattern.
Blood may also be cast off a bloodstained weapon. Such staining it is often apparent on the walls and
ceiling in indoor situations. The direction and origin of the backswing is often clearly discernible. The
number and sequence of swings may also be discernible.
Low velocity blood spatter (slide 22-28)
Blood source subjected to a low velocity impact (less than 1.5 m/s or 5 ft per second) e.g. free-falling
blood under the influence of gravity,
blood cast off from fist shoe, weapon (slide 29-39), dripping, splashing
Blood spots mostly 4-8 mm in diameter
Dripped blood (slide 37-40)
When blood trips into itself, many small droplets are projected upwards into the air and impact around the
central irregular blood pool at in the right angles and obliquely, resulting in surrounding circular and oval
blood at splatter patterns.
Splashed blood (slide 41)
Blood volumes exceeding 1 ml, travelling at a velocity of less than 5 ft per second ( m/s) or subjected to a
minor impact, squirted (slide 42-44), thrown against a vertical surface (slide 45), or stamped in (slide 46-49)
Results in an irregular central splashing pattern, surrounded by directional elongated blood spots.
Arterial spurting (slide 50-53)
Medium velocity blood spatter (slide 54-57)
Blood source subjected to a medium velocity impact (about 7.5-30 m/s pr 25-100 ft/s)
e.g. blows with a baseball bat, hammer, axe or similar instrument
Blood spots typically about 4 mm in diameter
High velocity blood spatter (slide 58-59)
Blood source subjected to a high velocity impact (>7.5 m/s or 100 ft/s) e.g. gunshot or a high velocity
machinery
Blood spots typically less than 1 mm in diameter (a fine mist)
Back spatter results from blood being blown back from entry wound towards the firer and weapon
(slide 60-
61)
Forward spatter is usually more florid, arising from the exit wound and passes in the same direction as the
projectile (slide 62-66)
Wipe patterns (slide 67)
Object moves through a wet bloodstain. Feathered edge suggests direction
Transfer patterns (slide 68)
Wet, bloodied object contacts a secondary surface
Transfer from hand, fingers, shoes, weapon, hair (slide 69-70).
Transfer to walls, ceilings, clothing, bedding.
Produces mirror-image of bloodied object
Flow pattern (slide 71-78)
Blood flows horizontally & vertically, altered by contours, obstacles. Often ends in a pool