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Bloodstain EvidenceBloodstain EvidenceMay reveal:May reveal:
• 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
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Liquid BloodLiquid Blood• Physical properties
– viscosity
– surface tension
– specific gravity
• Behaves as a projectile in motion– biology, physics, maths
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Blood lab 1
• We measured the diameter of blood drops from different heights
• Whiteboard now:– What was the independent variable?– What was the dependent variable?– What type of graph did we use and why?
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Surface TensionSurface Tension
• Resistance to penetration & separation• Surface acts to reduce surface area
• Smallest SA to Volume ratio is offered by sphere
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Dripping BloodDripping Blood
Blood drop grows until Wt (G) > S.T.
Single drop breaks free (teardrop shape)
Surface tension pulls in vertically
Shape settles into sphere (0.05 ml)
Does not break up until impact
And horizontally
Blood trickles downwards
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Drop size
.
Standard drop size 50ul (0.05ml)
.
Rapid bleeding gives slightly larger drop
Shaking/movementcasts off smaller drops
.
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Terminal Velocity v Distance Fallen(metric)
Terminal velocity as a function of distance fallen (metric)
0
1
2
3
4
5
6
7
8
0 1 2 3 4 5 6 7
Fall distance (m)
Ter
min
al v
elo
city
(m
/s)
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Terminal Velocity v Distance Fallen(imperial)
Terminal velocity as a function of distance fallen (imperial)
0
5
10
15
20
25
30
0 2 4 6 8 10 12 14 16 18 20
Distance fallen (f)
Te
rmin
al v
elo
cit
y (f
/s)
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Free Falling Blood Droplets
.
50 ul4.6 mm
7.5 m/s
4.2 to5.4 m
0.5 ul2.12 mm
2.4 to 3 m
.4.6 m/s
0.12 ul1.32 mm
0.84 to 1 m
.3.3 m/s
0.06 ul1.1 mm
0.5 to 0.65 m
.2.2 m/s
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Shape & Size of BloodspotShape & Size of Bloodspot
• Depends mostly on nature of target surface– texture (rough or smooth)– porous or non porous
• Size is related to distance fallen, provided:– standard 50 ul drop of blood
• There is little change in spot diameter beyond a fall distance of 1.2 m
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Height FallenHeight Fallen
Single drops of blood falling from fingertip onto smooth cardboard from various heights.No change in diameter beyond 7 ft.
Adapted from Introduction to Forensic Sciences,W. Eckert, CRC, 1997
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Blood Lab 2
• We looked at how different surfaces affect the blood splatter
• Whiteboard now:– What was the independent variable?– Dependent variable?– Control?– What kind of graph did you use and why?
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Effect of Target SurfaceEffect of Target Surface
. .... .Spreads out smoothly ST of spreading edge is
broken by irregular surface
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Experiments with Falling Blood DropletsExperiments with Falling Blood Droplets
blood
dropper
ruler
Terazzo floor
whiteboard
rough paper towel
paper
Fabric (theatre green)
Height
Target Surface
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Height/Surface
Single drop of blood falling from various heights (m) onto various surfaces
smooth floor paper towel fabric
0.5
1
2
3
0.5
1
2
3
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Blood Lab 3
• We looked at direction of blood splatter
• Whiteboard now:– What was the independent variable?– Dependent variable?
– CSI video
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Blood Lab 4
• We looked at how angle has an effect on blood drops
• Whiteboard now:– Independent and dependent variable?– Name 2 characteristics you noticed of the blood
spatter relative to angle
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Angle of Angle of ImpactImpact
90
10
70
2030
60 50 40
Adapted from Introduction to Forensic Sciences,W. Eckert, CRC, 1997
80
Gravitational dense zoneat lower edge
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Wave Cast-offWave Cast-offDavid Sadler:David Sadler:
.Parent drop
wave cast-off
Tail of wave cast-off points back to parent drop
Tail of elongated stain points in direction of travel
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Point of ConvergencePoint of Convergence16
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Point of ConvergencePoint of Convergence5 ml blood squirted from a syringe from height of 1 m
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Point of OriginPoint of Origin
length
width
Angle of impact = arc sin W/L
Distance from point of convergence
Hei
ght a
bove
poi
nt o
f co
nver
genc
e
Origin
85 60 45 30
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Tracing Origin of Tracing Origin of BloodspotsBloodspots
• Point of convergence method– 2 dimensional image
• Point of origin method– adds 3rd dimension to image
• In practice:– use of string & protractor at scene
– use of computer at laboratory
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Blood SpatterBlood Spatter
• Low velocity (5 f/s, 1.5 m/s)– e.g. free-falling drops, cast off from weapon
• Medium velocity (25 - 100 f/s, 7.5 - 30 m/s)– e.g. baseball bat blows
• High velocity (>100 f/s, 30 m/s)– e.g. gunshot, machinery
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Herbert Leon MacDonell,
Laboratory of Forensic Science,P.O. Box 1111,Corning,New York,14830,USA
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Cast-off from WeaponCast-off from Weapon
• First blow causes bleeding• Subsequent blows contaminate weapon with
blood• Blood is cast-off tangientially to arc of upswing
or backswing• Pattern & intensity depends on:
– type of weapon
– amount of blood adhering to weapon
– length of arc
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Downswing of HammerDownswing of Hammer
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Cast-off from WeaponCast-off from Weapon
ceiling
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Overhead swing with bloodied metal bar26
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Cast-off Pattern (1/2)Cast-off Pattern (1/2)
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Cast off Pattern (2/2)Cast off Pattern (2/2)1
2
3
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Cast off Pattern (2/2) Cast off Pattern (2/2) ? Sequence? Sequence
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Cast off Pattern (2/2) Cast off Pattern (2/2) ? Sequence? Sequence 1
(4 spots)
2(3 spots)
3(2 spots)
If weapon does not pick up more blood, spatter from subsequent backswings becomes progressively less.In practice weapon picks up more blood with each successful blow.
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Three overhead swings with hatchet
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Cast-off & medium velocity spatter32
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Cast-off & medium velocity spatter 233
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Cast-off PatternCast-off Pattern? Object? Object
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Cast-off PatternCast-off Patternfrom Handfrom Hand
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Cast-off pattern from bloodied hand swung in front of target
6” ruler
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.... ...
..
...
Drip PatternDrip Pattern
• Free-falling drops dripping into wet blood• Large irregular central stain• Small round & oval satellite stains
.. .......
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Drip 1:Drip 1:
Blood dripping into itself from height of 1 m (8 drops)
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Drip 2Drip 2
Blood dripping into itself from height of 1 m (8 drops)
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Dripping onto steps
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Splash onto vertical surface
6” ruler
10 ml blood thrown 1 m onto a vertical target surface
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Stamping in blood 1
Area seen in close-up in next slide
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Stamping in blood Close-up of heel area
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Stamp 1Stamp 1
Blood pool (10 drops) before stamping
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Stamp 2Stamp 2
Blood pool (10 drops) after stamping49
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Arterial Spurt PatternArterial Spurt Pattern• Blood exiting body under arterial pressure• Large stains with downward flow on
vertical surfaces• wave-form of pulsatile flow may be
apparent
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Small arterial spurt
spatter
broken pottery
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Neck incisions (scene)52
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Neck incisions
Thyroid cartilage
Probe in carotid artery
‘Hesitation’ injuries
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Medium Velocity Medium Velocity Blood SpatterBlood Spatter
• Blood source subjected to MV impact– (25 - 100 f/s, 7.5 - 30 m/s)
• Spot diameter: mostly 1 - 4 mm• Blows with weapon (e.g. baseball bat)
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Medium velocity blood spatter.Point of impact 15 cm in front of vertical target surface
6” ruler
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Flick 1:Flick 1:
Blood flicked between middle finger & thumb onto a vertical smooth surface from a distance of 15 cm
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Flick 2:Flick 2:
Blood flicked between middle finger & thumb onto a vertical smooth surface from a distance of 15 cm
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High Velocity Blood SpatterHigh Velocity Blood Spatter• Blood source subjected to HV impact
– > 100 f/s, 30 m/s
• Fine mist: spot size < 0.1 mm• Small mass limits spread to 1 m• !Some larger droplets reach further• Gunshot
– back-spatter from entry wound– forward spatter from exit wound
• High speed machinery
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Gunshot: back& forward spatterBloodstained foam held just above target surface.
Back-spatter on entry
Forward spatteron exit
bullet
Bullet passing L to R just above sheet
Bullet enters foam
bullet exits foam
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Gunshot Back SpatterGunshot Back Spatter
• Arises from entrance wound• Passes back towards weapon & shooter• Seen only at close range of fire• Seen on:
– inside of barrel
– exterior of weapon
– hand, arm, chest of shooter
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Back spatter on steadying
hand
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Gunshot Forward SpatterGunshot Forward Spatter
• Arises from exit wound• Passes forwards in same direction as shot• More copious than back-spatter• Can be seen at any range of fire• Seen on nearby surfaces, objects, persons
– especially on wall behind victim
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Forward spatter (5 ms after bullet impacted at 1000 f/s)
2.5 cm
blood soaked target
bullet
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6” ruler
Forward spatter onto target placed 15 cm behind point of HV bullet impact (bullet passing towards screen) 1
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Forward spatter (closer view)65
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Forward spatter (closest view)
5 mm
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Wipe PatternsWipe Patterns• Object moves through a wet bloodstain• Feathered edge suggests direction
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Transfer PatternsTransfer Patterns
• Wet, bloodied object contacts a secondary surface
• Transfer from:– hand, fingers
– shoes, weapon
– hair
• Transfer to:– walls, ceilings
– clothing, bedding
• Produces mirror-image of bloodied object
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Transfer from hair (hair-swipe) 1
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Transfer from hair (hair-swipe) 2
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Flow PatternsFlow Patterns• Blood flows horizontally & vertically• Altered by contours, obstacles• Often ends in pool
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Flow pattern72
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Trapped!
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Stabbing 1
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Stabbing 275
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Blood flow on shirt
Horizontally to R side
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Pattern on shirt77
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Bloodspots on trousers78