FORS-2107EL Study Guide - Quiz Guide: Staccato, Sole Markings, Comparison Microscope
27 Jun 2018
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FORS-2107 - Quiz 1 Review
Blood Stain Analysis
Bloodstain Pattern Analysis
-Forensic Importance: What we are looking for in a forensic sense:
-Location of original blow
-Number of blows
-Type and velocity of weapon used
-Movement and directionality of assailant/ victim/ objects, while they were shedding
blood
Basic Principles of BPA
-1) Blood will behave according to the law of physics
-2) The size of a projected bloodstain has a direct relationship to the speed at which it
is traveling
-3) You can tell the direction a bloodstain was moving by its shape
-4) Bloodstain patterns are predictable and reproductive
Surface Tension
-The force that pulls the surface molecules, of the drop, towards the interior of liquid
and therefore decreases the surface area, therefore the larger the surface area =
greater surface tension
-When a force is applied to a mass of blood, it breaks the blood into the droplets that
fly through the air as spheres
Stages of Impact
-1) Contact and collapse (looks like a hat)
-Drop collapses from the bottoms up when it hits
-The drop will be pushed outwards into a rim (like a hat shape)
-2) Displacement (looks wobbly outside rim)
-Dimples and spines begin to form based on the irregularities of the impact surface
-This occurs due to the texture of the surface that the blood is hitting, different
surfaces cause different amounts and patterns of spines
-3) Dispersion (looks like a bowl)
-Most of the volume of blood is forced to the rim
-Creates satellite spatter that fall (flies) off the rim
-Blood falling from a height or high velocity can
overcome its natural cohesiveness and appears as
spiking patterns around the original drops
-4) Retraction (looks like final stain shape)
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-Surface tension will be trying to keep the droplet together
-Satellite spatters with enough momentum can form completely separate stains
-Droplet can burst if the force pulling it apart is great enough
Categories of Bloodstains
Passive bloodstains (dripping)
-Created by the force of gravity acting alone
-Low velocity stains
-Further subdivided into
-Drops: just drops everywhere
-Drips: more drops from standing up, which will have a parent stain with satellites off
of the parent stain
-Pool: When pooling occurs, due to blood being collected in one region
Projected Bloodstains (flung)
-Occurs when energy has been transferred to the blood source
-Higher velocity = smaller drops
-Low Velocity Impact stains (gravity)
-Up to 5 feet/second, size = 4 mm and greater
-Medium Velocity Impact stains (beatings/stabbing)
-Force of 5-25 feet/sec, size = 1 - 4 mm
-High velocity/Misting (gun shots, explosions, mechanical accidents)
-Force can be greater then 100 feet/sec, size = <1mm
-Misting can occur, which looks like it came out of a spray can
-Caveats (things to look out for)
-Dry quickly
-Does not travel far
-Often hidden/missed in textured surfaces (carpeting)
-May exhibit back spatter as well as forward spatter
Relationships
-higher force = smaller drops
-larger height = larger size of stain (diameter of drop increases (with consistent
volume every time))
-Higher velocity = smaller drops
Surface Texture
-Degree of distortion occurs because of surface texture, not height
-Smooth/hard surfaces : uniform border, shape will be more circular
-Rough/textured surfaces: irregular border, shape will be more elliptical
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Cast-off Bloodstains
-Blood released from a blood bearing object in motion
-First blow breaks skin, and does not create spatter due to no blood on the weapon
yet
-# of blows = # of tracks +1
Arterial Gush/ Spurt
-When carotid artery gets cut and blood gushes out as the heart pumps, this will leave
behind staccato like marks
Expirated Blood
-Blood that is projected from lungs/mouth/nose or open “sucking” chest wounds
-Will be mixed with mucus/saliva, which causes lighter stains
-Characteristically will have a bubble pattern
Transfer Bloodstain
-Swipe Transfer Pattern
-A pattern is created from a wet bloody object moving across a clean surface
-This is when the surface is clean and the object has the blood on it
-Ex. hair from a bloody head being dragged
-Wipe Transfer pattern
-Created when an object moves through wet blood stain
-This is when the surface has blood on it and the object is clean
-Skeletonize bloodstain
-Caused when the outside of a blood stain dries faster then the inside, and an
object moves through it which will move or remove the wet blood in the middle,
this is a type of wipe
Directionality
-The satellite end of the drop will point in the direction of travel
-angle of impact = arcsin width/length
-The larger the angle of impact, the more circular the drop is
-The angle is measure from the surface upwards
Detecting Blood at Crime Scene
-Latent blood
-Light source
-High intensity, UV, ALS
-This will make blood look really really black
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Document Summary
Forensic importance: what we are looking for in a forensic sense: Movement and directionality of assailant/ victim/ objects, while they were shedding blood. 1) blood will behave according to the law of physics. 2) the size of a projected bloodstain has a direct relationship to the speed at which it is traveling. 3) you can tell the direction a bloodstain was moving by its shape. 4) bloodstain patterns are predictable and reproductive. The force that pulls the surface molecules, of the drop, towards the interior of liquid and therefore decreases the surface area, therefore the larger the surface area = greater surface tension. When a force is applied to a mass of blood, it breaks the blood into the droplets that. 1) contact and collapse (looks like a hat) Drop collapses from the bottoms up when it hits. The drop will be pushed outwards into a rim (like a hat shape)
