blood spatter

FSB05blo o d spatte rProperties of bloodTeacher Background InformationBlood is considered to be a fluid. A fluid is a substancewith no fixed shape and is subject to externalpressure. A fluid can be either a liquid or a gas. Aliquid is a fluid that has a fixed volume while a gas is afluid that can expand indefinitely.ViscosityViscosity is defined as a fluid’s resistance to flow. Themore viscous a substance is, the more slowly it willflow. The SI unit for viscosity is the Pascal second. Fluidviscosity is compared to water that has a viscosity ofone. Blood is thicker than water and is viscous primarilydue to the cellular component (see FSB04). Theviscosity of some common substances, including blood:Figure 1: A water strider standing on water.Citation: Water strider: David Cappaert, www.insectimages.orgLiquid Viscosity (mP·s -1 )Milk (25 o C) 3Blood (37 o C) 3-4Glycerin (20 o C) 1420Mercury (15 o C) 1.55Water (20 o C) 1.0Water (100 o C) 0.28http://hypertextbook.com/physics/matter/viscosity/In Figure 1, the surface tension of the water allows thewater strider to walk on the water without sinking.This is because the upward force from surface tensionbalances the insect’s weight.Definition of surface tension: the surface tension γ isthe magnitude F of the force exerted parallel to thesurface of a liquid divided by the length L of the lineover which the force acts:γ = F_LSurface tension is measured in force per unit length:newtons per metre: (N·m-1). The old unit is dynes per cm.Surface tensionSurface tension is the force that pulls the surfacemolecules towards the interior of a liquid, decreasingthe surface area and causing the liquid to resistpenetration or separation.Surface tension is the tendency of the surface of aliquid to contract to the smallest area possible. Thefluid is able to do this as the cohesive forces arestronger on the surface of liquids as there are noneighbouring molecules above. As a result there arestronger attractive forces between molecules and theirnearest neighbours on the surface; the surface tensionforce actually exerts an upward force. Surface tension islike having an elastic film over the surface.The surface tension of some common liquids:Liquid Surface tension N·m -1Benzene (20 o C) 0.029Blood (37 o C) 0.058Glycerin (20 o C) 0.063Mercury (20 o C) 0.47Water (20 o C) 0.073Water (100 o C) 0.059http://www3.interscience.wiley.com:8100/legacy/college/cutnell/0471713988/ste/ste.pdf

FSB05blo o d spatte rProperties of bloodSurface tension is important in bloodstain patternanalysis as;••the gravitational force must overcomethe surface tension of blood beforea drop of blood can fall, anddrops of blood remain intact as they movethrough the air due to surface tension.larger droplets.] Droplets do not “break up” whilst inmotion; another force would need to be applied tocause the droplets to further divide. The oscillationsgenerally have no effect on the resulting spatterpattern except for instances where there are only afew stains and they are present on surfaces less than100cm from the source.ImpactWhen a droplet of blood strikes a horizontal surfaceat 90 o it produces a circular stain. If the surface textureis smooth, such as glass or a polished tile, the surfacetension will hold the droplet in the circular pattern.Essentially the surface influences the outflow. Surfacetension ensures that the droplet collapses uniformlyhowever the smooth surface means that the rimoutflow is uniform.Figure 2: Complimentary effects of adhesion,cohesion and surface tension on a single blooddroplet.Image courtesy UWA PhD research student Mark Reynolds.DensityDensity is defined as mass per unit volume. Thedensity of water is 1000 kg/m 3 . The density of bloodis proportional to the total protein concentration orcellular component of blood and is influenced onlyto a minor extent by other ions, gases etc. that aredissolved in the plasma.The density of blood plasma is approximately 1025kg/m 3 and the density of blood cells circulating inthe blood is approximately 1125 kg/m 3 . The averagedensity of whole blood for a human is about 1060kg/m 3 .Blood DropletsThe application of a force to a mass of blood causesthe mass to break up into droplets. As a blood droplettravels through the air it retains a spherical shape dueto surface tension. Smaller drops (1mm diameter andless) are almost perfect spheres while larger dropsoscillate due a range of other forces acting on thedroplet. [Smaller droplets do oscillate but the timerequired to dampen the oscillations is far less thanFigure 3: Several blood droplets that have fallen ontoa rough surface.Image courtesy DUIT Multimedia: Paul Ricketts.If the droplet falls onto a rough surface such ascardboard, carpet or concrete it will produce anirregular and distorted stain pattern. The roughsurfaces results in an irregular rim outflow.

FSB05blo o d spatte rProperties of bloodPhases of impactThere are 4 distinct phases of impact:1) Contact /collapseThe droplet contacts the target surface and collapsesfrom the bottom up. The part of the drop that has not yetcollided with the surface remains as part of the sphere.in contact with the surface and more blood is forcedinto the rim.The angle of impact affects the collapse as it definesthe nature of the rim and the blood flow into it. Forexample: if the droplet impacts at 90 o the blood flowinto the rim is equal on all sides. If the impact angle ismore acute, the blood flows into the area of the rimopposite the direction from which the droplet came.2) DisplacementIn this stage, the blood droplet has collapsed againstthe target surface and nearly all of the blood hasmoved from the centre of the droplet to the rim. Theactual area of displacement will be the same size asthe 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 tothe 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 pushedinto a rim on contact with a receiving surface.As the collapse occurs, the blood that has come incontact with the surface is forced outward creating arim. The rim gets bigger as more of the droplet comesFigure 6: Displacement phase of a blood droplet in a90 o impact.Image used with permission from Tom Bevel & Ross Gardner,June 2006.

FSB05blo o d spatte rProperties of bloodThe surface texture is important. Surface tension isresponsible for keeping the shape of the droplet asit moves through the air. When the droplet hits thetarget surface, the ‘skin’ of the droplet, created bysurface tension shifts its shape. The droplet doesn’tactually burst.If the surface is rough, the blood flows irregularly intothe rim so the spines or dimples that form will alsobe irregular in shape. This will result in a distorted orasymmetrical shape.3) DispersionIn this phase, most of the blood is forced into the rim.The spines and dimples continue to rise upward andin a direction opposite to the original momentum. Asthe amount of blood in the rim and spines increasesthey become unstable.4) RetractionThe last phase results from the effect of surfacetension attempting to pull the droplet back. If theforces trying to pull the droplet apart are overcome bysurface tension, the resulting stain will be reasonablycircular and symmetrical in shape. If the forces pullingthe droplet apart overcome the surface tension,the droplet will ‘burst’ and create an irregular stainpattern.An excellent animation showing the impact behaviourof a blood droplet (November 2006).http://www.nfstc.org/links/animations/images/blood%20spatters.swfHeightThe higher the droplet falls from the ‘more’ bloodsatellite spatter occurs. Blood spatter is a broad termessentially meaning blood distributed through the airin the form of droplets. Satellite spatter, or spatter onthe receiving surface may or may not be formed.If two similar sized droplets fall from different heightsthe resulting stains have different sizes. E.g. a dropletfalling from 10cm will produce a different stain thana droplet falling from 100cm. The stain diameterfrom the 100cm height will be larger than the patternfrom the 10cm height. The reason is that the velocityof the droplet will be greater the longer the dropletis airborne [until it reaches terminal velocity.] Abovea fall distance of 2.2m there is little change in thediameter of the blood spot.Figure 7: Early dispersion phase of a blood dropletimpacting at 90 o .Image used with permission from Tom Bevel & Ross Gardner,June 2006.Force, Velocity and Droplet SizeThe size and appearance of the bloodstains dependson the force that was used to create them. When anobject comes into contact with blood, the force of theobject moves the blood. The blood must respond tothis energy transfer in some fashion. The response isoften by the distribution of blood through the air inthe form of droplets.Velocity is measured in meters per second. At a crimescene there may be evidence of low, medium or highvelocity blood spatter or a combination of these. Forexample, dripping blood (low velocity) has a velocityof 1.5 metres per second. Blood droplets producedfrom a bullet shot from a gun will have much greaterenergy and will travel faster.

FSB05blo o d spatte rProperties of bloodLow velocity blood spatterA low velocity force is usually the result of blooddripping from a person who is still, walking orrunning. Blood drops may be free falling and onlymoving due to the force of gravity. At low velocitieslarger bloodstains are produced. Sometimes lowvelocity bloodstains are a result of weapon cast-off offrom blood dripping from a victim.Dripping blood often falls at a 90 0 angle and formsa round bloodstain that is often 4mm in diameter orlarger: up to approximately 10mm. If droplets are,however, falling from a moving object or person(walking or running) they fall to the ground at anangle (see angle of impact) and the direction of themovement can be established.Identifying Blood Trail MotionFigure 9: Passive bloodstains falling onto a smoothsurface at approximately 90°Image courtesy UWA PhD research student Mark Reynolds.Medium velocity blood spatterA medium velocity force moves blood betweenfive and 50 metres per second and the resultingbloodstains at 90 o are between one and threemillimetres in size. The size of the bloodstain dependson the angle of impact with the receiving surface. Anoblique stain can be greater than 10mm but would belong and thin. Medium velocity blood spatter mightresult from blunt force trauma, for example, beatingwith fists, baseball bats, whips, bricks or hammers.Medium velocity blood spatter can also occur when abody collides with rounded or edged surfaces.Droplets dripping from a moving object or person do notdrop straight down. As they are in motion themselves, theyfall to the ground at an angle.Blood-trail motion is defined by considering thedirectionality of the individual droplets present in the bloodtrail 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.

FSB05blo o d spatte rProperties of bloodHigh velocity blood spatterA high velocity force moves blood greater than 50metres per second and the bloodstains are usuallysmaller than 1mm and appear as fine spray or misting.High velocity blood spatter can be caused by highspeedmachinery such as chain saws and woodchippers.Figure 12: Spines, scallops and satellite spatter helpto identify the path of the blood droplet.Image used with permission from Tom Bevel & Ross Gardner, June 2006Figure 11: Spatter deposited on a wall as a result of agunshot.Image courtesy Stuart James, February 2007.DirectionCrime scene investigators can determine the directionthat a blood droplet was travelling in as dropletsimpact surfaces in a consistent manner. The dropletwill keep moving along the same path that it wastravelling before hitting the surface. When it impactsa surface, the blood in the droplet moves outwardsduring the collapse phase creating either an ellipticalor circular stain.A crime scene investigator will look at other featuresof the bloodstain to determine which direction theblood droplet was travelling in. Bloodstains alsousually have features such as satellite stains, scallopsor spines. The stain will have a higher number of thesefeatures on one side. This is due to the way the dropletcollapses on impact. As discussed previously, bloodflows into the area of the rim opposite the directionfrom which the droplet came. In many instances thedimples on the rim break slightly from the dropletstructure creating spines, scallops or if it breaksentirely away, satellite stains.The long axis of the stain (major axis) provides anindication of the direction the droplet was travelling inprior to contact with the receiving surface and hencethe direction that it came from. The droplet alwaystravels in the long axis, but it is sometimes difficult totell the actual direction as shown in Figure 12.Figure 13: Scallops, spines and satellite stains arealways in the direction of travel.Image used with permission from Tom Bevel & Ross Gardner, June 2006The pointed end of the bloodstain always points inthe direction of travel.

FSB05blo o d spatte rProperties of bloodAngle of ImpactThere is a relationship between the length andwidth of a bloodstain and the angle at which thedroplet impacts on a surface. It is therefore possibleto calculate the angle of impact on a flat surface bymeasuring the length and width of a stain.The angle of impact is the acute angle that isformed between the direction of the blood dropand the surface it strikes. This is an importantmeasure because it is used to determine the area ofconvergence and the area of origin.Figure 15: The measurement of the length and widthof stains.Image used with permission from Tom Bevel & Ross Gardner, June 2006Calculating the angle of impactThe angle of impact formula relies on the relationshipsthat exist between the angles of a right triangleand the length of its sides. These are trigonometricfunctions called sine, cosine and tangent.Figure 14: The angle of impact of a blood droplet ona receiving surface.Image used with permission from Tom Bevel & Ross Gardner, June 2006Imagine a right triangle formed between the dropletand the target surface as the droplet strikes. A blooddroplet 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 alphaangle.When a droplet of blood impacts a surface at 90 o ,the bloodstain will be circular. The more the angle ofimpact decreases, the more the stain is an ellipse. Theangle of impact can be measured by the degree towhich the shape of the drop changes from a circle toan ellipse.An excellent animation showing the angle of impact(November 2006).http://www.nfstc.org/links/animations/images/blood%20spatters.swfWhen measuring the length and width of a stain, nopart of the spines, tails or satellite spatter are includedin the measure. Round the stain to an elliptical shapewhen making measurements.Figure 16: The relationship of the droplet to animagined right angle.Image used with permission from Tom Bevel & Ross Gardner, June 2006

FSB05blo o d spatte rProperties of bloodThe diagram below (Figure 17) represents a stain thathas impacted on a surface.An exampleWidth = 3mmLength = 5mmSine i = width / lengthSine i = 3mm / 5mm = 0.6Figure 17: The width and length of a bloodstain canbe used to calculate the angle of impact.Image used with permission from Tom Bevel & Ross Gardner, June 2006As a result, we have two known quantities from thecrime 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 bythe 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, ORAngle = 37 oThe “inverse sine” or the arc sine function on ascientific calculator (ASN) converts the ratio to anangle.Inverse Sine (ASIN) i = Angle of ImpactThe steps are:••Accurately measure the width and lengthof a given bloodstain. This should bemeasured to the nearest millimetre.Divide the width of the stain by the length of thestain 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 calculatorInverse Sine i (0.6) = 36.8The angle of impact is between 36-37 oIt is important to note that this method gives anestimate of the impact angle rather than a preciseresult. The accepted variance is between 5-7 o .Computer fitting of theoretical ellipses has refinedthe measurement process to sub-degree levels ofaccuracy.

FSB05blo o d spatte rProperties of bloodArea of ConvergenceConsider a simplified crime scene where there aretwo elliptical bloodstains on a floor, forty centimetresapart. Lines are drawn from the centre of the long axisof each bloodstain and extended until the two linesfrom the separate stains meet. The point where thelines meet is called the Area of Convergence. (NB _this is also called the POINT of convergence – for ourpurposes the term will be the AREA of convergenceas accuracy is not sufficient to determine the actualPOINT).Figure 19: Measuring the distance from thebloodstain to the area of convergence.Before drawing lines it is important to determine thedirectionality of the bloodstain. The lines must bedrawn away from the direction of travel towards theorigin.Always work via the centre of the long axis and extendthe line from the back of the bloodstain.In addition to two stains having a coincidental intersectingpoint, it is also possible to have several patterns overlap.If this condition is not considered it might well result in amistaken point of convergence.Figure 18: The area of convergence.Image used with permission from Tom Bevel & Ross Gardner, June 2006This area of convergence is possibly the source ofboth bloodstains, but the path crossover may also becompletely coincidental if the two stains were createdby unrelated events.In the figure below there are 3 stains with differentangles of impact. When lines are drawn from thestains, (the centre of the long axis of the stain) thelines converge at an area (of convergence).Figure 20: Establishing the direction of travel.Area of OriginAt a crime scene with several bloodstains, crimescene investigators attempt to determine the originof the blood. In essence the investigator is trying todetermine from which location in a 3-dimensionalspace the blood originated, from 2- dimensionalmeasurements. Figure 21 below attempts to show thepoint in space where the paths converge.

FSB05blo o d spatte rProperties of bloodDefining the Area of Origin by GraphingA graph is prepared that has the following features..2.3.The X-axis represents the target plane andgraphs the distance from the back-edgeof the stain to the area of convergence.The Z-axis represents the height abovethe target plane – in this examplethe target plane is the floor.The scales of both axes, X and Zare scaled the same (cm).The base of each stain’s present position, the point in twodimensionalspace where the paths converge (c), and theirpoint of origin (o), define another right angle.Do the following.4.Mark on the X-axis of the graph theconvergence distance (cm) for each stain.Figure 21: A representation of the area of originestablished from 2-dimensional calculations.Image used with permission from Tom Bevel & Ross Gardner, June 2006Calculation MethodsThe angle of impact and length of the convergenceline can be graphed for each stain and the area oforigin (of the blood) established OR it can be donemathematically through the relationships that exist ina right triangle OR it can be done using a protractorand string.Whichever method is used for the calculation, theinitial steps for all methods are the same:••Identify stains that have a commonarea of convergence.Draw lines through the central long axis ofthe stain away from the direction of travel.• Identify the area of convergence.••Measure the distance (cm) from the back edgeof the stain to the area of convergence.Calculate the angle of impact of each ofthe stains (measure width and length ofstains in mm and apply the formula).• Use a minimum of 3 stains.5.6.7.Using a protractor, draw a line from themark on the X-axis, at the calculatedangle of impact, to the Z-axis.Repeat this procedure for each stain.The area at which the lines from the X-axisconverge on the Z-axis establishes the probableheight of the area of origin. See below.Once the angle of impact has been calculated and thedistance from each stain to the area of convergencehas been measured, either of 3 methods can be used.Figure 22: A graph showing the method forestimating the area of convergence.10

FSB05blo o d spatte rProperties of bloodDefining the Area of Origin withthe Tangent Function.The same steps as above are followed to determinethe:• Area of convergence (AOC)• Distance from the stain to the AOC•Angle of impact of the selectedstains – a minimum of 3 stains.The following formula is used to determine the pointof origin.TANi = H/Di = angle of impactD = distance from stain to area of convergenceH = unknown distance above target surfacebLine bc = H - height above the target: unknown.Line ca = D – distance to the area of convergence:known.i = angle of impact: known.TAN i = H/DTo solve for unknown HH = TAN i * DDefining the Area of Origin by Triangulation.The same steps as above are followed to determinethe:• Area of convergence (AOC)• Distance from the stain to the AOC•Angle of impact of the selectedstains – a minimum of 3 stains.Apparatus• Ring stand• Protractor• String• Masking tape• 1m rule• pencilMethod• Place the ring stand on the area of convergence••Write the calculated angle ofimpact next to each stain.Using string, masking tape and a protractor,raise the string to the calculated angleand attach it to the ring stand.• Do the same for a minimum of 3 stains.•The place on the ring stand where the stringfrom each stain meets is the ‘area of origin’.• Measure the height of the area of origin.ciaFor example:Distance to the AOC = D = 30cmAngle of impact = i = 35 oH = TAN i * DH = 0.7002 * 30cm= 21cmNB: the value for the TAN of angle 35 can be foundfrom the Table of Trigonometric Function or by using ascientific calculator.11

FSB05blo o d spatte rProperties of bloodLimitationsThe methods described above have limitations but areable to give an investigator a good approximation ofthe origin. This helps to identify the general locationwhere an application of force to a source of bloodoccurred.Crime scene investigators now tend to use computersoftware applications to analyse blood stain patternsincluding area of origin calculations however manyinvestigators prefer to use traditional methods. Theirchoice of method depends on a range of factors.The information from area of origin calculations canbe used to verify or refute various claims made abouta crime scene. For example, if all of the blood spatterevidence points to a certain height that equates toa area low to the ground this would not back upa suspect’s claim that it was self-defence from astanding position.The scenario in this program of work requires studentsto analyse stains and calculate the area of origin ofbloodstains on one target surface which is a wall.Students will then review 3 statements: suspect,victim and witness and determine which statementverifies the forensic evidence.Bloodstains in other parts of the room (floor, walls,stove and ceiling) are not measured in the activitybut the characteristics of the spatter can be used tofurther support or refute a statement.Referenceshttp://www3.interscience.wiley.com:8100/legacy/college/cutnell/0471713988/ste/ste.pdfhttp://hypertextbook.com/facts/2004/MichaelShmukler.shtmlhttp://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.12