Properties of blood. Teacher Background Information. FSB05 blo o d spatte r

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1 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: Figure 1: A water strider standing on water. Citation: Water strider: David Cappaert, Liquid Viscosity (mp s -1 ) Milk (25 o C) 3 Blood (37 o C) 3-4 Glycerin (20 o C) 1420 Mercury (15 o C) 1.55 Water (20 o C) 1.0 Water (100 o C) 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. 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. The surface tension of some common liquids: Liquid Surface tension N m -1 Benzene (20 o C) Blood (37 o C) Glycerin (20 o C) Mercury (20 o C) 0.47 Water (20 o C) Water (100 o C) cutnell/ /ste/ste.pdf

2 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 90 o 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/m 3. 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/m 3 and the density of blood cells circulating in the blood is approximately 1125 kg/m 3. The average density of whole blood for a human is about 1060 kg/m 3. 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. 2

3 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 90 o 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.. Figure 5: A diagram showing blood being pushed into a rim on contact with a receiving surface. 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 Figure 6: Displacement phase of a blood droplet in a 90 o impact. Image used with permission from Tom Bevel & Ross Gardner, June

4 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). 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. Figure 7: Early dispersion phase of a blood droplet impacting at 90 o. Image used with permission from Tom Bevel & Ross Gardner, June Force, Velocity and Droplet Size 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. 4

5 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 90 0 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 90 o 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.. Figure 10: Spatter deposited on a wall as a result of a blunt force beating. Image courtesy UWA PhD research student Mark Reynolds. 5

6 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. Figure 11: Spatter deposited on a wall as a result of a gunshot. Image courtesy Stuart James, February 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. The pointed end of the bloodstain always points in the direction of travel. 6

7 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. 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. When a droplet of blood impacts a surface at 90 o, 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). 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. 7

8 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. 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 = 37 o 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 o 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 5-7 o. Computer fitting of theoretical ellipses has refined the measurement process to sub-degree levels of accuracy. 8

9 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. 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). Figure 20: Establishing the direction of travel. 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. 9

10 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. The Z-axis represents the height above the target plane in this example the target plane is the floor. The scales of both axes, X and Z are scaled the same (cm). 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. Do the following. 4. Mark on the X-axis of the graph the convergence distance (cm) for each stain. Figure 21: A representation of the area of origin established from 2-dimensional calculations. Calculation Methods 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 Using a protractor, draw a line from the mark on the X-axis, at the calculated angle of impact, to the Z-axis. Repeat this procedure for each stain. 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. 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. 10

11 Defining the Area of Origin with the Tangent Function. The same steps as above are followed to determine the: Area of convergence (AOC) Distance from the stain to the AOC Angle of impact of the selected stains a minimum of 3 stains. The following formula is used to determine the point of origin. TANi = H/D i = angle of impact D = distance from stain to area of convergence H = unknown distance above target surface b Line 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/D To solve for unknown H H = TAN i * D Defining the Area of Origin by Triangulation. The same steps as above are followed to determine the: Area of convergence (AOC) Distance from the stain to the AOC Angle of impact of the selected stains a minimum of 3 stains. Apparatus Ring stand Protractor String Masking tape 1m rule pencil Method 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. c i a For example: Distance to the AOC = D = 30cm Angle of impact = i = 35 o H = TAN i * D H = * 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. 11

12 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 Bevel, T. & Gardner, R.M Bloodstain pattern analysis. CRC Press Ltd, LLC. James S.H., Kish, P.E. & Sutton, T.P 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