Understanding Blood Spatter | Fresno Criminal Defense Attorney
Blood Drop Flight Dynamics
Since blood is a fluid that has as viscous consistency, an external force applied to a blood spot will result in a predictable sequence of events. Blood drops are held together by cohesive forces that produce a surface tension within and on the external surface of each drop. Surface tension is the force that pulls the surface molecules of a liquid together, decreasing the surface area of the drop, and forming a skin that helps the liquid resist penetration. Surface tension is the force that pulls a droplet into its absolute smallest volume. The surface tension of blood is less than that of water. Fluids that are more viscous in nature have slow flow characteristics; the greater the viscosity of a fluid, the more slowly it will flow. Blood is actually six times more viscous than water and has a slightly higher specific gravity. Both viscosity and specific gravity are features that provide the stability and resistance seen in exposed blood drops. To create blood spatters, the surface tension of a blood drop must be overcome through the application of an external force.
A passive drop of blood in air (such as blood dripping from a small wound) occurs when the volume and mass of a drop increases to an extent that that gravitational pull on the drop overcomes the cohesive forces of the blood source. When a droplet forms, it takes on a teardrop shape as it pulls away from the source. When falling through the air, the shape of a blood drop is directly related to the molecular cohesive forces acting upon the surface of the drop, causing it to take on a spheroidal shape. While the drop is falling, forces impinging on the droplet cause the drop to oscillate and the shape changes repeatedly from spheroid to spheroidal oblong. Since the droplet oscillation is a dynamic process, the oscillation eventually decays and the droplet eventually remains spheroidal. This damping of oscillations is a relatively quick process. The amount of oscillation within a droplet tends to vary with the size of the droplet, with larger droplets exhibiting more oscillation than smaller droplets. Oscillations can be enhanced when falling droplets collide, however it is thought that these kinds of collisions are infrequent and if collisions do occur they do not cause droplets to break up further.
The amount of blood needed for a passive drop to form depends on the type of surface and the surface area from which the blood originated. An average volume of a drop of blood is about 0.05 ml, which forms a sphere about 4.56 mm in diameter while in the air.
A blood drop falling through the air will continue to accelerate until the forces opposing the drop equal the force of the gravitational pull, and the falling drop will reach terminal velocity. Studies conducted by MacDonnell determined the average terminal velocity speed for a falling blood drop of blood to be 25.1 feet /second. This velocity is reached in a maximum falling distance of 20-25 feet. In addition, the size of an individual droplet determines the path of the droplet in free flight. As air resistance is inversely proportional to the droplet size, a larger droplet will encounter less air resistance than a smaller droplet and will therefore travel further in free flight. If two different sized droplets are projected in the same direction, at the same velocity, the smaller droplet will quickly lose velocity and fall to the ground before the larger droplet.
Impact Angle Effects
The shape of a bloodstain on a surface is related to the angle at which the blood droplet hits the target surface. If the angle of impact of a falling drop is 90° then the bloodstain will usually be circular in shape. Bloodstains striking a surface at angles more acute than 90° are usually elliptical in shape. The width and length of each stain is directly related to the angle of impact. If the width to length ratio (width/length) is less than 1.0, the ratio is equal to the sine of the angle of impact; therefore, the inverse sine is equal to the angle of impact. For a circular bloodstain the width and the length are equal and therefore the ratio is equal to 1.0, which corresponds to angle of impact of 90°. If a target is in motion when the droplet impacts the target, the target motion creates a wave cast-off action that results in a droplet hitting the target at an angle less than 90° and producing a pattern that mimics a droplet striking a stationary target at a more acute angle. These types of patterns are generally associated with fewer satellite spines and spatters.
Droplet Dynamics on Impact
To create smaller droplets or spatter from a volume of blood the surface tension of the primary stain must be disrupted. As a blood droplet falls through the air the forces of gravity alone aren't sufficient to overcome and disrupt the surface tension of the drop. It doesn't matter how far the drop falls, the spot will not break up until something disrupts the surface tension. When a falling blood droplet strikes a surface, the resulting spot has a diameter that varies with the volume of the drop, the texture of the surface it impacts, and to some extent, the distance fallen. The physical nature of the target surface struck by the blood drop will have a large impact on the resulting satellite or spatter droplets formed. A hard, smooth, non-porous surface generally produces a spot with little, if any, spatter, whereas a rough textured surface such as wood can produce many spatters. These types of target surfaces have small protuberances and irregularities that produce spatter with spiny or serrated edges and irregularly shaped parent stains.
Extensive studies have shown that there are four general phases of impact that are independent from surface effects and angle of impact.
Phase 2 Phase 1
Contact and Collapse: When a droplet first hits a target surface the droplet begins to collapse from the bottom upward. The parts of the sphere that are not in immediate contact with the surface remain intact until contact with the target surface occurs. As the spot collapses, the blood at the collapse point is forced outwards to create a rim around the droplet. This boundary grows as the spot collapses. The angle of impact will affect the boundary by determining the nature of the rim and the flow of blood into it. Impacts occurring at acute angles force the blood from the collapsed area into a directional flow at the rim opposite to the direction of impact creating the elliptical shape of a directional stain. Although target surface characteristics can influence the outflow of a stain during the collapse phase, surface characteristics have a more significant effect in the displacement phase.
Displacement: during this phase the sphere has collapsed against the target surface and the majority of the blood volume of the droplet has been displaced into the boundary of the resulting stain. At this point the surface tension has still not been overcome and the stain is essentially still a single mass that has just had massive shifts in its shape. Dimples and short spines, which are formed when the blood is displaced from the central portion of the stain, can be seen on the boundary rim of the stain. Although most of the blood volume is now in the boundary rim, the stain is still intact as a single unit. The area of the stain in this phase defines the overall dimensions of the final stain, excluding any spines formed.
The angle of impact only has an effect on the nature of the dimples. Dimples will form around the boundary rim but acute angle impacts result in dimples being formed only in the forward edge of a developing rim. Dimples that are present in the displacement phase may result in the formation of an individual spatter or spine. During this phase, the target surface texture plays a significant role in the development of spines and spatters, since irregularities and protuberances in the target surface disrupt the surface tension causing blood to flow irregularly into the rim boundary resulting in differences in the rim boundary producing spines and spatters of different volume.
Uniformly sized blood drops, dropped from different heights onto a smooth hard surface will produce bloodstains with increasing diameters. When dropped over a range of 6 inches to 7 feet, the diameters of the resultant spots generally range from 13 to 21 mm. This diameter is related to the velocity at which the spot is falling. At heights beyond 7 feet, a falling spot will have reached its terminal velocity by the time the spot impacts the surface so the resulting stain diameter doesn't change significantly beyond this range. Extrapolating spot diameter to determine the distance fallen shouldn't be attempted at a crime scene because the size of the drop is usually not known.
Phase 3 Dispersion: in the dispersion phase, blood is forced into the boundary rim and dimples that rise upward and opposite to the direction of the original momentum. As the volume of blood in the rim and dimples becomes unstable, inertia breaks the structures apart resulting in the creation of satellite spatter that traditionally forms the crown or blossom characteristic of stains that strike a surface close to 90°. Stains striking the target surface at a more acute angle produce wave cast-off spatter where the blossom effect occurs more in the form of a wave on one side of the rim. The wave creates spine like structures that in turn may create further satellite droplets.
Retraction: this is the final phase of stain development and appears to result from the effect of surface tension trying to pull the fluid of the stain back into a single form. Inertial forces present in the moving fluid are overcome by surface tension and the liquid retracts, as the two different forces compete; inertia pushes the forming spines away from the parent stain and surface tension tries to retract the liquid in the spine. In very acute angle impacts, the forces of inertia overcome surface tension and satellite spots will detach.
In liquid–liquid impacts (a blood droplet falling into another volume of blood) the same four phases occur. However, in the displacement phase, even more blood is forced into the expanding boundary as blood from both the droplet and the stain flow into the rim. This results in a less defined displacement phase and an earlier development of the dispersion phase. The blossom and dimples are more pronounced and the dimples will result in satellite spatter.
Ripple effect of liquid-liquid impact
Motion and Directionality
The ability to determine motion and directionality from bloodstain patterns can provide an investigator with information that may help him more fully understand the specific events occurring during an incident, potentially helping the investigator establish where an event began, and where it ended. Bloodstain pattern analysis can provide valuable information on:
- The sequence of events
- Droplet directionality
- Blood trail motion
In the investigation of bloodstain patterns, it's always important to remember that the largest amounts of blood are most often found at the site where an attack or affray ended. When an attack is first initiated there may only be small amounts of blood loss, which increases with subsequent physical damage to the victim. The more physical damage occurring, the more blood loss. As blood loss proceeds, shock and physical damage may decrease a victim's ability to escape and to remain mobile. When processing a crime scene, locations with limited or small amounts of blood spatter very often lead to areas with greater amounts of spatter or staining. As a victim sustains different wounds, spatter patterns may change but an increase in bloodstaining is usually be evident.
Sequence of Events
Consider a typical sequence of events occurring during a physical attack:
The first wound sustained by the victim may result in a small laceration or cut, producing a few blood droplets from damaged superficial veins. This might result in small amounts of spatter being deposited onto surfaces adjacent to the wound, at the location where the victim received the first blow. In this scenario, the victim is still able to move or escape to another area, where they receive a second blow. This assault produces spatter from the blood present from the first wound, and possibly inflicts more physical damage to the first wound, resulting in widening of the initial wound and a subsequent increase in blood flow to the area, with related further blood loss. At this point the victim may try to protect themselves against subsequent blows or be dazed by subsequent blows, causing decreased movement and mobility. This results in the victim staying longer at that particular location and the further accumulation of blood at that site. Throughout an attack, the victim's hands and clothing usually become stained with blood, which in turn may be transferred to walls, flooring and surrounding items. The more blood lost during the attack, the more staining that will be found. Using the staining patterns at the scene, it may be possible to determine the flow of the incident and the events that likely occurred.
One type of scenario where large amounts of blood are not likely to be found would be in a situation where the body has been removed from a primary scene to a secondary scene after bleeding has ceased. In this situation the absence of blood is very significant and in itself is an indication of sequencing actions.
An analyst can also use geometric stain parameters to determine the direction of flight before a blood drop impacted an object or surface. These parameters can be derived from the stain edge characteristics of individual stains and can provide very useful information for an investigation. Remember, blood impacts a surface in a defined way resulting in a stain with characteristics that can indicate the directionality of the impacting droplet. These characteristics, and therefore droplet directionality, are usually pretty obvious to the investigator, unless the droplets impact on carpeting or irregular, porous surfaces, whereby determination of directionality can become quite difficult.
As a droplet impacts a target surface, inertia keeps the droplet moving in its original direction. Unless the droplet impacts a surface at 90°, the resulting stain will have a long axis and a short axis (major and minor axes). The long axis is always aligned with the direction of the stain. The long axis defines two possible flight paths for the stain, but it is the presence of scallops, spines and spatters that help define which of the two directions the droplet is traveling. Also, as a general rule, the narrow end of an elongated bloodstain usually points in the direction of travel.
With stains that impact at angles between 75° to 90°, spines and satellite stains may occur all around the stain, however, as the angle of impact decreases, the spines and satellites tend to become more prevalent on one side of the stain compared to the other. The greatest concentration of spatters will be on the side following the direction of travel. Interpretation of impact angle for more circular stains needs to be conducted carefully since the long and short axis may be difficult to resolve. As the angle of impact decreases below 40°, the stain becomes much more elliptical, and the nature of outflow of the stain usually produces a single satellite stain. Satellite stains resulting from impact at acute angles have directionality that usually follows that of the parent stain, although minor redirection may occur due to surface target irregularities and texture.
As an injured individual moves around, or as bloody items are moved around a scene, then blood trails may be produced. Blood droplets break free form the blood source, fall, and strike the surrounding floor and surfaces traveling with the same momentum, and generally in the same direction, as the item from which they fell. The combination of gravity and momentum cause the droplets to hit their target surface at varying angles. Stains in a blood trail show evidence of this angle and directionality, enabling an investigator to determine in which direction a trail leads.Blood Trails
The shape of the stain may be able to give an indication of the speed at which the source item of the blood was traveling. As the speed of horizontal movement increases, each droplet falls with greater forward momentum and angles of impact become more acute; creating stains that are even more elliptical. However, increased horizontal speed also increases horizontal air resistance. (Think of spitting straight ahead while riding your bike.) As the vertical drop distance increases, the forward moving blood drop encounters air resistance for a longer period of time which counters the forward movement of the blood, pushing the blood in the opposite direction, subsequently decreasing the angle of impact, producing stains that are less elliptical.
Motion and Directionality (continued)
Wipes and Swipes
Blood wipes and swipes also play an important role in determining motion at a scene. As items become bloody and stains are formed, events at the scene might still be unfolding. These ongoing events may disturb previously deposited stains. The evidence of motion is more obvious in wipes formed from preexisting stains, or when a blow creates spatter that is later disturbed. Spatter has specific boundaries that are obviously disturbed as the stain is wiped in the same direction as the item disturbing it. The direction of motion in such instances is quite obvious.
When swipes are formed, the direction of motion might not be so obvious, and generally depends on how the bloody object came into contact with the target, since the leading and trailing boundaries of the stain might be similar or very different. If the leading and trailing edges are similar then there may be no means of establishing the direction in which the swipe occurred.
In most cases, direction of motion may be defined by assessing the thinning of a stains appearance or color. As blood is deposited on the target, the amount of blood available for swiping and smearing against the target decreases, resulting in the final portion of the swipe having thinning characteristics. The thinning effect may be accompanied by trailers of stain leading away from the parent stain, left behind as the bloody object loses contact with the target surface. This phenomenon is known as feathering. Feathering may be seen in both leading and trailing boundaries, something often seen with hair swipes, and can make interpretation and determination of directionality very difficult for an investigator. Feathering on one side of a stain is generally considered to be a significant characteristic that enables directionality to be determined.
Transfer Pattern Repetition
Repetitive pattern transfers can also be used to determine the occurrence of motion at a scene. A repetitive pattern occurs when an item becomes bloodied and then comes in repeated contact with a given target. Bloody hands, feet, shoes, and socks usually cause these kinds of patterns and can occur when a person steps into a pool or puddle of blood. As the person walks, blood is deposited onto the target surface each time the shoe contacts the target. This continues until the blood source is depleted. Directionality can be determined from the basic pattern if the item itself has directionality, such as a footprint or handprint, or as the pattern dims or diminishes as the number of contacts increases. When trying to determine the object used to make the pattern, the stains created later in the series are often are clearer in detail than those at the beginning of the series, and may provide individualizing characteristics of use to the investigator.
Blood flow patterns can also provide a lot of information about motion. Since blood flow follows the fundamental laws of physics and gravity, flows that indicate body movement after the onset of blood flow may be of interest to the investigator. Irregular flow patterns, or patterns that are different from those expected may provide information relating to earlier positioning of the victim. Irregular flow may occur due to capillary action, or if the flow is dammed by the presence of a large object, which may in some instances be the body or body parts, the direction of flow may follow the contour of the object. If the object or body is later moved then the flow pattern might seem out of place with what was expected.
Blood is transferred when a wet, bloody object comes into contact with another object or surface, resulting in a blood transfer pattern that mirrors the original surface. In reality it takes very little blood to produce a stain and to transfer it to other objects. These types of patterns can be created by bloody footwear, fingerprints, hairs, and fabrics. Pattern transfer stains are often overlooked or not given much consideration when interpreting blood staining at a crime scene, but can often provide class or identifying characteristics that may be of significant use to the investigator. Pattern transfer stains have the most evidentiary value when they can provide information that helps identify the object that caused the transfer. In most situations the information obtained can be very limited and will be reported as being consistent or inconsistent with a particular item.
Most pattern stains include class characteristics, but with the appropriate expertise and experience, a tool mark examiner or fingerprint analyst may be able to match individualizing characteristics, but they rarely obtain enough information to make an absolute identification. Class characteristics may be sufficient to correlate the pattern information with objects recovered at the scene. Analysts often need to conduct comparison experiments to determine whether an object is consistent or not with a recovered pattern. Experimentation however, can never recreate the same pattern since the same amount of blood, the orientation of the weapon and the target surface can never be exactly reconstructed.
Pattern Transfer Evaluation
An analyst can take two different approaches when evaluating pattern transfers. They can:
- Look for a unique pattern
- Look for a distinct pattern or defect in the stain
- Compare this distinct characteristic or defect with any class characteristics in the standard or suspect weapon/item
- Look for class matches but try and individualize it if possible
- Remember, exact matches are unlikely because the stain would have been created under changing and complex conditions
- Compare with a range of possible sources to screen out unlikely objects
- Always get others' opinions to minimize subjectivity in the analysis
Another approach is to:
- Create a range of standards for comparison – this may be a group of suspect items recovered from the scene
- Compare these standards with the stain and look for class characteristics
- “Screen positive” samples are compared by making clear transparencies of the object and the satin and then evaluating them using a light box and overlaying the transparencies
All patterns are created through some dynamic event but the resulting stains can be significantly affected by the dynamics of the event. Changing the orientation of the weapon or item, relative to the target can result in very different stains.
Repetitive Patterns and Prints
The most common transfer patterns encountered at crime scenes are fingerprints, handprints, footwear prints and footprints. These prints can help the analyst define motion that occurred at the scene, and by combining this information with repetitive pattern transfers, the investigator can possibly determine the specific actions and movements of the individuals involved.
Hair swipes are classic transfer patterns that are often found at crime scenes. These stains usually contain v-like patterns, called bifurcations that are highly characteristic of hair transfer patterns. Pattern transfers should always be evaluated and photographed early in scene processing, since post event transfers can occur very easily when police, first responders and medical personnel move items during scene or victim processing. Removing or moving wet objects often causes some form of stain transfer. Personnel present at the scene may also track blood as they move around during processing or walkthrough. Movement of the victim can also result in stain transfer, stain alteration and inadvertent blood flow.
Pools and Standing Blood
Flow patterns occur from the change in the shape and direction of a wet bloodstain due to the influence of gravity or movement. During a crime scene investigation the investigator needs to differentiate and determine those stains resulting from passive blood flow or those from active blood flow. Passive flow is blood flow created by gravity alone, with no circulatory action involved and is usually influenced by body position. Investigatively, these stains usually have little significance unless they point to the movement or rearrangement of a body after death. Active blood flow occurs when the body is still alive and bleeding occurs. Active flow can help an analyst or investigator in placing the victim at different locations in the scene while events unfolded.
Blood flow always follows gravity, so flow marks on a victim's body can provide useful information as to the body's position and whether the body has been moved at any time. During evaluation of blood stain patterns the investigator should always be aware of the possibility of abnormal blood flow patterns. Interruption of a flow pattern can also be significant in an investigation. At the point of interruption an object stemming the flow may leave a transfer pattern. If the flow stops and blood congeals then this can provide information on the timing and the sequence of events. In some cases the analyst may find pools or standing blood at the scene. Considering the drying time, the volume of the blood and the clotting time, significant information about the overall incident can be gleaned from such stains.
The time it takes for a stain to dry, is an important parameter in timing events that may have occurred at a scene. Generally a dried ring forms around a wet blood droplet (skeletonization) within 50 seconds of it hitting the target surface. This outer ring remains in place even if the rest of the stain is wiped away. The complete drying time for a droplet depends on the nature of the target surface and the environment surrounding the stain. Some droplets can take as long as 20 minutes to completely dry. Skeletonization can also provide information as to whether a stain was disturbed or not shortly after hitting the target surface.
Clotting events can also give an indication of the time lapsed between blood shed and discovery of a stain. Considering the three phases of clotting: initiation, formation and retraction; initiation begins between 10 seconds and 1.5 minutes after bloodshed. Clot formation, the point where if the clot is disturbed there is no flow back, begins 5 to 20 minutes after bloodshed. Retraction, which is separation of the serum from the fibrin clot, occurs 30 minutes to 1.5 hours after blood shed. Serum is the fluid portion of blood that remains after clot formation; this fraction does not include clotting factors and formed elements. Serum itself can cause staining often seen as a clear yellowish stain with a shiny surface appearing around a bloodstain after the blood retracts due to clotting. Retraction time is affected by temperature, humidity, the nature of the target surface and air movement, but primarily the target surface and ambient temperature.
The volume of blood in a standing stain or pool can be important in the absence of a body. The volume present can help determine whether the victim could survive the apparent severity of the wound or wounds. There have been cases where the volume of blood present at a scene has indicated that the victim is undoubtedly dead. In this situation the volume of blood can only be estimated since a number of factors will affect the amount of blood present including surface characteristics, the amount of congealed or clotted blood and the absorbency of the surrounding area. Experimental estimates on blood volumes of characteristic stain have included processes of weighing dried blood stains, stain crusts and dried clots. Alternatively, wet volume estimates have been used to estimate stain volume. In this process the analyst tries to recreate a similar sized stain, measures the area of the stain and then estimates the volume within the stained area. Using a similar target substrate the analyst pours blood into the area while monitoring the size of the stain as it develops to provide a rough estimate of the amount of blood it would take to create that stain. These methods are not without their limitations.
Pools & Standing Blood: Splashed Blood
Blood splashes occur when large volumes of blood (> 1 mL) meet low velocity forces or fall freely to a surface. Splashes have large central areas and elongated peripheral spatters. Secondary spatter (ricochet) can occur when blood is deflected from one surface to another. If bleeding is severe enough, the movement of the victim or assailant can also produce splashing. Blood dripping into blood can create characteristic patterns that may be useful in determining the events at a scene.
Drip patterns are bloodstain patterns resulting from blood dripping into blood which typically occurs when a bleeding person stays in one position long enough for dripping to occur. This pattern has two distinct characteristics: random satellite spatter that surrounds the pool; and the formation of an inverted fan shape on any adjacent vertical surfaces (such as an adjacent wall). As blood droplets fall into a pool of blood, they form large bloom-like structures.
Satellite spatters detach from the bloom and are projected outward from the pool creating small spatters, 1-2 mm in length, that have varying angles of impact. This spatter can radiate outwards several feet from the pool. Droplets that fall into the pool create a rolling or wave effect within the pool that sends the spatters in random directions. These satellite spatters are often small enough that they are easily confused with other impact spatter. As droplets detach from the bloom, they follow traditional parabolic flight paths and will impact any vertical surface in the close vicinity. The closer the wall or vertical surface, the earlier the droplet will be interrupted in its parabolic trajectory. This will create a pattern on the vertical surface that resembles an inverted fan; the base of the fan is the point closest to the dripping action. Drips may indicate a person was at the one location for a few moments, but the actual duration is generally difficult to estimate. These patterns are different from blood trails, which provide evidence of continuous motion.
Clothing and Fabric Stains
When analyzing bloodstains, the clothing of an alleged assailant can provide very important information. Analysis of stains on a garment can provide valuable information on how the blood was deposited on the garment. In most cases the analyst sees these types of stain once evidentiary items are submitted to the laboratory for further analysis. The analyst has a role to determine whether the stain and patterns created result from the events at the crime scene and not from handling of the items during scene processing or from transport of the items from the scene to the laboratory. Saturation stains are irregularly shaped stains caused by contact with a significant blood source such as pooled blood or blood flow. Large saturated stains usually cause the most transfer in these circumstances, especially when bodies are wrapped up in body bags and manipulated for transport. These types of stains should be documented and recorded before the body is bagged.
Clothing examination needs to take place in a laboratory area that has enough space to lay out the items and to manipulate them. The area should be covered with clean white butcher's paper and the analyst must wear gloves during the examination to prevent cross contamination (and for health and safety reasons). Selected stains should be recovered for serology tests after the blood stain patterns are examined and recorded since cutting up the stain will alter the stain pattern. Blood stain patterns can often be difficult to see on clothing items due to the color and weave of the fabric. The presence of decorative patterns and the absorbency of the fabric can lead to stain distortion. The greatest distortion occurs with very absorbent fabrics with course weaves; fabrics with tighter weaves tend to show less distortion. The age and condition of the fabric (new or washed) can also alter stains.
Blood deposited passively includes transfer, flow patterns, saturation stains and drips; Blood deposited actively includes impact spatter, arterial spurts, expirated stains, and castoff.