What tells you the direction of travel of a blood droplet?

Blood behaves not unlike spilled water droplets, and the speed at which the droplets travel when they strike a surface — known to analysts as a target — affects their shape. This speed, combined with angle and surface characteristics, also determines how far blood droplets skip or bounce after meeting a barrier.

One pattern of slow-moving blood, called "drips," occurs after an injury, and has a relatively large footprint of 0.16 inches (4 millimeters) or more. Drips, which result from blood dripping onto blood, can fall from a bleeding nose or wound, or a motionless, bloodied weapon or object. A moving object produces what's known as a cast-off pattern. Other low-velocity patterns include blood pooling around a victim's body and impressions left by bloody objects. This latter phenomenon, called a transfer, sometimes retains the shape of the object that made it [source: Wonder].

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At the other end of the scale are the tiny droplets caused by blood traveling at high speeds. These are usually caused by gunshot wounds, but they can also result from explosions, power tools or high-speed machinery. These fast-moving drops leave stains measuring less than 0.04 inches (1 millimeter) across.

Bullet wounds can produce both back and front spatters. Back spatter, or blowback, refers to blood exiting the entrance wound in the direction opposite the impact [source: Dutelle]. Actually, thanks to Newton's Third Law of Motion, back spatter can result from other impacts and traumas as well.

Investigators dealing with such small drops must rule out other sources of blood spray, such as respiration or pinhole arterial pressure [source: Wonder]. Forward spatter, in the direction of the impact, occurs only in the case of an exit wound [source: Dutelle].

Between these extremes lies a range of medium-sized droplets. Typically measuring 0.04 to 0.16 inches (1 to 4 millimeters), they can be caused by a blunt object such as a bat or a fist, or can result from stabbing, cast-offs or even bloody coughs [sources: Dutelle, Wonder].

Several factors complicate their analysis. For example, during a beating or stabbing, arterial damage can cause the subject to bleed faster or to spurt blood, the latter creating what's known as a projected pattern [source: Dutelle].

In addition to spatters, analysts look for voids, aka blockages. In the case of a high-density spatter, these gaps in the pattern indicate that something in the way, potentially the assailant, caught some of the victim's blowback.

Drop size is only one aspect used in analyzing blood spatters. Next, we'll look at the shapes of spatters and how analysts use strings, trigonometric functions and computer programs to map out a blood-spattered crime scene.

The Way of the Gun

In close-range gunshot, hot gunpowder exiting the gun barrel can bruise, burn or penetrate the skin in a powder-stippling pattern, which analysts can use to estimate distance [source: Hueske]. Back spatter can cause external and internal muzzle staining, but in unusual cases the internal stain can also result from the cooling of explosive gases released by the gunshot, which suck the victim's blood into the gun's muzzle [source: Evans].

Often found at the scenes of violent crimes, the analysis of bloodstains can provide vital clues as to the occurrence of events. Though bloodstain pattern analysis (BPA) can be a subjective area of study at times and often reliant on the experience of the investigator, the idea that blood will obey certain laws of physics enables the examination of blood at an incident scene and on items of evidence to offer at least an insight into what was likely to have occurred.

The successful interpretation of bloodstain patterns may provide clues as to the nature of the offence, the possible sequence of events, any disturbance to the scene that may have occurred, and even the position of individuals and objects during the incident. It may prove beneficial in refuting or corroborating eyewitness accounts.

Types
The appearance of a bloodstain can depend on a number of factors, including the velocity at which it was travelling, distance travelled, the amount of blood, the angle of impact, and the type of target onto which it lands.

Single Drop
These bloodstains typically refer to blood drops that have fallen vertically, whether it be from an injured person or another object, and landed onto another surface. As a blood drop falls perpendicular to a surface it maintains a spherical form until impacting. The size and appearance of this stain will depend on a number of factors. The volume of a single drop of blood will vary depending on the quantity of blood present and the surface area available from which the drop is falling. As would be expected, a larger surface area would allow for larger drop of blood to form before falling. The height from which the blood falls will affect the size of the stain, with greater heights tending to result in larger bloodstains. Furthermore, the target surface itself will cause an effect, with absorbent surfaces usually producing smaller stains than non-absorbent targets. The nature of the target can alter the appearance of the stain. For instance, a rough target surface can result in increased distortion to the stain and even satellite stains, which are additional stains radiating outwards. A drop of blood falling into an existing bloodstain will result in a drip pattern.

Impact Spatter
This type of bloodstain is the result of a forceful impact between an object and wet blood, causing the blood to break into smaller droplets. A greater force will typically produce smaller droplets, with the density of blood drops decreasing moving further away from the initial blood source. The study of impact spatter may provide insight into the relative position of individuals and objects during an incident and the nature of the incident.

Cast-Off Stain
Cast-off bloodstains occur when centrifugal force causes blood drops to fall from a bloodied object in motion. Similarly, cessation cast-off patterns may result from the sudden deceleration of an object. In this instance, the blood flung from a blood-stained object, such as a weapon, may produce characteristic patterns of numerous individual blood drops forming a curved or straight line. If an object is repeatedly moved, each subsequent swing will result in less cast-off as less blood remains on the object. Bloodstains produced in this fashion can be particularly difficult to interpret as there is a great deal of possible variation in patterns produced. However depending on the nature of the motion of the bloodied object, cast-off blood will at least produce relatively linear stains.

Transfer Bloodstains
Transfer or contact stains result when a bloodied surface comes into contact with another surface, transferring blood to that secondary target. The study of this type of bloodstain can prove particularly beneficial in establishing a sequence of events at the incident scene and tracing the movement of objects or individuals. In some cases it may even be possible to establish what object the transfer stain was likely to be caused by, for instance if a particular pattern is produced that can be traced to a blood-bearing object. Similarly, such bloodstains may be left by the hands of an individual, thus opening the possibility of fingerprint evidence.

Projected Pattern/Arterial Damage Stain
This type of bloodstain results from the discharge of pressurised blood onto a target surface, for instance the ejection of blood from a punctured artery. Areas of the body in which wounding may cause arterial bloodstains include the carotid artery, the radial artery in the wrist, the femoral artery in the inner thigh, the brachial artery in the arm, temporal regions of the head, and the aorta (though damage to the aorta is less likely due to increased protection of the chest cavity). Blood is expelled from the artery as the heart continues to pump and, as the blood travels, it breaks up into smaller individual droplets. Bloodstains produced will usually represent the beating of the heart as blood is expelled in periodic spurts. The resulting bloodstains can vary depending on a variety of factors, including whether the victim was stationary or moving as blood was being ejected, where on the body the injury occurred and the extent of the wound. If a wound is smaller in size, naturally smaller blood drops will be produced, which can subsequently be expelled further from the injury site than larger blood drops.

Pool Stains
Pooling bloodstains refer to the accumulation of blood on a particular surface, generally from prolonged bleeding from a wound or accumulation of arterial blood. If a body is not present at the incident scene, depending on the quantity of blood present, it may even be possible to roughly estimate whether the victim is likely to be dead or alive based on how much blood they have lost.

Insect Stains
These are bloodstains resulting from insect activity. The presence of insects such as flies at an incident scene, particularly one involving blood, is not uncommon (see the forensic entomology page). Flies may feed on blood and tissues at the scene and then, following regurgitation or excretion, produce small circular stains known as flyspeck. This minute stain could be mistaken for alternative bloodstains, such as expirated blood. Furthermore, small additional stains may be caused by insects walking through a stain, thus spreading the blood.

Expiration Stains
Often associated with injury to the respiratory tract, this type of bloodstain is caused by blood being coughed or otherwise expelled from the mouth. The stains will often be slightly diluted in appearance due to the additional presence of saliva or mucous. When blood is expirated from the mouth, it will often produce a pattern of small, round stains that could be likened to a fine mist.

Examination of Bloodstain Patterns
Various factors must be taken into account in order to successfully interpret a bloodstain. The surface onto which the blood is found may have had an effect on the behaviour and appearance of the stain. For instance, a bloodstain pattern may appear different if landing on an absorbent surface such as fabric as oppose to tile or plastic. Studying the state of the bloodstain may be able to shed light onto how much time has passed since the blood was shed, as over time blood will naturally coagulate (the process by which liquid blood turns into a gelatinous substance through various clotting factors). Furthermore, the extent of drying or coagulation will depend on the quantity of blood present – for instance a single drop will dry significantly faster than a large pool of blood. During this process of coagulation serum stains may be formed, which occur when the serum (liquid portion of the blood) separates.

Bloodstains at an incident scene may not always be visible to the naked eye, either due to low amounts of blood present or an individual cleaning in attempts to remove signs of bloodshed. Despite the use of cleaning reagents or even attempting to cover the stains with paint, detectable traces will generally remain, which can be visualised using various chemicals or specialised light. Although blood will not fluoresce under UV light like some bodily fluids, it will significantly darken, thus enhancing its visibility. Furthermore, certain chemical reagents can be used to visualise latent bloodstains. These tests, such as luminol and phenolphthalein, generally work by reacting with a constituent of blood to produce some kind of chemiluminescence. However it should always be remembered that these chemical reagent tests are often presumptive, meaning that they can only indicate that the stain is possibly blood. In reality, other substances may react with the reagent in the same way.

A lack of a bloodstain can be just as revealing. The absence of blood in a continuous bloodstain is known as a void, and may suggest that something or someone was present in that area when the bloodstain was caused. This could indicate an object present at the time of the incident has been removed from the scene, or an individual (or even multiple individuals) were present in specific locations when blood was shed.

It can easily be incorrectly assumed that blood found at an incident scene belongs to a victim, however it must be taken into account that some bloodstains may have resulted from the perpetrator being injured at some point. Either way, the information available from the presence of bloodstains is not limited to bloodstain pattern analysis, but also DNA analysis. See the DNA analysis page for more information.

Point of Origin – Directionality and Angle of Impact
In the reconstruction of an incident scene involving bloodstains, it is often beneficial to establish the point of origin of bloodstains, based on directionality and angle of impact. The examination of certain bloodstains may allow for the determination of the direction of travel of blood as it impacted the target. Whereas a drop landing perpendicular to a surface (depending on the type of surface) will tend to produce a more circular pattern, those landing at an angle will result in an elongated stain. The tapered end of this stain will generally point in the direction in which the droplet was travelling. Small amounts of blood may break away from the parent stain entirely – these are known as satellite stains.

Although it may be possible to estimate area of origin purely through visual observation of bloodstain patterns, in some instances trigonometry may be utilised to determine a more precise point of origin. Depending on the type of bloodstain pattern, it may be possible to establish the angle at which a blood droplet hit a target, referred to as the angle of impact. By measuring the ratio of the width of the bloodstain to the length, it can be possible to calculate the angle of impact. If the angle of impact of multiple bloodstains is established, it may be possible to determine the area of convergence (the point where lines of travel from multiple stains meet) through stringing techniques and establish the area of origin.

Documentation and Collection
Documentation of bloodstain evidence will most typically be carried out using photography, including photographs of the wider scene along with close-up images of particular bloodstains. A ruler or other form of scale may be placed in the photograph in order to give perspective as to the size of a bloodstain. Sketches and even videos may also be utilised for further documentation. Collection of bloodstain evidence can be a complex matter, as the evidence will not likely be confined to a small object that can be easily removed from the scene. After rigorous documentation of the evidence, ideally the bloodstains themselves will be collected. This can involve simply removing objects from the scene or, more problematically, sections of carpet or large pieces of furniture. Evidence removed should be packaged in such a way that the stains are not altered or damaged. Collection of blood evidence for the purpose of DNA profiling will generally be conducted using a swab.

Jackson, A. R. W, Jackson, J. M., 2011. Forensic Science. Essex: Pearson Education Limited.

Scientific Working Group on Bloodstain Pattern Analysis. [online] Available at: [http://www.swgstain.org]

White, P. C., 2004. Crime Scene to Court: The Essentials of Forensic Science. Cambridge: The Royal Society of Chemistry.

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