A bloodstain holds a wealth of information for forensic investigators. These patterns, left behind at a scene, act as silent witnesses, revealing significant details about an event. By examining their characteristics, forensic scientists can piece together the sequence of actions that led to the bloodshed. This study moves beyond simply identifying blood to understanding the dynamics of its deposition.
The Nature of Bloodstains
Blood exhibits specific physical properties that govern its behavior when it leaves the body and forms patterns. Surface tension is a property that causes the surface molecules of a liquid to be pulled together, minimizing the surface area and forming a “skin” that resists penetration. This force causes blood drops to maintain a spherical shape while in flight, similar to how water beads on a car surface.
Viscosity, a measure of a liquid’s resistance to flow, also plays a role. Blood is approximately four to six times more viscous than water, influencing how it spreads upon impact and the characteristics of resulting stains. Cohesion, the attraction between like molecules, works with surface tension to keep blood drops intact until an external force overcomes these bonds. These properties provide the foundational understanding for interpreting blood patterns.
How Bloodstains Form
Various mechanisms and forces contribute to distinct bloodstain patterns. Gravity is a primary force, causing blood to drip downward when no other external force is applied, leading to passive drops or pools. A blood drop detaches when gravitational pull overcomes the cohesive forces holding it to the source.
Impact is another common mechanism, where a force applied to liquid blood causes it to disperse into smaller droplets, known as spatter. The amount of force and the volume of blood involved influence the size and quantity of these spatters. Projection, such as arterial spurting or cast-off, occurs when blood is ejected under pressure or flung from a moving object. Transfer patterns arise when a blood-bearing surface comes into contact with another surface, leaving a recognizable imprint.
Types of Bloodstain Patterns
Bloodstain patterns are categorized by their appearance and the forces that created them. Passive bloodstains form primarily by gravity, with little to no external force. Examples include circular drops, which result from blood falling straight down, and pools, which form when blood accumulates. Flow patterns occur when liquid blood moves due to gravity on an inclined or uneven surface.
Spatter bloodstains are created when an external force is applied to a blood source, causing droplets to disperse. Impact spatter results from an object striking liquid blood, such as during a beating or gunshot. Cast-off spatter occurs when blood is flung from a moving object, in a linear or curvilinear pattern, indicating the arc of motion. Expirated spatter, fine mist-like stains, is produced when blood from an internal injury is expelled through the nose or mouth, containing air bubbles.
Transfer bloodstains are formed when a blood-covered object contacts a clean surface, leaving an impression. Wipes happen when an object moves through an existing wet bloodstain, altering its appearance. Swipes involve a bloodied object moving across a clean surface, leaving a pattern that indicates relative motion. Contact patterns are direct imprints, like bloody fingerprints or shoeprints. Altered bloodstains show changes due to factors like clotting, dilution, or insect activity, which can provide information about the time elapsed since deposition.
Unlocking Clues from Bloodstains
Forensic analysts interpret bloodstain patterns to reconstruct the events of a crime, a field known as Bloodstain Pattern Analysis (BPA). Directionality, indicated by the shape of a bloodstain, reveals the direction of travel. As a blood drop strikes a surface at an angle, the stain becomes more elliptical, with a “tail” pointing in the direction of travel.
The angle of impact, the acute angle at which a blood drop strikes a surface, can be determined by measuring the width and length of an elliptical stain. A nearly circular stain indicates a 90-degree impact, while more elongated stains suggest shallower angles. The point of origin, the three-dimensional location from which the blood originated, is determined by tracing multiple spatter stains back to a common area. This involves calculating the angle of impact for several stains and using string or computer models to find where their trajectories converge in space.
The sequence of events can be inferred from overlapping patterns or different stain types. For instance, a wipe pattern over a dried blood pool indicates movement occurred after the initial blood deposition and drying. BPA requires specialized training and expertise in fluid dynamics, physics, and mathematics to interpret these patterns and provide insights into a crime’s dynamics.