Module 2: Bloodstain Pattern Analysis: Drops, Target Surface, Size, Shape & Directionality

Overview of bloodstain pattern analysis.
- An essential part of crime scene investigation.
- Provides insights into the nature of the event behind the blood stains.
Key aspects considered in bloodstain pattern analysis:
- Location: Where the stains are found at the scene.
- Shape: The geometric form of the bloodstains.
- Size: Dimensions of the bloodstains, important for analysis.
- Distribution: How the stains are spread across the scene.
- Physical characteristics: Other observed features of the blood stains.

Bloodstain patterns play a significant role in understanding what happened at a crime scene.
Recognizing classifications of stains is crucial, even for those not in-depth trained in bloodstain pattern analysis.
Proper documentation of blood stain scenes is imperative for credible analysis.

Basic understanding of blood as a fluid and its predictable behavior under physical forces.
Blood stain analysts like Dexter study patterns produced under controlled conditions, known as known patterns, and compare them against unknown patterns observed at crime scenes.
Predictability in blood behavior allows analysts to:
- Determine impact angles.
- Recognize directional flight.
- Identify alterations from environmental effects.

Blood is subjected to internal and external forces, acting in predictable ways.
Surface tension: The force that pulls surface molecules toward the liquid's interior.
- Decreases surface area and resists penetration by external objects.
- Blood’s surface tension is slightly lower than that of water.
- Liquid mercury has a surface tension almost ten times greater than blood.

External forces must overcome the surface tension of blood to create spatters.
Blood drop shapes in the air are influenced by molecular cohesive forces, resulting in a spheroid configuration (perfectly round).
Misconceptions addressed:
- Blood does not form in a teardrop shape.
A blood drop falls when its volume and mass exceed the cohesive forces holding it together.

The surface type influences the volume of the blood drop:
- Example: Blood drops from a fingertip are larger than those from a hypodermic needle but smaller than those from a baseball bat.
Typical blood drop volume is reported as 0.05 milliliters with an average diameter of 4.56 millimeters.
Blood drops achieve terminal velocity as the downward gravitational force balances air resistance.
- Terminal velocity for a 0.05 milliliter drop reaches approximately 25.1 feet per second after falling 20 to 25 feet.

Diameter of blood stains depends on various factors including:
- Volume of the drop.
- Surface texture of the target.
- Distance fallen (height).
Experimentation shows that blood drops from a height produce increasing stain diameters:
- Measurement range from 13 to 21.5 millimeters for free-falling blood from 6 inches to 7 feet.
Beyond 7 feet, stains do not show a significant increase in diameter.

Blood drops do not break into smaller droplets unless the surface tension is disrupted.
Characteristics of impact surfaces:
- Hard smooth surfaces (like glass): minimal distortion at the edges.
- Linoleum: may show scalloping on edges.
- Wood or concrete: causes greater distortion and may produce secondary droplets (satellites).

The geometry of bloodstains permits the determination of their flight direction.
An elongated bloodstain's narrow end points towards the direction of travel.
Establishing a point of convergence:
- By tracing lines through the long axes of several bloodstains.
- Defines the area where blood originated in 2D (X and Y axes).
Area of origin in 3D can be visualized by establishing impact angles and projecting back to a common axis.
- Normal stringing method used in analysis.

Angle of impact affects bloodstain shape:
- At 90 degrees: stain appears circular.
- Angles less than 90 degrees: elliptical bloodstains form.

Encourage viewing videos demonstrating how surface texture affects blood drops.
Resources provided to assist visual understanding of bloodstain behavior on various surfaces.