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BLOOD. Drops and Spatter. Surface Tension. The liquid in a drop of blood is held together by a cohesive force known as surface tension Surface tension keeps a drop of blood in a spherical shape as it falls
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BLOOD Drops and Spatter
Surface Tension • The liquid in a drop of blood is held together by a cohesive force known as surface tension • Surface tension keeps a drop of blood in a spherical shape as it falls • If any of the blood overcomes the cohesive force and separates from the main drop of blood, it will form secondary droplets known as satellites
Satellite Blood Drop
Blood Drops • Blood drops are subject to the same principles of physics as any free-falling object • The force of a free-falling object can be expressed by the formula: F = ma where F = force m = mass a = acceleration due to gravity (9.8 m/s2)
Blood Drops (cont.) • The longer an object is falling, the greater the speed of descent and thus the greater the kinetic energy • In actual crime scene analysis, the most important variable to consider and try to control is the contact surface itself • When striking a surface, the blood will leave a pattern that is very much dependent on the type and nature of the landing surface
Blood Drops (cont.) • The blood drop will not break apart on a hard and less porous surface (i.e. glass) • A soft and porous surface (i.e. concrete) causes the blood drop to break apart GLASS CONCRETE CARDBOARD
Blood Drops (cont.) • The shape of a blood drop can indicate the distance from which the blood fell and the angle of impact • Round drops: • The drop struck the surface at a 90o angle • If the edges of the drop are smooth, it indicates a low velocity • If the edges are ragged, it indicates a high velocity
Blood Drops (cont.) • Elongated drops: • The drop struck the surface at an angle greater than 90o • The larger the angle, the more elliptical the droplet • The tail of a drop indicates the direction the blood travels • Smaller drops are produced during active situations (i.e. struggle) • Very small drops indicate that the wound was produced by a powerful force (i.e. gunshot)
Directionality of Blood Drops • When blood comes into contact with another surface, the blood tends to stick or adhere to it • As a result, the point of impact may appear to be darker and wider than the rest of the drop • Momentum tends to keep blood moving in the direction it was traveling, but cohesion keeps most of the blood as one drop
Directionality of Blood Drops (cont.) • As blood droplets move away from their source, they elongate and produce a thinner tail-like appearance • The tail points in the direction of blood’s movement • Satellites may appear in front of the moving droplet of blood Direction Of Blood Movement
Lines of Convergence • If there are at least two drops of blood spatter, the location of the source of blood can be determined • By drawing straight lines down the long axis of the blood spatter and noting where the lines intersect, a point of convergence can be determined
Height of Blood Drop • Once the point of convergence has been determined, the angle of the blood drop can be used to determine the height at which the blood drop originated • Measure the length and width of the drop • Use the following formula to find the angle: sin x = width/length Length Width
Height of Blood Drop (cont.) • Once the angle has been determined, use this and the length from the point of convergence to the blood drop to determine height • Use the following formula: • tan x = height/distance to pt of convergence • where x = angle of blood drop
Example • A drop of blood found at a crime scene measures 5 mm by 15 mm. The point of convergence is found to be 120 cm away. At what height, in cm, was the wound that caused the blood droplet? What is the height in ft?