Objectives
By the end of this activity, you will be able to:
A blood-spatter pattern is created when a wound is inflicted and blood leaves the body. This pattern can help reconstruct the series of crime-scene events surrounding a shooting, stabbing, or beating. Recall that blood forms droplets as it falls from a wound. A drop of blood that falls on a flat surface will not totally flatten out—the blood drop will have a domed surface. The reason for this shape is the internal cohesive nature of blood. Blood tends to pull together because of cohesion and resists flattening out on a surface. The result is that the surface of the blood is elastic, giving the top of the blood spatter a rounded appearance.
If any of the blood does overcome cohesion and separates from the main droplet of blood, it will form small secondary droplets known as satellites.
Materials
1 bottle of simulated blood
White Paper
Meter sticks
Rulers
Drop cloth
Safety Precautions
Procedure
By the end of this activity, you will be able to:
- Prepare reference papers of blood spatter dropped from varying heights.
- Compare and contrast the blood spatter produced from different heights with regard to size, shape, and number of satellites.
- Distinguish between the parent drop and satellites.
- Distinguish between satellites and spines.
- Analyze the results of your experiment, and prepare a summary of the effect of height on blood-spatter stains.
A blood-spatter pattern is created when a wound is inflicted and blood leaves the body. This pattern can help reconstruct the series of crime-scene events surrounding a shooting, stabbing, or beating. Recall that blood forms droplets as it falls from a wound. A drop of blood that falls on a flat surface will not totally flatten out—the blood drop will have a domed surface. The reason for this shape is the internal cohesive nature of blood. Blood tends to pull together because of cohesion and resists flattening out on a surface. The result is that the surface of the blood is elastic, giving the top of the blood spatter a rounded appearance.
If any of the blood does overcome cohesion and separates from the main droplet of blood, it will form small secondary droplets known as satellites.
Materials
1 bottle of simulated blood
White Paper
Meter sticks
Rulers
Drop cloth
Safety Precautions
- Cover the floor in the work area with a drop cloth.
- Simulated blood may stain clothing and surfaces.
Procedure
- Spread the drop cloth on the floor of the work area.
- Cut each piece of paper in half so you have a total of 6 sheets measuring 5.5in x 8.5in.
- Label the top-right corner of each paper with the height of the blood drop and your numbers.
- Place the labeled papers on the drop cloth.
- One of the partners holds a meter stick vertically over the top of the paper. The other partner holds the dropper of blood 30 cm above the paper, and aiming toward the top of the paper, releases one drop of blood.
- Add a second drop on the same paper from the same height. Avoid dropping the blood in the same location.
- Repeat this process, dropping blood from heights of 50, 100, 150, 200, and 250 cm. Remember to prepare a new paper for each height.
- Do not stack the papers until the blood has dried. Put them in a location designated by Mr. U.
- Measure the diameter at the widest part of each drop of each of the spatter patterns to the nearest millimeter and record the data in the Data Table. Do not include any satellites or spines in your measurement.
- Determine the average diameter for the blood spatter for each height, and record it in the Data Table.
- Prepare a graph comparing the effect of height on the average diameter of the blood drop. Your graph should have a title, labeled x-axis and y-axis, and an appropriate scale.
Questions
- Is there a relationship between the height from which the blood is dropped and the size of the blood-spatter droplets? Support your conclusion with data from your experimentation.
- True or False: As the height from which the blood is dropped increases, the size of the blood spatter continues to increase. Support your answer with data from your experimentation.
- Blood is dropped from heights of 30 cm and 250 cm. Compare and contrast the outer edges of blood droplets produced from these two heights.
- Examine the blood spatter produced by dropping blood from the six different heights. Is there a relationship between the height from which the blood is dropped and the number of satellites produced? Support your answer with data from your experimentation.
- Compare your results with those of your classmates.
- Were your results similar to those of your classmates? If not, how did they differ?
- If someone accidentally dropped two or more drops of blood in the same location, what effect would it have on the blood-spatter pattern?
- If blood were dropped from a roof, would the diameter of the bloodstain be larger than the diameter of the bloodstain dropped from 250 centimeters? Support your claim with data from your experimentation or from research.
- A drop of blood will continue to pick up speed until it reaches its terminal velocity.
- What is terminal velocity?
- What factors affect the terminal velocity of a substance?
- What is the terminal velocity of blood?
- How far does blood need to fall until it reaches its terminal velocity?
- What effect does the surface of a target have on the shape of the blood-spatter stain? Design an experiment to test the effect of the target surface on the shape of the bloodstain. Your experiment should investigate four different types of surfaces.
- Using the terms adhesion and cohesion, provide an explanation for the variety of shapes of bloodstains found on different target surfaces.