Key concepts
Physics
Viscosity
Blood

Introduction
With Halloween just around the corner, pretend-bloody scenes are just a block away. Whether it is dripping vampire teeth or a leaky bandage, fake blood is sure to bring characters to life—so to speak.

Although chocolate syrup might make convincing fake blood in black-and-white pictures, it is not so believable in full color—or in real-life encounters. In this activity science helps you engineer your latest product: good-looking (and tasty!) fake blood.

Background
Several physical quantities, including the viscosity and color, determine the look of a fluid. Viscosity is a measure of the thickness and stickiness of a fluid. It quantifies its resistance to flow. Together with its density, the viscosity of blood determines how blood flows through blood vessels—or out of your finger after a paper cut. At normal body temperature (37 degrees Celsius), blood is four times more viscous than water but only slightly denser. As a result, it flows more slowly and it is stickier than water. Corn syrup or honey, on the other hand, are far more viscous (more than thousand times) than blood. They are also denser. Diluting these with water will reduce their viscosity.

To get the color of human blood right seems easy—it is always red when we see it. The red color comes from red blood cells, which carry oxygen from the lungs to tissues and organs. They turn bright red when exposed to oxygen.

Clotting (or coagulating) blood has a different look. The chemical reactions taking place that clot blood make it look darker. It is also thicker and more gel-like.

Now that you know a little about what blood looks like, you are better equipped to create fake blood.

Materials

  • Three small cups or glasses (more if you want to test more recipes)
  • Flour
  • Water
  • Corn syrup
  • Chocolate syrup
  • Red and blue liquid food coloring (Use it carefully, food coloring can stain. Work on a protected surface, and clean up any accidental spills right away.)
  • Cutting board or plate (preferably white or a light color)
  • Three small spoons (teaspoon-size ones work well)
  • Measuring spoons
  • Sink or bowl with water to rinse cups and the cutting board
  • Towel to dry off the cutting board
  • Protected area that will not be stained by food coloring
  • Old clothing that can be stained by food coloring
  • Paper and pencil (optional)
  • Other ingredients to try, such as ketchup, coffee grounds, maple or pancake syrup, cornstarch (optional)

Preparation

  • Protect your workspace and clothes—the fake blood can be sticky and difficult to wash off.
  • Gather your ingredients at your workspace.

Procedure

  • Mix one tablespoon of flour in two tablespoons of water in one cup. (You can keep track of all of your recipes with a pencil and paper if you like. This will allow you to easily re-create your best fake blood again!)
  • Measure three tablespoons of corn syrup into a second cup.
  • Measure three tablespoons of water into a third cup.
  • Add two drops of red food coloring to each cup and watch how the food coloring disperses over the fluids. Does the red food coloring disperse in the same way in all three liquids?
  • Use small spoons to mix the red food coloring with the other ingredients in all three cups. Be sure to use a dedicated spoon with each cup.
  • Use the small spoons to take approximately one-quarter teaspoon of red liquid from each cup and place the liquids as separate blobs in a row near the edge of a cutting board or plate. Lift the side of the surface with the blobs so the board is tilted. Now watch how the liquids run down. Do all three run down with the same speed? Do some leave a trace behind whereas others do not? Which liquid do you think would make the best fake blood?
  • You probably noticed that water runs way too fast to mimic blood well. Discard the contents of this cup. While you are at the sink, rinse off the cutting board and wipe it dry so it is ready to perform more tests.
  • Reuse this now-empty third cup and in it, pour half of the contents from each of your other two cups. (If you'd rather start over for the third cup, here is the recipe: half a tablespoon of flour, one tablespoon of water, one and a half tablespoons of corn syrup, all mixed together with two drops of red food coloring. If you are keeping track of your recipe, note what you have added.)
  • Perform your test again, placing small amounts of the three liquids on top of the cutting board or plate, tilting the board and watching how the liquids run down. Do any of these mimic how blood might run? Which one would you say comes closest?
  • If you found that one recipe mimicked blood better than the others, continue with that recipe. If you did not find a reasonable match, proceed with using the third cup with the flour, corn syrup, water and food coloring. You can still adjust how your fake blood flows later in the activity.
  • In addition to flowing like real blood, its color needs to mimic the color of real blood. Human blood is always red. Oxygen-rich blood, like the blood from a finger cut or a bloody nose, is bright red. Blood turns dark red when it has delivered its oxygen to tissue and organs. Blood drawn by a nurse has this color. Note that the blood vessels you see through your skin only appear blue; this is because light reflecting off them is filtered by your skin before it reaches your eye. If you opened up your skin, the blood vessels and the blood inside them would look red. Which type of blood would you like to create?
  • To get a realistic look, add a tiny bit of blue food coloring and/or a little chocolate syrup. Note that the blue food coloring is very strong. It is best to make a drop of blue food coloring on your cutting board or plate, dip the tip of your mixing spoon in it, and use this spoon to stir the tiny amount of liquid in your cup. It is better to add gradually, because once you have too much blue, it is hard to correct. A tiny bit of blue or brown will mimic the bright, oxygen-rich blood. Add a little more for the darker blood that is low in oxygen.
  • Place a blob on the top of your cutting board or plate, tilt the board and watch it flow. Does it look realistic? Are the flow and the color right? To improve your fake blood, add corn syrup, flour, water, food coloring (red or blue) or chocolate syrup—one at a time. Check the results after each addition and, if you are noting your recipe, do not forget to keep track of what you added.
  • As a last test, try your blood on your skin, teeth or other surface where the food coloring will not stain (or can stain, such as on a costume you are preparing for Halloween). Does it look like real blood?
  • Extra: To mimic dried blood, add more chocolate syrup to make it thicker and darker—clotting blood has a darker red color with a brown hue and is thicker, more gel-like. You can also add a spoon of coffee grounds to mimic the scaly appearance of the crust on healing wounds.
  • Extra: Test other ingredients you find around the kitchen for their ability to mimic blood. See how they flow, how they can add to the color. Some ideas include ketchup, red beet juice, maple syrup and peanut butter.

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Observations and results
The main ingredients of your final recipe probably consisted of corn syrup diluted with water and thickened with flour. This particular mixture resembles the flow of blood quite well because it has a similar viscosity, or resistance to flow. You probably noticed the red-colored corn syrup is too viscous, it flows too slowly and leaves a thick trace behind. The red water was way too runny and left almost no trace on the board. The flour helped to thicken the mixture, but did not get the viscosity quite right.

The recipe to obtain the most realistic color depends on the exact hue of food coloring used. Most often, a tiny bit of blue yields a more realistic look. The brown from chocolate syrup often provides a good addition in color, especially if you want to mimic blood that is low in oxygen. It also reduces the transparency of the fake blood.

Now that you have some experience, you might just find more great recipes using other food items available in your kitchen. Enjoy your realistic (and edible) creation—and happy Halloween!

Cleanup
Wash all equipment with soapy water.

More to explore
Marble Race—in Liquid!, from Scientific American
What Is Viscosity?, from Princeton University
What's Blood?, from KidsHealth
Crime Scene Science: Solve Mysteries by Investigating Drops, from Scientific American

This activity brought to you in partnership with Science Buddies

Science Buddies