Tie Dye Milk

Age

Elementary School

Format

Hands-on

Materials

   Petri dish
   Whole milk
   Toothpick
   Liquid soap
   Food coloring
   A basin for disposal of the used milk
   Wash basin 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. If young children are allowed to perform the demonstration, watch them carefully to make sure they do not consume any of the materials. Make sure they do not keep the toothpicks - they are sharp and could be dangerous to small children.

Preparation

Pour a thin layer of milk into a petri dish, making sure that the milk covers the entire bottom of the petri dish.

Demonstration

Add a drop of food coloring to the layer of milk. Wet one end of the toothpick with a small amount of soap. Touch the soap to the milk near the food coloring to disturb the food coloring. It is not necessary to move the toothpick around in the milk! In fact, this will actually prevent you from demonstrating the concept this demonstration is meant to show – the decrease in surface tension should generate all the motion for you.

You may have audience members perform the demonstration. Try placing a few different colors around the milk to watch them mix.

When a demonstration is completed, pour the milk into the disposal basin and rinse off the dish in the wash basin.

What to Say

Ask the audience if they know what milk is made of. Give them hints until someone says that it's mostly water (cow's milk is almost 90% water), then make sure that they know that there are a variety of fats, proteins, vitamins, and other nutrients in milk.

Explain the concept of surface tension and show the audience how it keeps the drop of food coloring from dispersing throughout the milk. Describe surfactants. Then add the soap to demonstrate what happens with lower surface tension.

Ask the audience what would happen if we used fat free milk or water for the experiment. Since there are no fat molecules to disrupt, there would not be nearly as much of a change in surface tension (although the surfactant in the soap does cause some decrease in surface tension in pure water) – the colors would not move very much!

Why It Is

Cow's milk is approximately 87% water, 5% lactose (a carbohydrate), 4% protein, and 4% fat (Ref. 1). Fat is in an emulsion in milk – liquid fat is dispersed within water.

Molecules in a liquid are held together by intermolecular “cohesive” forces. Molecules in the center of a volume of liquid are surrounded on all sides by other liquid molecules, but molecules on the surface have a lot of area where there are no neighboring molecules. Surface molecules bond more strongly to the neighbors they do have than do molecules that are surrounded on all sides. Therefore there is a strongly-connected “film” at the surface of liquids that makes it hard for anything (such as food coloring) to move around on the surface.

The primary cleaning agents in common soaps are detergents composed of surfactants that break up grease. Surfactant molecules have hydroophobic (water “fearing”) and hydrophilic (water “loving”) ends. Essentially, the hydrophobic end is attracted to fat and protein molecules very strongly – it sticks itself in between chains of fats and proteins by breaking the bonds between them. The hydrophilic end attaches itself to water, so that the surfactant helps carry away fats and dissolve them into the water.

When the coloring is first added to the milk, it is held together in a single drop by the high surface tension provided by the water and the tightly-bonded fats and proteins. The soap lowers the surface tension of the water itself and also breaks up the fats and proteins, so the surface tension of the milk is lowered significantly. The area of the milk not near the soap then has a higher surface tension which pulls on the area of the milk with the soap and coloring, which is why we see the coloring radiating away from the soap.

Real Life Examples

Using soap to clean greasy dishes or to clean your body of its naturally secreted oils. Surfactants in soap help to break off grease molecules and dissolve them in water so they may be washed away.