Detergent in Milk

Food coloring in a milk bath swirls eradically in response to a detergent droplet.

Equipment

  • pie dish to hold milk

Reagents

  • milk
  • food coloring
  • detergent

Presentation

  1. The dish of milk is placed under the document camera.
  2. Drops of foodcoloring are dripped around the surface of the milk.
  3. A drop of detergent is dripped into the milk causing the food coloring to swirl about in the dish.

Details

Milk is a complex solution of water and suspended proteins and fat globules.  In fact, the reason it is white is that it scatters all visible light due to the suspended particles.  In this demo, the food coloring provides an aid to the eye of how the detergent molecules are interacting with the milk particles.  The detergent is amphiphilic, causing it to only partly dissolve into water, and to spread across the surface, pushing the food coloring dyes across the surface.  The hydrophobic tails of the detergent molecules interact favorably with the fat globules in milk allowing the coloring to mix below the surface.  This generates convection emanating outward from the detergent and rising upward from below the detergent drop leading to the swirling effect.  

Safety

Special Safety Notes

  • no safety hazards
milk
The detergent on a Q-tip pushing the colors around.