Make Color Droplets Chase One Another, the Reaction Between High and Low Concentrations

According to biology, microorganisms have developed something called chemotaxisin in order to survive, which is a kind of taxis. It refers to the movement of cells, bacteria and other unicellular or multicellular organisms in response to a chemical stimulus. This time, we are going to use the interaction between molecules to emulate chemical chemotaxis, and the tools we need are very simple.

Things required:

  • A piece of glass, about 10 x 10 cm, thinner is preferred but not necessary
  • A bottle of propylene glycol
  • Water
  • A few plastic cups
  • A few pipettes
  • Food coloring
  • A gas torch



  1. First, make propylene glycol solutions of 1%, 20% and 40% concentration and dye them with food coloring.
  2. Next, evenly heat the glass with gas torch for 15 to 20 seconds, keep it dry and clean.
  3. Next, use pipettes to absorb the propylene glycol solutions of different levels of concentration, then release them on the glass in small droplets


Something amazing is happening. These droplets are chasing one another.

What’s happening is, due to the different levels of concentration, these droplets will have different rates of evaporation, thus causing convection. The difference in surface tension also causes molecules to lose the balance among themselves, which is why higher concentrated liquid is chasing lower concentrated liquid.