If you go to YouTube and search “milk, food colouring and detergent”, you will find lots of videos of a colourful activity aimed at the elementary school level. Typically little or no science is mentioned. The pattern is created when a detergent-dipped Q-tip touches drops of food colouring in a plate of milk — or just by adding a drop directly as was done above. The result is beautiful movements of swirling colour. Not only is it fun to watch (and do) — this demonstration is an opportunity to delve deeper into the chemistry of surfactants.
The three photos here show milk in a Christmas tree shaped platter. There are drops of red and green food colouring in the first photo (right). The middle photo shows a drop of detergent just as it is added to the milk. The third photo (right) shows the swirling effect from the addition of detergent. No Q-tips were harmed in this demonstration.
Jean Duhamel, University of Waterloo gave the following explanation:
The surfactant is a surface active molecule that coats the milk surface. Although the surfactant is initially concentrated inside the Q-tip, it diffuses from the Q-tip to become more dilute at the surface of the milk. This process is entropy*-driven. When the surfactant moves from the surfactant-rich Q-tip to the surfactant-poor milk surface, the energy of the surfactant molecules becomes increasingly spread out (or dispersed). In other words, the entropy of the combined system (surfactant plus milk) increases. Ultimately, the surfactant occupies the maximum area at the milk surface. It does so by pushing the food coloring away, allowing the surfactant to spread.
From a thermodynamic standpoint, we could write the following transition:
State 1 (concentrated surfactant in the Q-tip, low entropy) → State 2 (surfactant diluted at the milk surface, high entropy)
Since ΔG(1→2) = ΔH(1→2) – TΔS(1→2), the massive increase in entropy going from State 1 to State 2 yields a large positive value for ΔS and thus a negative ΔG value. The process occurs spontaneously.
*Entropy measures the dispersal or “spreading out” of energy. In general, energy spontaneously disperses or spreads out unless it is prevented from doing so.
**According to thermodynamics, a process that occurs at constant temperature and pressure is spontaneous if DG is negative.