The Marangoni effect is a jungle full of amazing surface tension animals
Every day we encounter wonderful phenomena driven by differences in surface tension in liquids, but we ignore them. This is unfortunate because their beauty, importance and complexity are something that we should teach kids to notice and enjoy.
The drama of soap touching water
In our previous episode we introduced the concept of surface tension force. It is a cumulative effect of electrostatic forces between fluid molecules. Kids are typically introduced to the topic of surface tension through two phenomena: soap bubbles (that we will discuss in future episodes) and pepper and soap experiment that we will discuss here.
The recipe for this experiment is simple: sprinkle pepper onto the surface of water and then touch the surface with a drop of liquid soap. The pepper will quickly move away from the touching point.
A typical explanation given to kids is that soap breaks down the surface tension of water. But if you think for a moment, this explanation does not make much sense. First, soap also has surface tension, just not as strong as the one in water. It would be more precise to say that the strength of surface tension has changed. Second, how is that this change in the surface tension produces such a strong fluid flow far away from the soap?
When a drop of detergent is added to the water surface, two things happen. First, the soap molecules orient themselves such that they show their “water loving” side (i.e., the side with electrostatic attraction to water) toward water, while also experiencing attraction forces between themselves. Second, the water tries to restore its surface by pulling water molecules to the sides and pushing the soap up (recall that when we will talk about insects walking on water). However, since the soap molecules are attracted to water, while they also refuse to be sucked into the water interior, the soap is dragged together with the water flow.
This makes the soap molecules deposited over an increasingly larger surface area, forming a thin film over the water surface, while water keeps moving to restore its equilibrium surface. If something does not physically stop further movement (i.e., walls of the container or repulsion of compressed pepper dust), this process will continue until the soap molecules form a monolayer over the water surface (a single layer of molecules can spread over an amazingly large surface).
The magnificent beasts of Marangoni effect
Even before the molecular understanding of surface tension forces, scientists were able to measure surface tension forces and build mathematical descriptions of their behaviour. They noticed how a fluid flow is generated when the surface tension force is perturbed. The flow will be in the direction away from the area of lower tension force. The first detailed descriptions of such flows started in the mid-19th century. Today, this type of fluid flow is called the Marangoni effect after an Italian physicist who analysed this phenomenon in 1865.
An extreme case of soap taking over control of the water surface are soap bubbles. A mesmerizing dance of rainbow colours on the bubble surface is actually Marangoni flows. Changes in these colours enabled the first direct measurement of molecular size in 1913.
We encounter the Marangoni effect every day almost everywhere where fluids are present. It participates in many phenomena and exploring them can be fun for everyone. But first we need to notice those phenomena. It’s like a jungle of Marangoni effects out there, where kids are shown just one animal – pepper and soap on water. Now imagine discovering a whole zoo of weird and beautiful phenomena driven by the Marangoni effect: drying of whiskey or coffee or salty water drops, tears of wine, Marangoni blossom, swimming droplets, Marangoni propulsion, dancing colours of soap bubbles, Marangoni bursting, etc. These are all Marangoni jungle animals caused by interactions between fluids, or differences in temperature or concentrations. We will explore them in future episodes.