Back in 2011 chemists were baffled when they dropped a single droplet of dichloromethane (DCM) into a beaker of water. DCM is a solvent and commonly used as a paint stripper or degreaser. However, it is not miscible (proportionally mixed) with water and as it has a higher density than water you would expect it to sink in water. To the surprise of a team of chemists, led by Professor Olivier Steinbock, this is not what happened and instead – when the droplet of DCM touched the surface of a soapy water-antiseptic solution – it began to spin. The video shows the spinning droplet, looking very similar to a spiral galaxy.
The droplet of DCM separates with part of it spreading out creating a film that acts as a boat to support the rest of the droplet. This gradient in surface tension results in mass transfer, known as the Marangoni effect, and thus creates a force on the system that can lead to motion – spin. This spin is not propelled by an external source, as Prof. Steinbock states “These droplets are self-propelled”. This differential gradient continues to change as the system fragments and evolves until at a critical point it changes from a state of symmetry to one of chaos. A European team of scientists recently published a paper in Physical Review describing and exploring this phenomenon further.
Understanding the details of the evolutionary process in such a simple system could be extremely interesting and important to the study of all systems across all scales of physics.