When drops pulsate and draw flowers…

How does a drop of oil behave when placed on water? Most often it spreads. But its chemical properties can also cause it to change the surface of the water at a later date, resulting in unexpected behaviour. Véronique Pimienta, a faculty staff in the IDeAS team at the Softmat laboratory, and hydrodynamicists from IMFT have discovered and explained the spectacular and particularly aesthetic evolution of a drop of dichloromethane, a very common solvent, obtained by adding a small quantity of the same surfactant to the water and to the drop. The results of their research, which could help to improve the effectiveness of therapeutic targeting techniques, are published on 26 February 2018 in Nature Communications.

Carlo Marangoni established in 1865 that a drop deposited on a surface of water will spread if the surface tension between the water and the air exceeds the sum of the drop/air and drop/water surface tensions. In accordance with this criterion, the drop of dichloromethane begins to spread and an expanding circular film forms around it. As dichloromethane is highly volatile, this film tends to evaporate and a bead forms at its periphery. Deformations gradually appear on this bead, which eventually breaks up, generating a ring of droplets. The story could end after this already unusual event. But in this case, the dichloromethane dispersed on the surface of the water when the droplets detach alters the system’s spreading capacity and the film shrinks.

End of the story? No! As the dichloromethane continues to evaporate and solubilise in the water thanks to the action of the surfactant (cetyltrimethylammonium bromide), the surface regenerates and the spreading conditions are reset: everything is in place to allow the drop to begin successive pulsations. And these pulses are of exceptional amplitude and regularity. However, the dichloromethane continued to evaporate and, in a highly reproducible way, star branches corresponding to excess film thickness emerged around the drop when the film was spread for the fourth time.
Just an incident? Not at all… These deformations locally modify the speed of retraction of the film, whose contours adopt a zig-zag shape around the star branches. Lines of micro-droplets then form along these branches and meet at the tip of each one, where they merge before being ejected and forming rays. A ring of droplets, branches lined with micro-droplets and spokes? A single drop is transformed into a flower! And more will follow in subsequent pulses.

The mysteries of this evolution have been revealed thanks to a close dialogue between experiments and theoretical models. It is by comparing the predictions of these hydrodynamic models with observations that we have been able to outline the genesis of this unusual flower. However, there is still a long way to go to obtain a complete and predictive model of this remarkable self-organisation process, which intimately links hydrodynamics, chemistry and phase changes.

This way of ejecting drops along star branches is reminiscent of microfluidic devices in which micro-droplets, which constitute individual reactors, circulate in micro-channels. So how can we take advantage of this original ejection process? By using the mother drop as a vector for forming particle assemblies. In this way, the introduction of polymeric compounds into the drop can find applications in therapeutic vectorisation, leading to the formation of objects whose geometry enables better interaction with the tissues to be treated. Similarly, judicious doping of the droplet with nanoparticles may open up new prospects for the design of new electronic or optical micro-devices.

The CNRS has devoted one of its episodes of the “Zeste de science” series for the general public to these phenomena.

To find out more, read:

  • the interview of Véronique Pimienta in the Dépêche du Midi journal;
  • the article in the magazine “Pour la Science”.