What is the inverted Cheerios effect, and why does it matter?

The phenomenon of a famous breakfast cereal clumping together when floating in a milk bowl is known as the ‘Cheerios effect’, and in recent years it has inspired a new set of manufacturing technologies for advanced materials, and helped physicists understand the gravitational collapse of galaxies.

Now, a group of scientists has described the interaction of liquid drops on soft solid surfaces as an ‘inverted Cheerios effect’, which can be tweaked so that the droplets move towards or away from each other.

The inverted Cheerios effect, identified in a paper published in Proceedings of the National Academy of Sciences, describes how the roles of liquid and solid are interchanged when liquid droplets rest on a solid — but soft — surface.

And just as the classical Cheerios effect spawned new technologies, the newly discovered inverted Cheerios effect may open up new opportunities in engineering and the life sciences.

“Tuning the movement of liquid droplets could have implications for the performance of engineering technologies which rely on drops of water and other liquids,” said co-author Dr Lorenzo Botto from Queen Mary University of London's School of Engineering and Materials Science.

“For example, the physical phenomena we have highlighted in this paper suggest ways to design surfaces that prevent fogging or control heat transfer; for instance, to create car windows that are always transparent despite high humidity or surfaces that improve heat management in conditioners or boilers. By making surfaces softer or harder, and changing the thickness of the soft layer, we will be able to control how the drops coalesce and spread on the substrate.”

The scientists suggest the interactions of the liquid particles can be tuned to repel each other or move towards each other by changing the thickness and softness of the substrate.

Dr Botto said: “While the science is quite young, there are exciting implications of our work not just limited to engineering. For example, quantifying the forces at play when drops sit on a soft layer will also help us understand how cells interact with each other and with the soft tissues on which they live.”