Cheerios effect

Settled in 2005 by Dominic Vella, L. Mahadevan (then at Cambridge/Harvard).

The clumping of floating cereal, bubbles, or pollen at the edges of a liquid surface is an everyday observation with no single 'discoverer.' In 2005, Dominic Vella and L. Mahadevan gave the first pedagogically clear, quantitative treatment in the American Journal of Physics, deriving the interfacial-deformation forces (a competition between buoyancy and capillarity) responsible for the mutual attraction or repulsion of floating objects and giving the effect its enduring nickname. The paper turned a breakfast-table curiosity into a standard classroom demonstration of capillary interactions relevant to self-assembly of small particles at fluid interfaces.

The same physics that clumps cereal at the bowl's edge governs the self-assembly of micro- and nano-particles at interfaces.

Explanations that circulated

• Sometimes attributed purely to surface tension 'pulling' objects together; Vella & Mahadevan showed buoyancy (the weight of each object deforming the interface) is the dominant force for typical floating objects, with surface tension playing a secondary role, and that the interaction can be either attractive or repulsive depending on relative densities and meniscus curvature.

References

• The 'Cheerios effect', D. Vella, L. Mahadevan, American Journal of Physics, 2005