Cheerios effect

In fluid mechanics, the Cheerios effect is a colloquial name for the phenomenon of floating objects appearing to either attract or repel one another.

[1] The effect is observed in small objects which are supported by the surface of a liquid.

In addition, the same attractive or repulsive effect can be observed between objects and the wall of the container.

Once again there are two possibilities: the interface between the liquid and the container wall is either a concave or a convex meniscus.

Buoyant objects will be attracted in the case of a concave meniscus and repelled for convex.

This deformation of the liquid surface, combined with the net upwards or downwards force experienced by each object, is the cause of the Cheerios effect.

[2] The same principle holds at the side of the container, where the surface of the liquid is deformed by the meniscus effect.

If the container is wetting with respect to the liquid, the meniscus will slope upwards at the wall of the container, and buoyant objects will move towards the wall as a result of travelling upward along the surface.

By contrast, non-buoyant floating objects will move away from the walls of such a container for the same reason.

When such objects come close to each other they rotate in the plane of the water surface until they find an optimum relative orientation then move toward each other.

[3][4][5] Writing in the American Journal of Physics, Dominic Vella and L. Mahadevan of Harvard University discuss the Cheerios effect and suggest that it may be useful in the study of the self-assembly of small structures.

is the Bond number, and is a nondimensional factor in terms of the contact angle