The “Cheerios effect”

TLDR

Objects floating at a liquid–gas interface interact via interfacial deformation and gravity, and this effect has applications in nature and laboratory settings. The study emphasizes that buoyancy dominates over capillary suction for small particles, demonstrated in a classroom experiment that derives the conditions for attraction or repulsion. The authors quantify interaction forces and present a simple dynamical model to describe the attraction or repulsion between floating particles. The model’s predictions are validated by experiments showing mutual attraction of two identical spherical particles.

Abstract

Objects that float at the interface between a liquid and a gas interact because of interfacial deformation and the effect of gravity. We highlight the crucial role of buoyancy in this interaction, which, for small particles, prevails over the capillary suction that often is assumed to be the dominant effect. We emphasize this point using a simple classroom demonstration, and then derive the physical conditions leading to mutual attraction or repulsion. We also quantify the force of interaction in particular instances and present a simple dynamical model of this interaction. The results obtained from this model are validated by comparison to experimental results for the mutual attraction of two identical spherical particles. We consider some of the applications of the effect that can be found in nature and the laboratory.