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The Cheerio Effect

Bubbles trapped at the interface between a liquid and a gas rarely rest. Over a timescale of several seconds to minutes, long-lived bubbles move towards one another and, when contained, tend to drift towards the exterior walls. The sceptical reader may readily verify these claims by pouring themselves a glass of bubbly in the family room, beer in the bar, or soap bubbles in the kitchen or bathroom and following the motion of those bubbles at the surface - particularly those at the periphery of the glass. This phenomenon has even been affectionately dubbed the `Cheerios effect' after the observation that breakfast cereals floating in milk often clump together or stick to the walls of the breakfast bowl. Despite being a subject with enormous potential for simple, reliable party tricks, the technological implications of the Cheerios effect are far from frivolous. This is an effect due to the surface tension between the milk and the air: the air-milk interface does not like to be deformed (just like an elastic sheet resists deformation) but at the same time, gravity is pulling on the individual Cheerios. At some deformation of the interface, the two effects cancel so that we have a deformed interface and the single object is in equilibrium. This balance can be illustrated quite dramatically by floating a single drawing pin on water, as shown below.

Figure 1: A single drawing pin floating on water illustrating the balance between surface tension and the weight of the floating object. This picture was described as the `phenomenal anti-intuitively floating-upside-down thumbtack' by the blog of the Annals of Improbable Research.

Related Article

  • The ‘Cheerios Effect’,
    D. Vella and L. Mahadevan, American Journal of Physics, 73, 817-825, 2005. Click here for PDF version of article.

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   Last Updated: January 6, 2013

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