Why do cereal flakes that are floating in milk tend to float towards each other once they get a couple of centimetres apart?

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  • Posted 5 years ago
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The primary explanation for why two cereal flakes attract each other in milk is due to surface tension. Surface tension causes the milk to form a small valley around each flake. As the two flakes move closer together, these valleys meet to form a natural depression between the flakes, and they simply slide downhill towards each other.

Photographer: Nyana Stoica | Source: Unsplash

Using two leaves in a pond as our example, it is said that the leaves essentially “take a random walk around the pond” until they get close enough to one another. Each leaf is pressing on the surface of the water; this weight deforms the surface tension of the water around the leaf, and the surface of the water “dips if the leaves are close enough to one another causing the leaves to effectively lean towards one another in this slight indentation.

To illustrate this point, picture stretched rubber, onto which two marbles are placed. Each marble presses the surface of the rubber downwards, causing the marbles to roll towards one another.

It could also be implied that the movement of the flakes might not be as random as the leaves.

Average motion of the particles and surface tension are functions of temperature, temperature of the milk would play a role. Others factors like small air currents in the air, ripples from the spoon, sugar or other coatings on the cereal could all affect the movement of the flakes.

Bumping the table could work as well as hitting your spoon on the bowl. Also, the cereal flakes would automatically retain some inherent motion as they are dropped into the bowl. On a slippery surface (such as milk) it takes great care to ensure that two things start out at rest relative to each other.

At the same time, a similar question was embroiling the newsgroup: why do tea-leaves tend to congregate in the middle (and bottom) of a tea-cup?

The consensus was that when stirred, tea-leaves group in the middle because of the lesser frictional force on the inside edge of the tea-leaves (slower currents) than on the outside edge of the leaves (where the currents are swifter).

The bond between the flakes once they have joined, but that the initial attraction of cereal flakes might be explained by the same tendency to 'meet in the middle' as the tea-leaves. Noting that the pressures of stirring or pouring the milk will tend to drive the liquid either clockwise or counterclockwise.

One could assume that these currents are maintained for quite some time. Since the currents on the outside of the flake (bowl side) are greater, you would have a push towards the middle, I would also suspect that the flake turns slowly about its centre in the same angular direction as the milk in the bowl.

Judging from the two contributions that follow, the United Kingdom seems to

be the epicentre of cereal-attraction research. We received this report from

Robert J Hill, of Krisalis Software, who wanted to test whether, if a cold bowl of

milk was warmed slightly, convection would occur (i.e. heat rising from the edge

of the bowl pushing the flakes together):

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