Have you ever observed the foam in the sink and in the coffee cup?
have you ever observed the foam in the sink and coffee cup? No matter what the initial state is, after standing still for a period of time, bubbles tend to become two states: either together or on the side. Why?
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in fact, many small objects floating on the water tend to gather together, a phenomenon often referred to as the "cheerios effect", because the same is true of cereal circles soaked in breakfast milk.
generally speaking, this phenomenon stems from the curved liquid surface and the force acting on small floating objects in the vertical direction, and specifically, it will be discussed in two cases.
there are two situations in which small objects float on the water, the first is like a small bubble, immersed in the water and lifted by buoyancy, and the second is like paper clips and coins placed carefully flat on the surface of the water. they are not floated by buoyancy, but are supported by surface tension. In both cases, the liquid level near the small object will bend, as shown in the following picture, the first liquid level rises near the object, and the second decreases.
Let's first look at small bubbles. Due to buoyancy, small bubbles always tend to go to the highest liquid level, and it is this property that makes small bubbles seem to attract each other-because the liquid level near other small bubbles is always a little higher than the baseline.
and the same reason can explain why the foam is glued to the edge of the coffee cup. Many of the cups are made of glass or ceramic, and the surfaces of these materials have a good affinity with water, so the liquid level is a little higher than in other places (see image below). As a result, small bubbles that want to float as much as possible will also gather here.
while small objects such as paper clips and coins that float on surface tension, the situation is the opposite. These small objects hold down the local liquid level, and under the action of gravity, they tend to slide to the lowest liquid level. So when two coins on the water meet, they also get close to each other: because the liquid level near the object is lower, gathering can help them stay at the lowest level.
thus it can be seen that small floating objects tend to gather with each other in both cases, but for different reasons. And if buoyant objects such as small bubbles encounter objects floating by surface tension, such as coins, they will not gather together, but will move away from each other (see chart below).
as to whether the floating small object will stick to the edge, it also depends on whether the bending direction of the liquid surface close to the container coincides with that of the small object. For example, small bubbles in a hydrophilic glass container will be glued to the edge, while coins will not, but if the container is treated with a super-hydrophobic material, the whole situation will be reversed. Although
is a very common phenomenon in life, and the principle is not complicated, this "cereal circle phenomenon" has not been discussed in depth for a long time. The explanations in this article are all from a paper from a few years ago, in which there are very detailed discussions and calculations. If you are interested, you can learn more here: https://arxiv.org/pdf/cond-mat/0411688.pdf