Why do some objects tend to sink after some time in water even if they float at the start?

Question

I have observed this phenomenon in swimming pools: I have seen many dead insects floating on the surface, but after some time some they tend to sink down without any external influence. Why does this exactly happen? It even happens with paper: When it is fully immersed in water, after some time, it overcomes surface tension and buoyant force and sinks down. Do paper/insects gain more density?

Answer 1

In order for an object to float, its density has to be less than or equal to the density of the liquid.

The objects you describe sink after a while because a sufficient fraction of the volume of air originally in each object, due to its porosity, is replaced by water which has a higher density than air. The end result is the overall density of the object becomes greater than the density of water after water replaces air in the object.

Hope this helps.

Answer 2

There are different mechanisms that can possibly be relevant here:

• The object is floating because its less dense than water on account of enclosed or separated air (or other gases).

  • Due to contact with water it slowly fills up with water making it more dense and thus sinking. Typical examples are a sponge or a boat with a small hole.

  • Due to random interactions such as wind, the enclosed air gets reduced. A typical example would be a boat which gets filled by waves.

  • Due to decay processes the enclosure breaks. A typical example would be a boat which rots until it just breaks.

• The object is floating on account of surface tension. Typical examples are water striders. Here, a slow wetting of the object or random interactions (e.g. due to waves and wind) can destroy the effect.

  You can experience this yourself with the classical experiment with the floating paper clip. Here, surface tension is the only thing keeping the clip afloat. Usually, the clip will sink at some point – which is when the surface tension “breaks”. In contrast to the above effects, this happens suddenly rather than gradually.

Often you have a combination of several mechanisms, e.g., an object that requires some enclosed air and surface tension to float.