If we have an object submerged in water with a density greater than it then it will sink while objects with less density will float. This is found in many scientific articles and videos.. but why?
Particularly speaking, How do we understand this in a molecular picture? If we think of it using a free body diagram it makes sense but I can't imagine why it is so if we look at it from a microscopic scale.
At a macroscopic level, buoyancy is the result of the increase of pressure with depth. This creates a net force on a submerged object which is equal and opposite to the weight of the volume of liquid that it displaces. Hence if a fully submerged object displaces an amount of liquid with a weight that is greater than itself then it will float; if not then it will sink.
You can also think of buoyancy in terms of energy. If you start with an object is partially submerged and push it down slightly so that an additional volume $\delta V$ is submerged, then the same volume $\delta V$ of liquid is displaced upwards. The object loses potential energy but the displaced liquid gains potential energy. If the potential energy gained by the displaced liquid is greater than the potential energy lost by the object then it must require a force to push the object down (otherwise we would have a perpetual motion machine). This force is the net buoyancy force on the object (buoyancy force minus object's weight).
One interesting consequence of this is that the level at which an object floats is independent of the force of gravity. If you put a rubber duck in a bathtub on the moon (and put the bathtub in a sealed environment at a pressure of one atmosphere), the duck will float with exactly the same proportion out of the water as on earth. The smaller weight of the duck and the smaller weight of displaced water cancel out.
At a microscopic level pressure is the result of collisions between the atoms or molecules of the liquid and the atoms or molecules of the submerged object. Gravity causes an increase of liquid density with depth; in other words there are more atoms per unit volume in the liquid. This results in more collisions per unit time with the submerged object as you go deeper in the liquid, which at a macroscopic level means greater pressure.
The simpler bond that I know is: 2 Hydrogen nuclei + 1 electron and is analised in elementary books of QM as Griffiths. The water molecule itself is a much more complicated case, and the nature of the bond between molecules is a step further.
But I think that one of the conclusion for the simpler model is also valid here: There is a sharp potential function that defines an equilibium distance between the molecules. Any attempt to decrease it, results in a very strong separating force. And there is also an attracting force when trying to separate them.
So, they behave as oscillators around an equilibrium position. At the surface, only the collisions of air molecules tend to compress the water molecules, so the potential energy of the oscillators is small.
For points below the surface, they are compressed also by the weight of the water above. The potential energy grows, and any collision to other molecules or to a submerged body is stronger.
So, the force on a submerged body is greater below it than above it, even for a pratically incompressive fluid as water.
