Why does pressure have a tendency to contract surfaces rather than expand them?

Question

I had a asked a question about six months back what direction forces due to pressure must be pointing and I got a pretty good answer for it (here). Thinking a bit more deeply about the answer, I started getting confused how the pressure always wants to contract a body rather than expand it?

THe following equation is written in the mentioned answer:

$${\bf F}_{net}=-\int_S p{\bf n} dS$$

If I recall correctly, we consider the normal vector as pointing outside from the surface we are integrating, hence for a small area element , we are saying the pressure would push it inwards. But why/ how must pressure push inwards rather than outwards?

More simply put, why does pressure push rather than pull?

Note : I do understand we can simply say that the most direct answer is that "It is simply what we observe in real life" but why exactly is it this which we observe in real life?

Answer 1

It's easy to imagine a situation in which a billion billion billion molecules are bouncing off a surface essentially simultaneously. This happens any time we place a solid in a fluid, for example. Each strike would deliver a "kick" of momentum if the solid surface were allowed to move inward, and in fact this is precisely how pressure is defined: that intensive variable that tends to transfer energy by shrinking a volume. (Put another way, one of the energy term in the fundamental thermodynamic relation is $-P\,dV$, where the pressure $P$ is the conjugate variable to the volume $V$.)

It's harder to imagine a situation in which a billion billion billion molecules adhere to and tug on a surface. In this situation, however, you would see an equitrixial normal tensile stress, i.e., a "negative pressure" or tendency for the object to expand.