Reason for fluid pressure

Question

What is the physical reason that high pressure regions (say, of air or water), want to expand into low pressure regions? What force is causing them to "equilibrate" the pressure?

The only thing I can think of is that in liquids (or gasses) all molecules are repelling each other, meaning that in the absence of other forces (like gravity) they would disperse as far apart from each other as possible. But it doesn't seem like that could be true of all gasses and liquids (that the molecules repel each other). For example, water is polar and I'd think the positive/negative sides of the molecules would attract. Not to mention Van der Walls force. Is there some repulsive force that overcomes all other attractive forces, or is there something else entirely that I'm missing?

I'm interested in a mechanical/physical reason that this should happen.

Answer 1

Interactions between the molecules of the gas are not required. In fact ideal gases are modeled as if the molecules have zero interaction.

They do however move and interact with the container. That is sufficient to explain the behavior.

Imagine that you have a vessel with two identical halves that are connected by a small portal that can be opened and closed. Put in gas so that one half is pressurized more. Assuming each half has the same volume, and the gas is all at the same temperature, then the only way for it to have more pressure is to have more molecules inside.

Now, open the portal. Even though none of the molecules of this ideal gas push each other, there will be more that reach the portal from the higher pressure side than do from the lower pressure side. Over time, this leads to a bulk flow in the same direction.

In thinking about this more, this explanation says that the mere fact that there are more molecules in one container causes bulk flow into the "low pressure" continainer. But: why is there flow at all?

The idea isn't "there are more molecules" and you're done. The idea is:

• There are more molecules (but all the molecules are the same)
• they have the same temperature (which means the average speed is the same)
• leading to the idea that there are more collisions per area on the high-pressure side

If one side has a thousand molecules and one has a million molecules, the one with more will have a greater chance of striking a specific-sized target over a period of time.

Now instead of a random target, let's pick the valve barrier (still closed at the moment). We count the molecular collisions against that barrier for one second. We'll find that the high-pressure side has more collisions than the low pressure side.

So what happens if we remove the barrier (open the valve)? It means that on average, more molecules will reach the barrier from the high pressure side than reach it from the low pressure side in the same period of time. With no barrier in place, most of these molecules will go through the valve to reach the other side. That difference in rate of molecules reaching the valve is what leads to the bulk flow.

Answer 2

According to the second law of thermodynamics,entropy of an isolated system tends to increase. Considering the high pressure region and low pressure region as an isolated system, its total entropy goes up, making fluid flow from high pressure to low pressure to increase the disorder(entropy of the system).This behavior follows from statistical models of fluids and their random motions.