Imagine a large diameter piston filled with water connected to a small funnel. When you press on the piston slowly but with considerable force the water will move very quickly from the funnel in form of a jet. But how is it possible on a molecular level?
Water molecules are constantly moving about in the piston with various speeds and directions bumping into each other and exchanging momentum like billiard balls, however water molecules from the funnel are moving uniformly at great speed.
I want to know how it is possible for slow molecules to be adding momentum to the ones that are already moving faster than the average. In billiard ball analogy slow moving ball moving in the same direction would never catch up with the faster one to further increase its momentum and if it was moving in the opposite direction then it could only receive momentum from the faster one and therefore only slow it down.
Now I imagine that this question probably sounds silly but I can't find any answer after searching for it, so I decided to ask here.
Answer
Adjacent molecules in a liquid all repel each other because of the electron clouds that surround the nuclei that they contain. In that sense these molecules never even 'touch' each other (at least not in the intuitive sense of the word).
When you apply pressure to the liquid you're squeezing them into a (very slightly) smaller volume, thereby increasing the repulsive forces between them. Now allow an outlet (your funnel or the hole in the milk carton of the previous answer) and these increased repulsive forces now propel molecules through the outlet in a macroscopic flow. The higher the pressure, the more the volume is decreased (and thus inter-molecular distances are reduced), the more the repulsive forces are increased and the higher the macroscopic flow rate through the outlet.
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