How does hydraulic equilibrium work on the particulate level?
The membrane has plenty of holes that are a little bit bigger than small molecules like water. Bigger molecules or bigger ions like usual solutes cannot go through these holes. So water has the tendency to cross the membrane both ways. When a water molecule hits a hole in the membrane from the pure water side, it can cross it. Nothing prevents it. When a solute molecule hits a hole of the same membrane, it blocks the passage for a couple of microseconds. No water can go through this hole from the solution side of the membrane. So on average, more water can cross the holes of the membrane in the direction water to solution, then backwards.
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