Osmosis (excluded volume dynamics).
The upstream initial salt concentration is not nil (and arbitrary chosen) and the downstream salt concentration is nil. Salt entities throw the membrane and go to downstream because of the salt concentration gradient. Some water molecules go from downstrem to upstream at the beginning of the simulation run because there is also a gradient of water concentration. But the total quantity of throwed molecules is low because the flux of salt and water are almost equal.
Example 1, case of osmosis with a non selective membrane :