We have two types of particles, represented here as balls and squares.
In this simulation, the balls and squares have the exact same density, and they interact through perfectly elastic collisions.
The barrier in the center only allows the balls to pass through.
The square particles are unable to cross the barrier.
All the square particles are stuck on the right side of the barrier.
Notice how the particles on the right side rise to a greater height than the particles on the left side.
This behavior can also be observed with real fluids in a laboratory.
For example, the balls might represent water molecules, and the squares might represent dissolved salt ions.
The barrier would be a semi-permeable membrane that only allows water molecules to pass through.
Some people try to attribute the rise in the fluid on the right due to attractive forces between the salt ions and the water molecules.
However, in this simulation there are no attractive forces whatsoever, yet the fluid on the right rises to a greater height anyway.
Also, in a real fluid, the fluid will still rise as shown here, even if you replace the salt ions with any other particles unable to pass through the membrane, regardless of the extent to which water is attracted to these particles.
Therefore, there is clearly something else going on.
Let us consider a situation with zero gravity.
In the beginning we have approximately the same number of total particles on both sides of the barrier.
At the top of the screen, we will keep track of the number of particles on each side of the barrier.
There is a net flow of balls from the left side to the right side.
As before, the balls are able to pass through the barriers but the squares are not.
As before, all the square particles are stuck on the right side of the barrier, but the balls are able to freely pass back and forth.
As we approach an equilibrium condition, the total number of particles on the right will be greater than the total number of particles on the left.
As a result, the pressure on the walls on the right side of the container will be higher than the pressure on the walls of the left side of the container.
This is what caused the rise in fluid in the previous example.
This example and the previous example are what we call "Osmosis."
Osmosis is not simply caused by the diffusion of the particles.
This is because when we think of the behavior of these balls, we need to keep in mind their interactions with the square particles, and we therefore can’t just think of the balls as diffusing evenly throughout the container as if the square particles are not present.
On the other hand, this is an example of diffusion.
Here, there are only balls present.
Therefore, these balls will eventually be spread out equally on both sides of the barrier, if we wait long enough.
Everything shown in this video is just a consequence of the motions and collisions of the particles.
In all the simulations shown in this video, there were never any magical forces sucking the balls from one side of the container to the other.
Similarly, when creating these simulations, we do not need to add an entropy generator to the algorithm.
Entropy increases on its own, and this is responsible for all the phenomena discussed in this video.
This is the beauty of physics.
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