Explaining Diffusion

Diffusion is a random (chaotic) process that leads to the uniform distribution of free-moving particles. The term is most commonly encountered in chemistry and biology, but it plays an important role in everyone’s daily lives. In general, diffusion can be thought of as the process of “spreading out”.

Before considering the science, think about a big gymnasium floor – smooth, polished, and vast. Then think about a bucket of marbles – cat’s eyes for the sake of argument. Visualize what happens if that bucket is dumped out onto the floor. Marbles scatter every which way, colliding with one another and scattering all across the floor. If they were all collected, individual marbles might end up in different places, but the overall result is the same – marbles scattered everywhere. No matter how many times the experiment is repeated, they never bounce around and gather themselves up into a pile. They always spread out – or diffuse. Like most analogies, this isn’t a perfect one. The floor still has friction, and the marbles come to a stop before they can be distributed uniformly. It is, however, enough to start the ball (or marble) rolling, so to speak.

Diffusion is usually discussed as it happens in fluids – liquids or gasses. An easy visual example is to take a fish tank full of water, and to let fall one drop of food coloring into it. (Assuming the dye is non-toxic, the fish shouldn’t mind, right?) The food coloring immediately begins to snake out and spread through the water, creating fascinating patterns for a time, until it spreads so thinly that it is distributed throughout the water evenly, and the whole tank has a slightly colored hue.

It is obvious that diffusion occured. The question is, “How did it occur?” In any fluid, be it water, air, blood, or the cytoplasm in your cells, the molecules are in constant chaotic motion. They move basically in a straight line until they encounter other molecules or forces that cause them to bounce off in a new direction. With billions of molecules, the motions of a individual molecules aren’t predictable. The overall motion, however, is a lot easier to predict. Think about that drop of color when it first falls into the tank. Two things are happening at that point: molecules of dye are colliding with one another, and molecules of water are banging into the dye molecules around the edges. In short, the dye molecules are getting knocked about in all directions, and as a result, they start to spread out. It might seem that since they’re getting hit from all sides, they would tend to stay put, but this isn’t the case. The collisions are of varying intensities, so the dye ends up moving in different directions. Just like the marbles, the dye molecules will never be able to re-converge into a single droplet as a result of this process. Since there are more dye molecules around the drop to begin with, they are knocked away from the drop far more often than dye molecules from outside, where they are more scarce, are knocked back towards the center of the drop. In a fluid, heat keeps the molecules in motion, so unlike with the marbles, the system never comes to a stop, and the dye is able to spread out to complete uniformity. Even then, the molecules keep bouncing around, but with each following a random path, the average effect is to maintain uniformity.

Anything that dissolves in water diffuses in the same manner, whether you see it or not. Stirring and heating speed the process, as they make the molecules move more rapidly and distribute faster, but given enough time, diffusion still occurs in cold, still water. The same happens in other liquids as well. Your blood, for example, carries gasses through your body – gasses that enter and exit by diffusion at your lungs. The process is the same in gasses. Look up at a factory smokestack one day, and watch the motion of the smoke plume. It billows and drifts, spreading out until it seems to thin and disappear. It hasn’t disappeared of course. It has simply diffused to the point that the individual particles blend in with the rest of the air. That’s one reason air pollution is easy to ignore, unless you live somewhere where the pollutants get trapped, like a valley. Then it all collects in the valley, as in the fishbowl – spreading out to a uniform haze, but not escaping into the atmosphere at large as rapidly as it is churned out by cars, factories, and other sources.

Diffusion can also occur across a membrane, if that membrane has large enough holes to allow the particles or molecules to pass. This is the diffusion that biologists tend to be concerned with – the crossing of molecules across a cell’s membrane. The membrane serves as a barrier to some things, but anything that is small enough to cross it will do so, treating the fluids on either side as a single fluid space. That means that if you were to put some food coloring outside a cell, and the dye molecules were smaller than the holes in it’s membrane, the color would also spread into the cell as well as throughout the surrounding liquid. If you were then to place that colored cell into a fluid that was colorless, the dye would continue to diffuse, this time spreading from the cell to the fluid, until uniformity was reached.

As fun as dying cells may be, cells use diffusion as a transport mechanism for things like oxygen, carbon dioxide, and water. As blood passes by a cell, and the blood has a lot of oxygen in it, but the cell does not, some oxygen spreads to the cell. If the cell has a carbon dioxide buildup, it passes that excess on to the blood, again by diffusion. Water passes freely through cell membranes as well, keeping them as much in equilibrium with the surrounding environment as possible. For cells, the great thing about making use of diffusion is that it is a natural process, and the cell doesn’t have to use any energy to make it happen.

For those who missed out on technical terms in the rest of this article, diffusion is closely related to Brownian motion and entropy. For beginners who want to learn more, those are your suggested reading topics. For those of you who have had enough, just remember – anytime something disperses on its own – that’s diffusion.