Before we can begin talking about what diffusion is, we have to first define the term concentration. Concentration is the amount stuff within a specified space. It is similar to density, though density usually deals with much smaller scales like the spaces between atoms. Concentration can be applied to anything, so let’s apply it to bunnies!diffusion-768x646

Let’s say you are looking at a field with 3 bunnies in it, you might say, “This is not a very concentrated field of bunnies.” Now if there were 500 bunnies in that field, you’d probably shout, “This is a very concentrated field of bunnies!” If those fields were only divided by a fence, and you open a gate in the fence, you’re right to assume that the bunnies from the concentrated field will wander into the empty field. You just predicted diffusion.

Diffusion is the natural tendency to move from a high concentration to a lower concentration and it applies to atoms, energy, bunnies, whatever. If you have a balloon filled with water, there is a high concentration of water in a relatively small space. When you pop that balloon, the water will diffuse outward to the areas of lower concentration, which is all around it. If you set a plate of cookies in a room, eventually the whole room will smell of cookies. Some of the exterior atoms of the cookies will diffuse into the air, dissolved in gaseous water. There is a high concentration of cookie smell near the cookies so the molecules spread out to the rest of the room.

Diffusion can occur in liquids as well, if you’ve ever poured one of those concentrated flavor juices into a cup of water, you’ve seen diffusion in action. The color spreads to all parts of the glass, puffing out like clouds. Diffusion lowers the concentration as much as possible until it finally reaches equilibrium.

Equilibrium is the term we use when the concentration is the same throughout a space. In the juice example, all the liquid becomes the same color. In the example with the bunnies, equilibrium means there is a relatively equal number of bunnies in both fields.

When we are looking at energy and gases, things will tend to diffuse in all directions. A scented candle will diffuse its smell in all directions outward. When we are thinking about matter that remains connected to the high concentration, it diffuses away and branches out in the process. The branches and roots of trees are a great example of this.

Your blood vessels and neurons branch outward to reach all parts of your body. Rivers falling down a mountain will branch into different streams. Even lightning will branch outward as it travels to another cloud or the ground. In fact, diffusion is the reason lightning even exists!

Lightning is caused by a buildup of electrons in a cloud due water molecules rubbing together and transferring their electrons. The lightning strike is a quick way to find equilibrium when the charge of one clouds builds up too much. The reason we see lightning is because the atoms in the path of lightning are temporarily superheated into a plasma state. The high energy atoms will will emit photons to lower the atom’s energy back to its gaseous state. The sudden spike of energy causes pockets with no air at all and the diffusion of gas particles collapsing back into this area is what causes thunder. Think of the air escaping from between your palms when you clap your hands.

Speaking of weather, air heated by the sun diffuses to places of cooler air and because of the movement of warm air, the colder air moves to replace it creating cyclical currents. This aspect of diffusion causes the global winds as well as the ocean currents. Ocean and air currents are only slightly different because the oceans are blocked off by land while the air has fewer obstacles in its way.

Diffusion is happening all around you: open up a window and feel the diffusing air flow through your hair as the air moves toward equilibrium. Watch your mirror defog after a shower as your fan lowers the concentration of water in the air, pulling more water off of the mirror to replace it. Your world works in specific and predictable ways; learn the basics and you can learn to master the world around you.