What is Brownian Motion?
Brownian motion, also known as pedesis, is defined as the random movement of particles within fluids, such as liquids or gases. Since the movement is random, Brownian motion can only be loosely predicted using probabilistic models.
The first observations of Brownian motion were not actually by Robert Brown, the Scottish botanist for whom the phenomenon was named. Instead, it was Titus Lucretius Carus, an ancient Roman philosopher, who first noticed and recorded the random movements occurring with dust particles in the air in his poem titled “On the Nature of Things”. Similarly, in 1827, Robert Brown detected this random motion through studying the movement of pollen in water under a microscope.
Since then, the phenomena has been studied by several noteworthy scientists in history such as Albert Einstein, who, conceptually, explained the reasons of Brownian motion, and Jean Perrin, who not only proved Einstein’s theories with experimentation, but won the Nobel Prize in Physics for his efforts.
What Causes Brownian Motion?
Brownian Motion is caused by collisions between microscopic particles such as atoms and molecules within any fluid and the particles of interest. Robert Brown could not completely explain this phenomenon due to the lack of understanding of atomic theory at the time. In fact, while John Dalton, an English chemist, had already discerned the possible structure of the atom by the time that, it wasn’t until the late 1800s, long after Brown’s studies, that studying atomic theory became mainstream.
Since Brownian motion occurs because of collision, any property that impacts the speed of a particle within a fluid or the density of that fluid will affect the random motion of that particle. Below are just a few of those properties.
Atomic movement is based on energy, which could either be added to or taken out of any system. When energy is removed or emitted by an atom, the atom’s movement slows down. On the other hand, if energy is provided, the atom’s movement increases. These principles follow whether the state of matter is a solid, liquid or gas and, therefore, affects how a particle placed within that liquid or gas behaves.
According to position of the fluid, there could be changes in pressure, which change fluid velocity. For example, we use sea level as a baseline for atmospheric pressure. At sea level, liquid molecules behave in a certain fashion. But the lower you go, the more that gravity and, by extension, the weight of the water above impacts the liquid molecules, making the water denser and forcing the liquid to be more compact.
Conversely, the more you rise into our atmosphere, the less air molecules there are since most are being held closer to sea level by gravity. In the atmosphere, this difference in molecule densities creates a gradient in pressure, which results in the phenomenon of wind.
Placing any substance at any particular elevation below or above sea level can impact the motion of that substance.
A fluid’s viscosity, or thickness, is another instance in which the quality of the motion of fluid molecules changes. Viscosity is also defined as the factor that causes a fluid to resist flow, making its role in a particle’s Brownian Motion very clear. A viscous fluid flows less, slowing the atomic movement and, therefore, causing less collisions with any particles placed within it.
Brownian Motion Examples
Since diffusion is universal among all of the properties that effect pedesis, we can use the central example of an ink droplet in water to explain how these properties impact behavior.
If an ink droplet was placed within different beakers, each containing water with different temperatures, they will diffuse through the water at different speeds. When an ink droplet enters water that is near freezing temperature, the atoms will move more slowly, and, therefore, less collisions will occur. In these instances, the concentrated droplet would diffuse more slowly. The opposite occurs when an ink droplet is placed in water that is near boiling. Because the atoms are far more excited, there are more collisions with the ink, causing that droplet to diffuse far more with more randomness.
Density differences within both air and liquid can create wind and water currents. It would be one thing to study the motion of an ink droplet in a container of water, but what if that ink droplet was added to a flowing body of water? The differences in the Brownian motion become readily apparent in these scenarios, since particles tend to experience far more collisions in a current than in still fluids.
Naturally, adding a droplet of ink into a semi-fluid such as honey would be different than adding it into a container of water. This is because a fluid like honey is similar to a fluid that’s been cooled, albeit for different reasons. In a viscous fluid, there are molecular bonds in place, which prevent the liquid from flowing freely. This creates somewhat of a lattice that makes it harder for the droplet to disperse, and therefore, easier to predict with a probabilistic model.