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Exploring the past of diffusion: How did early scientists understand this phenomenon?
Diffusion is a fundamental natural phenomenon that refers to the spontaneous movement of matter (such as atoms, ions, molecules, and energy), usually from an area of high concentration to an area of low concentration. This concept not only plays an important role in physics, but also extends to many fields such as chemistry, biology, sociology, economics and data science. While the process of diffusion cuts across multiple disciplines, early scientists’ exploration of the phenomenon undoubtedly laid the foundation for our modern understanding.
The word diffusion comes from the Latin "diffundere", meaning "to spread, to scatter", reflecting its essentially random and unpredictable nature.
The diffusion of gases was first systematically studied by the British chemist Thomas Graham in the early 19th century. His observations showed that when gases of different properties come into contact, they do not separate into layers based on density, but instead diffuse into each other and remain closely mixed. This discovery not only challenged the understanding of gas behavior at the time, but also laid the foundation for the later diffusion theory.
"Gases of different properties, when in contact, do not arrange themselves according to their density, with the heavy at the bottom and the light at the top, but diffuse spontaneously into each other, maintaining an even mixture."
Next, Adolf Fick proposed Fick's law of diffusion in 1855, which remains an important cornerstone of diffusion research today. Fick believed that the diffusion flux is inversely proportional to the concentration gradient, that is, diffusion is the natural result of the rapid movement of matter from an area of high concentration to an area of low concentration. This concept can not only describe the diffusion behavior between gases, but can also be applied to the diffusion of liquids and solids.
Another important discovery in the 19th century was Brownian motion, the random movement of tiny particles in a liquid. This phenomenon was described by British scientist Robert Brown in 1827, and its microscopic mechanism was subsequently studied in depth by Albert Einstein and other scientists, leading to the development of modern diffusion theory.
Brownian motion revealed how matter diffuses through random behavior, which had a profound impact on physics research at the time.
Diffusion was not limited to gases in the past, but also extended to solid materials. In the late 19th century, William Chandler Roberts-Allston conducted systematic studies on diffusion in metals, in particular the diffusion of gold in lead. This research advances the theory of diffusion in solids and shows that atomic defects, such as vacancies and inserted atoms, are crucial for diffusion processes in crystals.
In the context of chemistry and materials science, diffusion is not only the movement of fluid molecules through a porous solid, but also involves different types of diffusion mechanisms. Molecular diffusion occurs when the probability of molecular collisions is greater than that of collisions with pore walls. Knudsen diffusion occurs when the pore diameter is comparable to or smaller than the mean free path of the diffusing molecules. In this case, the diffusion rate of the molecules decreases significantly, which allows scientists to distinguish different types of diffusion in the process.
Diffusion models and their laws are still widely used in many fields including medicine, engineering, and environmental science. The background and observations of early scientists not only helped us construct the basic theory of diffusion, but also promoted our subsequent understanding and application of this phenomenon.
Diffusion is a stochastic process and its complexity remains a challenge in current research. On the other hand, the richness of this concept provides a broad application space for researchers in various fields. We can't help but ask: Will future technology give us a deeper understanding of diffusion and the randomness behind it?
