Diffusion—the process by which small particles disperse within a fluid medium—describes a wide range of phenomena, such as the spreading of pollen and dust in the atmosphere and the mixing of two liquids. A team in Germany has now been able to follow individual atoms diffusing through a thin gaseous medium. They find that just a single collision is enough to bring an atom close to equilibrium with other atoms in the medium. The results could help in modeling diffusion in rarefied environments, such as interstellar space.
Diffusion was first explained at the microscopic level by Albert Einstein, who showed in a 1905 paper that a diffusing particle should follow a random, meandering path, called Brownian motion, owing to collisions with the molecules of the surrounding medium. In early studies, the particles were much larger than the molecules, so billions of collisions were typically needed to change the particle’s path. “This can be compared to the situation of a cargo ship in a ball pit,” says Artur Widera of the University of Kaiserslautern in Germany.
For relatively large particles, one need not track every collision. Instead, the collective effect of impacts can be modeled as a randomly fluctuating force, along with a viscosity that accounts for a particle’s energy loss to the surroundings. Combining these two effects in a modified Newtonian equation, called the Langevin equation, allows researchers to calculate many of the particle’s properties of interest, such as how its average velocity evolves over time.
But what if the individual collisions are more significant—if the diffusing particle is more like another ball than a ship? That, after all, is more like the situation when gases or liquids mix. To track the effect of individual collisions, Widera and colleagues studied the diffusion of just a few cesium (Cs) atoms within a rarefied cloud of several thousand rubidium (Rb) atoms.
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