A Does gravity affect Brownian Motion?

AI Thread Summary
Passive diffusion rates and Brownian motion behave differently on the International Space Station (ISS) compared to Earth due to the absence of significant convection in microgravity. In microgravity, hot air does not rise, affecting how particles like pollen move. The random walk of particles may indeed have a slowed velocity in space, raising questions about biological adaptations to these conditions. Chandrasekhar's 1943 paper discusses the effects of gravity on particle motion, providing insights into how gravity influences Brownian motion. Understanding these dynamics is crucial for comprehending biological processes in microgravity environments.
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Brownian motion of free falling ISS vs Brownian Motion on Earth
I know passive diffusion rates behave differently on the International Space Station relative to Earth (video of a contained flame experiment burning up there.) However, does the random walk of pollen particles etc. have slowed velocity in comparison to that on Earth? Has been bugging me for a while, as I was wondering how our biology deals with slower Brownian mtion if so...
 
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Chandrasekhar wrote an excellent paper on Stochastic Problems in Physics and Astronomy in 1943. You can find his paper in N. Wax's Book in Dover, "Selected Papers..." To make a long story short, in an early chapter, Chandrasekar demonstrates the exponential atmpsphere is the steady state solution for a falling particle with viscous damping rebounding from a fixed surface (the ground). The nice feature of the paper is that it also demonstrates the transient (i.e. time dependent) solution showing how the equilibrium solution is approached. Wax's book has many good papers along these lines.
 
dedocta said:
Summary:: Brownian motion of free falling ISS vs Brownian Motion on Earth

video of a contained flame experiment burning up there.
That has less to do with diffusion differences and more to do with the fact that convection operates differently in microgravity. Basically, hot air rises in a gravitational field because it is less dense than cold air. In microgravity, this buoyant force doesn’t exist or is much smaller, so that the hot carbon dioxide generated by a flame does not rise away from the flame to make way for fresh air to sustain the reaction.
dedocta said:
Summary:: Brownian motion of free falling ISS vs Brownian Motion on Earth

However, does the random walk of pollen particles etc. have slowed velocity in comparison to that on Earth? Has been bugging me for a while, as I was wondering how our biology deals with slower Brownian mtion if so...
You can model this straightforwardly by adding a gravity term to the Langevin equation
$$\dot{\mathbf{v}}=-\gamma\mathbf{v}+\sigma\mathbf{\xi}(t)-\mathbf{g}$$
Gravity will pull denser particles in the direction of the gravitational source, but if the noise term is large (at higher T, for instance), or the drag term is large (at higher density, for instance), then gravity will become irrelevant.
 
update on my earlier post. N.B. the page 57 of the Chadrasekhar paper concerning gravity effects on Brownian motio
 
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